For Official Use ENV/JM/TG/RD(2006)1/REV

Organisation de Coopération et de Développement Economiques 

Organisation for Economic Co-operation and Development 05-Apr-2006 

___________________________________________________________________________________________

_____________ English - Or. English 

ENVIRONMENT DIRECTORATE 

JOINT MEETING OF THE CHEMICALS COMMITTEE AND 

THE WORKING PARTY ON CHEMICALS, PESTICIDES AND BIOTECHNOLOGY 

 

Test Guidelines Programme 

PROJECTS FOR DEVELOPING TEST GUIDELINES ON PHOTOTRANSFORMATION IN WATER 

AND ON SOIL SURFACES 

18th Meeting of the Working Group of National Coordinators of the Test Guidelines Programme 

16-18 May 2006, Bern, Switzerland 

Ms. Laurence MUSSET 

Tel: +33 (0)1 45 24 16 76; Fax: +33 (0)1 45 24 16 75; Email: laurence.musset@oecd.org 

JT03207035 

Document complet disponible sur OLIS dans son format d'origine 

Complete document available on OLIS in its original format 

For Official Use ENV/JM/TG/RD(2006)1/REV English - Or. English
ENV/JM/TG/RD(2006)1/REV 

2

This room document is provided for information, to facilitate the discussion of the projects 

related to the phototransformation of chemicals in water and on the soil. It includes six annexes: 

Annex 1 is Part 1: Direct Photolysis of the second revised draft of the Test Guideline Phototransformation 

of Chemicals in Water (November 2004); 

Annex 2 is Part 2: Indirect Photolysis of the second revised draft Test Guideline Phototransformation of 

Chemicals in Water (November 2004); 

Annex 3 is the compilation of comments received before revising the draft Test Guideline 

Phototransformation of Chemicals in Water (November 2004); 

Annex 4 is the second draft of the Test Guideline Phototransformation of Chemicals on Soil Surface 

(February 2004) 

Annex 5 is the compilation of comments received before the preparation of the second draft Test Guideline 

Phototransformation of Chemicals on Soil Surface (February 2004) 

Annex 6 is a third draft of the Test Guideline Phototransformation of Chemicals on Soil Surface prepared 

by BIAC (July 2005) 
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DRAFT DOCUMENT 

November 2004 August 2000

ANNEX 1 

OECD GUIDELINE FOR TESTING OF CHEMICALS

PROPOSAL FOR A NEW GUIDELINE

Phototransformation of Chemicals in Water -– Part 1: Direct and Indirect Photolysis

INTRODUCTION

1. This guideline provides guidance for conducting phototransformation in water studies to determine 

the potential effects of solar irradiation on chemical pollutants in surface water. Such studies determine 

phototransformation kinetics, products, and product pathways resulting from both either direct and (as an 

option or if applicable) or indirect aqueous photolysis. It is based on current and /or proposed methods 

(references 1 through 10) and on relevant literature on environmental aqueous photochemistry. Part 1 of 

this guideline deals with direct photolysis.

2. A glossary of definitions (11) and symbols/units are provided in Annexes 1 and 2, respectively. 

ENVIRONMENTAL SIGNIFICANCE AND USE

3. Chemical pollutants and/or their transformation products that are potentially susceptible to direct 

and/or indirect photolysis include those that are transported to surface water by direct point discharge 

and/or by runoff from urban and/or rural areas. Chemical pollutants in surface water also often originate 

from transformation of other chemicals via hydrolysis, photolysis, and biotransformation. 

4. Direct photolysis in natural water involves the transformation of a chemical resulting from the 

direct absorption of a solar photon (references 12 through 18). Indirect photolysis in natural water 

sometimes involves the transformation of a chemical due to energy transfer from naturally occurring 

photosensitizers in electronically excited triplet states (12)(13)(19)(20). However, indirect photolysis more 

often involves the transformation of a chemical due to reactions with transient oxidants such as hydroxyl 

radicals, molecular oxygen in a singlet electronic state (“singlet oxygen”), and peroxy radicals (references 

12 and 22 through 32). Both triplet state photosensitizers and transient oxidants result from the absorption 

of solar photons by chromophoric dissolved organic matter (CDOM) and nitrate ion (12)(21)(32)(33)(34). 

5. Direct photolysis can be an important dissipation pathway for some chemical pollutants that 

exhibit significant light absorption above the 2905 nm cutoff of solar irradiation at the earth’s surface. 

Indirect photolysis can also be an important dissipation pathway for some chemical pollutants that come in 

contact with photosensitizers in electronically excited triplet states or with short-lived photochemically 

generated oxidants such as hydroxyl radicals and singlet oxygen. In some cases both direct and indirect 

photolysis can contribute significantly to the dissipation of a chemical in natural waters. 

6. The direct and indirect phototransformation of chemicals in natural water bodies is a complex 

process which depends on a number of factors such as: 
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 (a) the chemical structure and electronic absorption spectrum of the chemical; 

 (b) the concentration, composition, and absorption spectra of CDOM (from which 

 photosensitizers and singlet oxygen arise); 

 (c) the concentration of nitrate (the primary source of hydroxyl radicals); and 

 (d) the solar photon flux spectrum to which the chemical, CDOM and nitrate are exposed. 

7. The solar photon flux spectrum to which a chemical, CDOM, and nitrate are exposed depend on 

numerous factors including the latitude, season, and the physical and chemical properties of the water body 

(12)(18)(41 35)(42 36). Important properties of the water body which contribute to the attenuation of solar 

irradiation with depth include the concentration and composition of CDOM and suspended particulates. 

The mixing and stratification properties of the water will help determine the horizontal and vertical 

distribution of the chemical, CDOM, and nitrate. 

8. The results of phototransformation in water studies are used in conjunction with physical chemical 

properties and data from other studies (abiotic hydrolysis; biotransformation; adsorption/desorption) to 

help assess the overall environmental transformation and transport of chemical pollutants. The results of 

phototransformation in water studies are also used to help develop inputs for environmental fate computer 

models, and to help develop protocols for conducting other aquatic laboratory and/or aquatic field studies. 

PRINCIPLE OF THE STUDY

Scope and approach

9. This part of the test guideline is designed to provide some or all of the following data and/or 

information depending upon specific needs: 

• The quantum yield and resulting estimated direct photolysis rate constants for test chemicaltest 

substances for various types of water bodies (defined by depth and light attenuation), seasons, and 

latitudes of interest. 

• Direct photolysis rate constants for test chemicaltest substances determined in the laboratory using 

filtered xenon arc lamp or sunlight irradiation and extrapolated to natural water. A xenon arc lamp 

filtered to remove irradiation < 2905 nm will be referred to throughout the rest of the document as a 

filtered xenon arc lamp. 

•The net (combined direct and indirect) and indirect photolysis rate constants of one or more test chemicals 

in a standard simulated natural water (containing humics as a surrogate for CDOM and in some cases, 

nitrate) and/or in one or more natural waters. 

• The transformation pathway and the identities, concentrations, and rates of formation and decline of 

major phototransformation products resulting from direct photolysis. and/or indirect photolysis. 

10. This guideline is designed as a tiered approach. Each tier is triggered by the results of the previous 

tier. The tTiered approaches are proposed separately for direct and indirect photolysis tests ais shown in 

Figures 1 and 2, respectively (see Annex 3). 
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Pseudo first-order kinetics of direct and indirect photolysis

11. The rate of decline of a test chemicaltest substance in either a direct or indirect photolysis study 

is generally assumed to follow pseudo first-order kinetics (e.g., time series data are fit to a pseudo firstorder kinetics model): 

d C

dt k C [ ] = − [ ]

 [eq. 1] 

 The integrated form of equation 1 is: 

 

[ ] [ ]exp( ) C C kt = − 0

 [eq. 2] 

 The ln transformed version of equation 2 is: 

 

ln[ ] ln[ ] C C kt = − 0 [eq. 3] 

 where 

c C = test chemicaltest substance concentration at time t (mol/L) 

c 0C0 = initial test chemicaltest substance concentration at time t = 0 

 k = pseudo first-order rate constant (1/unit of time) 

 t = time 

t1/2 = half-life

The pseudo first-order rate constant is determined by using non-linear regression to fit data to 

equation 2 or using linear regression to fit data to equation 3. 

The half-life can be determined by substituting the pseudo first-order rate constant into the 

following equation: 

t

k d i

1 2

2

/

( )

ln = or [eq. 4] 

 

where where

kd is the direct photolysis rate constant and ki is the indirect photolysis rate constant

t1/2 = half-life.

Other kinetic models may be applied if they better fit the kinetic data.

Note: In the equations provided in this guideline, units for half-lives, pseudo first- order rate constants 

and photon fluxes are (for illustrative purposes) given in days, 1/day, and photons/cm2

•day, 

respectively. However, other units of time such as hours or seconds may be used as long as there 

is consistency within the same equation. Units of concentration are given in mol/L. 

Direct photolysis 

12. To undergo transformation via direct photolysis, a chemical molecule must first absorb a photon.

of light. The absorption of a photon by a molecule results in a transition from an electronic ground state to 
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an electronically excited state of the molecule. To be absorbed by the molecule, the energy of the photon 

(which is inversely proportional to its wavelength) must correspond to the difference between the ground 

and a possible excited electronic state of the molecule (12). Within the UV-Visible wavelength of interest 

(2905-800 nm), photons energies range from 150 kJ/mol at 800 nm to slightly greater than 400 kJ/mol at 

2905 nm (11)(43 37). 

13. The absorption of a photon is a necessary, but often not a sufficient condition, for a molecule to 

undergo transformation via direct photolysis (12)(14)(43 37). The absorbed energy must first be sufficient 

to cause the transformation via bond cleavage, rearrangement, oxidation, or reduction. Then, 

phototransformation to form new molecular structures must compete with other possible deactivation 

processes such as quenching, other radiationless processes, and radiative processes. Consequently, the 

fraction of photon excited molecules that actually undergo phototransformation (i.e. e.g. the quantum 

yield) is generally much less than 1 (usually < 0.1 and sometimes < 0.01) (14)(23 1). The mechanistic 

aspects of phototransformation reactions at a molecular level are beyond the scope of this guideline (see 

reference 44 38 for further information). 

14. It can be shown that for an optically dilute solution (absorbance A < 0.02) of a chemical in pure 

water exposed to polychromatic irradiation above the cutoff of solar irradiation at the earth’s surface of 

2905 nm, the direct photolysis rate constant (assuming the quantum yield is independent of wavelength) is 

given by (12)(14through 18) (See Annex 4: Derivation of selected equations): 

 

k

j

l

D

I d

sys

= ∑ 2 3

0

295

800 . φ ε λ λ

 [eq. 5] 

where: 

 kd = direct photolysis rate constant (1/day) 

φ = the quantum yield (independent of wavelength) 

ελ = molar absorption coefficient (L/mol•cm; M-1 •cm-1 ) at wavelength λ

 I0λ = photon flux (photons/cm2 •day) over a 1 nm interval centered at wavelength λ

Dsys = depth of irradiated system (cm) = volume of irradiated system/incident area 

 l = light pathlength (cm) 

j = conversion factor to make units consistent with the L/mol•cm unit of the molar 

absorption coefficient and the mol/L unit of concentration = 6.02 x 1020

(L/cm3

)(photons/mol) 

Note: The term (l/Dsys) in equation 5 approximately cancels out for rectangular photolysis cells exposed 

to irradiation from a filtered xenon arc lamp and for which Dsys = Dcell . However, that would not 

be true for cylindrical photolysis cells or for any photolysis cells exposed to solar irradiation 

where the pathlength l has a more complicated dependency on Dcell. Methods for determining the 

pathlength of cylindrical cells exposed to filtered xenon-arc lamps are discussed in references 12 

and 17. Pathlength determinations as a function of the depth of a system, Dsys exposed to solar 

irradiation are discussed in references 12 and 18. In addition, note that if the quantum yield is not 

independent of wavelength, it must remain inside of the summation in equation 5. 

15. Equation 5 is applicable to solar as well as filtered xenon arc lamp irradiation. However, the 

following equation is often used to determine the direct photolysis rate constant for optically dilute 

solutions of test chemicaltest substances in pure water in photolysis cells or in near surface, clear natural 

water exposed to solar irradiation (2)(12) (See Annex 4: Derivations of selected equations): 
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k L d solar ( ) = φ ε ∑ λ λ

295

800

 [eq. 6] 

 where: 

 kd = direct photolysis rate constant (1/day) 

φ = quantum yield (independent of wavelength) 

ελ = molar absorption coefficient (L/mol•cm) at wavelength λ

Lλ = average daily solar photon flux irradiance (mmol photons/cm2•day) over 

wavelength interval ∆λ centered at wavelength λ

16. Equations 1, 5 and 6 show that direct photolysis kinetics are theoretically pseudo-first order only if 

the photon flux remains constant over time. Photon fluxes from filtered xenon arc lamps at any given 

wavelength remain, in general, relatively constant over time. This is one of the primary advantages in using 

filtered xenon arc lamps. However, solar photon fluxes at any given wavelength vary cyclically over 24-

hour periods. Nevertheless, the data obtained from studies conducted in natural sunlight (in which the rate 

constant reflects an average over the entire study duration) generally do fit a single, pseudo-first order rate 

constant model reasonably well. The reason is that changes in the average pseudo first-order rate constant 

over time as the study progresses is generally relatively small and not systematic. This is particularly valid 

for studies conducted for one to several weeks or for 2-4 hours mid-day. It may not be as valid for studies 

lasting 4 hours to several days or for those conducted beyond 30 days. 

Summary of the proposed tiered approach for direct photolysis 

Tier 1: Theoretical screen

17. Estimate a maximum possible direct photolysis rate constant for the test chemicaltest substance

in the near surface of a clear natural water as follows. Measure the test chemicaltest substance molar 

absorption coefficients from 2905 nm to 800 nm, and use tabular solar irradiance for summer and 400 or 

500 latitude over the same wavelength interval. Estimate a maximum possible direct photolysis rate by 

assuming the quantum yield in equation 6 is equal to one and by substituting the molar absorption 

coefficients and tabular solar irradiance values, Lλ, into the equation (2)(12). Determine if the estimated 

maximum possible direct photolysis rate constant would theoretically result in estimated direct photolysis 

losses greater than 50% of an initial concentration over 30 days/nights. If maximum possible losses are < 

50%, no further direct photolysis work is performed, but proceed to Indirect Photolysis Tier 1 

(experimental /theoretical screen)., if required. If the maximum possible losses > 50%, proceed to Direct 

Photolysis Tier 2 (experimental screen) and range finding) or to Direct Photolysis Tier 3 (determination of 

the quantum yield).

Tier 2 - Experimental screen and range finding

18. Approximately determine the rate of decline in the concentration of the test chemicaltest 

substance and corresponding direct photolysis rate constant in buffered pure water exposed to a filtered 

xenon arc lamp (recommended) or sunlight. Determine if the approximate direct photolysis rate constant 

would result in estimated direct photolysis losses equal to or greater than to 20% of an initial concentration 

over a period equivalent to 30 days/nights in the near surface of a clear natural water body exposed to 

summer sunlight. If estimated losses are < 20%, no further direct photolysis work is performed, but 

proceed to Indirect Photolysis Tier 1 (experimental/theoretical screen.), if required. If estimated losses 

≥ 20%, proceed to Direct Photolysis Tier 3 (determination of the quantum yield). If the determination of 
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the quantum yield is not required, proceed to Direct Photolysis Tier 4 (kinetics study). 

Tier 3 - Determination of the quantum yield and its use to estimate direct photolysis rate constants

19. Determine the quantum yield for direct photolysis of the test chemicaltest substance in buffered 

pure water using monochromatic irradiation (12), polychromatic artificial irradiation (8) or sunlight (2).

The pseudo first-order rate constant for direct phototransformation and thus the life-time of a test substance 

in water can be calculated from the quantum yield (18)(52). Once the quantum yield has been determined, 

use it as input to c Computer programs such as GCSOLAR (18) or ABIWAS (41 35)(42 36) to may help to 

estimate direct photolysis rates for the test chemicaltest substance applicable to any types of surface waters 

(defined by depth and light attenuation), seasons, and latitudes of interest (2). If the Direct Photolysis Tier 

4 study is potentially applicable, d Determine if the direct photolysis rate constants estimated from the 

quantum yield would result in estimated direct photolysis losses greater than or equal to 20% of the initial 

concentration over a period equivalent to 30 days/nights in the near surface of a clear natural water body 

exposed to sunlight. If the estimated losses are < 20%, no further direct photolysis work is required but 

proceed to Indirect Photolysis Tier 1 (experimental/theoretical screen)., if required. If the estimated losses 

are > 20%, proceed to the Direct Photolysis Products Tier 4. kinetics study.

Tier 4 -– Identification ofKinetics study on the parent and, if applicable, major transformation products

20. Estimate Determine the rate of formation and decline of phototransformation products in the 

concentration of the test chemicaltest substance and corresponding direct photolysis rate constant in 

buffered pure water exposed to a filtered xenon arc lamp (recommended) or sunlight. If applicable, also 

Isolate and identify/quantify and determine the rates of formation and decline of the major transformation 

products. In this guideline, major transformation products are defined as those accounting for ≥ 10% of the 

applied test substance chemical in an individual sample at any sampling time. 

Indirect Photolysis 

21. To undergo transformation via indirect photolysis, the test substance must: 

•receive energy from a photosensitizer in an excited electronic triplet state such as various ones generated 

from the solar irradiation of CDOM (12)(14)(19)(20), or

•react with a photochemically generated oxidant such as a hydroxyl free radical, a singlet oxygen, or a 

alkylperoxy free radical (references 12 and 21 through 32). Singlet oxygen and alkylperoxy free 

radicals appear to be generated primarily from the solar irradiation of CDOM in oxygenated natural 

waters. Some hydroxyl free radicals also appear to originate from the solar irradiation of CDOM. 

However, the major source of the hydroxyl free radical in natural waters appears to generally be the 

solar irradiation of nitrate (21).

22. In infrequent cases where the nitrate to nitrite ratio is less than 100 in aerobic surface waters, the 

photolysis of nitrite (which is present in much lower concentrations than nitrate) may also contribute 

significantly to the formation of hydroxyl radicals because “nitrite photoproduces OH radicals about 50 

times more rapidly than nitrate” (39).

23. The transformation of chemical pollutants in surface waters may also occur via photoredox 

reactions with soluble and insoluble Fe (III) species, which are commonly present at significant 

concentrations in many surface waters (40)(41).

24. Indirect photolysis kinetics are actually closer to being second-order than first-order because the 

rate of decline of the test chemical is generally proportional to the product of the test chemical and 
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sensitizer or photooxidant concentrations (12)(20)(21). However, although transient photooxidant and 

excited photosensitizer concentrations are very low (typically below 1 x 10-10 M), they can quickly attain 

quasi steady-state concentrations in irradiated solutions thereby leading to pseudo first-order kinetics 

(12)(20)(21).

25. In many cases, the net pseudo first-order rate constant for indirect photolysis will reflect the 

reaction of different molecules with more than one type of sensitizer and/or oxidant:

k k S or indirect j nd j ss = ∑ ( ) () 2 ( ) Ox j(ss)

j=1

j=n

[eq. 7]

where:

kindirect = net pseudo first-order indirect photolysis rate constant (1/day)

kj(2nd) = second order rate constant for the reaction with sensitizer j or oxidant j (L/mol•d)

Sj(ss) = steady state concentration of sensitizer j (mol/L)

Oxj(ss) = steady state concentration of oxidant j (mol/L)

Summary of the proposed tiered approach for indirect photolysis 

Tier 1 - Experimental and/or theoretical screen

26. Perform an experimental screen in simulated natural water (SNW) containing humics as a 

surrogate for CDOM, and/or in a simulated natural water containing nitrate as well as humics (SNW-N) 

and/or in one or more natural waters (NW) and in buffered pure water (PW) (3). Specifications for PW 

appear in paragraph 68-a. Approximately determine the rate of decline in the concentration of the test 

chemical in SNW, and/or SNW-N, and/or NW exposed to sunlight to determine if indirect photolysis 

represents a potentially significant transformation pathway. The use of a xenon lamp light source for 

indirect photolysis studies is not recommended at this time. Determine if the estimated indirect (not net) 

photolysis losses over the equivalent of 30 days/nights of sunlight exposure is greater than or equal to 20% 

of an initial concentration of the test chemical. If the estimated indirect photolysis losses are < 20%), no 

further indirect photolysis work is performed. If the estimated indirect photolysis losses are > 20%, 

proceed to the Indirect Photolysis Tier 2 (kinetics study).

27. In some cases, it may be possible to supplement the Tier 1 experimental screen with a theoretical 

screen based on using structure activity relationships (SARs) to estimate rate constants for reaction with 

photooxidants and information on the typical concentrations of photooxidants in surface water (21).

Tier 2 - Kinetics study on the parent and (if applicable) major transformation products

28. Determine the rate of decline in the concentration of the test chemical and its corresponding net 

and indirect photolysis rate constants in SNW, and/or SNW-N and/or one or more NWs irradiated with 

sunlight and in PW. If data on transformation products for indirect photolysis are applicable, 

identify/quantify major transformation products and determine their rates of formation and decline. In this 

guideline, major transformation products are defined as those accounting for > 10% of the applied test 

chemical (percent of applied radioactivity or molar basis) during any part of the study.
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APPLICABILITY OF THE TEST GUIDELINE 

21.29. This test guideline is applicable to chemical substances for which analytical methods with 

sufficient accuracy and precision are available and validated (if required). If data are needed on 

transformation products, adequate quantitative analytical methods must also be available/developed for 

them. 

22.30. If a non-ionic test chemicaltest substance is somewhat susceptible to hydrolysis, the test may still 

be applicable as long as a dark control is used to account for the hydrolysis rate (2)(4)(5). However, 

photolysis tests should be conducted at a pH at which the hydrolysis rate of the chemical is minimized 

(5)(45 42). The rate constant determined from the dark controls should be identical to the hydrolysis rate 

constant and can be subtracted from the overall rate constant determined from the irradiated solutions to 

give the direct photolysis rate constant. However, in some cases, the desired test substance chemical may 

undergo rapid hydrolysis (i.e., hydrolysis half-life of < 1 day) within the environmentally relevant pH 

range of 45-9 at 20-25oC (46). In such cases, the photolysis study should instead be conducted on each the

major hydrolysistic product(s) (> 10% of applied) as the test substance). 

23.31. The test is not applicable to highly volatile compounds such as fumigants and some organic 

solvents. However, in some cases, where the Henry’s Law constant of the test chemicaltest substance

and/or anticipated transformation products may indicate the potential for fairly substantial volatilization 

rates from water, the study may still be applicable provided that precautions are taken. Such precautions 

include using minimal head-space, adequate sealing in the photolysis cells to minimize volatile losses 

during the study (2)(4) or trapping. Trapping volatiles becomes more difficult, but preferred, when 

employing sacrificial sampling on numerous small photolysis cells than when employing aliquot sampling 

on a smaller number of much larger test vessels. However, with the proper equipment and experimental 

set-up, it can be done as shown by Ruzo (473). 

INFORMATION ON THE TEST SUBSTANCE 

24. Unlabelled or labelled test substances can be used to measure the rate of 

phototransformation. Labelled mareial is required for studying the pathway of phototransformation 

and for establishing a mass balance. 14C-labelling is recommended but other isotopes, such as 13C, 

15N, 32P, may also be useful. As far as possible, the label should be positioned in the most stable 

part(s) of the molecule1. The chemical and radiochemical purity of the test substance should be at 

least 95 %.

25.32. Before carrying out any of the phototransformation tests, the following information on the test 

substance should be known, if available: 

 (a) Solubility in water [OECD Guideline 105].

 (b) Solubility in organis solvents 

 (cb) Vapor pressure [OECD Guideline 104] 

 (dc) Henry’s Law Constant2*

 

1

 For example, if the test substance contains one ring, labelling on this ring is required; if the test substance contains 

two or more rings, separate studies may be needed to evaluate the fate of each ring and to obtain suitable information 

on formation of phototransformation products.

2*

 Henry’s Law constant estimated from solubility in water and vapour pressure or measured. Henry’s Law constants 

for some chemicals may also be found in: http://webbook.nist.gov/chemistry
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 (ed) Abiotic hydrolysis as a function of pH [OECD Guideline 111] 

 (fe) n-octanol/water partition coefficient [OECD Guidelines 107 and 117] 

 (gf) pKa(s) of ionisable substances [OECD Guideline 112] 

(g) Chemical nomenclature (IUPAC and CAS)

(h) Chemical Abstract Registry Number of the test substance 

(i) Common name(s), if any

(j) Chemical structure

(k) Position(s) of radiolabelled atom(s) or other molecular tags (if applicable)

(l) Isomer purity (if applicable)

 

Note: The temperatures at which the physical chemical properties listed above in the previous 

paragraph were determined should generally be reported. within the range of (25 ± 5)°C and 

should be specified.

33. If available, the solubility in non-aqueous solvents may be useful in selecting a co-solvent or 

extractant. Also, the following optional information may be useful for indirect photolysis studies conducted 

in natural water: 

- Aerobic - Anaerobic Transformation in Aquatic Sediment Systems 

[OECD draft Guideline 308]

- Adsorption / Desorption [OECD Guideline 106]

26.34. The n-octanol/water partition coefficient and aqueous solubility supply some information on the 

potential of the test chemicaltest substance to significantly adsorb to glassware and reaction vessels. 

However, any additional information on the glassware adsorption potential of the test chemicaltest 

substance should be known to ensure that such characteristics are taken into account in preparing glassware 

and reaction vessels for the study.

27. An appropriate analytical method of known accuracy, precision and sensitivity for the 

quantification and identification of the test substance and its phototransformation products should be 

available. The analytical detection limit for the test substance and its phototransformation products should 

also be known (see paragraph 31).

ANALYTICAL METHODS AND QUALITY CRITERIA

35. The experimenter should develop methods with appropriate accuracy (recovery), precision 

(reproducibility), and adequate limits of detection (LOD) and quantification (LOQ). In many instances, 

validation of the analytical methodology may be required. In those cases, the experimenter should be 

responsible for ensuring that the analytical methods have been validated. 

36. The analytical methods that may be used include liquid scintillation counting (LCS) and 

radiochromatography (radiolabelled substances), high-performance liquid chromatography (HPLC), gas 

chromatography (GC), gas-liquid chromatography (GLC), and capillary electrophoresis (CE), etc. TLC can 

also be used for radiolabelled test chemicals, but only if adequate resolution can be achieved and there is 

no decomposition of the reference standards in the silica plates.

37. When necessary to identify and/or confirm transformation products, the separation methods listed 

in the preceding paragraph may be coupled with spectrometric methods such as mass spectrometry (MS) 

and/or Fourier-Transform Infrared spectroscopy (FT-IR). Examples of such couplings include as GC/MS, 

HPLC/MS, LC/MS-MS, GC/FT-IR or any other adequate techniques. 
ENV/JM/TG/RD(2006)1/REV 

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38. Reference chemicals of potential transformation products may be used for identification (e.g. by 

co-chromatography). When radiolabelled test substance is used, it may be necessary to use radiolabelled 

reference chemicals. However, as indicated in the previous paragraph, the identities of major 

transformation products need to be confirmed preferably by an independent analytical chemistry method 

such as with MS. Transformation products that cannot be identified from reference chemicals should be 

elucidated using, for example, MS, NMR and infrared spectroscopy.

39. Parameters that can influence the accuracy and/or precision of the determination of chemical 

concentrations in aqueous media include the stability of the chemicals in samples during storage, extraction 

efficiency, analytical separation efficiency in mixtures, and the magnitude of the concentrations with 

respect to analytical detection and quantification limits. 

40. Analytical instrumentation should be periodically calibrated within applicable concentration 

ranges according to the manufacturer’s specifications. If samples are stored prior to analysis, storage 

stability tests may need to be performed depending upon the storage method and duration. The accuracy 

and precision of the analytical method should be checked within the expected concentration range. 

Accuracy can be determined from the recovery percentages of spiked samples. Precision can be determined 

by replicate analyses of an appropriate percentage of the samples.

41. Regardless of the chosen analytical method, analytical recoveries for radiolabelled test chemicals 

should fall within 90% and 110% of the applied radioactivity. For non-radiolabelled test chemicals, 

analytical recoveries of 85 to 115% are desirable.

42. For radiolabelled materials, total mass balance should fall within that range 90% to 110% of the 

total applied radioactivity (42).

REFERENCE SUBSTANCES

28. Reference substances should be used for the characterisation and/or identification of 

phototransformation products by spectroscopic and chromatographic methods.

QUALITY CRITERIA

Recovery

29. Extraction and analysis of, at least, duplicate samples immediately after the addition of the test 

substance gives a first indication of the repeatability of the analytical method and of the initial recoveries 

for the test substance. Recoveries for later stages of the experiments are given by the respective mass 

balance obtained with labelled material. Recoveries should range from 90 to 100 % for labelled chemicals 

(48) and from 70 to 110 % for non-labelled chemicals (7).

Repeatability and sensitivity of analytical method

30. Repeatability of the analytical method (excluding the initial extraction efficiency) to quatify test 

substance and phototransformation products can be checked by duplicate analysis of the same extract , 

incubated long enough for formation of phototransformation products.
ENV/JM/TG/RD(2006)1/REV 

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31 The limit of detection (LOD) of the analytical method for the test substance and for the 

phototransformation products should be at least 0.01 mg/kg water (as test substance) or 1 % of the applied 

dose whichever is lower. The limit of quantification (LOQ) should also be specified.

Accuracy of transformation data

32 Regression analysis of the concentration of the test substance as a function of time gives the 

appropriate information on the reliability of the phototransformation curve and allows the calculation of 

the confidence limits for half-lives (in case of pseudo first-order kinetics) or DT50 values and, if 

appropriate, DT90 values.

TEST MATERIALS

Test vessels

33.43. All glassware and reaction vessels/photolysis cells used in the photolysis studies should must be 

sterilized by autoclaving (2)(3)(4)(12) or any other suitable non-chemical method. (2). Chemical 

sterilization is not recommended since chemical reagents may leave residues that could absorb in the 2905

to 800 nm region and/or behave as photosensitizers. 

Test substance application

34. The test substance should be directly dissolved in the aqueous media at a concentration which 

should not exceed half its solubility. For test substances with low aqueous solubility, a co-solvent may be 

used to prepare the test solution. However, the amount of co-solvent used must be as little as possible and 

preferably not exceed 1% by volume in the test solution (45). Co-solvent percentages up to 10% may be 

used, but if more than 10% co-solvent is required to dissolve the test substance, the study should not be 

performed. The selected co-solvent must not solvolyse the test substance, must not absorb in the 290 to 

800 nm region, and must not be a photosensitizer. Acetone is an example of a solvent that is a 

photosynthesizer, and should not be used. Acetonitrile is a generally recommended co-solvent.

35. The initial concentration of the test substance for quantum yield determinations and direct 

photolysis studies should be such that the absorbance at any wavelength above 290 nm (maximum 

absorption wavelength included) is less than 0.02. These optically dilute, also referred to as “optically thin” 

solutions generally have a concentration of < 10-4 M. Nevertheless, the initial concentration of the test 

substance should be at least 10 times greater than the quantification limit of the analytical method.

36 Because oxygen can affect the rates and even pathways of photodegradation reactions, test 

solutions should be saturated with air at the beginning of the study (4). A buffer solution that has been 

autoclaved should be given sufficient time to aerate prior to dispensing into the vessels. It is not 

recommended to autoclave small amounts of buffer in individual vessels because driving the air out may 

change the volume. 

44. If radiolabelling is used, test chemicals should be labelled at one or more atoms within each 

stable or relatively stable portion of the molecule. For chemicals containing one or more aromatic rings, 

one or more atoms in each ring are usually labelled. In addition, one or more atoms on both sides of any 

linkage thought to be readily susceptible to cleavage should be radiolabelled (42). Radiolabelling is 

particularly important when there is the need to identify transformation pahtways. Radiolabelling does not 

have to be limited only to 14C. For example, in some cases, 32P can be used in place of or in addition to 14C 

(42). If mass spectroscopy is used, stable isotopes such as 15N can also be used as molecular tags. 
ENV/JM/TG/RD(2006)1/REV 

14

45. Radiolabelled test substances should have radiochemical and chemical purities of 95% or greater 

(42). Non-radiolabelled test substances should have a chemical purity of 95% or greater (42). 

46. If the test chemical consists of more than one geometrical and/or optical isomer, the percentage 

represented by each isomer should be provided. 

47. Pesticide testing should generally be conducted with the active ingredient. The same generally 

applies to pharmaceutical chemicals. However, in some instances it may be more appropriate to use a 

formulated product.

48. If the potential loss of the test chemical by volatilization or adsorption to test vessels is of 

concern or if the identification and quantification of major transformation products are of interest, 

radiolabeling should be used. 

Test media for direct photolysis studies

37.49. Guidance for preparing test media for direct and indirect photolysis respectively is provided in 

Annex 5. 

Test media for direct photolysis

38.50. For direct photolysis studies, the test media should be sterilized aqueous buffer solutions at the 

appropriate pH (for sterilisation methods see paragraph 33). The water should be “pure water” (PW). The 

buffers should be identical or comparable to those used in the hydrolysis studies (46) and should not absorb 

between 2905 and 800 nm or be photosensitizers. Also, the buffers should not adversely affect the 

solubility of the test substance. chemical. 

39.51. For If the non-ionisable test substances the tests should be conducted at a pH at which the test 

substance is hydrolytically most stable in the pH range 4-9. chemical is non-ionizable and is approximately 

stable to abiotic hydrolysis at pH 7, the buffer used in the direct photolysis study should maintain a pH of 

approximately 7. However, if the hydrolysis rate of a non-ionizable test chemical at pH 7 is significant, but 

its hydrolysis rate at pH 5 or 9 is much lower, the buffer used should maintain a pH (within the 5 to 9 

range) at which the hydrolysis rate is minimized (42). 

40.52. If the test chemicaltest substance is appreciably ionic anywhere within a 45 to 9 pH range, the 

study should be conducted in one or more aqueous buffers at any of the pH 4 5, 7, and/or 9. Selection of 

the pH (s) at which the study should be conducted depends on the electronic absorption spectra at each pH. 

Spectra showing absorption bands between 2905 and 800 nm or shorter wavelengths tailing above 290 

5 nm together with estimates of maximum photolysis rates (Tier 1: Direct Photolysis) are the criteria for 

selecting the pH(s). Refer to Tier 1, Direct Photolysis for further details.

53. For test chemicals with low aqueous solubility, a co-solvent may be used to prepare the test 

solution. However, the amount of co-solvent used must be as little as possible and preferably not exceed 

1% by volume in the test solution (42). Co-solvent percentages up to 10% may be used, but if more than 

10% co-solvent is required to dissolve the test chemical, the study should not be performed. The selected 

co-solvent must not solvolyse the test chemical, must not absorb in the 295 to 800 nm region, and must not 

be a photosensitizer. Acetone is an example of a solvent that is a photosynthesizer, and should not be used. 

Acetonitrile is a generally recommended co-solvent.

Test media for indirect photolysis studies
ENV/JM/TG/RD(2006)1/REV 

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54. The selection of test media for indirect photolysis studies depends on the purpose of the study. 

The preparation of test media is based on the procedure given in OPPTS 835.5270 guideline (3). Three test 

media may be used for these studies: (1) SNW (Simulated Natural Water); SNW-N (Simulated Natural 

Water with Nitrate); and (3) NW (Natural Water). SNW-N may be appropriate for areas where surface 

waters show strong evidence of nitrate pollution associated with the use of fertilizer and other nitratebearing substances. Nitrate has been known to photochemically produce hydroxy radical (21)(22)(26). NW 

may be important in situations when the understanding of the photochemical behavior of chemicals in 

actual natural surface waters is of interest.

55. Humic acid or other humic materials may be obtained commercially (Aldrich, Fluka, etc.). 

Humic and fulvic acid extracted from soils or natural surface water may be used as surrogates for aquatic 

humic acids (23)(44). Composite data for typical humic acids are available from the International Humic 

Substances Society (45). The UV-VISIBLE spectra of the selected humic materials in the test media 

(natural water and/or buffered SNW) should be provided. The absorption spectra of some humic materials 

are shown in reference 46.

Light sources

41.56. For direct photolysis studies the recommended light source should be a filtered xenon arc lamp 

capable of simulating natural sunlight in the 2905 to 800 nm region or sunlight (49 7). For indirect 

photolysis studies, the only recommended light source at this time is sunlight. The photon flux of the 

filtered xenon arc lamp as a function of wavelength should be determined at least at the beginning and the 

end of the study using one or more actinometers or and a spectral radiometer. if available. If sunlight is 

used, average daily solar photon fluxes (Lλ) as a function of wavelength should be determined for the 

season and latitude closest to those of the experiment using solar irradiance tables and a spectral 

radiometer., if available. 

42.57. If a filtered xenon arc lamp is used, the irradiation photon flux incident on the test vessels should 

be adequate to cause substantial decline of the test substance chemical over a several day experimental 

period. The transformation rate constant and corresponding half-life obtained with a filtered xenon arc 

lamp can be adjusted to solar equivalents from a comparison of solar photon flux to lamp photon flux 

because of the linear relationship between the transformation rate and photon flux (see equation 14). 

43.58. If sunlight is used, the study should be conducted during the season when and at the latitude 

where the concentration of the test substance chemical is expected to be highest in the environment. For 

pesticides, that would be the season when and the latitude where it is typically applied. 

44.59. If a filtered xenon arc lamp (see Annex 6) and sacrificial sampling of multiple test vessels are 

used, an appropriate experimental set-up should be used to ensure that all of the test vessels receive the 

same incident irradiation flux. Self-contained, commercially available xenon photolysis units in the 

projection or “working plane” configuration produce uniform fluxes (50 48)(51 49a). A commonly used 

configuration is the “merry-go-round” photochemical reactor (12). Merry-go-round photochemical reactors 

are commercially available (5149a)(5149b). Another example of an experimental set-up is shown reference 

52 0 (see Annex 6).. An experimental set-up that includes trapping of volatiles appears in references (47 3)

and is also shown in Annex 6.

Other photolytic equipment and reagents 

45.60. Photolytic equipment includes: 
ENV/JM/TG/RD(2006)1/REV 

16

• Optical cells of appropriate material (generally quartz) and path length for measuring UV-VIS spectra. 

• Appropriate optical filters to cut-off radiation below 2905 nm and above 800 nm. 

• When monochromatic irradiation is used in the determination of the quantum yield, the appropriate 

filter systems must be used to isolate the desired wavelength (12)(49 7). Monochromatic irradiation at 

the desired wavelength can also be achieved with a monochromator in conjunction with a suitable 

polychromatic light source such as a mercury or xenon arc lamp. An example of a useful optical bench 

is shown in Annex 6. (Filters for isolating the 313 and 366 nm wavelengths from a mercury arc lamp 

are discussed in reference 12). 

• Specially designed reaction cuvettes/photolysis cells to contain test solutions (exposed and dark 

control) and chemical actinometers, when needed. The recommended material is quartz. 

• Appropriate apparatus designed to contain the light source, filters, sample, holders, reaction 

vessels/actinometers such as the photochemical “merry-go-round” reactor. Several appropriate designs 

are commercially available to accommodate specific needs. shown in Part 1, Annex 6.

• Photometer or spectral radiometer to measure photon flux as a function of wavelength. 

46.61. Photolytic reagents include: 

• All of the water used in the direct photolysis tests must be of high purity and free of any absorbing or 

photosensitizing chemical species. For example, the U.S.EPA OPPTS 835.2210 (2) recommends using 

reagent water meeting ASTM Type IIA standards, as described in ASTM D1193-99 Standard 

Specification for Reagent Water (see Annex 5: Test Media). “Milli-Q” water is one example of high 

purity water. In this guideline, the water used in the preparation of stock, test and buffer solutions is 

referred as “pure water”.

• Appropriate chemical reagents to be used as actinometers. 

•Commercially available, certified humic material for indirect photolysis studies (Fluka; Aldrich).

•Analytical grade NaNO3

• Appropriate aqueous buffers such as phosphate for pH 7, acetate for pH 4 5 and borate for pH 9 for 

compatibility with hydrolysis studies. Phosphate buffers are usually recommended for direct photolysis 

studies(2)(12) but only cover. pH range 5 to 8. Caution should be applied when using phosphate

buffers because they might promote catalytic effects.

Analytical equipment and chemical reagents

47.62. In addition to standard laboratory equipment, the following is required: 

• Spectrophotometer to acquire spectral data in the UV-VIS range. For example, single beam, diode 

array instruments that allow sensitive, rapid acquisition of digitized spectral absorbance data. 

• Appropriate analytical instrumentation to measure the concentration of the test substance in exposed 

and dark control samples, such as HPLC, GLC, TLC, etc. 

• Suitable instrumentation for identification purposes, such as MS, HPLC/MS, GC/MS, LC/MS, GCFTIR, NMR, etc. 

• When radiolabelled test substances are used, liquid scintillation counters (LSC) and instrumentation 

for radiochromatography should be available. 

• Appropriate extraction apparatus. 

• Equipment to sterilise reaction vessels and other glassware used in the preparation of stock and test 

solutions. 

48.63. Chemical reagents include: 

• Reagent grade chemicals and solvents free of any photosensitizer. 
ENV/JM/TG/RD(2006)1/REV 

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• Scintillation liquid (for radiolabelled chemicals).

64. Analytical references include:

•Test substance reference standards for co-chromatography and/or for other analytical methods.

•Potential transformation products (including hydrolysis products) reference standards for the 

determination of products, if applicable.

TEST METHODOLOGY

Test conditions

49.65. The temperature during photolysis studies may vary by as much as 10 degrees but should be 

maintained within the range of 20 to 30 C. (25 ± 5)oC and monitored periodically.

50.66. The pH of buffered solutions should be maintained within +0.15 pH units. The pH should be 

checked during the course of the study, if feasible. 

51.67. Dark control solutions should be used for all quantum yield determinations and for all direct and 

indirect photolysis studies and should be kept at the same temperature as the irradiated samples. 

68. The concentration of the chemical in the test media should never exceed half its solubility. When 

a co-solvent is needed, it should preferably not exceed 1% by volume in the test solution and never exceed 

10% by volume. 

69. The initial concentration of the test chemical for quantum yield determinations and direct 

photolysis studies should be such that the absorbance at any wavelength above 295 nm (maximum 

absorption wavelength included) is less than 0.02. These optically dilute, also referred to as “optically 

thin”, solutions generally have a concentration of < 10-4 M. Nevertheless, the initial concentration of the 

test chemical should be at least 10 times greater than the quantification limit of the analytical method. 

70. The concentration of the test chemical in simulated (SNW, SNW-N) or natural water (NW) test 

media should be equivalent to that of optically dilute test solutions in pure water/buffer solutions required 

for direct photolysis studies.

71. Because oxygen can affect the rates and even pathways of photodegradation reactions, test 

solutions should be saturated with air at the beginning of the study (4). A buffer solution that has been 

autoclaved should be given sufficient time to aerate prior to dispensing into the vessels. It is not 

recommended to autoclave small amounts of buffer in individual vessels because driving the air out may 

change the volume. 

Study duration, sampling, and replication 

52.72. The study duration should be sufficient to determine the DT75 (two half-lives if decline is firstorder) of the test chemicaltest substance and (if applicable) the formation and decline of major 

transformation products until they have declined to < 25% of their maximum concentration. However, the 

maximum study duration should not exceed the equivalent of 30 days/nights of sunlight exposure during an 

appropriate season and at an appropriate latitude. The “DT75" is defined as the time required for 75% of the 

initial concentration of test chemical to disappear.
ENV/JM/TG/RD(2006)1/REV 

18

53.73. Based upon equating photons absorbed by the test chemicaltest substance per unit volume, the 

maximum direct photolysis study duration (using continuous filtered xenon arc lamp irradiation) that is 

equivalent to 30 days/nights of sunlight exposure will be only a few days and is given by (see Annex 4):.

 

Max direct photolysis duration days j

D

l

L

I

cell

xenon

xenon

. ()

( )

 =

∑

∑

30 295

800

0

ε

ε

λ λ

λ λ

 [eq. 78] 

 where: 

 30 = 30 days/nights of solar irradiation 

Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area 

 lxenon = light pathlength in cell exposed to a filtered xenon arc lamp

 j = conversion factor (for details see eq. 9)

 and other terms same as defined for equations 5 and 6. 

Note: For rectangular box cells, the term Dcell/lxenon cancels out because lxenon = Dcell. However, that will 

not be true for cylindrical cells (12). 

54.74. Although equation 78 allows for flexibility in choosing an experimental photon flux from a 

filtered xenon arc lamp, practical considerations somewhat limit the range of experimental fluxes that 

should be used. In general, it is recommended that the experimental photon flux from a filtered xenon 

lamp be adjusted to be somewhat comparable to that of sunlight (at noon for short several hour studies or 

24 hour average for several day to weeks studies). The reason is the experimental flux should not be so 

intense that photolytic processes other than those that would occur under natural sunlight are initiated. 

Also, if the parent is susceptible to hydrolysis at the experimental pH and the hydrolytic products are 

susceptible to direct photolysis, time should be allowed for them to form. 

55.75. A preliminary range-finding study should be performed to help determine the number of samples 

to be collected and the timing of their collection. For linear and non-linear regressions on test chemicaltest 

substance data in definitive or upper tier tests, the minimum number of samples collected should be five 

and seven, respectively. The greatest number of samples taken should generally be during the first part of 

the study where the slope of the concentration versus time series for the test chemicaltest substance is 

generally greatest. If an accurate determination of the rates of formation and decline of major 

transformation products is desired, the number of samples taken should generally be greater and somewhat 

more dispersed throughout the study duration. The number of sampling times can be less in range finding 

or lower tier tests. 

56.76. Either aliquot or sacrificial sampling can be used. Sacrificial sampling of entire photolysis cells 

at each sampling time is often preferred over withdrawing aliquots of test solutions at each sampling time. 

The reason is that it helps to maintain aseptic conditions and allows for mass balance determinations at 

each sampling time. The use of sacrificial sampling is particularly necessary in cases where mass balance 

determinations are needed such as when the test chemicaltest substance and/or its transformation products 

are volatile and/or adsorb strongly to the photolysis cells (5). Future sampling descriptions in this 

document will be for the generally preferred sacrificial sampling, but aliquot sampling from larger 

irradiated and dark control reaction vessels is acceptable as long as the resulting mass balance is adequate 

and aseptic conditions are properly maintained. 

57.77. Replicates (preferably a minimum of two three) of each experimental determination of kinetic 

parameters are recommended to determine variability and reduce uncertainty in the determination of 
ENV/JM/TG/RD(2006)1/REV 

19

kinetic parameters. However, the uncertainty in the determination of kinetic parameters can also be 

reduced by increasing the number of sampling intervals within each experiment, particularly when the rates 

of formation and decline of major transformation products are being determined. In such cases, a minimum 

of two replicate determinations of kinetic parameters may be adequate. With generally limited resources, 

the researcher needs to weigh the pros and cons of experimental replication versus increased numbers of 

sampling times per experiment in designing the study. 

Calculations

58.78. Basic calculations/equations, including the following one used to determine whether going to 

higher tiers is triggered, are included within the body of the guideline: 

% loss over 30 days / nights solar = −− = 100 1 30 { } exp[ ] ( ) ( ) k td solar [eq. 89] 

 where: 

ksolar = pseudo first-order direct or indirect photolysis rate constant for test chemicaltest 

substance exposed to solar irradiation (1/day) 

59.79. Calculations/equations for the kinetics of the formation and decline of major photoproducts, and

accounting for photobleaching are optional. The necessary non-linear regressions can be readily performed 

with the use of commercially available spreadsheets and/or statistical software. 

Determination of the photon flux from a filtered xenon arc lamp

60.80. The photon flux from a filtered xenon arc lamp can be determined with the use of low and/or 

high optical density actinometers alone or preferably in conjunction with a spectral radiometer (531). The 

advantage in using a spectral radiometer to determine the photon flux is convenience, as it avoids changing 

actinometers. However, spectral radiometers are expensive and are not always readily available. 

Furthermore, even if a spectral radiometer is generally used to determine the photon flux, it should be 

periodically calibrated against photon fluxes determined with actinometers. Calibration is necessary 

because of the difference between the exposure surface of a spectral radiometer and that of the photolysis 

cuvette or tubes used to contain the actinometers (53 1). 

61.81. By determining the monochromatic first-order direct photolysis rate constant for a low optical 

density actinometer of known quantum yield in PW in a photolysis cell irradiated with monochromatic 

irradiation, the incident photon flux at the wavelength of the irradiation can be determined from the 

following equation (53 1; see Annex 4 3 - Equation Derivations): 

I j D

l

k

xenon

cell

xenon

d act

act act

0 2 3 λ

λ

λ φ ε ( )

( )

( ) . =

 [eq. 910]

where:

 I0λ (xenon) = photon flux (photons/cm2 •day) over a 1 nm interval centered at wavelength 

 j = conversion factor to make units consistent with the L/mol•cm unit of the molar 

 absorption coefficient and the mol/L unit of concentration = 

 6.02 x 1020 (L/cm3

) (photons/mol) 
ENV/JM/TG/RD(2006)1/REV 

20

 kdλ (act) = monochromatic direct photolysis rate constant (1/day, pseudo first-order) 

of an 

 actinometer exposed to monochromatic irradiation at wavelength λ

φ(act) = the quantum yield of the actinometer 

ελ = molar absorption coefficient (L/mol•cm; M-1•cm-1) of the actinometer at 

 wavelength λ 

 Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area 

 lxenon = light pathlength in cell exposed to a filtered xenon arc lamp 

 

62.82. Equation 910 only gives the photon flux at one wavelength, but it can be used at different 

individual wavelengths wherever another monochromatic direct photolysis rate constant is determined for 

one or more low optical density actinometers. The determination of the photon flux at several different 

individual wavelengths spread throughout the wavelength range in which an estimate of the photon flux is 

desired should be sufficient to determine the photon flux from a filtered xenon arc lamp as a continuous 

function of wavelength. Because photon flux irradiation spectrum from a filtered xenon arc lamp is 

smooth, interpolation from known photon fluxes at several different wavelengths can be readily done (53 

1). 

63.83. By determining the polychromatic zero order direct photolysis rate constant for a high optical 

density actinometer of known quantum yield in PW in a photolysis cell irradiated with a filtered xenon arc 

lamp, the sum of the incident photon flux over the absorption spectrum of the actinometer above 2905 nm 

can be determined from the following equation (53 1) (see Annex 4):

I jD k xenon

cell

act

d act

initial

final

0λ

λ

λ

φ ( ) ( )( = =

=

∑ zero order)

 [eq. 101]

where: 

 (λ=initial) = initial wavelength of actinometer absorption (if > 2905 nm) 

 (λ=final) = final wavelength of the actinonmeter absorption (if < 800 nm) 

 I0λ = photon flux over a 1 nm interval centered at wavelength λ (photons/cm2 

•day) 

j = conversion factor to make units consistent with the L/mol•cm unit of the 

molar 

 absorption coefficient and the mol/L unit of concentration = 

 6.02 x 1020 (L/cm3

)(photons/mol) 

kd_(act) (0 order)= monochromatic direct photolysis rate constant (zero order) of an actinometer 

 exposed to monochromatic irradiation at wavelength λ

(mol/L•day) 

 Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area (cm) 

φact = the quantum yield of the actinometer 

 

64.85. By using more than one high optical density actinometer with absorption bands at different 

wavelengths, the sum of the photon flux over any desired range including the entire range between 2905

and 800 nm can be determined. Further experimental details of procedures for determining the photon flux 

from and for calibrating xenon arc lamps are provided in references (41 37)(53 1).
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21

Performance of direct photolysis study

Tier 1 - Theoretical screen. 

65.86. For a test substance chemical which does not ionize significantly within a pH range of 4 5 to 9, a 

single UV-Visible absorption spectrum should be measured at a single pH within the 4 5 to 9 range where 

the hydrolysis rate, if any, is minimized. If the chemical is stable to abiotic hydrolysis, the spectrum should 

be measured at pH 7. The pH at which a direct photolysis study on a non-ionic test substance chemical is 

conducted should be the same as the pH at which the spectrum is determined. For a test substance chemical 

that is appreciably ionized anywhere within a 4 5 to 9 pH range, UV- Visible absorption spectra should be 

measured at pH 4 5, 7, and 9.

66.87. Electronic absorption spectra of the test substance chemical should be obtained in appropriate 

solvent systems at desired pHs with a spectrometer capable of recording UV-VISIBLE spectra between 

2905 and 800 nm. Because the higher energy (shorter wavelength) bands of many chemicals may 

sometimes tail into wavelengths above 2905 nm, it is recommended that the spectrum be recorded 30 to 40 

nm below 2905 nm. The procedure to record, assign absorption band maxima, band-widths and determine 

decadic molar absorption coefficients has been described elsewhere (OECD Guideline 101). To obtain an 

adequate absorption spectrum, it may sometimes be necessary to enhance the solubility of the test 

substance chemical by using aqueous methanol, ethanol, or acetonitrile. It is recommended that UVVISIBLE spectra of test substance chemicals be also obtained in the pure neat co-solvent. 

67.88. If the test substance chemical absorbs above the 2905 nm cutoff of solar irradiation at the earth’s 

surface with ελ> 2 L/mol•cm (M-1cm-1) over at least 15 nm, estimate a maximum possible direct photolysis 

rate by assuming that the quantum yield is equal to one in equation 6 (2): 

k L d (max) = =

=

∑ ε λ λ

λ

λ

295

800

 [eq. 112] 

 where: 

 kd(max) = maximum possible direct photolysis rate constant (1/day) 

ελ = molar absorption coefficient (L/mol•cm; M-1 cm-1) at wavelength λ

Lλ = average daily solar photon flux (mmol photons/cm2

•day) over wavelength interval 

 centered at wavelength λ. 

68.89. In estimating the maximum possible photolysis rate constant, the average daily solar photon 

fluxes Lλ substituted into equation 112 should be those for summer at 40° or 50° latitude. Values of Lλ can 

be readily obtained from tables (2)(12). As an alternative to the use of equation 112 and tables of Lλ, the 

maximum direct photolysis rate constant can be readily estimated by inputting molar absorption 

coefficients and a quantum yield equal to one into computer programs such as GCSOLAR (18) or 

ABIWAS (41 35)(42 36). 

Criteria for deciding whether to go to Direct Photolysis Tier 2 or Tier 3

69.90. Determine if the maximum possible direct photolysis rate constant for the test substance chemical 

in the near surface of a clear natural water exposed to an average daily solar photon flux would result in 

estimated direct photolysis losses > 50% of an initial concentration over 30 days /nights of sunlight 

exposure by substituting kmax(solar) for k(solar) in equation 89. 
ENV/JM/TG/RD(2006)1/REV 

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70.91. If estimated direct photolysis losses (assuming a maximum direct photolysis rate constant) are 

less than 50% of the initial concentration over a 30-day period direct photolysis is considered to be an 

insignificant process. In such a case, no further direct photolysis work is performed. If required, Go go to 

Indirect Photolysis Tier 1 (experimental /theoretical screen). 

71.92. If estimated direct photolysis losses (assuming a maximum direct photolysis rate constant) are 

greater than or equal to 50% of the initial concentration over a 30-day period, go to Direct Photolysis Tier 

2 (experimental screen and range finding) or Direct Photolysis Tier 3 (determination of the quantum yield). 

Tier 2 - Experimental screen and range finding test for higher tiers

72.93. Dissolve the test substance chemical in sterilized, buffered pure water and place an optically 

dilute solution in photolysis test vessels. Expose at least half of the test vessels to a filtered xenon arc lamp 

or sunlight and allow an equal number or fewer of the test vessels to serve as dark controls. To account for 

volatile losses of test substance and /or transformation products, test vessels should be attached to suitable

traps for the collection of organic volatiles and CO2.

73.94. Determine the concentrations of the test substance chemical in the irradiated test vessels and in 

the dark control test vessels at adequate enough sampling intervals to approximately determine the direct 

photolysis rate constant of the test substance. chemical.

74.95. At each sampling interval, sacrifice an irradiated photolysis cell for analysis. The number of 

sampling intervals for which a dark control is also sacrificed for analysis should depend upon a prior 

knowledge of the rate of hydrolysis of the test substance chemical at the experimental pH. For example, if 

the test substance chemical is susceptible to appreciable rates of hydrolysis at the experimental pH, a dark 

control cell should be sacrificed for analysis each time an irradiated cell is sacrificed. However, if the test 

substance chemical is stable to hydrolysis at the experimental pH, a minimum of 3 sampling intervals for 

the dark control will be adequate. 

75.96. If the data follow first-order kinetics, compute irradiated and dark control rate constants for the 

test substance chemical by using linear regression on equation 3 or non-linear regression on equation 2. 

76.97. Once the irradiated and dark control rate constants for the test substance chemical have been 

determined, compute the approximate laboratory direct photolysis rate constant from the following 

equation: 

kkk d screen irradiated dark ( ) = − [eq. 123] 

Estimation of direct photolysis rate constants in near surface clear natural water from laboratory values

77.98. Estimate a direct photolysis rate constant for the test chemicaltest substance in near surface clear 

natural water exposed to average daily solar photon fluxes Lλ _ from the direct photolysis rate constant in a 

photolysis cell exposed to continuous photon fluxes from a filtered xenon arc lamp using the following 

equation (assuming Dcell the pathlength l; see Annex 4): 
ENV/JM/TG/RD(2006)1/REV 

23

k j D

l

k L

I

d solar

cell

d xenon

xenon

( )

( )

( )

. =

∑

∑ 23

295

800

0

ε

ε

λ λ

λ λ

 [eq. 134] 

 where: 

kd(solar) = average daily direct photolysis rate constant (1/day) for the test chemicaltest substance

in the near 

 surface of clear natural water exposed to average daily solar irradiation fluxes Lλ

kd(xenon) = direct photolysis rate constant (1/sec) for the test chemicaltest substance in buffered 

pure water in 

 photolysis cells exposed to constant filtered xenon arc lamp fluxes I0λ

ελ(chem) = molar absorptivity of the test chemicaltest substance (L/mol•cm; M-1 cm-1) 

Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area (cm) 

lxenon = light pathlength in cell exposed to a filtered xenon arc lamp 

I0_(xenon) = incident filtered xenon arc lamp photon flux (photons/cm2

•day) over a 1 nm interval 

 centered at wavelength λ

Lλ = 24 hour average daily solar photon flux over wavelength interval λ centered at 

 wavelength λ for the weakly absorbing near surface of clear natural water 

bodies in 

 (mmol photons/cm2•day) 

j = conversion factor = 6.02 x 1020 (L/cm3)(photons/mol) 

78.99. To use equation 134, the photon flux from the filtered xenon arc lamp that is incident on the 

photolysis cells (I0λ) must be determined as a function of wavelength using actinometers or and (if 

available) a spectral radiometer. The solar photon fluxes as a function of wavelength for the near surface of 

clear natural waters (Lλ) for various mid-seasons and latitudes are readily available (2)(12). 

Criteria for deciding whether to go to Direct Photolysis Tiers 3 and/or 4

79.100. Determine if the screen direct photolysis rate constant for the test substance chemical in the near 

surface of a clear natural water exposed to sunlight would result in estimated direct photolysis losses >

20% of an initial concentration over 30 days /nights of sunlight exposure by substituting kd(screen)(solar) for 

k(solar) in equation 89. 

80.101. If estimated direct photolysis losses (assuming the screen direct photolysis rate constant) are less 

than 20% of the initial concentration over a 30-day light/dark sunlight exposure period, direct photolysis is 

considered to be an insignificant process. In such a case, no further direct photolysis work is performed. If 

required, p Proceed to the Indirect Photolysis Tier 1 screen. 

81.102. If estimated direct photolysis losses (assuming the approximate direct photolysis rate constant) 

are greater than or equal to 20% of the initial concentration over a 30-day light/dark sunlight exposure 

period, proceed to Direct Photolysis Tiers 3. and/or 4. If determining the quantum yield of the test 

chemical and using it to estimate direct photolysis rate constants for types of water bodies, seasons, and 

latitudes is required, proceed to Direct Photolysis Tier 3. Otherwise, proceed to direct photolysis Tier 4 to 

perform a more definitive kinetics study. 
ENV/JM/TG/RD(2006)1/REV 

24

Tier 3 - Determination of the quantum yield and estimations of direct photolysis rate constants

82.103. Determine the quantum yield of the test substance chemical in buffered solutions using 

monochromatic irradiation (12), filtered xenon-arc lamp polychromatic irradiation (8) or sunlight 

polychromatic irradiation (2). Each has advantages and disadvantages compared to the others (see Annex 

7). 

104. The main advantage in using monochromatic irradiation to determine the test chemical quantum 

yield of the test chemical is that (as will be shown later by equations 19 through 21) it eliminates the need 

to directly measure the incident monochromatic irradiation. However, a disadvantage is that it will take 

longer to determine the quantum yield with monochromatic irradiation than with the use of polychromatic 

irradiation because the actinometer and the test chemical are absorbing photon fluxes at just one 

wavelength instead of over multiple wavelengths. 

105. The main advantage in using polychromatic irradiation (filtered xenon arc lamp or sunlight) 

instead of monochromatic irradiation to determine the test chemical quantum yield is that the time required 

to determine the quantum yield may be substantially shorter (8). The reason is due to the absorption of 

photon fluxes by the actinometer and test chemical over multiple wavelengths instead of just one 

wavelength. The main disadvantage in using polychromatic irradiation to determine the test chemicaltest 

substance quantum yield is that unlike with monochromatic irradiation (as will be shown later by equations 

20 and 21), its use requires the determination of the photon flux.

106. The advantage in using sunlight instead of a filtered xenon arc lamp, if polychromatic irradiation 

is used to determine the quantum yield, is that tables of average solar intensities as a function of season and 

latitude are readily available. Also, if sunlight is used, it eliminates the necessity of having a 

monochromatic or polychromatic artificial light source. However, if sunlight is used, an actinometer with 

an adjustable quantum yield is needed so the direct photolysis rate of the actinometer can be adjusted to be 

comparable to that of the test chemical under the same irradiation conditions (2)(12). Parallel exposure of 

the test chemical and actinometer to sunlight over comparable time frames minimizes errors due to changes 

in solar flux caused by fog, varying cloud cover, etc. (12)(51).

107. An example of an adjustable quantum yield actinometer is the p-nitroacetophenone - pyridine 

(PNAP-PYR) actinometer (52). For a 1.0 X 10-5 M p-nitroacetophenone solution, the quantum yield of the 

actinometer in experiments carried out with monochromatic irradiation at 313 nm can be adjusted by 

varying the concentration of pyridine in the solution according to the following equation (2)(12):

φ act= • 0 0169 .[ ] pyridine [eq. 15]

where:

φact = quantum yield of the 1.0 X 10-5 M p-nitroacetophenone

[pyridine] = molar concentration of pyridine

108. Suitable chemical actinometers used in aqueous photochemical studies include the following:

Actinometer System Literature Citation

(a) p-nitroacetophenone/pyridine 52

(b) p-nitroanisole/pyridine 26, 52

(c) valerophenone 53

(d) ferrioxalate 19, 23, 37, 54, 55
ENV/JM/TG/RD(2006)1/REV 

25

(e) o-nitrobenzaldehyde 37, 52

(f) Reinecke’s salt 12

(g) aberchrome 540 56, 57

109. The first two actinometers listed in the above table have adjustable quantum yields and are 

therefore particularly useful when solar irradiation is used (12). The first three actinometers listed in the 

table are low optical density actinometers and the last four are high optical density actinometers. The 

characteristics of those actinometers including the optimal wavelength range for their use are discussed in 

references 12 and 58. Other chemical actinometers are described in the IUPAC document on chemical 

actinometers (58).

Determination of the quantum yield using monochromatic irradiation 

83.110. Place solutions of the test substance chemical and the selected optically low density actinometer 

in separate but identical photolysis cells. Irradiate at least half of the photolysis cells with monochromatic 

irradiation of wavelength λ and let an equal or fewer number (depending upon the hydrolytic stability of 

the test chemicaltest substance) serve as dark controls. Determine the concentration of the test substance in 

the irradiated cells and in the dark controls at adequate enough sampling intervals to determine the 

monochromatic direct photolysis rate constant of the test chemicaltest substance (kd_(chem)) and of the 

actinometer (kdλ(act)) at the wavelength λ of irradiation. At each sampling interval, sacrifice an irradiated 

photolysis cell for analysis. The number of sampling intervals for which a dark control is also sacrificed for 

analysis should depend upon a prior knowledge of the rate of hydrolysis of the test substance chemical at 

the experimental pH. 

84.11. The decline of a test substance chemical or actinometer concentrations at low optical density 

should follow pseudo first-order kinetics so that the monochromatic irradiated and the dark control rate 

constants can be determined by using linear regression on equation 3 or non-linear regression on equation 

2. The decline of a test substance chemical or actinometer at high optical density should follow zero-order 

kinetics so that the monochromatic irradiated and the dark control rate constants can be determined by 

using linear regression on the following zero-order kinetics equation: 

C C kt = − 0 [eq. 146] 

85.112. Once the monochromatic irradiated and dark control rate constants for the test substance 

chemical and the actinometer have been determined, the monochromatic direct photolysis rate constant of 

the test substance chemical (kd_(chem)) and of the actinometer kdλ(act) at wavelength λ are given by: 

kk k d chem or irradiated chem or act dark chem or act λλ λ ( ) ( )( ) ( )( ) act = −

 [eq. 157] 

Note: In a sterilized aqueous buffer, the dark control rate constant (kdark) should be equal to the 

hydrolysis rate constant at the pH of the buffer. 

86.113. Determine the molar absorption coefficients of the test substance chemical (ελ(chem)) and (if a low 

optical density actinometer is being used) the actinometer (ελ(act)) at the wavelength λ of the 

monochromatic irradiation using the Beer-Lambert law assuming that ελC is >> than the attenuation 

coefficient αλ of the test medium: 
ENV/JM/TG/RD(2006)1/REV 

26

ε λ

λ

(

( )

( ) [ ] chem

chem act

chem act

A

l C or act)

 or 

 or 

=

 [eq. 168] 

 where: 

ελ (chem) = molar absorption coefficient of the test substance chemical or actinometer 

 L/mol•cm; M-1cm-1) 

 Aλ(chem or act) = absorbance of test substance chemical or actinometer solution at wavelength λ

 L = light pathlength (cm) generally equivalent to Dcell

 [C(chem or act)] = concentration of test substance chemical or actinometer solution (mol/L) 

87.114. The quantum yield of an optically dilute test substance chemical determined using 

monochromatic irradiation and a low optical density (low absorbance) actinometer is given by (12)(49 7) 

(See Annex 4): 

φ φ

ε

ε

λ λ

λ λ

chem act

d chem act

d act chem

k

k = ( )()

()( ) [eq. 179] 

 where: 

φchem = the quantum yield of the test substance chemical

φact = the quantum yield of the actinometer 

kdλ(chem) = monochromatic direct photolysis rate constant (1/day, pseudo first-order) of a test 

 substance chemical exposed to monochromatic irradiation at wavelength λ

 kdλ(act) = monochromatic direct photolysis rate constant (1/day, pseudo first-order) of an 

 actinometer exposed to monochromatic irradiation at wavelength λ

ελ(chem) = molar absorption coefficient (L/mol•cm; M-1 cm-1) of the test substance chemical at 

 wavelength λ 

ελ (act) = molar absorption coefficient (L/mol•cm; M-1 cm-1) of the actinometer at wavelength λ 

88.115. The quantum yield of an optically dilute test substance chemical determined using 

monochromatic irradiation and a high optical density (high absorbance) actinometer is given by (12)(49 7).

(See Annex 4): 

φ φ

ε λ

λ

λ

chem

act

chem cell

d chem

D d act zero

k

k = 2 3. ( )

( )

( )( order) [eq. 1820] 

 where: 

φchem = the quantum yield of the test chemicaltest substance

φact = the quantum yield of the actinometer 

 kd_(chem) = monochromatic direct photolysis rate constant (1/day, pseudo first-order) of a test 

 chemical exposed to monochromatic irradiation at wavelength λ

 kd_(act)(zero) = monochromatic direct photolysis rate constant (mol/L•day, zero-order) of an 
ENV/JM/TG/RD(2006)1/REV 

27

 actinometer exposed to monochromatic irradiation at wavelength λ

 Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area (cm) 

89.116. The quantum yield of a test chemicaltest substance at high optical density (high absorbance) 

determined using monochromatic irradiation and a high optical density (high absorbance) actinometer is 

given by (12)(49 7). See Annex 4): 

φ φ λ

λ

chem act

d chem zero

d act zero

k

k = ( )( )

( )( ) [eq. 1921] 

 where: 

φchem = the quantum yield of the test chemicaltest substance

φact = the quantum yield of the actinometer 

 kd_(chem)(zero) = monochromatic direct photolysis rate constant (mol/L•day, zero-order) of a test 

 chemical exposed to monochromatic irradiation at wavelength λ

 kd_(act)(zero) = monochromatic direct photolysis rate constant (mol/L•day, zero-order) of an 

 actinometer exposed to monochromatic irradiation at wavelength λ

90.117. As can be seen from equations 179, 1820, and 1921, it is not necessary to determine the photon 

flux if monochromatic irradiation is used to determine the test chemicaltest substance quantum yield. 

Determination of the quantum yield using polychromatic irradiation (lamp or sunlight)

91.118. The quantum yield of an optically dilute test chemicaltest substance using a low optical density 

(low absorbance) actinometer and sunlight or a polychromatic filtered xenon arc lamp can be determined 

from equations 202 and 213, respectively (2)(6)(12)(See Annex 4): 

 

φ φ

ε

ε

λ λ

λ

λ

λ λ

λ

chem act λ

d chem

d act

act

nm

nm

chem

nm

nm

k

k

L

L

= 



 





























=

=

=

=

∑

∑

( )

( )

( )

( )

295

800

295

800

 [eq. 202] 

φ φ

ε

ε

λ λ

λ λ

chem act

d chem

d act

act xenon

chem xenon

k

k

I

I

= 



 





























∑

∑

( )

( )

() ( )

()( )

0

295

800

0

295

800

 [eq. 213] 

 where: 

 

 kd(chem) = direct photolysis rate constant (1/day, pseudo first-order) of the test chemicaltest 

substance

 kd(act) = direct photolysis rate constant (1/day, pseudo first-order) of the actinometer 

φchem = quantum yield of the test chemicaltest substance (dimensionless ratio) 

φact = quantum yield of the actinometer (dimensionless ratio) 

ελ(chem) = molar absorption coefficient (L/mol•cm; M-1 cm-1) of the test chemicaltest substance

at 
ENV/JM/TG/RD(2006)1/REV 

28

 wavelength λ

ελ(act) = molar absorption coefficient (L/mol•cm; M-1 cm-1) of the actinometer at 

 wavelength λ

 Lλ = average daily solar photon flux (mmol photons/cm2

•day) over wavelength interval 

 ∆λ centered at wavelength λ

 I0λ(xenon) = incident filtered xenon arc lamp photon flux (photons/cm2

•day) over a 1 nm 

 interval centered at wavelength λ

92.119. As can be seen from equations 202 and 213, it is necessary to independently and accurately 

determine the incident photon flux as a function of wavelength when polychromatic irradiation is used to 

determine the test chemicaltest substance quantum yield. 

93.120 The determination of the test chemicaltest substance quantum yield using polychromatic 

irradiation and an actinometer are discussed in detail in (2) for the use of sunlight, and in (8) for the use of 

a filtered xenon arc lamp. 

Dependence of the quantum yield on wavelength

121. Most of the direct photolysis equations involving polychromatic irradiation in this document and 

in the literature are presented with the quantum yield taken outside of the integral or summation over 

different wavelengths. The reason is that in aqueous solutions, the quantum yields of test chemicals are (in 

general) approximately independent of wavelength, even across different absorption bands. In aqueous 

solutions, molecules in higher excited electronic states tend to undergo radiationless decay to their first 

excited state before any photoreaction occurs (12). If photoreaction does occur, it generally occurs in 

competition with other deactivation processes while the molecule is in its first (not higher) excited 

electronic state. 

122. However, the quantum yield may sometimes differ between absorption bands (6)(14). Therefore, 

it may sometimes be necessary to determine the quantum yield at least one wavelength for each adsorption 

band within the 295-800 nm range using monochromatic irradiation. The wavelength chosen for each 

absorption band should correspond to the maximum absorption. In cases where the quantum yield varies 

with wavelength, the quantum yield should be brought inside any integrals or summations over 

wavelengths.

123. In cases where the quantum yield remains constant within the same absorption band, but varies 

across different absorption bands, equations 5 and 6 for the direct photolysis rate constant can be modified 

to give:

k I d i ii

i begin

i end

i

i n

= ∑ ∑ =

=

φ ε λ λ

λ

λ

0

1 ()

( )

[eq. 24]

k L d i

i

i n

i i

i begin

i end

= =

=

∑ ∑ φ ε λ λ

λ

λ

1 ()

( )

[eq. 25]

where:

φI = quantum yield for absorption band i
ENV/JM/TG/RD(2006)1/REV 

29

εiλ = molar absorption coefficient (L/mol•cm; M-1 cm-1) at wavelength λ within 

absorption band i

I0iλ = photon flux per nm (photons/cm2 •sec/nm) at wavelength λ within absorption 

band i

λI = wavelength interval in nm generally = 1 nm

Liλ = average daily solar photon flux irradiance (mmol photons/cm2•day) over 

wavelength interval ∆λ centered at wavelength λ within absorption band i

I = indice for different absorption bands up to the maximum of n bands

iλ(begin) = wavelength at the beginning of absorption band i

iλ(end) = wavelength at the end of absorption band i

Use of the quantum yield to estimate direct photolysis rate constants

94.124. The pseudo first-order rate constant for direct phototransformation and thus the life-time of a test 

substance in water can be calculated from the quantum yield (18)(52). Once the quantum yield of a test 

chemical has been determined, it should be used as input into c Computer programs such as GCSOLAR 

(18) or ABIWAS (41 35)(42 36) to estimate direct photolysis rate constants for seasons, latitudes, and 

water body depths and light attenuations of interest. GCSOLAR and ABIWAS can be used to estimate 

direct photolysis rate constants at any given time in near surface water or at any specified depth. The 

programs can also average rate constants over any given time and/or depth interval of interest. 

95.125. Although programs such as GCSOLAR or ABIWAS are readily available, equation 5 is often 

used to estimate the direct photolysis rate constant for optically dilute solutions of test chemicaltest 

substances in pure water in photolysis cells or in the near surface of clear natural water exposed to solar 

irradiation (2)(12).

Criteria for deciding whether to go to direct photolysis Tier 4

96.126. A Direct Photolysis Tier 4 kinetics study should be performed if identification/kinetics data on 

the major transformation products are applicable and if the direct photolysis rate constant(s) for the parent 

test chemicaltest substance (estimated from its quantum yield) indicate(s) that 

direct photolysis is a potentially significant process. 

97.127. Determine if the direct photolysis rate constant (estimated from the quantum yield) for the test 

chemicaltest substance in shallow natural water exposed to sunlight would result in estimated direct 

photolysis losses > 20% of an initial concentration over 30 days /nights of sunlight exposure by 

substituting kd(solar) for k(solar) in equation 89. 

98.128. If estimated direct photolysis losses are less than 20% of the initial concentration over a 30 day 

light/dark sunlight exposure period, direct photolysis is considered to be an insignificant process. In such a 

case, no further direct photolysis work is performed.. If of interest, pProceed to indirect photolysis Tier 1. 

Proceed to direct photolysis Tier 4 if the estimated direct photolysis losses are greater than or equal to 20% 

of the initial concentration over a 30 day sunlight exposure period.:

Estimated direct photolysis losses are greater than or equal to 20% of the initial concentration over a 30 

day light/dark sunlight exposure period, and

•The identification of major transformation products and/or the determination of their rates of formation 

and decline are required. 
ENV/JM/TG/RD(2006)1/REV 

30

Tier 4 Identification of - Kinetics study on the test chemical and (if applicable) major transformation 

products

Experimental determinations of direct photolysis rate constants in the laboratory

99.129. In direct photolysis Tier 4, the decrease in the concentrations of the test chemical the test 

substance is determined in buffered pure water is exposed to a filtered xenon arc lamp or sunlight. If 

applicable, Tthe rates of formation and decline of major transformation products are also determined, if 

possible. Major phototransformation products are isolated and identified.. For the purposes of this 

guideline, a major transformation product is one accounting for > 10% of the applied radioactivity (or 

molar basis) in an individual sample at any time during the study. 

100.130. If a flow-through system is used to trap volatile test chemicaltest substances and/or their major 

transformation products, the following considerations should be made. The air should be passed through 

microbial filters to keep the samples sterile. Moist air should be used to prevent evaporation. The system 

should be checked for sterility at the study termination.

101.131. Place the test solution in photolysis test vessels. At least half the test vessels should be exposed 

to a filtered xenon arc lamp or sunlight and an equal or fewer number of the test vessels should serve as 

dark controls. 

102.132. Determine the concentrations of the transformation products in the irradiated test vessels and in 

the dark control test vessels at adequate enough sampling intervals. to determine the direct photolysis rate 

constant of the test chemical.

103.133. At each sampling interval, sacrifice an irradiated photolysis cell for analysis. The number of 

sampling intervals for which a dark control is also sacrificed for analysis should depend upon the rate of 

hydrolysis of the test chemicaltest substance at the experimental pH.

134. f the data follow first-order kinetics, compute irradiated and dark control rate constants for the 

test chemical by using linear regression on equation 3 or non-linear regression on equation 2.

135. Once the irradiated and dark control rate constants for the test chemical have been determined, 

compute the laboratory direct photolysis rate constant from the following equation:

kk k d irradiated dark = − [eq. 26]

136. If a filtered xenon arc lamp is used for irradiation, estimate an approximate average daily direct 

photolysis rate constant for the test chemical in shallow natural water exposed to average daily solar 

photon fluxes Lλ from equation 14.

137. In direct photolysis Tier 4, the decrease in the concentrations of the test chemical is determined in 

buffered pure water exposed to a filtered xenon arc lamp or sunlight. If applicable, the rates of formation 

and decline of major transformation products are also determined. For the purposes of this guideline, a 

major transformation product is one accounting for > 10% of the applied radioactivity at any time during 

the study.

138. If a flow-through system is used to trap volatile test chemicals and/or their major transformation 

products, the following considerations should be made. The air should be passed through microbial filters 

to keep the samples sterile. Moist air should be used to prevent evaporation. The system should be checked 
ENV/JM/TG/RD(2006)1/REV 

31

for sterility at the study termination. 

139. The following equation can be used to estimate the direct photolysis rate constant for the test 

chemical in shallow natural water exposed to one or more different sunlight conditions from those in the 

study (see Annex 3 - Equation Derivations):

k k

L

L

sun shallow sun shallow water

nm

nm

nm

( ) nm

( )

( )

 water)2 = =

=

=

=

∑

∑ 1

2

295

800

1

295

800

ε

ε

λ λ

λ

λ

λ λ

λ

λ

[eq. 27]

where:

kd(sun shallow water)2 = estimated direct photolysis rate constant in the near surface of clear natural 

water exposed to solar irradiation fluxes (Lλ(2)) different from solar irradiation 

fluxes (Lλ(1)) used in the experiment.

kd(sun shallow water)1 = estimated direct photolysis rate constant in the near surface of clear natural 

water exposed to the experimental solar irradiation fluxes.

Lλ(2) = average daily solar photon fluxes different from those closest 

(Lλ(1)) to the experimental conditions.

Lλ(1) = average daily solar photon fluxes closest to the experimental ones 

based on the season and latitude at which the study was performed.

140. Compute direct photolysis half-lives by substituting the estimated direct photolysis rate constants 

into equation 4.

Major transformation products (if required)

104.141. Identify and quantify major phototransformation products using analytical methods and criteria 

indicated in paragraph s 27. 39 through 43.

105.142. In cases where data follow first-order kinetics and the parent test chemicaltest substance rapidly 

transforms, use the same first-order methods applied to the test chemicaltest substance to also estimate 

first-order direct photolysis rate constants and corresponding half-lives for any major primary 

transformation products formed from the parent which are still at substantial concentrations (e.g. at > 10% 

of applied) after the parent has degraded to negligible levels. 

143. Equation 14 for estimating sunlight rate constants from filtered xenon arc lamp rate constants and 

equation 27 for estimating a sunlight rate constant for a given season and latitude from a sunlight rate 

constant for a different season and/or latitude are also applicable to any first-order photolysis rate constants 

determined for any major transformation products. 

106.144. In cases where primary or secondary transformation products are being substantially formed over 

the same time period they are being substantially lost, their direct photolysis rate constants and 

corresponding half-lives cannot be readily estimated as they were for the parent. However, as an option, 

attempts can be made to estimate first-order rate constants for the formation and decline of major 

transformation products using non-linear regression to fit concentration versus time series data to various 

equations derived from assumed transformation pathways.

Performance of indirect photolysis tiers 
ENV/JM/TG/RD(2006)1/REV 

32

Tier 1 for indirect photolysis - Experimental and/or theoretical screen

145. Perform a screen in simulated water (containing humics as a surrogate for CDOM and when 

applicable, nitrate) or natural water to determine if indirect photolysis represents a potentially significant 

transformation pathway. 

146. Place solutions of the test chemical in simulated natural water (SNW) and/or simulated natural 

water with nitrate (SHW-N) and/or natural water (NW) and in buffered pure water (PW) into equal 

numbers of photolysis cells. Expose at half of the photolysis cells containing each type of solution to 

sunlight and let an equal or smaller number of the photolysis cells containing each type of solution serve as 

dark controls.

147. The use of a filtered xenon arc lamp for indirect photolysis studies is not recommended at this 

time. The complexity of indirect photolysis processes does not allow straightforward extrapolation of 

results from filtered xenon arc lamps to sunlight equivalents and may not be feasible (2)(12). 

148. Determine the concentrations in the irradiated photolysis cells and in the dark control photolysis 

cells at adequate enough sampling intervals to determine the irradiated and dark control rate constants of 

the test chemical in SNW and/or SNW-N and/or in NW and in PW. At each sampling time, sacrifice an 

irradiated photolysis cell. The number of sampling intervals for which a dark control is also sacrificed for 

analysis should depend upon the stability of the test chemical in the dark control at the experimental pH. 

Assuming the decline of the test chemical follows first-order kinetics, determine the irradiated and dark 

control rate constants in the irradiated and dark control solutions by using linear regression on equation 3 

or non-linear regression on equation 2.

149. Once the irradiated and dark control rate constants for the test chemical in SNW and/or SNW-N

and/or NW and in PW have been determined, the net photolysis rate constants, the direct photolysis rate in 

PW, and the indirect photolysis rates constants can be respectively determined from the following 

equations:

kk k pX X irrad X dark = − ( ) () [eq. 28]

kk k d PW PW irrad PW dark () ( ) ( ) = − [eq. 29]

k kk iX approx pX d PW ( ) () = − [eq. 30]

where:

X = SNW, SNW-N, or NW

Note: Equation 30 is only approximate because the direct photolysis rate constant in PW (kd(PW)) will be 

greater than the direct photolysis rate constant in SNW, SNW-N or NW (kdX) due to light 

attenuation (12). However, the approximation is adequate for a screen. Also, unlike indirect 

photolysis Tier 2, this indirect photolysis Tier 1 screen does not take into account the 

photobleaching of humic solutions in photolysis cells by sunlight as reported by Winterle and 

Mill (59).

150. Determine if the approximate indirect photolysis rate constant for the test chemical in shallow 

natural surface water would result in estimated indirect photolysis losses > 20% of an initial concentration 

over 30 days/nights of sunlight exposure by substituting kiX(approx) for k(solar) in equation 9.
ENV/JM/TG/RD(2006)1/REV 

33

151. If estimated indirect photolysis losses (assuming an approximate indirect photolysis rate 

constant) are less than 20% of the initial concentration over a 30 day/night sunlight exposure period, 

indirect photolysis is considered to be an insignificant process. In such a case, no further indirect 

photolysis work is performed.

152. If estimated indirect photolysis losses (assuming an approximate indirect photolysis rate 

constant) are greater than or equal to 20% of the initial concentration over a 30-day light/dark sunlight 

exposure period, indirect photolysis is considered to be a potentially significant process. Proceed to 

Indirect Photolysis Tier 2 to perform a more definitive kinetics study.

153. Mill (21) has summarized structural activity relations (SARs) for estimating second-order rate 

constants for the reaction of organics with photooxidants in water. He also summarized typical 

concentrations of various photooxidants generally found in the top layers of surface water exposed to 

sunlight. In some cases, it may be possible to supplement or even replace the experimental screen with a 

theoretical screen based on using structural activity relationships (SARs). These SARs estimate rate 

constants and information on the typical concentrations of photoxidants in surface water. Although a 

theoretical screen involving one or two potential photooxidants may indicate that going to Tier 2 is 

justified, it is doubtful that one can indicate that going to Tier 2 is not justified. The reason is that a 

theoretical screen will generally cover only one or two of the large number of possible types of indirect 

photolysis reactions. Further details on using structure activity relations for estimating rate constants for 

the reaction of test chemicals with photoxidants in water such as the hydroxyl radical and singlet oxygen 

are provided in reference 51.

Tier 2 - Experimental determination of rate constants (and identification/quantification of major 

transformation products, when required)

154. In the indirect photolysis Tier 2, the decrease in the concentration of the test chemical is 

determined in simulated natural water (SNW) and/or simulated natural water with nitrate (SNW-N) and/or 

one or more natural surface waters (NW) and in buffered pure water (PW) exposed to sunlight. If 

applicable, the identities and rates of formation and decline of major transformation products are also 

determined. For the purposes of this guideline, a major transformation product is one accounting for > 10% 

of applied at any time during the study.

155. The performance of Tier 2 differs from that in Tier 1 in several ways including:

•The number of sampling times is greater.

•The direct photolysis constant in humic water is estimated by inputing the chemical’s quantum yield and 

the attenuation constants of the humic solution into GCSOLAR or ABIWAS. 

•If applicable, the rates of formation and decline of major transformation products are determined.

•If a study lasts for several days or more such that “photobleaching” of the humic solutions in photolysis 

cells may be substantial in some cases, it can (as an option) be accounted for as indicated in Leifer (12) 

and OPPTS 835.5270(2). 

156 Dissolve the test chemical in SNW and/or SNW-N and/or NW and in PW and place the solutions 

in equal numbers of photolysis cells. Expose at least half of the photolysis cells containing each type of 

solution to sunlight and let an equal or lesser number of the photolysis cells containing each type of

solution serve as dark controls. 

157. Determine the concentrations in the irradiated cells and in the dark controls at adequate enough 

sampling intervals to determine the irradiated and dark control rate constants of the test chemical in 

simulated natural water (SNW), and/or simulated natural water with nitrate (SNW-N), and/or natural 
ENV/JM/TG/RD(2006)1/REV 

34

surface water (NW), and in buffered pure water (PW). At each sampling interval, sacrifice an irradiated 

test vessel for analysis. Sacrifice dark controls for analysis at an equal or fewer number of sampling 

intervals 

158. Determine the irradiated and dark control rate constants of the test chemical in SNW and/or 

SNW-N and/or NW and in PW by using linear regression on equation 3 or non-linear regression on 

equation 2.

159. Once the irradiated and dark control rate constants for the test chemical in SNW and/or SNW-N 

and/or NW and in PW have been determined, the net photolysis rate constants in those waters and the 

direct photolysis rate constants in PW can be determined from equations 28 and 29.

160. If the same light source and photon flux, test vessels, and experimental set-up are used for the 

definitive determination of the direct photolysis rate constant in PW in the Tier 4 Direct Photolysis study, a 

separate determination of the direct photolysis rate constant in PW in the Tier 2 Indirect Photolysis study 

as just described is not necessary. 

Determination of direct photolysis rate constant in SNW, and/or SNWN, and/or NW from 

one in PW

161. Determine attenuation constants as a function of wavelength for all experimental waters (SNW, 

SNW-N, and/or NW and PW) from the following equation:

α λ λ( ) ( ) = A

l [eq. 31]

where:

α(λ) = light attenuation constant as a function of wavelength λ (1/cm)

A(λ) = absorbance of test medium as a function of wavelength λ

l = light pathlength in cell (cm) 

162. Estimate average daily direct photolysis rate constant for the test chemical in near surface PW 

and in SNW and/or SNW-N, and/or NW by inputting its quantum yield and attenuation constants for PW 

and for SNW, and/or SNW-N, and/or NW into GCSOLAR or ABIWAS. If the quantum yield of the test 

chemical was determined in Direct Photolysis Tier 3, use it as input. Otherwise, estimate the quantum yield 

by substituting the experimental direct photolysis rate constant in PW and ελLλ into equation 6 and 

rearranging to solve for the quantum yield. 

163. Estimate the direct photolysis rate constant in SNW and/or SNWN and/or NW in a photolysis 

cell from the following equation:

k

k

k

k dX

dX GCSOLARorABIWAS

d PW GCSOLARorABIWAS

d PW = ( )

( ) ( )

( )(exp)

[eq. 31]

where: 

X = SNW, SNW-N, or NW
ENV/JM/TG/RD(2006)1/REV 

35

kdX = direct photolysis rate constant in SNW, SNWN, or NW in a photolysis cell

kdX(GCSOLAR) = GCSOLAR or ABIWAS estimated average daily direct photolysis rate constant 

in near surface SNW, SNW-N, or NW

kd(PW)(GCSOLAR) = GCSOLAR or ABIWAS estimated average daily direct photolysis rate constant 

in PW

kd(PW)(exp) = experimentally determined direct photolysis rate constant in PW in a photolysis 

cell

164. Once the net and direct photolysis rate constants in SNW and/or SNW-N and/or NW have been 

determined, the indirect photolysis rate constant in those waters can be estimated from the following 

equation:

kkk iX pX dX = − [eq. 32]

where X = SNW, SNW-N, or NW

165. The methods used in the identification and kinetics analysis of transformation products formed 

during indirect photolysis studies are comparable to those formed during direct photolysis studies.

Method of accounting for “photobleaching”

166. If a study lasts for several days or more, the decline of the test chemical in irradiated SNW, 

SNW-N, or NW may not be first-order due to the “photobleaching” of humic solutions. Photobleaching in 

photolysis cells that has been studied and reported by Winterle and Mill (54). 

167. Photobleaching does not appear to be a significant process in the actual environment because 

CDOM appears to be generated at much faster rates than the rates of its decline through photobleaching 

(51). However in the laboratory, the photobleaching of humic solutions in photolysis cells by sunlight 

somewhat complicates a more definitive determination of the indirect photolysis rate. Because of 

photobleaching, the indirect photolysis rate constant for a test chemical in SNW, SNW-N, or NW 

solutions in photolysis cells exposed to sunlight decreases over time. If that occurs, there are two options:

168. When photobleaching begins to become substantial, the study can be terminated. In such 

cases, only data obtained from the study prior to substantial photobleaching should be used in the 

analysis. The study can be continued by accounting for photobleaching.

Extrapolation from cell or near surface to the same water over a deeper depth, and from 

experimental to other solar photon fluxes 

169. Unfortunately, methods of extrapolation to greater depths and light attenuation and to other solar 

conditions are not as straightforward for indirect photolysis as they are for direct photolysis, even when 

irradiation is restricted to sunlight. The reason isthat indirect photolysis may involve the absorption of solar 

flux by many different types of sensitizers with different absorption spectra and by nitrate. Therefore, the 

results of an indirect photolysis study conducted in cells exposed to natural sunlight are only applicable to 

the near surface and the same set of solar conditions as the experimental ones. 

REPORTING OF DATA AND RESULTS

170. The degree of presentation of results depends on the complexity and purpose of the study, that is, 

the number of tiers. The reporting should follow the IUPAC recommendations (63).
ENV/JM/TG/RD(2006)1/REV 

36

Direct photolysis data and results

Tier 1 

• Provide the UV-VISIBLE spectrum or spectra of the test chemicaltest substance as the molar 

absorption coefficients (L/mol•cm or M-1cm-1) on the y-axis versus wavelengths (nm) on the x-axis. 

• Report the estimated maximum photolysis rate constant and associated direct photolysis losses over a 

30 days /nights sunlight exposure period. Provide the equations and calculations used to estimate 

them. Provide the rationale for selecting the season and latitude of the average daily solar photon 

fluxes (Lλ) used in the calculations.Report whether or not the results of the calculations trigger Direct 

Photolysis Tiers 2 and/or 3. 

Tier 2 

• If a filtered xenon arc lamp was used, provide the incident photon flux in a graph as the photon flux 

(photons/cm2

•sec) on the y-axis versus the wavelength (nm) on the x-axis. 

• If sunlight was used, provide tabular values of solar photon fluxes for the season and latitude closest to 

those of the study. 

• Provide table(s) and graph(s) which show the concentrations of the test chemicaltest substance in 

irradiated and dark control solutions versus time. Show the best fit regression lines on the graph(s) 

along with the data points. 

• Report the irradiated, dark control, and approximate direct photolysis rate constant of the test 

chemicaltest substance along with their corresponding half-lives. Provide the equations, calculations, 

and regression coefficients used to compute them together with regression n and r2 values, and 

standard errors of coefficients (for example, slope, intercept). 

• Based upon the approximate direct photolysis rate constant, report the associated estimated direct 

photolysis losses over a 30 days /nights sunlight exposure period. Provide the equations and 

calculations used to estimate them. Provide the rationale for selecting the season and latitude of the 

average daily solar photon fluxes (Lλ) used in the calculations. Report whether or not the results of the 

calculations trigger Direct Photolysis Tiers 3 and/or 4. 

Tier 3 

• For the determination of the test chemicaltest substance quantum yield with the use of an actinometer, 

provide table(s) and graph(s) showing the concentrations of the test chemicaltest substance and 

actinometer in irradiated and dark control solutions versus time. Show the best fit regression lines on 

the graph(s) along with the data points. 

• Report the irradiated, dark control, and direct photolysis rate constants of the test chemicaltest 

substance and actinometer. Provide the equations, calculations, and regression coefficients used to 

compute them together with regression n and r2 values, and standard errors of coefficients (for 

example, slope, intercept). 

• Report the quantum yield of the test chemicaltest substance. Provide the equation and calculation used 
ENV/JM/TG/RD(2006)1/REV 

37

to compute it. 

• Report estimated direct photolysis rate constants for the test chemicaltest substance in natural water 

that are estimated from the test chemicaltest substance quantum yield for seasons, latitudes, and water 

body types of interest. Provide and/or describe the equations and calculations or computer program 

(e.g. GCSOLAR or ABIWAS) and inputs to estimate them. 

Tier 4

•If a filtered xenon arc lamp was used, provide the incident photon flux in a graph as the photon flux 

(photons/cm2•sec) on the y-axis versus the wavelength (nm) on the x-axis.

•If sunlight was used, provide tabular values of solar photon fluxes for the season and latitude closest to 

those of the study.

• Provide table(s) and graph(s) showing the concentrations of the test chemical and (if applicable) major 

phototransformation products concentrations and mass balance determinations in irradiated and dark 

control solutions versus time. Show the best fit regression lines on the graphs along with the data 

points. 

•Report the irradiated, dark control, and direct photolysis rate constants of the test chemical along with 

their corresponding half-lives. Provide the equations, calculations, and regression coefficients used to 

compute them together with regression n and r2 values, and standard errors of coefficients.

• If applicable, report the rate constants for the formation and decline of the major transformation 

products. Report the identity of the major phototransformation products (> 10% of the applied dose). 

Provide the transformation pathway model (see next paragraph), equations, calculations, and 

regression coefficients used to compute them together with regression n and r2 values, and standard 

errors of coefficients. 

•Provide chemical structures and names of the test chemical and (if applicable) major transformation 

products in both irradiated and dark control solutions. Provide graphical presentations of postulated 

transformation pathways in both the irradiated and dark control solutions.

Indirect photolysis data and results

Tier 1

•Assuming sunlight was used for indirect photolysis as recommended, provide tabular values of solar 

photon fluxes for the season and latitude closest to those of the study.

•Provide important characteristics of all simulated and/or natural waters used including DOC, humic 

content (if simulated), pH, dissolved oxygen, nitrate. Include the UV-VISIBLE spectrum of the test 

media from 295 to 800 nm. For natural water, include site of collection.

•Provide table(s) and graph(s) showing the concentrations of the test chemical in irradiated and dark 

control solutions versus time. Show the best fit regression lines on the graph(s) along with the data 

points.

•Report the irradiated, dark control, and approximate net, direct, and indirect photolysis rate constants of 
ENV/JM/TG/RD(2006)1/REV 

38

the test chemical along with their corresponding half-lives.. Provide the equations, calculations, and 

regression coefficients used to compute them together with regression n and r2 values, and standard 

errors of coefficients.

•Report the approximate indirect (or indirect plus hydroxyl) photolysis rate constant and associated 

indirect photolysis losses over a 30 days/nights sunlight exposure period. Provide the equations and 

calculations used to estimate them. Provide the rationale for selecting the season and latitude of the 

average daily solar photon fluxes (Lλ) used in the calculations. Report whether or not the results of the 

calculations trigger Tier 2 Indirect Photolysis.

Tier 2

•Provide tabular values of solar photon fluxes for the season and latitude closest to those of the study.

•Provide important characteristics of all simulated and/or natural waters used including DOC, humic 

content (if simulated), pH, dissolved oxygen, and nitrate. The UV-VISIBLE spectrum (295 to 800 nm) 

of the test media should be included. For natural water, include the site of collection.

•Provide table(s) and graph(s) showing the concentrations of the test chemical and (if applicable) major 

transformation product concentrations and mass balance determinations in irradiated and dark control 

solutions versus time. Show the best fit regression lines on the graphs along with the data points.

•Report the irradiated, dark control, and net, direct, and indirect photolysis rate constants of the test 

chemical along with their corresponding half-lives. Provide the equations, calculations, and regression 

coefficients used to compute them together with regression n and r2 values, and standard errors of 

coefficients. If applicable and possible, supply the same information for major transformation 

products. 

•If applicable, report the rate constants for the formation and decline of the major transformation products. 

Provide the transformation pathway model (see next paragraph), equations, calculations, and 

regression coefficients used to compute them together with regression n and r2 values, and standard 

errors of coefficients.

• Provide chemical structures and names of the test chemical and (if applicable) major transformation 

products in both irradiated and dark control solutions. Provide graphical presentations of postulated 

transformation pathways in both the irradiated and dark control solutions.

Additional reporting requirements

General information

• Title of the study, author(s), performing laboratory (name and address), laboratory report identification 

number, date of completion. In some instances, it may be necessary to identify who is submitting the 

study, their address and any other identification number. 

• Name(s) of the test substance, chemical nomenclature, Chemical Abstracts Registry Number (CAS 

Reg. No.). A tabular format may be used to present this infomation, but it is optional. 

• Physical and chemical properties applicable to the test substance. As an option, these physical and 

chemical properties may be listed in a table. 
ENV/JM/TG/RD(2006)1/REV 

39

• Report if the test substance is the primary chemical or any transformation product resulting from 

abiotic hydrolysis and/or biotransformation. Report if the test substance is an industrial chemical, a 

pharmaceutical chemical, a pesticide, etc. 

• For radiolabelled test substances, the radiolabelled element and isotope, radiolabelling position(s), 

chemical and radiolabel purity, and specific activity should be reported. 

Experimental information 

• Standard laboratory equipment and reagents. 

• Analytical method(s) used for identifying and quantifying the test substance. The report should include 

a brief description of the method, reference standards, instrumentation, accuracy and precision, LOD 

and LOQ. Results of independent laboratory validation (if required). 

• Quality Control (QC) results such as analytical recoveries and relative percent standard deviations for 

replicate analyses. 

Specific experimental information related to photolysis (60) 

• Experimental conditions, reagents, reaction vessels and other information relevant to each of the tiers 

comprising the study. This may include: 

 - Temperature 

 - Solvent and/or buffers used for preparing stock and test solutions for direct photolysis, 

including dark control samples. 

 - Co-solvent and percent by volume used, if any. 

 - Nominal and actual concentration of test substance in the test media 

• Absorption Spectroscopy: 

 - Description of instrumentation 

 - Pathlength and material of optical cells 

 - Spectral resolution 

 - Concentration of test chemicaltest substance in solution 

 - Co-solvent and percent by volume used, if any 

 - Temperature and pH 

• Specific information on reaction vessels: 

 - Material (type of glass, quartz, etc) 

 - Geometry of the reaction vessels and positioning of the reaction vessels with respect to the 

light source. 

 - Any significant optical properties of the vessel material (wavelength cut-off, etc.) 

 - Illustration of the reaction vessel(s) arrangement and/or a photograph is encouraged to be 

included in the report. 
ENV/JM/TG/RD(2006)1/REV 

40

• Xenon-light source: 

 - Manufacturer, model 

 - Filters and their purpose 

 - Measured spectrum of the light source between 295 and 800 nm at at least the beginning and 

end of the study. 

 - Comparison of the spectrum of the light with that of natural sunlight for the conditions 

required for specific tiers. 

 - Intensity (radiant flux, fluence rate) at sample (J m-2s

-1; photons m-2s-1) and area irradiated. 

• Irradiation dose (fluence) (J m-2): 

 - State if continuous or intermittent irradiation was used 

 - If continuous irradiation, state radiant flux (intensity) and time 

 - If intermittent irradiation, state radiant flux, cycle duration and number of cycles 

• Quantum Yields: 

 - Experimental conditions (temperature, test media, concentration of test substance in test 

media 

 - Description of reaction vessels (diagram and/or photograph) 

 - Specify if monochromatic or polychromatic irradiation was used and source 

 - Monochromatic wavelength and bandwidth used. 

 - Intensity (radiance flux) 

 - Photometric methods, if any 

 - Chemical actinometer used (criteria for selection, concentration of actinometer in test media, 

absorption spectrum of the actinometer in the test media). 

 - Positioning of the actinometer with respect to the test substance 

• Computational methods for: 

 - Calculation of molar absorption coefficients (ελ) utilizing Beer-Lambert’s relationship 

 - Maximum photolysis reaction rate assuming φ = 1 (direct and/or indirect photolysis) 

 - Experimental photolysis reaction rate (direct and/or indirect photolysis) 

 - Quantum yield calculations (quantum yield should be expressed as a fraction and not as a 

percent)

 - GCSOLAR or ABIWAS calculation of reaction rates at different latitudes, seasons, time of 

the day, water bodies. 

 - Half-lives or DT50, DT75, and DT90 values and appropriate associated confidence intervals. 

Interpretation and Discussion of results

171. The purpose of conducting the study should be discussed in terms of the potential for the test 

chemical to be present in surface water. For example, chemicals can be transported to surface water in 

numerous ways including by direct point discharge into water bodies, runoff, spray drift or atmospheric 

deposition.

172. The depth of the discussion depends on the complexity of the study. Regardless of the 

complexity of the study, the discussion should be focused on the effect sunlight may have on the rate and 

pathway of transformation of the test chemical in aqueous environments.
ENV/JM/TG/RD(2006)1/REV 

41

173. The potential for the test chemical to undergo direct and/or indirect photolysis should be 

discussed. When applicable, the rate constants for irradiated and dark control test chemical should be 

compared to distinguish between hydrolytic and photolytic transformations. When possible, discuss the 

relative contribution(s) of direct and indirect photolysis to the overall net photolysis rate.

174. When quantum yields are determined, the calculated rate constants at different latitudes, seasons 

and for water bodies with depths and light attenuation coefficients of interest should be used to discuss 

how sunlight would affect the transformation rate of the test chemical at the time when the concentration of 

the chemical in surface water is expected to be the maximum.

175. When transformation products are identified and quantified in direct and indirect photolysis 

studies, photolytic products should be distinguished from hydrolytic products. When feasible, potential 

hydrolytic and/or photolytic stability of primary transformation products should be included in the results. 

When available, the rates of formation and decline of primary transformation products should be discussed. 

In turn, transformation products from direct and indirect photolysis studies should be compared to 

distinguish products unique to each. 

176. When studies are conducted with both SNW and SNW-N, the potential contribution of hydroxyl 

radical should be assessed by comparing the results in the two types of water. 

177. Sources of error and uncertainty such as losses of the test chemical and/or transformation 

products due to volatilization and/or adsorption to glassware should be discussed.

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C.J.M. 1985. Chemosphere, 14: pp 53- 77. 

61.53. Mill T. 2000. Documents on Calibration of Lamp and Indirect Photoreaction Estimation 

Methods, prepared for and submitted to the U.S. Environmental Protection Agency, Office of Pesticide 

Programs under PR Number 9W-2901-NASA, February 22, 2000. 

62.54. Dulin D. and Mill T. 1982. Development and evaluation of sunlight actinometers. Environ. Sci. 

Technol. 16(11): pp. 815-820. 

63.55. Zepp, R.G., Gumz, M.M., Miller, W., and Gao, H. 1998. Photoreaction of Valerophenone in 

Aqueous Solutions. J. Phys. Chem. A, 102: pp. 5716- 5722. 

64.56. Murov et al. 1993. Chemical Actinometry. In Handbook of Photochemistry. 2nd ed. Marcel 

Decker Inc., New York. pp. 298-305. 

65.57. Millet et al. 1998. Abiotic Degradation of Halobenzonitriles: Investigations of the Photolysis in 

Solution. Ecotoxicol. Environ. Safety. 41: pp. 44-50. 

66.58. Heller, H. and Langan J. 1981. A new reusable chemical actinometer. EPA Newsletter 71. 

67.59. Bahnemann, D. 1993. Ultra-small metal oxide particles: preparation, photophysical 

characterization and photocatalytic properties. Isr. J. Chem. 33: pp. 115. 

60. Gauglitz G. and Hubig S. 1984. Photokinetische Grundlagen moderner Aktinometer. Z. Phys. 

Chem. N. F. 139, 237-246.

61. Gauglitz G. and Hubig S. 1985. Chemical actinometry in the UV by azobenzene in concentrated 

solutions: A convenient method. J. Photochem. 30. 121-125.

68.62. IUPAC. 1989. Chemical Actinometry. Prepared for publication by H.J. Kuhn, S.E. Braslavsky, 

and R. Schmidt. Pure & Appl. Chem., Vol. 61, No. 2, (1989) pp. 187- 210. 

25.Winterle J and Mill T. 1985. Toxic substances process data generation and protocol development. 

Work assignment 18: Indirect Photoreaction Protocol. Submitted to Office of Toxic Substances, 

U.S. Environmental Protection Agency. EPA Contract No. 68-03-2981.
ENV/JM/TG/RD(2006)1/REV 

46

70.63. IUPAC. 1984. Recommended Standards for Reporting Photochemical Data (1984). Pure & 

Appl. Chem. 56: pp 939- 934. 

26.AWWA. 1991. Standard Methods for the Analysis of Water and Wastewater. American Water Works 

Association, 6666 West Quincy Avenue, Denver, Colorado 80295, USA. 

http://www.awwa.org/stanmeth/contents.htm

72.Mill T, Mabey W, Winterle J, Davenport J, Barich V, Dulin D, Tse D, and Lee G. 1982. Design and 

validation of screening and detailed methods for environmental processes. Final Report. Submitted to 

Office of Toxic Substances, U.S. Environmental Protection Agency. EPA Contract No. 68-01-6325.
ENV/JM/TG/RD(2006)1/REV 

47

ANNEX 1

Definitions and Units

Test substance: any substance, whether the parent compound or relevant transformation products

Transformation products: all substances resulting from biotic or abiotic transformation reactions of the 

test substance including CO2.

Half-Life, t0.5 is the time taken for 50% transformation of a test substance when the transformation can be 

described by first-order kinetics; it is independent of the concentration.

DT50 (Disappearance Time 50) is the time within which the concentration of the test substance is reduced 

by 50%; it is different from the half-life t0.5 when transformation does not follow first-order kinetics.

DT75 (Disappearance Time 75) is the time within which the concentration of the test substance is reduced 

by 75%.

DT90 (Disappearance Time 90) is the time within which the concentration of the test substance is reduced 

by 90%.

The limit of detection (LOD) is the concentration of a substance below which the identity of the substance 

cannot be distinguished from analytical artefacts.

The limit of quantification (LOQ) is the concentration of a substance below which the concentration 

cannot be determined with an acceptable accuracy.

 The definitions and units thereafter are taken from the Glossary of terms used in photochemistry 

(IUPAC Recommendations 1996) published in Pure and Applied Chemistry, 68, 2223-2286, 1996. 

Commonly used and SI units are provided. Cited with permission from the International Union of Pure and 

Applied Chemistry 

Absorbance (A), [Dimensionless] - The logarithm to the base 10 of the ratio of the spectral radiant power 

of incident, essentially monochromatic radiation (I= ∫λ I λ dλ) to the radiant power of transmitted radiation 

(Pλ): 

A = log(Pλ

o

 /Pλ) = -log T 

In practice, absorbance is the logarithm to the base 10 of the ratio of the special radiant power of light 

transmitted through the reference sample to that of the light transmitted through the solution, both observed 

in identical cells. T is the (internal) transmittance. 

The terms “absorbancy”, “extinction” and “optical density” should no longer be used. 

Absorption Coefficient (a), [m-1 or cm-1] - Absorbance divided by the optical pathlength, l. The term 

“absorptivity” is not recommended

Actinometer - A chemical system or physical device which determines the number of photons in a beam 

integrally or per unit time and it is commonly applied to devices used in the ultraviolet and visible 

wavelength ranges. Examples of a chemical actinometers are iron(III) oxalate solutions. Examples of 

physical devices are bolometers, thermopiles and photodiodes. Actinometers give a reading that can be 

correlated to the number of photons detected. 
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48

Beer-Lambert Law - The absorbance of a beam of collimated monochromatic radiation in a homogeneous 

isotropic medium is proportional to the absorption pathlength, l, and to the concentration, c. This law holds 

only under the limitations of the Lambert Law and for absorbing species exhibiting no concentration 

dependent aggregation. The law can be expressed as, 

A = log(Pλ

o

 /Pλ) = ε c l

or 

Pλ = Pλ

o

 10 -εcl

where the proportionality constant, ε, is called the molar (decadic) absorption coefficient.

Units: For l in cm and c in mol dm3

 or M, ε will result in dm3mol-1cm-1 or M-1cm-1. These are the 

commonly used units. The SI units for ε is m2mol-1. 

Chromophore- That part of a molecular entity consisting of an atom or group of atoms in which the 

electronic transition responsible for a given spectral band is approximately localized. 

Depth of Penetration (of light), [m] - The inverse of the absorption coefficient. If the decadic absorption 

coefficient, a, is used, the depth of penetration (1/a) is the distance at which the spectral radiant power, Pl

decreases to one tenth of its incident value, Pλ

o

. 

Einstein - One mole of photons. It is not an IUPAC-sanctioned unit. It is sometimes defined as the energy 

of one mole of photons. 

Energy Transfer - From a phenomenological point of view, the term is used to described the process by 

which a molecular entity absorbs light and a phenomenon originates from the excited state of another 

molecule. In mechanistic photochemistry, the term has been reserved for the photophysical process in 

which an excited state of a molecular entity (the donor) is deactivated to a lower-lying state by transferring 

energy to a second molecular entity (the acceptor), which is thereby raised to a higher energy state. The 

excitation may be electronic, vibrational, rotational or translational. The donor and acceptor may be two 

parts of the same molecule, in which case the process is called intramolecular energy transfer. 

Excited State - A state of higher energy than the ground state of a chemical entity. In photochemistry an 

electronically excited state is usually meant. 

Frequency (v or ω), [sec-1 or rad*sec-1] - The number of waveperiods per unit time. The linear frequency, v, 

is the number of cycles per unit time. For the angular frequency, the symbol ω (= 2 πv) is used. 

Ground State - The lowest energy state of a chemical entity. In photochemistry, ground electronic state is 

usually meant. 

Half-width (of a band) - The full width of a spectral band at a height equal to half of the height at the band 

maximum. Also known as full width at half maximum (FWHM). The dimensions of band width should be 

either inverse length (wavenumbers, ) or inverse time (frequencies) so that the values give an indication of 

the energies. Note the hyphen in half-width. Half bandwidth has the meaning of half-width at half 

maximum. 

Hyperchromic Effect - Increase in the intensity of a band due to substituents or interactions with the 

molecular environment. Hypochromic effect is the opposite to hyperchromic effect. 

Intensity- Traditional term for photon flux, irradiance or radiant power (radiant flux). In terms of an object 
ENV/JM/TG/RD(2006)1/REV 

49

exposed to radiation, the term should now be used only for qualitative descriptions. 

Intensity (l) (of a light source)- Same as radiant intensity. 

Intensity (of a spectral feature)- Describes the magnitude of the particular feature in the spectrum. 

Irradiance (E), [W m-2] - The radiant flux or radiant power, P, of all wavelengths incident on an 

infinitesimal element of surface containing the point under consideration divided by the area of the element 

(dP/dS,, simplified expression: E= P/S when the radiant power is constant over the surface area 

considered). E = ∫Eλ dλ, where Eλ is the spectral irradiance at wavelength λ. For a parallel and 

perpendicular incident beam not scattered or reflected by the target or its surrounding fluence rate (Eo) is 

an equivalent term. 

Lambert Law - The fraction of light absorbed by a system is independent of the spectral radiant power ( 

Pλ

o

 ). This law holds only if Pλ

o

 is small, scattering is negligible, and multiphoton processes, excited state 

populations, and photochemical reactions are negligible. 

Lamp - A source of incoherent radiation. 

Molar Absorption Coefficient, Molar Decadic Absorption Coefficient - 

[dm3

mol-1cm-1, M-1cm-1 or dm3mol-1mol-1] 

Absorbance divided by the absorption pathlength, l, and the concentration, 

.ε = [1 / c l] log(Pλ

o

 /Pλ) = A / c l: 

The term molar absorptivity for molar absorption coefficient should be avoided. 

Photochemical Reaction - This term is generally used to describe a chemical reaction caused by absorption 

of ultraviolet, visible, or infrared radiation. 

Photochemistry - The branch of chemistry concerned with the chemical effects of light (far UV to IR). 

Photodegradation - The photochemical transformation of a molecule into lower molecular weight 

fragments, usually in an oxidation process. This term is widely used in the destruction (oxidation) of 

pollutants by UV-based processes. 

Photolysis - A light-induced bond cleavage. 

Photon - The quantum of electromagnetic energy at a given frequency. This energy, E = hv is the product 

of the Planck (h) constant and the frequency of the radiation (v). 

Photooxidation - Reactions induced by light. Common process are: 

• The loss of one or more electrons from a chemical species as a result of photexcitation of that species; 

• The reaction of a substance with oxygen under the influence of light. When oxygen remains in the 

product this latter process is also called photooxygenation. Reactions in which neither the substrate nor 

the oxygen are electronically excited are sometimes called “photoinitiated oxidations”.

Photoreductions - Reduction reactions induced by light. Common process are: 

• Addition of one or more electrons to a photoexcited species; 

• The photochemical hydrogenation of a substance. Reactions in which the substrate is not electronically 

excited are sometimes called “photinitiated reductions”. 
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50

Photosensitization - The process by which a photochemical or photophysical alteration occurs in one 

molecular entity as a result of initial absorption of radiation by another molecular entity called a 

photosensitizer. In mechanistic photochemistry the term is limited to cases in which the photosensitizer is 

not consumed in the reaction. 

Primary Photochemical Process (Primary Photoreaction)- Any elementary chemical process undergone by 

an electronically excited molecular entity yielding a primary photoproduct.

Primary (Photo)product - The first observable chemical entity which is produced in the primary 

photochemical process and which is chemically different from the reactant. 

Quantum (of radiation) - An elementary particle of electromagnetic energy in the sense of the waveparticle duality. 

Quantum Yield (φ) - The number of defined events which occur per photon absorbed by the system. The 

integral quantum yield is: 

φ = number of events/ number of photons absorbed 

For a photochemical reaction,

φ = amount of reactant consumed or product formed/amount of photons absorbed 

The differential quantum yield is Φ = d[x]/dt /n, where d[x]/dt is the rate of change of a measureable 

quantity, and n is the amount of photos (mol or its equivalent einstein) absorbed per unit time. Φ can be 

used for photophysical processes or photochemical reactions. 

Radiant Energy (Q), [J] - The total energy emitted, transferred or received as radiation of all wavelengths 

in a defined period of time (Q= ∫ Qλ dλ ). It is the product of radiant power, and time, t:Q= P t when the 

radiant power is constant over the time considered. 

 Radiant Exposure (H), [J m-2] - The irradiance, E, integrated over time of irradiation (∫Edt, simplified 

expression H= E t when the irradiance is constant over the time considered). 

Radiant Intensity (I), [W sr -1] - Radiant (energy) flux or radiation power, P, at all wavelengths per unit 

solid angle, ω. The radiant power emitted in a given direction by a source or an element of the source in an 

infinitestimal cone containing the given direction divided by the solid angle of the cone (dP/dω, simplified 

expression: I = P/S when the radiant power is constant over the surface area considered). 

Radiant Power (P), [J s-1, or W] - Power emitted, transferred or received as radiation. 

Singlet Molecular Oxygen - The oxygen molecule (dioxygen), O2 , in an excited singlet state. The ground 

state of oxygen is a triplet 3

Σg

-

). The two metastable singlet states derived from the ground state 

configuration are 1

∆g and 1Σg

-

. 

The IUPAC discourages the use of the term “singlet oxygen” alone, without mentioning the oxygen 

species (in this case, dioxygen) to avoid confusion with 1S and 1D excited states of the oxygen atom. 

Singlet State - A state having a total electron spin quantum number equal to 0. 

Solvent Shift - A shift in the frequency of a spectral band of a chemical species arising from its interaction 
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51

with its solvent environment. Bathochromic shift, also known to as “red shift”, refers to the shift of a 

spectral band to lower frequencies (longer wavelenghts) as result of interaction with the solvent

Hypsochromic shift, also known as “blue shift”, refers to the shift of a spectral band to higher frequencies 

(shorter wavelengths) as a result of interaction with the solvent.. 

Spectral Irradiance (Eλ), [W m-3, or W m-2 nm-1] - Irradiance, E, at wavelength λ per unit wavelength. 

Triplet State - A state having a total soin quantum number of 1. 

Wavelength (λ )- The distance, measured along the line of propagation, between two corresponding points 

on adjacent waves. The wavelength depends on the medium in which the wave propagates 

Wavenumber (σ ,υ), [m-1 or cm-1]- The reciprocal of the wavelength, λ, or the number of waves per unit 

length along the direction of propagation. 

Xenon Lamp - An intense source of ultraviolet, visible and near-infrared light produced by electrical 

discharge in xenon under high pressure. 
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52

ANNEX 2 

Symbols and Units Related to Photochemistry 

Symbol Name SI Units Common Units

A Absorbance Dimensionless Dimensionless 

a Decadic absorption 

coefficient 

m-1 cm-1

D Attenuance Dimensionless Dimensionless 

H0 Fluence J m-2 Same 

E0 Fluence rate W m-2

E Irradiance W m-2

ε

Molar (decadic) 

absorption coefficient 

m-2 mol-1 cm-1dm3

mol-1 or

cm2

mmol-1 

Sometimes L mol-1cm-1

 φ Quantum yield Dimensionlessa Dimensionlessa

Q Radiant energy J 

H Radiant exposure J m-2

I Radiant intensity W sr-1

P W 

I Spectral radiance 

intensity 

W sr-1m-1 W sr-1 nm-1

λ Wavelength m nm 

σ; υ

Wavenumber m-1 cm-1
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53

ANNEX 3

Figure 1: Proposed test scheme for direct photolysis

Tier 1: Theoretical Screen

• Measurement of Absorbance 

• Estimation of Max. Photolysis Rate

Decision Criteria:

Photolysis Loss ≥50 % 

Over 30 Days?

yes

n

Tier 2: Experimental Screen (optional)

Determination of Photolysis Rate 

Constant over 30 Days

Decision Criteria:

Photolysis Loss ≥20% in 30

Days Sunlight Exposure?

yes

no

Tier 3: Quantum Yield Determination

and Calculation of Photolysis Rates and 

Life-Times

Decision Criteria:

Photolysis Loss ≥20% in 30

Days Sunlight Exposure?

yes

no

Go to Indirect

Photolysis Tier 1

yes

Tier 4: Isolation / Identification

of Major Phototransformation Products

(≥10% of dose applied)
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54

Tier 1 - Theoritical Screen

1.If the test chemical absorbs solar irridiation > 295 nm, estimate a maximum direct photolysis rate 

constant Kd(max) in shallow natural water by assuming the quantum yield φ = 1; Kd(max) = Σ ελLλ

2.Determine if the maximum direct photolysis rate constant would result in estimated direct photolysis 

losses of ≥20% of an initial concentration over a 30-day/night sunlight exposure period using the 

following equation: C/C0 = exp [ - Kd(max) (30-days) ]

 Yes↓→No → Quit or go to Tier 1 Indirect Photolysis

Tier 2 - Experimental Screen

1.Measure the decline of the test chemical in sterilised buffer exposed to a xenon lamp or sunlight. If data 

fit a first order kinetics model, determine the apporximate direct photolysis rate.

2.Determine if approximate direct photolysis rate constant would result in estimated direct photolysis losses 

of ≥10% of an initial concentration from the following equation: C/C0 = exp [-Kd(max) (30 days)] If yes, 

go to next step; if no, quit or go to Tier 1 Indirect Photolysis.

3.Is there interest in determining the quantum yield and in estimating theoretical direct photolysis rate 

constant from the quantum yield?

 Yes↓→No → Go to Tier 4 Direct Photolysis

Tier 3 - Quantum Yield and Estimation of Theoretical Rate constant

1.Determine the quantum yield and use it to estimate theoretical direct photolysis rate constant for 

seasons, latitudes, elevations and types of water bodies of interest.

2.Is there interest in comparing experimental direct photolysis rate constants with theoretical ones and/or 

identifying/quantifying major transformation products?

 Yes↓→No → Quit or go to Tier 1 Indirect Photolysis

Tier 4 - Determine the Direct Photolysis Rate Constant

1.Measure the decline of the test chemical (and if of interest) the formation and decline of major 

transformation products in sterilised buffers exposed to a xenon lamp or sunlight.

2.If data fit a first order kinetics model, determine the direct photolysis rate constants and corresponding 

half-life. Otherwise, determine the DT50, DT75 and DT90 values from fitting data to a non-first order 

model. If practical and of interest, determine the same parameters for major transformation products.

3.Is there interest in indirect photolysis? If yes, go to Tier 1 Indirect Photolysis; if no, quit.
ENV/JM/TG/RD(2006)1/REV 

55

Figure 2: Proposed Test Scheme for Indirect Photolysis

Tier 1 - Screen

1.Measure the decline of the test chemical in synthetic water containing humics (and if applicable, nitrate) 

in buffered distilled water and estimate indirect photolysis rate constant Ki(approx).

2.Determine if the approximate indirect photolysis rate constant would result in estimated indirect 

photolysis losses of ≥10% of an initial concentration over a 30-day/night sunlight exposure period 

using the following equation: C/C0 = exp [- Ki(approx) (30 days)]

3.In some cases, it may be possible to supplement or even replace the experimental screen with a 

theoretical screen based on using structural activity relationships (SARs) to estimate second order rate 

constants and on information on the typical concentrations of photooxidation in surface water.

Yes↓→No → Quit 

Tier 2 - Definitive Indirect Photolysis Test

1.Measure the decline of the test chemical (and if of interest) the formation and decline of major 

transformation products in synthetic and/or natural waters and in buffered distilled water exposure to a 

xenon lamp or sunlight.

2.If data fit a first order kinetics model, estimate net, indirect, and direct photolysis rate constants and 

corresponding half-lives for the test chemical. Correct for photobleaching and attenuation effects.

3.If data fit a non-first order model, determine the DT50, DT75 and DT90 values for fitting data to a nonfirst order model. If practical and of interest, determine the same parameters for major transformation 

products.
ENV/JM/TG/RD(2006)1/REV 

56

ANNEX 4

Derivations of Selected Equations

Background equations from Leifer (12):

d C

dt j

I

jD

IF F

sys

 [ ]



 



 =− =−

λ

λ λλ λ

φ φ

(abs by chem) (abs by sys) (abs by chem) 0

 [eq. 34-1] 

d C

dt j

I

jD

IF F

sys

[ ]

=− =− ∑ ∑ φ φ

λ λλ λ

295

800

295

800

(abs by chem) (abs by sys) (abs by chem) 0

 [eq. 34-2] 

where: 

φ = quantum yield 

j = conversion factor to make units consistent with the L/mol•cm unit of the molar absorption 

coefficient and the mol/L unit of concentration = 

 6.02 x 1020 (L/cm3

)(photons/mol) 

Iλ(abs by chem) = photon absorption rate by the chemical per unit volume over a 1 nm interval centered at 

wavelength λ (photons/cm3

•day) 

Dsys = depth of irradiated system (cm) = volume of irradiated system/incident area 

I0λ = incident photon flux over a 1 nm interval centered at wavelength λ 

 (photons/cm2 •day) 

Fλ (abs by sys) = fraction of incident photon flux absorbed by the system 

Fλ (abs by chem) = fraction of incident photon flux absorbed by the chemical 

[ ] F

I I

I

I

I

t

C l

C l

λ

λ λ

λ

λ

α ε

λ

α ε

λ λ

λ λ

(abs by sys) = − = − = −

− +

0 − +

0

0

0

1 10

1 10

( [ ])

( [ ])

 [eq. 34-3] 

F

C

λ C

λ

λ λ

ε

α ε (abs by chem) = +

[ ]

[ ]

 [eq. 34-4] 

where: 

αλ = attenuation coefficient of the system at wavelength λ (1/cm) 
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57

ελ = molar absorption coefficient of the chemical at wavelength λ (L/mol•cm) 

l = light pathlength (cm) 

Inserting equations 34-3 and 34-4 into equation 34-1 gives: 

[ ] d C

dt jD

I

C

sys C

C l [ ] [ ]

[ ]

 ( []



 



 =− − •

+





 



 − +

λ

λ

α ε λ

λ λ

φ ε

α ε

λ λ

0 1 10

 [eq. 34-5] 

Summing equation 34-5 over all wavelengths from 2905 nm to 800 nm gives: 

[ ] d C

dt jD

I

C

sys C

C l [ ] [ ]

[ ]

( [] =− − • +





 



 − +

=

=

∑ φ ε

α ε λ

α ε λ

λ λ λ

λ

λ λ

0

295

800

1 10

 [eq. 34-6] 

For solar irradiation on an aqueous system (12), 

I t It It 0λ λλ ( ) solar d s () () () = +

 [eq. 34-7] 

lD t d sys ≅ sec ( ) θ

 [eq. 34-8] 

l D s sys = 12.

 [eq. 34-9] 

where: 

I0λ(solar) = total incident solar photon flux as a function of time 

Idλ (solar) = direct incident solar photon flux as a function of time 

I0λ (solar) = sky radiation incident solar photon flux as a function of time 

ld = average light pathlength for direct irradiation 

ls = average light pathlength for sky irradiation 

{ } [ ] [ ] dC

dt jD

I I

C

sys C d

C D

s

sys C D =− − + −

+





 



 − + − +

=

=

∑ φ ε

α ε λ

αε θ

λ

α ε λ

λ λ λ

λ

λ λ λ λ 1 10 1 10 1 2

295

800

( [ ]) sec ( [ ]) . [ ]

[ ]

 [eq. 34-10] 
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58

Derivation of equation 5 in the main text (12):

10-y = exp(-2.3y) [eq. 34-11] 

Therefore, 

1 10 1 2 3 1 − =− − + =− − { } − + ( [ ]) exp . ( [ ]) exp( ) α ε

λ λ

λ λ α ε C l Cl x

 [eq. 34-12] 

where: 

x Cl A =+ = 23 23 . [] . ( ) α ε λ λ

 [eq. 34-13] 

A = absorbance of the solution 

A series expansion of the exponential in equation 34-12 gives: 

1

2 3 4 5

2345

− − = + − + − + ⋅⋅⋅⋅⋅ exp( ) ! ! ! !x x

xxxx

 [eq. 34-14] 

For an optically dilute solution of a test chemicaltest substance in pure water or in shallow water or in the 

near surface of natural water such that the absorbance of the system is < 0.02, all of the higher terms in 

equation 34-14 are small compared to x such that equation 34-14 reduces to: 

1− −≅ exp( x) x

 [eq. 34-15] 

Combining equations 34-13 and 34-15 with equation 34-12 gives for an optically dilute solution of a test 

chemicaltest substance in pure water or clear shallow water or in the near surface of natural water: 

1 10 2 3 − ≅+ ( ) − + ( [ ]) . [] α ε

λ λ λ λ α ε C l C l

 [eq. 34-16] 

Substituting equation 34-16 into equation 34-6 and canceling out the resulting identical terms in the 

numerator and denominator gives for an optically dilute solution of a test chemicaltest substance in pure 

water or in clear shallow water or in the near surface of natural water: 

d C

dt j

l

D

I C sys

[] . = − [ ]















 =

=

∑ 2 3 0

295

800 φ ε λ λ

λ

λ

 [eq. 34-17] 

By comparing equation 34-17 to equation 1 in the main text, it can be seen that for a test chemicaltest 

substance in an optically dilute solution in pure water or in clear shallow water or in the near surface of 

natural water, the direct photolysis rate constant is given by: 
ENV/JM/TG/RD(2006)1/REV 

59

k

j

l

D

I d

sys

= ∑ 2 3 0

295

800 . φ ε λ λ

 [eq. 34-18] 

where equation 34-18 is identical to equation 5 of the main text 

Derivation of equation 6 in the main text (12)

By analogy to equation 34-16, it can be seen that for a solar irradiated, optically dilute solution of a test 

chemicaltest substance in pure water or in clear shallow water or in the near surface of natural water such 

that the absorbance of the system is < 0.02, 

1 10 2 3 − ≅+ − + ( [ ]) sec . ( [ ]) sec αε θ

λ λ

λ λ αε θ C D

sys

sys C D

 [eq. 34-19] 

 

1 10 2 3 12 1 2 − ≅+ − + ( [ ])( . ) . ( [ ])( . ) α ε

λ λ

λ λ α ε C D

sys

sys C D

 [eq. 34-20] 

Substituting equations 34-19 and 34-20 into equation 34-10 and canceling out the resulting identical terms 

in the numerator and denominator gives: 

d C

dt j Zt C [] . = − ()[ ]





 

 ∑  2 3φ ε λ λ

 [eq. 34-21] 

where: 

Zt I t I t λλ λ d s ( ) ( ) sec . ( ) = + θ 12

 [eq. 34-22] 

Equation 34-21 gives the direct photolysis rate as a function of time because the solar photon flux at any 

given wavelength varies over time as represented by Z(t) for an optically dilute solution. 

Z

Z t dt

hours λ

λ

=

∫ ( ) 0

24

24

 [eq. 34-23] 

Mill et al (1985) defined 24 hour average solar irradience values as: 

L

j λ λ = Z

2 3.

 [eq. 34-24] 

Substituting equation 34-24 into 34-21 gives the average direct photolysis rate over a 24-hour period: 

d C

dt L C [ ] [ ] 



 



 = − 



 

 φ ε ∑  λ λ

 [eq. 34-25] 
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60

By comparing equation 34-25 to equation 1 in the main text, it can be seen that for an optically dilute 

solution of a test chemicaltest substance in pure water or clear shallow water or in the near surface of 

natural water exposed to sunlight, the 24 hour average direct photolysis rate constant is given by: 

k L d solar ( ) = φ ε ∑ λ λ

295

800

 [eq. 34-26] 

where equation 34-26 is identical to equation 6 in the main text. 

Derivation of equation 7 8 in the main text:

Rearranging equation 34-2 gives the total photon absorption rate per unit volume by the test chemicaltest 

substance: 

 

I I jdC

dt () () abs by chem abs by chem

[ ] = =− ∑ λ

295 φ

800

 [eq. 34-27] 

where: 

Iλ(abs by chem) = photon absorption rate by the chemical per unit volume over a 1 nm interval centered at 

wavelength λ (photons/cm3

•day) 

I(abs by chem) = total photon absorption rate by the chemical per unit volume (photons/cm3

•day) 

Multiplying equations 34-18 and 34-25 by (-j / .) gives the total photon absorption rate per unit volume by 

the test chemicaltest substance in an optically dilute solution exposed to xenon arc lamp or solar 

irradiation, respectively: 

I

l

D

I C sys

( xenon abs by chem) xenon xenon

=295

= 















=

2 3 ∑ 0

800

. [] ( ) ε λ λ

λ

λ

 [eq. 34-28] 

I j LC (solar abs by chem) = solar





 

 ∑ ε  λ λ [ ]

 [eq. 34-29] 

The total number of photons absorbed per unit volume by a test chemicaltest substance in a low optical 

density solution over x days of xenon arc lamp irradiation or over 30 day/nights of solar irradiation are 

given respectively by: 

Total # photons abs by chem / cm days xenon ( ) 3 = X ∑ l

D

I C sys

xenon xenon ( . )[ ] ( ) 2 3 0 ε λ λ

 [eq. 34-30] 

Total # photons abs by chem / cm days / nights solar ( ) 3 = 



 

 30 j LC ∑  solar ε λ λ [ ]

 [eq. 34-31] 

The number of days (X) of constant irradiation by a filtered xenon arc lamp that is equivalent to 30 

days/nights of solar irradiation of a low absorbance water can be obtained by equating equations 34-30 and 

34-31, assuming that the average concentration over 30 days/nights of solar irradiation is equal to the 

average concentration over X days of constant irradiation by a xenon arc lamp, and solving for X: 

X days xenon = 



 





























= 



 





























∑

∑

∑

∑

30

2 3 13

0 0

j D

l

L

I

j

D

l

L

I

sys

xenon

sys

xenon

.

() ()

ε

ε

ε

ε

λ λ

λ λ

λ λ

λ λ
ENV/JM/TG/RD(2006)1/REV 

61

 [eq. 34-32] 

where equation 34-32 is identical to equation 78 in the main text. 

Derivation of equation 910 in the main text:

Substituting equation 34-16 for an optically dilute solution (applicable to a low optical density actinometer 

solution) into equation 34-5, rearranging and incorporating subscripts to designate the use of 

monochromatic irradiation from a xenon arc lamp (obtained with a monochromator) gives: 

d C

dt j

l

D

I C act

sys

act xenon

[ ] . [ ] () ( )





 



 = − λ

λ λ

φ ε 2 3

0

 [eq. 34-33] 

By comparing equation 34-33 to equation 1 in the main text, it can be seen that the monochromatic pseudo 

first order rate direct photolysis rate constant of a low optical density actinometer exposed to 

monochromatic irradiation from a xenon arc lamp is given by: 

k

j

l

D

I d act

sys

λ λλ act act xenon φ ε () () ( )

. = − 2 3 0

 [eq. 34-34] 

Rearranging equation 4-34 to solve for I0_(xenon) gives: 

I j D

l

k

xenon

sys d act

act act

0 2 3 λ

λ

λ φ ε ( )

( )

( ) . =

 [eq. 34-35] 

where equation 34-35 is identical to equation 910 in the text. 

Derivation of equation 101 in the main text

For a high optical density actinometer solution, 

10 0 − + ≅ ( [ ]) α ε λ λ C l [eq. 34-36] 

and 

ε λ λ [ ] C > > α [eq. 34-37)] 

such that 

ε

α ε

ε

ε

λ

λ λ

λ

λ

[ ]

[ ]

[ ]

[ ]

C

C

C

+ C ≅ = 1

 [eq. 34-38] 

Substituting equations 34-36 and 34-38 for a high optical density actinometer solution into equation 34-6, 

summing only over the absorption spectrum of the actinometer above 2905 nm, and using subscripts to 

designate the use of a filtered xenon arc lamp gives: 

d C

dt jD

I act

sys

xenon

initial

final [ ] ( ) = − ==

∑ φ λ

λ

λ

0

 [eq. 34-39] 
ENV/JM/TG/RD(2006)1/REV 

62

Equation 34-39 is a zero order equation such that the zero order direct photolysis rate constant of the high 

optical density actinometer is given by: 

k

jD

I d act zero order

act

cell

xenon

initial

final

( )( ) ( ) = =

=

∑ φ λ

λ

λ

0

 [eq. 34-40) 

Rearranging equation 34-40 gives: 

I jD k xenon

cell

act

d act

initial

final

0λ

λ

λ

φ ( ) ( )( = =

=

∑ zero order)

 [eq. 34-41] 

where equation 34-41 is identical to equation 101 of the main text 

Derivation of equation 134 in the main text:

Dividing equation 34-25 (for the pseudo first order rate constant of a test chemicaltest substance in an 

optically dilute solution exposed to sunlight) by equation 34-18 (for the pseudo first order rate constant of a 

test chemicaltest substance in an optically dilute solution exposed to a filtered xenon arc lamp) and 

rearranging gives: 

k j D

l

k L

I

d solar

cell

d xenon

xenon

( )

( )

( )

. =

∑

∑ 2 3

295

800

0

ε

ε

λ λ

λ λ

 [eq. 34-42] 

where equation 34-42 is identical to equation 134 in the main text 

Derivation of equation 179 in the main text:

By analogy to equation 34-34 for a low optical density actinonmeter, the monochromatic pseudo first order 

direct photolysis rate constant of a test chemicaltest substance in an optically dilute solution exposed to 

monochromatic irradiation from a xenon arc lamp is given by: 

k

j

l

D

I d chem

sys

λ λλ chem chem xenon φ ε ( ) ( )( )

. = − 2 3 0

 [eq. 34-43] 

Dividing equation 34-34 by equation 34-43, and rearranging gives: 

φ φ ε

ε

λ λ

λ λ

chem act

d chem act

d act chem

k

k = ( )()

()( ) [eq. 3-4444] 

where equation 34-44 is identical to equation 179 of the main text 

Derivation of equation 1820 in the main text:

Substituting equations 34-36 and 34-38 (for a high optical density actinometer solution exposed to 

monochromatic irradiation from a xenon arc lamp) into equation 34-5 gives: 

d C

dt jD

I act

cell

xenon

[ ] ( )





 



 = −

λ

λ

φ 0

 [eq. 34-45] 
ENV/JM/TG/RD(2006)1/REV 

63

Equation 34-45 is a zero order equation such that the monochromatic zero order direct photolysis rate 

constant of a high optical density actinometer is given by: 

k

jD

I act zero order

act

cell

λ λ xenon

φ ( )( ) ( ) = 0

 [eq. 34-46] 

Dividing equation 34-43 (for the monochromatic pseudo first order direct photolysis rate constant of a test 

chemicaltest substance in an optically dilute solution exposed to monochromatic irradiation from a xenon 

arc lamp) by equation 34-46 (for the monochromatic zero order direct photolysis rate constant of a high 

optical density actinometer exposed to monochromatic irradiation from a xenon arc lamp), and rearranging 

gives: 

φ φ

ε λ

λ

λ

chem

act

chem cell

d chem

D d act zero

k

k = 2 3. ( )

( )

( )( order) [eq. 34-47] 

where 34-47 is identical to equation 1820 in the main text 

Derivation of equation 1921 in the main text:

By analogy to equation 34-46 for a high optical density actinometer, the monochromatic zero order direct 

photolysis rate constant of a test chemicaltest substance at high optical density in a solution exposed to 

monochromatic irradiation from a xenon arc lamp is given by: 

k

jD

I chem zero order

chem

cell

λ λ xenon

φ

( )( ) ( ) = 0

 [eq. 34-48] 

Dividing equation 34-48 by equation 34-46 and rearranging gives: 

φ φ λ

λ

chem act

d chem zero

d act zero

k

k = ( )( )

( )( ) [eq. 34-49]

where equation 34-49 is identical to equation 1921 in the main text

Derivation of equation 202 in the main text:

Dividing equation 34-25 (for the pseudo first order rate constant of a test chemicaltest substance in an 

optically dilute solution exposed to sunlight) by equation 34-25 (for the pseudo first order rate constant of a 

low optical density actinometer solution exposed to sunlight), and rearranging gives: 

φ φ

ε

ε

λ λ

λ λ

chem act

d chem

d act

act

chem

k

k

L

L

=

∑

∑

( )

( )

( )

( )

295

800

295

800

 [eq. 34-50] 

where equation 3-50 is identical to equation 202 in the main text 

Derivation of equation 213 in the main text:

Dividing equation 34-18 (for the pseudo first order rate constant of a test chemicaltest substance in an 

optically dilute solution exposed to a filtered xenon arc lamp) by equation 34-18 (for the pseudo first order 
ENV/JM/TG/RD(2006)1/REV 

64

rate constant of a low optical density actinometer solution exposed to a filtered xenon arc lamp), and 

rearranging gives: 

φ φ

ε

ε

λ λ

λ λ

chem act

d chem

d act

act xenon

chem xenon

k

k

I

I

=

∑

∑

( )

( )

() ( )

( )( )

0

295

800

0

295

800

 [eq. 34-51] 

where equation 34-51 is identical to equation 213 in the main text 

Derivation of equation 27 In the main text:

Dividing equation 3-25 (for the pseudo first order rate constant for a test chemical in an optically dilute 

solution in shallow or near to the surface water exposed to sunlight conditions 2 by equation 3-25 (for 

sunlight conditions 1), and rearranging gives:

k k

L

L

d shallow solar d shallow solar

chem

chem

( )( ) ( )( )

( )

( )

2 1

2

295

800

1

295

= 800

∑

∑

ε

ε

λ λ

λ λ

[eq. 3-52]

where equation 3-52 is identical to equation 27 in the main text
ENV/JM/TG/RD(2006)1/REV 

65

ANNEX 5

Preparation of Direct Photolysis Test Media

Water meeting ASTM Type IIA standards is recommended to prepare buffer solutions for direct photolysis 

studies. This water is described in ASTM D 1193-99- Standard Specification for Reagent Water. This 

document can be obtained from ASTM. In the USA, this document can be obtained from: 

American Society for Testing and Materials (ASTM) 

1961 Race Street 

Philadelphia, Pennsylvania 19103 

For outside the USA, see http://www.astm.org/dist.htm 

A recommended procedure for preparing buffers for direct photolysis studies is given in USEPA OPPTS 

835.2210 Direct Photolysis Rate in Water by Sunlight (2). This guideline recommends: 

Preparing all buffer solutions at 25o

 C 

Using reagent grade chemicals free of impurities that could behave as photosensitizers 

For pHs in the range of 3 to 6, use NaH2PO4/HCl 

For pHs in the range of 6 to 8, use KH2PO4/NaOH

For pHs in the range of 8 to 10, use H3BO3/Na OH

Using procedures such as those described in the Handbook of Chemistry and Physics or by any 

well establish method. 

 Always checking the pH with a well calibrated pH meter 

 Using the minimum concentration of buffer 

To prepare the test solutions for the direct photolysis studies, add the buffer solution to the test 

chemicaltest substance to attain the desired pH. If necessary, adjust the pH with 1 M HCl or NaOH. 

It is highly recommended that the actual concentration of the test chemicaltest substance in the test solution 

be determined prior to starting the study. 

Preparation of Indirect Photolysis Study Test Media

1. SNW (Simulated Natural Water):

(a) Prepare an initial stock solution of humic acid, followed by filtration, preaging and dilution. The 

stock solution should have about 60 ppm DOC of humic acid and absorbance of approximately 

1.7 at 313 nm and 0.7 at 370 nm. Aging can be accomplished by exposing the stock humic acid 

solution in a 2-L flask for 3-4 days to direct sunlight (2). The absorbance of the solution is 

checked and used in estimating a dilution factor for preparing the test solution.

(b) Buffer the SHW at pH 7 with 0.010 M phosphate solution. 

(c) The SNW test media should have an absorbance of 0.05 at 370 nm and a dissolved organic 

carbon concentration of about 5 mg/L or ppm.

(d) The appropriate amount of the test chemical is introduced to the test media immediately prior to 

irradiation.
ENV/JM/TG/RD(2006)1/REV 

66

2. SNW-N (Simulated Natural Water with Added Nitrate):

(a) Prepare test media as for SNW, Steps a through c;

(b) Add nitrate as NaNO3 or KNO3. In freshwater streams of the United States, average nitrate 

concentrations range from less than 0.1 mg/L to approximately 4 mg/L with the highest 

concentrations in agricultural followed by urban areas (USGS 1993). The mean nitrate 

concentration in some lakes may range up to 9 ppm (22). Nitrate concentrations in the 

agricultural and urban areas of Europe often exceed 9 mg/L (German personal communication).

(c) The test chemical is introduced immediately prior to irradiation.

3. NW (Natural Water):

Natural surface water may be taken from lakes, rivers, ponds, streams, creeks or coastal water 

bodies where the test chemical is expected to be present. Proper water sample collection and 

characterization is required. The geographical location (including longitude and latitude), site 

characteristics and date of collection should be recorded.

Collection and Sampling

(a) Sample containers such as polyethylene bottles should be soaked in 10% HNO3 overnight, 

thoroughly washed, and then rinsed with distilled water at least three times.

(b) Samples are collected from the surface (top few centimeters) of the selected water body.

(c) Water samples that are not used shortly after collection should be stored at 4o C.

 

Characterization

In the laboratory: The dissolved organic carbon (DOC), pH, and nitrate/nitrite need to be 

determined (61). As an option, additional factors that influence indirect photolysis can be 

determined including diisolved oxygen, redox potential, alkalinity, turbidity, other major major 

anions (HCO3

-

, Cl -, SO4

2-

), major cations (Na+, K+, Ca2+, Mg2+) and other metal ions such as iron 

and manganese (61). 

All waters (SNW, SNW-N, and NW) prepared for the indirect photolysis test should be filter-sterilized 

with a 0.2 μm filter. The absorptivity should be computed at 350 nm (absorptivity = 

2.303 x absorbance/pathlength). Computing the absortivity at 350 nm in SHW, SHW-N, and NSW 

provides an estimate of CDOM. CDOM exhibits a much stronger correlation with indirect photolysis rates 

than DOC. 
ENV/JM/TG/RD(2006)1/REV 

67

ANNEX 6

EXAMPLE OF A SUNTEST APPARATUS EQUIPPED WITH A XENON ARC LAMP
ENV/JM/TG/RD(2006)1/REV 

68

EXAMPLE OF A MERRY-GO-ROUND APPARATUS FOR DIRECT PHOTOLYSIS STUDIES

OPTICAL BENCH FOR QUANTUM YIELD DETERMINATIONS
ENV/JM/TG/RD(2006)1/REV 

69

EXAMPLE OF A FLOW-TROUGH SYSTEM FOR DIRECT PHOTOLYSIS STUDIES

EXAMPLE OF A STATIC SYSTEM FOR DIRECT PHOTOLYSIS STUDIES
ENV/JM/TG/RD(2006)1/REV 

70

ANNEX 7

Advantages and Disadvantages of Monochromatic and Polychromatic irradiation

1. The main advantage in using monochromatic irradiation to determine the test substance quantum 

yield of the test substance is that (as will be shown later by equations 19 through 21) it eliminates the need 

to directly measure the incident monochromatic irradiation. However, a disadvantage is that it will take 

longer to determine the quantum yield with monochromatic irradiation than with the use of polychromatic 

irradiation because the actinometer and the test substance are absorbing photon fluxes at just one 

wavelength instead of over multiple wavelengths. 

2. The main advantage in using polychromatic irradiation (filtered xenon arc lamp or sunlight) 

instead of monochromatic irradiation to determine the test substance quantum yield is that the time 

required to determine the quantum yield may be substantially shorter (8). The reason is due to the 

absorption of photon fluxes by the actinometer and test substance over multiple wavelengths instead of just 

one wavelength. The main disadvantage in using polychromatic irradiation to determine the test substance

quantum yield is that unlike with monochromatic irradiation (as will be shown later by equations 20 and 

21), its use requires the determination of the photon flux.

3. The advantage in using sunlight instead of a filtered xenon arc lamp, if polychromatic irradiation 

is used to determine the quantum yield, is that tables of average solar intensities as a function of season and 

latitude are readily available. Also, if sunlight is used, it eliminates the necessity of having a 

monochromatic or polychromatic artificial light source. However, if sunlight is used, an actinometer with 

an adjustable quantum yield is needed so the direct photolysis rate of the actinometer can be adjusted to be 

comparable to that of the test substance under the same irradiation conditions (2)(12). Parallel exposure of 

the test substance and actinometer to sunlight over comparable time frames minimizes errors due to 

changes in solar flux caused by fog, varying cloud cover, etc. (12)(53).

4. An example of an adjustable quantum yield actinometer is the p-nitroacetophenone - pyridine 

(PNAP-PYR) actinometer (54). For a 1.0 X 10-5 M p-nitroacetophenone solution, the quantum yield of the 

actinometer in experiments carried out with monochromatic irradiation at 313 nm can be adjusted by 

varying the concentration of pyridine in the solution according to the following equation (2)(12):

φ act= • 0 0169 .[ ] pyridine [eq. 1]

 where:

φact = quantum yield of the 1.0 X 10-5 M p-nitroacetophenone

 [pyridine] = molar concentration of pyridine

5. Suitable chemical actinometers used in aqueous photochemical studies include the following:
ENV/JM/TG/RD(2006)1/REV 

71

ANNEX 7 (continued)

Actinometer System Literature Citation

(a)p-nitroacetophenone/pyridine 54

(b) p-nitroanisole/pyridine 28, 54

(c) valerophenone 55

(d) ferrioxalate 21, 25, 43, 56, 57

(e) o-nitrobenzaldehyde 43, 54

(f) Reinecke’s salt 12

(g) aberchrome 540 58, 59

(h) azobenzene 60, 61

(i) oxalic acid/uranyl sulphate 8

6. The first two actinometers listed in the above table have adjustable quantum yields and are 

therefore particularly useful when solar irradiation is used (12). The first three actinometers listed in the 

table are low optical density actinometers and the last four are high optical density actinometers. The 

characteristics of those actinometers including the optimal wavelength range for their use are discussed in 

references 12 and 62. Other chemical actinometers are described in the IUPAC document on chemical 

actinometers (62).
ENV/JM/TG/RD(2006)1/REV 

72

DRAFT DOCUMENT 

November 2004August 2000

ANNEX 2 

OECD GUIDELINE FOR TESTING OF CHEMICALS

PROPOSAL FOR A NEW GUIDELINE

Phototransformation of Chemicals in Water -– Part 2: Direct and Indirect Photolysis

INTRODUCTION

1. This guideline provides guidance for conducting phototransformation in water studies to determine 

the potential effects of solar irradiation on chemical pollutants in surface water. Such studies determine 

phototransformation kinetics, products, and product pathways resulting from both either direct and (as an 

option or if applicable) or indirect aqueous photolysis. It is based on current and /or proposed methods 

(references 1 through 10) and on relevant literature on environmental aqueous photochemistry. Part 2 of 

this guideline deals with indirect photolysis.

2. A glossary of definitions (11) and symbols/units are provided in Annexes 1 and 2 respectively. 

ENVIRONMENTAL SIGNIFICANCE AND USE

3. Chemical pollutants and/or their transformation products that are potentially susceptible to direct 

and/or indirect photolysis include those that are transported to surface water by direct point discharge 

and/or by runoff from urban and/or rural areas. Chemical pollutants in surface water also often originate 

from transformation of other chemicals via hydrolysis, photolysis, and biotransformation. 

4. Direct photolysis in natural water involves the transformation of a chemical resulting from the 

direct absorption of a solar photon (references 12 through 2018). Indirect photolysis in natural water 

sometimes involves the transformation of a chemical due to energy transfer from naturally occurring 

photosensitizers in electronically excited triplet states (12)(13)(2119)(2220). However, indirect photolysis 

more often involves the transformation of a chemical due to reactions with transient oxidants such as 

hydroxyl radicals, molecular oxygen in a singlet electronic state (“singlet oxygen”), and peroxy radicals 

(references 12 and 242 through 342). Both triplet state photosensitizers and transient oxidants result from 

the absorption of solar photons by chromophoric dissolved organic matter (CDOM) and nitrate ion 

(12)(231)(342)(353)(364)(37)(38)(39)(40). 

5. Direct photolysis can be an important dissipation pathway for some chemical pollutants that 

exhibit significant light absorption above the 295290 nm cutoff of solar irradiation at the earth’s surface. 

Indirect photolysis can also be an important dissipation pathway for some chemical pollutants that come in 

contact with photosensitizers in electronically excited triplet states or with short-lived photochemically 

generated oxidants such as hydroxyl radicals and singlet oxygen. In some cases both direct and indirect 

photolysis can contribute significantly to the dissipation of a chemical in natural waters. 

6. The direct and indirect phototransformation of chemicals in natural water bodies is a complex 

process which depends on a number of factors such as: 
ENV/JM/TG/RD(2006)1/REV 

73

 (a) the chemical structure and electronic absorption spectrum of the chemical; 

 (b) the concentration, composition, and absorption spectra of CDOM (from which 

 photosensitizers and singlet oxygen arise); 

 (c) the concentration of nitrate (the primary source of hydroxyl radicals); and 

 (d) the solar photon flux spectrum to which the chemical, CDOM and nitrate are exposed. 

7. The solar photon flux spectrum to which a chemical, CDOM, and nitrate are exposed depend on 

numerous factors including the latitude, season, and the physical and chemical properties of the water body 

(12)(18)(4135)(4236). Important properties of the water body which contribute to the attenuation of solar 

irradiation with depth include the concentration and composition of CDOM and suspended particulates. 

The mixing and stratification properties of the water will help determine the horizontal and vertical 

distribution of the chemical, CDOM, and nitrate. 

8. The results of phototransformation in water studies are used in conjunction with physical chemical 

properties and data from other studies (abiotic hydrolysis; biotransformation; adsorption/desorption) to 

help assess the overall environmental transformation and transport of chemical pollutants. The results of 

phototransformation in water studies are also used to help develop inputs for environmental fate computer 

models, and to help develop protocols for conducting other aquatic laboratory and/or aquatic field studies. 

PRINCIPLE OF THE STUDY

Scope and approach

9. This part of the test guideline is designed to provide some or all of the following data and/or 

information depending upon specific needs: 

•The quantum yield and resulting estimated direct photolysis rate constants for test chemicals for various 

types of water bodies (defined by depth and light attenuation), seasons, and latitudes of interest.

•Direct photolysis rate constants for test chemicals determined in the laboratory using filtered xenon arc 

lamp or sunlight irradiation and extrapolated to natural water. A xenon arc lamp filtered to remove 

irradiation < 295 nm will be referred to throughout the rest of the document as a filtered xenon arc 

lamp.

•The net (combined direct and indirect) and indirect photolysis rate constants of one or more test 

chemicaltest substances in a standard simulated natural water (containing humics as a surrogate for 

CDOM and in some cases, nitrate) and/or in one or more natural waters using a filtered Xenon arc 

lamp or sunlight irradiation.. A xenon arc lamp filtered to remove irradiation < 290 nm will be referred 

to throughout the rest of the document as a filtered Xenon arc lamp.

• The transformation pathway and the identities, concentrations, and rates of formation and decline of 

major phototransformation products resulting from direct photolysis and/or indirect photolysis. 

10. This guideline is designed as a tiered approach. Each tier is triggered by the results of the previous 

tier. The T tiered approach es are proposed separately for direct and indirect photolysis tests is as shown in 

Figures 1 and 2, respectively (see Annex 3.).
ENV/JM/TG/RD(2006)1/REV 

74

Pseudo first-order kinetics of direct and indirect photolysis

11. The rate of decline of a test chemical test substance in either an direct or indirect photolysis study 

is generally assumed to follow pseudo first-order kinetics (e.g., time series data are fit to a pseudo firstorder kinetics model): 

d C

dt k C [ ] = − [ ]

 [eq. 1] 

 The integrated form of equation 1 is: 

 

[ ] [ ]exp( ) C C kt = − 0

 [eq. 2] 

 The ln transformed version of equation 2 is: 

 

ln[ ] ln[ ] C C kt = − 0 [eq. 3] 

 where 

 C = test chemicaltest substance concentration at time t (mol/L) 

 C0 = initial test chemicaltest substance concentration at time t = 0 

 k = pseudo first-order rate constant (1/unit of time) 

 t = time 

t1/2 = half-life

The pseudo first-order rate constant is determined by using non-linear regression to fit data to 

equation 2 or using linear regression to fit data to equation 3. 

The half-life can be determined by substituting the pseudo first-order rate constant into the 

following equation: 

t

k d i

1 2

2

/

( )

ln = or [eq. 4] 

 

where t1/2 = half-life and

kd is the direct photolysis rate constant and ki is the indirect photolysis rate constant. 

Note: In the equations provided in this guideline, units for half-lives, pseudo first- order rate constants 

and photon fluxes are (for illustrative purposes) given in days, 1/day, and photons/cm2

•day, 

respectively. However, other units of time such as hours or seconds may be used as long as there 

is consistency within the same equation. Units of concentration are given in mol/L.

Other kinetic models may be applied if they better fit the kinetic data.

Direct photolysis 

12. To undergo transformation via direct photolysis, a chemical molecule must first absorb a photon 

of light. The absorption of a photon by a molecule results in a transition from an electronic ground state to 

an electronically excited state of the molecule. To be absorbed by the molecule, the energy of the photon
ENV/JM/TG/RD(2006)1/REV 

75

(which is inversely proportional to its wavelength) must correspond to the difference between the ground 

and a possible excited electronic state of the molecule (12). Within the UV-Visible wavelength of interest 

(295-800 nm), photons energies range from 150 kJ/mol at 800 nm to slightly greater than 400 kJ/mol at 

295 nm (11)(37).

13. The absorption of a photon is a necessary, but often not a sufficient condition, for a molecule to 

undergo transformation via direct photolysis (12)(14)(37). The absorbed energy must first be sufficient to 

cause the transformation via bond cleavage, rearrangement, oxidation, or reduction. Then, 

phototransformation to form new molecular structures must compete with other possible deactivation 

processes such as quenching, other radiationless processes, and radiative processes. Consequently, the 

fraction of photon excited molecules that actually undergo phototransformation (e.g. the quantum yield) is 

generally much less than 1 (usually < 0.1 and sometimes < 0.01) (14)(21). The mechanistic aspects of 

phototransformation reactions at a molecular level are beyond the scope of this guideline (see reference 38 

for further information).

14. It can be shown that for an optically dilute solution (absorbance A < 0.02) of a chemical in pure 

water exposed to polychromatic irradiation above the cutoff of solar irradiation at the earth’s surface of 295

nm, the direct photolysis rate constant (assuming the quantum yield is independent of wavelength) is given 

by (12)(14through 18) (See Annex 4: Derivation of selected equations):

k

j

l

D

I d

sys

= ∑ 2 3

0

295

800 . φ ε λ λ

[eq. 5]

where:

kd = direct photolysis rate constant (1/day)

φ = the quantum yield (independent of wavelength) 

ελ = molar absorption coefficient (L/mol•cm; M-1 •cm-1 ) at wavelength λ

I0λ = photon flux (photons/cm2 •day) over a 1 nm interval centered at wavelength λ

Dsys = depth of irradiated system (cm) = volume of irradiated system/incident area

l = light pathlength (cm)

j = conversion factor to make units consistent with the L/mol•cm unit of the molar 

absorption coefficient and the mol/L unit of concentration = 6.02 x 1020

(L/cm3)(photons/mol)

Note: The term (l/Dsys) in equation 5 approximately cancels out for rectangular photolysis cells exposed 

to irradiation from a filtered xenon arc lamp and for which Dsys = Dcell . However, that would not 

be true for cylindrical photolysis cells or for any photolysis cells exposed to solar irradiation 

where the pathlength l has a more complicated dependency on Dcell. Methods for determining the 

pathlength of cylindrical cells exposed to filtered xenon-arc lamps are discussed in references 12 

and 17. Pathlength determinations as a function of the depth of a system, Dsys exposed to solar 

irradiation are discussed in references 12 and 18. In addition, note that if the quantum yield is not 

independent of wavelength, it must remain inside of the summation in equation 5.

15. Equation 5 is applicable to solar as well as filtered xenon arc lamp irradiation. However, the 

following equation is often used to determine the direct photolysis rate constant for optically dilute 

solutions of test chemicals in pure water in photolysis cells or in near surface, clear natural water exposed 

to solar irradiation (2)(12) (See Annex 4: Derivations of selected equations):
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76

k L d solar ( ) = φ ε ∑ λ λ

295

800

[eq. 6]

where:

kd = direct photolysis rate constant (1/day)

φ = quantum yield (independent of wavelength)

ελ = molar absorption coefficient (L/mol•cm) at wavelength λ

Lλ = average daily solar photon flux irradiance (mmol photons/cm2•day) over 

wavelength interval ∆λ centered at wavelength λ

16. Equations 1, 5 and 6 show that direct photolysis kinetics are theoretically pseudo-first order only if 

the photon flux remains constant over time. Photon fluxes from filtered xenon arc lamps at any given 

wavelength remain, in general, relatively constant over time. This is one of the primary advantages in using 

filtered xenon arc lamps. However, solar photon fluxes at any given wavelength vary cyclically over 24-

hour periods. Nevertheless, the data obtained from studies conducted in natural sunlight (in which the rate 

constant reflects an average over the entire study duration) generally do fit a single, pseudo-first order rate 

constant model reasonably well. The reason is that changes in the average pseudo first-order rate constant 

over time as the study progresses is generally relatively small and not systematic. This is particularly valid 

for studies conducted for one to several weeks or for 2-4 hours mid-day. It may not be as valid for studies 

lasting 4 hours to several days or for those conducted beyond 30 days.

Summary of the proposed tiered approach for direct photolysis 

Tier 1: Theoretical screen

17. Estimate a maximum possible direct photolysis rate constant for the test chemical in the near 

surface of a clear natural water as follows. Measure the test chemical molar absorption coefficients from 

295 nm to 800 nm, and use tabular solar irradiance for summer and 400 or 500 latitude over the same 

wavelength interval. Estimate a maximum possible direct photolysis rate by assuming the quantum yield 

in equation 6 is equal to one and by substituting the molar absorption coefficients and tabular solar 

irradiance values, Lλ, into the equation (2)(12). Determine if the estimated maximum possible direct 

photolysis rate constant would theoretically result in estimated direct photolysis losses greater than 50% of 

an initial concentration over 30 days/nights. If maximum possible losses are < 50%, no further direct 

photolysis work is performed, but proceed to Indirect Photolysis Tier 1 (experimental/theoretical screen), if 

required. If the maximum possible losses > 50%, proceed to Direct Photolysis Tier 2 (experimental screen 

and range finding) or to Direct Photolysis Tier 3 (determination of the quantum yield).

Tier 2 - Experimental screen and range finding

18. Approximately determine the rate of decline in the concentration of the test chemical and 

corresponding direct photolysis rate constant in buffered pure water exposed to a filtered xenon arc lamp 

(recommended) or sunlight. Determine if the approximate direct photolysis rate constant would result in 

estimated direct photolysis losses equal or greater than to 20% of an initial concentration over 

30 days/nights in the near surface of a clear natural water body exposed to summer sunlight. If estimated 

losses are < 20%, no further direct photolysis work is performed, but proceed to Indirect Photolysis Tier 1 

(experimental/theoretical screen), if required. If estimated losses ≥ 20%, proceed to Direct Photolysis 

Tier 3 (determination of the quantum yield). If the determination of the quantum yield is not required, 

proceed to Direct Photolysis Tier 4 (kinetics study). 
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Tier 3 - Determination of the quantum yield and its use to estimate direct photolysis rate constants

19. Determine the quantum yield for direct photolysis of the test chemical in buffered pure water using 

monochromatic irradiation (12), polychromatic artificial irradiation (8) or sunlight (2). Once the quantum 

yield has been determined, use it as input to computer programs such as GCSOLAR (18) or ABIWAS 

(35)(36) to help estimate direct photolysis rates for the test chemical applicable to any types of surface 

waters (defined by depth and light attenuation), seasons, and latitudes of interest (2). If the Direct 

Photolysis Tier 4 study is potentially applicable, determine if the direct photolysis rate constants estimated 

from the quantum yield would result in estimated direct photolysis losses greater than or equal to 20% of 

the initial concentration over 30 days/nights in the near surface of a clear natural water body exposed to 

sunlight. If the estimated losses are < 20%, no further direct photolysis work is required but proceed to 

Indirect Photolysis Tier 1 (experimental/theoretical screen), if required. If the estimated losses are > 20%, 

proceed to the Direct Photolysis Tier 4 kinetics study. 

Tier 4 - Kinetics study on the parent and, if applicable, major transformation products

20. Determine the rate of decline in the concentration of the test chemical and corresponding direct 

photolysis rate constant in buffered pure water exposed to a filtered xenon arc lamp (recommended) or 

sunlight. If applicable, also identify/quantify and determine the rates of formation and decline of major 

transformation products. In this guideline, major transformation products are defined as those accounting 

for ≥ 10% of the applied test chemical at any sampling time.

Indirect Photolysis 

12.21. To undergo transformation via indirect photolysis, the test substance must: 

 

• receive energy from a photosensitizer in an excited electronic triplet state such as various ones 

generated from the solar irradiation of CDOM (12)(14)(2119)(220), or 

• react with a photochemically generated oxidant such as a hydroxyl free radical, a singlet oxygen, or a 

alkylperoxy free radical (references 12 and 231 through 342). Singlet oxygen and alkylperoxy free 

radicals appear to be generated primarily from the solar irradiation of CDOM in oxygenated natural 

waters. Some hydroxyl free radicals also appear to originate from the solar irradiation of CDOM. 

However, the major source of the hydroxyl free radical in natural waters appears to generally be the 

solar irradiation of nitrate (231). 

13 22. In infrequent cases where the nitrate to nitrite ratio is less than 100 in aerobic surface waters, the 

photolysis of nitrite (which is present in much lower concentrations than nitrate) may also contribute 

significantly to the formation of hydroxyl radicals because “nitrite photoproduces OH radicals about 50 

times more rapidly than nitrate” (4339). 

14.23. The transformation of chemical pollutants in surface waters may also occur via photoredox 

reactions with soluble and insoluble Fe (III) species, which are commonly present at significant 

concentrations in many surface waters (440)(451). 

15.24. Indirect photolysis kinetics are actually closer to being second-order than first-order because the 

rate of decline of the test chemical test substance is generally proportional to the product of the test 

chemical test substance and sensitizer or photooxidant concentrations (12)(220)(231). However, although 

transient photooxidant and excited photosensitizer concentrations are very low (typically below 1 x 10-10

M), they can quickly attain quasi steady-state concentrations in irradiated solutions thereby leading to 

pseudo first-order kinetics (12)(220)(231). 
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16.25. In many cases, the net pseudo first-order rate constant for indirect photolysis will reflect the 

reaction of different molecules with more than one type of sensitizer and/or oxidant: 

k k S or indirect j nd j ss = ∑ ( ) () 2 ( ) Ox j(ss)

j=1

j=n

 [eq. 57] 

where: 

 kindirect = net pseudo first-order indirect photolysis rate constant (1/day) 

 kj(2nd) = second order rate constant for the reaction with sensitizer j or oxidant j (L/mol•d) 

 Sj(ss) = steady state concentration of sensitizer j (mol/L) 

 Oxj(ss) = steady state concentration of oxidant j (mol/L) 

Summary of the proposed tiered approach for indirect photolysis 

Tier 1 - Experimental and/or theoretical screen

17.26. Perform an experimental screen in simulated natural water (SNW) containing humics as a 

surrogate for CDOM, and/or in a simulated natural water containing nitrate as well as humics (SNW-N) 

and/or in one or more natural waters (NW). and in buffered pure water (PW) (3). Specifications for PW 

appear in paragraph 68-a. Approximately determine the rate of decline in the concentration of the test 

chemical test substance in SNW, and/or SNW-N, and/or NW exposed to sunlight or a Xenon arc lamp to 

determine if indirect photolysis represents a potentially significant transformation pathway. The use of a 

xenon lamp light source for indirect photolysis studies is not recommended at this time. Determine if the 

estimated indirect (not net) photolysis losses over the equivalent of 30 days/nights of sunlight exposure is 

greater than or equal to 20% of an initial concentration of the test chemical test substance. If the estimated 

indirect photolysis losses are < 20%), no further indirect photolysis work is performed. If the estimated 

indirect photolysis losses are > 20%, proceed to the Indirect Photolysis Tier 2 (kinetics study). 

27. In some cases, it may be possible to supplement the Tier 1 experimental screen with a theoretical 

screen based on using structure activity relationships (SARs) to estimate rate constants for reaction with 

photooxidants and information on the typical concentrations of photooxidants in surface water (21).

Tier 2 - Kinetics study on the test substance parent and (if applicable) major transformation products

18.28. Determine the rate of decline in the concentration of the test chemicaltest substance and its 

corresponding net and indirect photolysis rate constants in SNW, and/or SNW-N and/or one or more NWs 

irradiated with a Xenon lamp or sunlight. and in PW. Determine if the indirect photolysis rate constants

would result in estimated losses greater than or equal to 20% of the initial concentration over a period 

equivalent to 30 days in the near surface of a clear natural water body exposed to sunlight. If the estimated 

losses are <20%, no further indirect photolysis work is required. If the estimated losses are ≥20%, proceed 

to Tier 3. If data on transformation products for indirect photolysis are applicable, identify/quantify major 

transformation products and determine their rates of formation and decline. In this guideline, major 

transformation products are defined as those accounting for > 10% of the applied test chemical (percent of 

applied radioactivity or molar basis) during any part of the study.
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Tier 3 – Identification of major phototransformation products

19. Estimate the rates of formation and decline of phototransformation products in SNW and/or SNWN and /or NW exposed to a filtered Xenon lamp or sunlight. Isolate and identify the major 

phototransformation products. In this guideline, major transformation products are defined as those 

acconting for ≥10% of the applied test substance in an individual sample at any sampling time.

APPLICABILITY OF THE TEST GUIDELINE 

20.29. This test guideline is applicable to chemical substances for which analytical methods with 

sufficient accuracy and precision are available and validated (if required). If data are needed on 

transformation products, adequate quantitative analytical methods must also be available/developed for 

them. 

21.30. If a non-ionic test chemical test substance is somewhat susceptible to hydrolysis, the test may still 

be applicable as long as a dark control is used to account for the hydrolysis rate (2)(4)(5). However, 

photolysis tests should be conducted at a pH at which the hydrolysis rate of the chemical is minimized 

(5)(46 2). The rate constant determined from the dark controls should be identical to the hydrolysis rate 

constant and can be subtracted from the overall rate constant determined from the irradiated solutions to 

give the direct photolysis rate constant. However, in some cases, the desired test chemicaltest substance

may undergo rapid hydrolysis (i.e., hydrolysis half-life of < 1 day) within the environmentally relevant pH 

range of 45-9 at 20-25o

C (47). In such cases, the photolysis study should instead be conducted on each the 

major hydrolytic product(s) (> 10% of applied) as the test substance). 

22.31. The test is not applicable to highly volatile compounds such as fumigants and some organic 

solvents. However, in some cases, where the Henry’s Law constant of the test chemicaltest substance

and/or anticipated transformation products may indicate the potential for fairly substantial volatilization 

rates from water, the study may still be applicable provided that precautions are taken. Such precautions 

include using minimal head-space, adequate sealing in the photolysis cells to minimize volatile losses 

during the study (2)(4) or trapping. Trapping volatiles becomes more difficult, but preferred, when 

employing sacrificial sampling on numerous small photolysis cells than when employing aliquot sampling 

on a smaller number of much larger test vessels. However, with the proper equipment and experimental 

set-up, it can be done as shown by Ruzo (483). 

INFORMATION ON THE TEST SUBSTANCE

23. Unlabelled or labelled test substances can be used to measure the rate of photolysis. Labelled 

material is required for studying the pathway of phototransformation and for establishing a mass 

balance. 14C-labelling is recommended but other isotopes, such as 13C, 15N, 32P, may also be useful. As 

far as possible, the label should be positioned in the most stable part(s) of the molecule2. The 

chemical and radiochemical purity of the test substance should be at least 95 %.

24.32. Before carrying out any of the phototransformation tests, the following information on the test 

substance should be known, if available: 

 

2

 For example, if the test substance contains one ring, labelling on this ring is required; if the test substance contains 

two or more rings, separate studies may be needed to evaluate the fate of each ring and to obtain suitable information 

on formation of phototransformation products. 
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80

 (a) Solubility in water [OECD Guideline 105]. 

 (b) Solubility in organic solvents

 (cb) Vapor pressure [OECD Guideline 104] 

 (dc) Henry’s Law Constant*

 (ed) Abiotic hydrolysis as a function of pH [OECD Guideline 111] 

 (fe) n-octanol/water partition coefficient [OECD Guidelines 107 and 117] 

 (gf) pKa(s) of ionisable substances [OECD Guideline 112] 

(g) Chemical nomenclature (IUPAC and CAS)

(h) Chemical Abstract Registry Number of the test substance 

(i) Common name(s), if any

(j) Chemical structure

(k) Position(s) of radiolabelled atom(s) or other molecular tags (if applicable)

(l) Isomer purity (if applicable)

 

Note: The temperatures at which the physical chemical properties listed above in the previous 

paragraph were determined should generally be reported.within the range of (25 ± 5)°C and 

should be specified.

33. If available, the solubility in non-aqueous solvents may be useful in selecting a co-solvent or 

extractant. Also, the following optional information may be useful for indirect photolysis studies conducted 

in natural water: 

- Aerobic - Anaerobic Transformation in Aquatic Sediment Systems 

[OECD draft Guideline 308]

- Adsorption / Desorption [OECD Guideline 106]

25.34. The n-octanol/water partition coefficient and aqueous solubility supply some information on the 

potential of the test chemical test substance to significantly adsorb to glassware and reaction vessels. 

However, any additional information on the glassware adsorption potential of the test chemicaltest 

substance should be known to ensure that such characteristics are taken into account in preparing glassware 

and reaction vessels for the study. 

26. An appropriate analytical method of known accuracy, precision and sensitivity for the 

quantification and identification of the test substance and its phototransformation products should be 

available. The analytical detection limit for the test substance and its phototransformation products should 

also be known (see paragraph 31).

ANALYTICAL METHODS AND QUALITY CRITERIA

35. The experimenter should develop methods with appropriate accuracy (recovery), precision 

(reproducibility), and adequate limits of detection (LOD) and quantification (LOQ). In many instances, 

validation of the analytical methodology may be required. In those cases, the experimenter should be 

responsible for ensuring that the analytical methods have been validated. 

36. The analytical methods that may be used include liquid scintillation counting (LCS) and 

radiochromatography (radiolabelled substances), high-performance liquid chromatography (HPLC), gas 

 

*

 Henry’s Law constant estimated from solubility in water and vapour pressure or measured. Henry’s Law constants 

for some chemicals may also be found in: http://webbook.nist.gov/chemistry
ENV/JM/TG/RD(2006)1/REV 

81

chromatography (GC), gas-liquid chromatography (GLC), and capillary electrophoresis (CE), etc. TLC can 

also be used for radiolabelled test chemicals, but only if adequate resolution can be achieved and there is 

no decomposition of the reference standards in the silica plates.

37. When necessary to identify and/or confirm transformation products, the separation methods listed 

in the preceding paragraph may be coupled with spectrometric methods such as mass spectrometry (MS) 

and/or Fourier-Transform Infrared spectroscopy (FT-IR). Examples of such couplings include as GC/MS, 

HPLC/MS, LC/MS-MS, GC/FT-IR or any other adequate techniques. 

38. Reference chemicals of potential transformation products may be used for identification (e.g. by 

co-chromatography). When radiolabelled test substance is used, it may be necessary to use radiolabelled 

reference chemicals. However, as indicated in the previous paragraph, the identities of major 

transformation products need to be confirmed preferably by an independent analytical chemistry method 

such as with MS. Transformation products that cannot be identified from reference chemicals should be 

elucidated using, for example, MS, NMR and infrared spectroscopy.

39. Parameters that can influence the accuracy and/or precision of the determination of chemical 

concentrations in aqueous media include the stability of the chemicals in samples during storage, extraction 

efficiency, analytical separation efficiency in mixtures, and the magnitude of the concentrations with 

respect to analytical detection and quantification limits. 

40. Analytical instrumentation should be periodically calibrated within applicable concentration

ranges according to the manufacturer’s specifications. If samples are stored prior to analysis, storage 

stability tests may need to be performed depending upon the storage method and duration. The accuracy 

and precision of the analytical method should be checked within the expected concentration range. 

Accuracy can be determined from the recovery percentages of spiked samples. Precision can be determined 

by replicate analyses of an appropriate percentage of the samples.

41. Regardless of the chosen analytical method, analytical recoveries for radiolabelled test chemicals 

should fall within 90% and 110% of the applied radioactivity. For non-radiolabelled test chemicals, 

analytical recoveries of 85 to 115% are desirable.

42. For radiolabelled materials, total mass balance should fall within that range 90% to 110% of the 

total applied radioactivity (42).

REFERENCE SUBSTANCES

27. Reference substances should be used for the characterisation and/or identification of 

phototransformation products by spectroscopic and chromatographic methods.

QUALITY CRITERIA

Recovery

28. Extraction and analysis of, at least, duplicate samples immediately after the addition of the test 

substance gives a first indication of the repeatability of the analytical method and of the initial recoveries 

for the test substance. Recoveries for later stages of the experiments are given by the respective mass 

balance obtained with labelled material. Recoveries should range from 90 to 100 % for labelled chemicals 

(49) and from 70 to 110 % for non-labelled chemicals (7).
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Repeatability and sensitivity of analytical method

29. Repeatability of the analytical method (excluding the initial extraction efficiency) to quatify test 

substance and phototransformation products can be checked by duplicate analysis of the same extract , 

incubated long enough for formation of phototransformation products.

30. The limit of detection (LOD) of the analytical method for the test substance and for the 

phototransformation products should be at least 0.01 mg/kg water (as test substance) or 1 % of the applied 

dose whichever is lower. The limit of quantification (LOQ) should also be specified.

Accuracy of transformation data

31. Regression analysis of the concentration of the test substance as a function of time gives the 

appropriate information on the reliability of the phototransformation curve and allows the calculation of 

the confidence limits for half-lives (in case of pseudo first-order kinetics) or DT50 values and, if 

appropriate, DT75 and DT90 values.

TEST MATERIALS

Test vessels

32.43. All glassware and reaction vessels/photolysis cells used in the photolysis studies must be 

sterilized by autoclaving (2)(3)(4)(12) or by any other suitable non-chemical method (2). Chemical 

sterilization is not recommended since chemical reagents may leave residues that could absorb in the 

295290 to 800 nm region and/or behave as photosensitizers. 

Test substance application

33. The test substance should be directly dissolved in the aqueous media at a concentration which

should not exceed half its solubility. For test substances with low aqueous solubility, a co-solvent may be 

used to prepare the test solution. However, the amount of co-solvent used must be as little as possible and 

preferably not exceed 1% by volume in the test solution (46). Co-solvent percentages up to 10% may be 

used, but if more than 10% co-solvent is required to dissolve the test substance, the study should not be 

performed. The selected co-solvent must not solvolyse the test substance, must not absorb in the 290 to 

800 nm region, and must not be a photosensitizer. Acetone is an example of a solvent that is a 

photosynthesizer, and should not be used. Acetonitrile is a generally recommended co-solvent.

34. The initial concentration of the test substance for indirect photolysis studies should be such that the 

absorbance at any wavelength above 290 nm (maximum absorption wavelength included) is less than 0.02. 

These optically dilute, also referred to as “optically thin” solutions generally have a concentration of < 10-4

M. Nevertheless, the initial concentration of the test substance should be at least 10 times greater than the 

quantification limit of the analytical method.

35. Because oxygen can affect the rates and even pathways of photodegradation reactions, test 

solutions should be saturated with air at the beginning of the study (4). A buffer solution that has been 

autoclaved should be given sufficient time to aerate prior to dispensing into the vessels. It is not 

recommended to autoclave small amounts of buffer in individual vessels because driving the air out may 

change the volume. 
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83

44. If radiolabelling is used, test chemicals should be labelled at one or more atoms within each 

stable or relatively stable portion of the molecule. For chemicals containing one or more aromatic rings, 

one or more atoms in each ring are usually labelled. In addition, one or more atoms on both sides of any 

linkage thought to be readily susceptible to cleavage should be radiolabelled (42). Radiolabelling is 

particularly important when there is the need to identify transformation pahtways. Radiolabelling does not 

have to be limited only to 14C. For example, in some cases, 32P can be used in place of or in addition to 14C 

(42). If mass spectroscopy is used, stable isotopes such as 15N can also be used as molecular tags. 

45. Radiolabelled test substances should have radiochemical and chemical purities of 95% or greater 

(42). Non-radiolabelled test substances should have a chemical purity of 95% or greater (42). 

46. If the test chemical consists of more than one geometrical and/or optical isomer, the percentage 

represented by each isomer should be provided. 

47. Pesticide testing should generally be conducted with the active ingredient. The same generally 

applies to pharmaceutical chemicals. However, in some instances it may be more appropriate to use a 

formulated product.

48. If the potential loss of the test chemical by volatilization or adsorption to test vessels is of 

concern or if the identification and quantification of major transformation products are of interest, 

radiolabeling should be used. 

Test media

36.49. Guidance for preparing test media for direct and indirect photolysis respectively is provided in 

Annex 5. 

Test media for direct photolysis

50. For direct photolysis studies, the test media should be sterilized aqueous buffer solutions at the 

appropriate pH. The water should be “pure water” (PW). The buffers should be identical or comparable to 

those used in the hydrolysis studies and should not absorb between 295 and 800 nm or be photosensitizers. 

Also, the buffers should not adversely affect the solubility of the test chemical. 

51. If the test chemical is non-ionizable and is approximately stable to abiotic hydrolysis at pH 7, the 

buffer used in the direct photolysis study should maintain a pH of approximately 7. However, if the 

hydrolysis rate of a non-ionizable test chemical at pH 7 is significant, but its hydrolysis rate at pH 5 or 9 is 

much lower, the buffer used should maintain a pH (within the 5 to 9 range) at which the hydrolysis rate is 

minimized (42). 

52. If the test chemical is appreciably ionic anywhere within a 5 to 9 pH range, the study should be 

conducted in one or more aqueous buffers at any of the pH 5, 7, and/or 9. Selection of the pH (s) at which 

the study should be conducted depends on the electronic absorption spectra at each pH. Spectra showing 

absorption bands between 295 and 800 nm or shorter wavelengths tailing above 295 nm together with 

estimates of maximum photolysis rates (Tier 1: Direct Photolysis) are the criteria for selecting the pH(s). 

Refer to Tier 1, Direct Photolysis for further details.

53. For test chemicals with low aqueous solubility, a co-solvent may be used to prepare the test 

solution. However, the amount of co-solvent used must be as little as possible and preferably not exceed 

1% by volume in the test solution (42). Co-solvent percentages up to 10% may be used, but if more than 
ENV/JM/TG/RD(2006)1/REV 

84

10% co-solvent is required to dissolve the test chemical, the study should not be performed. The selected 

co-solvent must not solvolyse the test chemical, must not absorb in the 295 to 800 nm region, and must not 

be a photosensitizer. Acetone is an example of a solvent that is a photosynthesizer, and should not be used. 

Acetonitrile is a generally recommended co-solvent.

Test media for indirect photolysis studies

37.54. The selection of test media for indirect photolysis studies depends on the purpose of the study. 

The preparation of test media is based on the procedure given in OPPTS 835.5270 guideline (3). Three test 

media may be used for these studies: (1) SNW (Simulated Natural Water); SNW-N (Simulated Natural 

Water with Nitrate); and (3) NW (Natural Water). SNW-N may be appropriate for areas where surface 

waters show strong evidence of nitrate pollution associated with the use of fertilizer and other nitratebearing substances. Nitrate has been known to photochemically produce hydroxy radicals

(231)(242)(286)(50). NW may be important in situations when the understanding of the photochemical 

behavior of chemicals in actual natural surface waters is of interest. 

38.55. Humic acid or other humic materials may be obtained commercially (Aldrich, Fluka, etc.). 

Humic and fulvic acid extracted from soils or natural surface water may be used as surrogates for aquatic 

humic acids (253)(5144). Composite data for typical humic acids are available from the International 

Humic Substances Society (5245). The UV-VISIBLE spectra of the selected humic materials in the test 

media (natural water and/or buffered SNW) should be provided. The absorption spectra of some humic 

materials are shown in reference 5346.

Light sources

39.56. For indirect photolysis studies the recommended light source should be a filtered xenon arc lamp 

capable of simulating natural sunlight in the 295290 to 800 nm region or sunlight (5447). For indirect 

photolysis studies, the only recommended light source at this time is sunlight. The photon flux of the 

filtered xenon arc lamp as a function of wavelength should be determined at least at the beginning and the 

end of the study using one or more actinometers and or a spectral radiometer. if available. If sunlight is 

used, average daily solar photon fluxes (Lλ) as a function of wavelength should be determined for the 

season and latitude closest to those of the experiment using solar irradiance tables and a spectral 

radiometer., if available. 

40.57. If a filtered xenon arc lamp is used, the irradiation photon flux incident on the test vessels should 

be adequate to cause substantial decline of the test chemical test substance over a several day experimental 

period. The transformation rate constant and corresponding half-life obtained with a filtered xenon arc 

lamp can be adjusted to solar equivalents from a comparison of solar photon flux to lamp photon flux 

because of the linear relationship between the transformation rate and photon flux (see equation 14). 

41.58. If sunlight is used, the study should be conducted during the season when and at the latitude 

where the concentration of the test chemical test substance is expected to be highest in the environment. 

For pesticides, that would be the season when and the latitude where it is typically applied. 

42.59. If a filtered xenon arc lamp (see part 1, Annex 6) and sacrificial sampling of multiple test vessels 

are used, an appropriate experimental set-up should be used to ensure that all of the test vessels receive the 

same incident irradiation flux. Self-contained, commercially available xenon photolysis units in the 

projection or “working plane” configuration produce uniform fluxes (5548)(5649a). A commonly used 

configuration is the “merry-go-round” photochemical reactor (12). Merry-go-round photochemical reactors 

are commercially available (5649a)(5649b). Another example of an experimental set-up is shown reference 

570 (see part 1, Annex 6). An eExperimental set-ups that includes trapping of volatiles appears in reference 
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(483)and are also shown in Part 1 Annex 6.

Other photolytic equipment and reagents 

43.60. Photolytic equipment includes: 

• Optical cells of appropriate material (generally quartz) and path length for measuring UV-VIS spectra. 

• Appropriate optical filters to cut-off radiation below 295290 nm and above 800 nm. 

•When monochromatic irradiation is used in the determination of the quantum yield, the appropriate filter 

systems must be used to isolate the desired wavelength (12)(47). Monochromatic irradiation at the 

desired wavelength can also be achieved with a monochromator in conjunction with a suitable 

polychromatic light source such as a mercury or xenon arc lamp. (Filters for isolating the 313 and 

366 nm wavelengths from a mercury arc lamp are discussed in reference 12).

• Specially designed reaction cuvettes/photolysis cells to contain test solutions (exposed and dark 

control) and chemical actinometers, when needed. The recommended material is quartz. 

•Appropriate apparatus designed to contain the light source, filters, sample, holders, reaction 

vessels/actinometers such as the photochemical “merry-go-round” reactor. Several appropriate designs 

are commercially available to accommodate specific needs. shown in Part 1, Annex 6.

• Photometer or spectral radiometer to measure photon flux as a function of wavelength. 

44.61. Photolytic reagents include: 

• All of the water used in the direct photolysis tests must be of high purity and free of any absorbing or 

photosensitizing chemical species. For example, the U.S.EPA OPPTS 835.2210 (2) recommends using 

reagent water meeting ASTM Type IIA standards, as described in ASTM D1193-99 Standard 

Specification for Reagent Water (see Annex 5: Test Media). “Milli-Q” water is one example of high 

purity water. In this guideline, the water used in the preparation of stock, test and buffer solutions is 

referred as “pure water”.

• Appropriate chemical reagents to be used as actinometers. 

• Commercially available, certified humic material for indirect photolysis studies (Fluka; Aldrich). 

• Analytical grade NaNO3

•Appropriate aqueous buffers such as phosphate for pH 7, acetate for pH 5 and borate for pH 9 for 

compatibility with hydrolysis studies. Phosphate buffers are usually recommended for direct photolysis 

studies(2)(12). 

• Analytical equipment and chemical reagents

45.62. In addition to standard laboratory equipment, the following is required: 

• Spectrophotometer to acquire spectral data in the UV-VIS range. For example, single beam, diode 

array instruments that allow sensitive, rapid acquisition of digitized spectral absorbance data. 

• Appropriate analytical instrumentation to measure the concentration of the test substance in exposed 

and dark control samples, such as HPLC, GLC, TLC, etc. 

• Suitable instrumentation for identification purposes, such as MS, HPLC/MS, GC/MS, LC/MS, GCFTIR, NMR, etc. 

• When radiolabelled test substances are used, liquid scintillation counters (LSC) and instrumentation 

for radiochromatography should be available. 

• Appropriate extraction apparatus. 

• Equipment to sterilise reaction vessels and other glassware used in the preparation of stock and test 

solutions. 
ENV/JM/TG/RD(2006)1/REV 

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46.63. Chemical reagents include: 

• Reagent grade chemicals and solvents free of any photosensitizer. 

• Scintillation liquid (for radiolabelled chemicals).

64. Analytical references include:

•Test substance reference standards for co-chromatography and/or for other analytical methods.

•Potential transformation products (including hydrolysis products) reference standards for the 

determination of products, if applicable.

TEST METHODOLOGY

Test conditions

47.65. The temperature during photolysis studies may vary by as much as 10 degrees but should be 

maintained within the range of 20 to 30  ̊C. (25 ± 5)oC and periodically monitored.. 

48.66. The pH of buffered solutions should be maintained within +0.15 pH units. The pH should be 

checked during the course of the study, if feasible. 

49.67. Dark control solutions should be used for all quantum yield determinations and for all direct and 

indirect photolysis studies and should be kept at the same temperature as the irradiated samples. 

68. The concentration of the chemical in the test media should never exceed half its solubility. When 

a co-solvent is needed, it should preferably not exceed 1% by volume in the test solution and never exceed 

10% by volume. 

69. The initial concentration of the test chemical for quantum yield determinations and direct 

photolysis studies should be such that the absorbance at any wavelength above 295 nm (maximum 

absorption wavelength included) is less than 0.02. These optically dilute, also referred to as “optically 

thin”, solutions generally have a concentration of < 10-4 M. Nevertheless, the initial concentration of the 

test chemical should be at least 10 times greater than the quantification limit of the analytical method. 

70. The concentration of the test chemical in simulated (SNW, SNW-N) or natural water (NW) test 

media should be equivalent to that of optically dilute test solutions in pure water/buffer solutions required 

for direct photolysis studies.

71. Because oxygen can affect the rates and even pathways of photodegradation reactions, test 

solutions should be saturated with air at the beginning of the study (4). A buffer solution that has been 

autoclaved should be given sufficient time to aerate prior to dispensing into the vessels. It is not 

recommended to autoclave small amounts of buffer in individual vessels because driving the air out may 

change the volume. 

Study duration, sampling, and replication 

50.72. The study duration should be sufficient to determine the DT75 (two half-lives if decline is firstorder) of the test chemical test substance and (if applicable) the formation and decline of major 

transformation products until they have declined to < 25% of their maximum concentration. However, the 
ENV/JM/TG/RD(2006)1/REV 

87

maximum study duration should not exceed the equivalent of 30 days/nights of sunlight exposure during an 

appropriate season and at an appropriate latitude. The “DT75" is defined as the time required for 75% of the 

initial concentration of test chemical to disappear.

73. Based upon equating photons absorbed by the test chemical per unit volume, the maximum direct 

photolysis study duration (using continuous filtered xenon arc lamp irradiation) that is equivalent to 30 

days/nights of sunlight exposure will be only a few days and is given by (see Annex 4).

Max direct photolysis duration days j

D

l

L

I

cell

xenon

xenon

. ()

( )

 =

∑

∑

30 295

800

0

ε

ε

λ λ

λ λ

[eq. 8]

where:

30 = 30 days/nights of solar irradiation

Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area

lxenon = light pathlength in cell exposed to a filtered xenon arc lamp

and other terms same as defined for equations 5 and 6.

Note: For rectangular box cells, the term Dcell/lxenon cancels out because lxenon = Dcell. However, that will 

not be true for cylindrical cells (12).

51.74. Although equation 8 allows for flexibility in choosing an experimental photon flux from a filtered 

xenon arc lamp, practical considerations somewhat limit the range of experimental fluxes that should be 

used. In general, it is recommended that the experimental photon flux from a filtered xenon lamp be 

adjusted to be somewhat comparable to that of sunlight (at noon for short several hour studies or 24 hour 

average for several day to weeks studies). The reason is the experimental flux should not be so intense that 

photolytic processes other than those that would occur under natural sunlight are initiated. Also, if the test 

substanceparent is susceptible to hydrolysis at the experimental pH and the hydrolytic products are 

susceptible to direct photolysis, time should be allowed for them to form. 

52.75. A preliminary range-finding study should be performed to help determine the number of samples 

to be collected and the timing of their collection. For linear and non-linear regressions on test chemicaltest 

substance data in definitive or upper tier tests, the minimum number of samples collected should be five 

and seven, respectively. The greatest number of samples taken should generally be during the first part of 

the study where the slope of the concentration versus time series for the test chemicaltest substance is 

generally greatest. If an accurate determination of the rates of formation and decline of major 

transformation products is desired, the number of samples taken should generally be greater and somewhat 

more dispersed throughout the study duration. The number of sampling times can be less in range finding 

or lower tier tests. 

53.76. Either aliquot or sacrificial sampling can be used. Sacrificial sampling of entire photolysis cells 

at each sampling time is often preferred over withdrawing aliquots of test solutions at each sampling time. 

The reason is that it helps to maintain aseptic conditions and allows for mass balance determinations at 

each sampling time. The use of sacrificial sampling is particularly necessary in cases where mass balance 

determinations are needed such as when the test chemical test substance and/or its transformation products 

are volatile and/or adsorb strongly to the photolysis cells (5). Future sampling descriptions in this 

document will be for the generally preferred sacrificial sampling, but aliquot sampling from larger 

irradiated and dark control reaction vessels is acceptable as long as the resulting mass balance is adequate 

and aseptic conditions are properly maintained. 
ENV/JM/TG/RD(2006)1/REV 

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54.77. Replicates (preferably a minimum of twothree) of each experimental determination of kinetic 

parameters are recommended to determine variability and reduce uncertainty in the determination of 

kinetic parameters. However, the uncertainty in the determination of kinetic parameters can also be 

reduced by increasing the number of sampling intervals within each experiment, particularly when the rates 

of formation and decline of major transformation products are being determined. In such cases, a minimum 

of two replicate determinations of kinetic parameters may be adequate. With generally limited resources, 

the researcher needs to weigh the pros and cons of experimental replication versus increased numbers of 

sampling times per experiment in designing the study. 

Calculations

55.78. Basic calculations/equations, including the following one used to determine whether going to 

higher tiers is triggered, are included within the body of the guideline: 

% loss over 30 days / nights solar = −− = 100 1 30 { } exp[ ] ( ) ( ) k td solar [eq. 69] 

 where: 

ksolar = pseudo first-order direct or indirect photolysis rate constant for test chemicaltest 

substance exposed to solar irradiation (1/day) 

56.79. Calculations/equations for the kinetics of the formation and decline of major photoproducts, and 

accounting for photobleaching are optional. The necessary non-linear regressions can be readily performed 

with the use of commercially available spreadsheets and/or statistical software. 

Determination of the photon flux from a filtered xenon arc lamp

57.80. The photon flux from a filtered xenon arc lamp can be determined with the use of low and/or 

high optical density actinometers alone or preferably in conjunction with a spectral radiometer (581). The 

advantage in using a spectral radiometer to determine the photon flux is convenience, as it avoids changing 

actinometers. However, spectral radiometers are expensive and are not always readily available. 

Furthermore, even if a spectral radiometer is generally used to determine the photon flux, it should be 

periodically calibrated against photon fluxes determined with actinometers. Calibration is necessary 

because of the difference between the exposure surface of a spectral radiometer and that of the photolysis 

cuvette or tubes used to contain the actinometers (581). 

58.81. By determining the monochromatic first-order direct photolysis rate constant for a low optical 

density actinometer of known quantum yield in PW in a photolysis cell irradiated with monochromatic 

irradiation, the incident photon flux at the wavelength of the irradiation can be determined from the 

following equation (581; see Annex 43 - Equation Derivations): 

I j D

l

k

xenon

cell

xenon

d act

act act

0 2 3 λ

λ

λ φ ε ( )

( )

( ) . =

 [eq. 710]

where:

 I0λ (xenon) = photon flux (photons/cm2 •day) over a 1 nm interval centered at wavelength 

 j = conversion factor to make units consistent with the L/mol•cm unit of the molar 
ENV/JM/TG/RD(2006)1/REV 

89

 absorption coefficient and the mol/L unit of concentration = 

 6.02 x 1020 (L/cm3

) (photons/mol) 

 kdλ (act) = monochromatic direct photolysis rate constant (1/day, pseudo first-order) 

of an 

 actinometer exposed to monochromatic irradiation at wavelength λ

φ(act) = the quantum yield of the actinometer 

ελ = molar absorption coefficient (L/mol•cm; M-1•cm-1) of the actinometer at 

 wavelength λ 

 Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area 

 lxenon = light pathlength in cell exposed to a filtered xenon arc lamp 

 

59.82. Equation 10 only gives the photon flux at one wavelength, but it can be used at different 

individual wavelengths wherever another monochromatic direct photolysis rate constant is determined for 

one or more low optical density actinometers. The determination of the photon flux at several different 

individual wavelengths spread throughout the wavelength range in which an estimate of the photon flux is 

desired should be sufficient to determine the photon flux from a filtered xenon arc lamp as a continuous 

function of wavelength. Because photon flux irradiation spectrum from a filtered xenon arc lamp is 

smooth, interpolation from known photon fluxes at several different wavelengths can be readily done 

(581). 

60.83. By determining the polychromatic zero order direct photolysis rate constant for a high optical 

density actinometer of known quantum yield in PW in a photolysis cell irradiated with a filtered xenon arc 

lamp, the sum of the incident photon flux over the absorption spectrum of the actinometer above 

295290 nm can be determined from the following equation (581) (see Annex 4):

I jD k xenon

cell

act

d act

initial

final

0λ

λ

λ

φ ( ) ( )( = =

=

∑ zero order)

 [eq. 811]

where: 

 (λ=initial) = initial wavelength of actinometer absorption (if > 295290 nm) 

 (λ=final) = final wavelength of the actinonmeter absorption (if < 800 nm) 

 I0λ = photon flux over a 1 nm interval centered at wavelength λ (photons/cm2 

•day) 

j = conversion factor to make units consistent with the L/mol•cm unit of the 

molar 

 absorption coefficient and the mol/L unit of concentration = 

 6.02 x 1020 (L/cm3

)(photons/mol) 

kd_(act) (0 order)= monochromatic direct photolysis rate constant (zero order) of an actinometer 

 exposed to monochromatic irradiation at wavelength λ

(mol/L•day) 

 Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area (cm) 

φact = the quantum yield of the actinometer 

 

61.85. By using more than one high optical density actinometer with absorption bands at different 

wavelengths, the sum of the photon flux over any desired range including the entire range between 295290

and 800 nm can be determined. Further experimental details of procedures for determining the photon flux 

from and for calibrating xenon arc lamps are provided in references (5837)(591).
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90

Performance of direct photolysis study

Tier 1 - Theoretical screen. 

86. For a test chemical which does not ionize significantly within a pH range of 5 to 9, a single UVVisible absorption spectrum should be measured at a single pH within the 5 to 9 range where the 

hydrolysis rate, if any, is minimized. If the chemical is stable to abiotic hydrolysis, the spectrum should be 

measured at pH 7. The pH at which a direct photolysis study on a non-ionic test chemical is conducted 

should be the same as the pH at which the spectrum is determined. For a test chemical that is appreciably 

ionized anywhere within a 5 to 9 pH range, UV- Visible absorption spectra should be measured at pH 5, 7, 

and 9.

87. Electronic absorption spectra of the test chemical should be obtained in appropriate solvent 

systems at desired pHs with a spectrometer capable of recording UV-VISIBLE spectra between 295 and 

800 nm. Because the higher energy (shorter wavelength) bands of many chemicals may sometimes tail into 

wavelengths above 295 nm, it is recommended that the spectrum be recorded 30 to 40 nm below 295 nm. 

The procedure to record, assign absorption band maxima, band-widths and determine decadic molar 

absorption coefficients has been described elsewhere (OECD Guideline 101). To obtain an adequate 

absorption spectrum, it may sometimes be necessary to enhance the solubility of the test chemical by using 

aqueous methanol, ethanol, or acetonitrile. It is recommended that UV-VISIBLE spectra of test chemicals 

be also obtained in the neat co-solvent.

88. If the test chemical absorbs above the 295 nm cutoff of solar irradiation at the earth’s surface 

with ελ> 2 L/mol•cm (M-1cm-1) over at least 15 nm, estimate a maximum possible direct photolysis rate by 

assuming that the quantum yield is equal to one in equation 6 (2): 

k L d (max) = =

=

∑ ε λ λ

λ

λ

295

800

[eq. 12]

where:

kd(max) = maximum possible direct photolysis rate constant (1/day)

ελ = molar absorption coefficient (L/mol•cm; M-1 cm-1) at wavelength λ

Lλ = average daily solar photon flux (mmol photons/cm2•day) over wavelength interval 

centered at wavelength λ.

89. In estimating the maximum possible photolysis rate constant, the average daily solar photon 

fluxes Lλ substituted into equation 12 should be those for summer at 40° or 50° latitude. Values of Lλ can 

be readily obtained from tables (2)(12). As an alternative to the use of equation 12 and tables of Lλ, the 

maximum direct photolysis rate constant can be readily estimated by inputting molar absorption 

coefficients and a quantum yield equal to one into computer programs such as GCSOLAR (18) or 

ABIWAS (35)(36).

Criteria for deciding whether to go to Direct Photolysis Tier 2 or Tier 3

90. Determine if the maximum possible direct photolysis rate constant for the test chemical in the 

near surface of a clear natural water exposed to an average daily solar photon flux would result in 

estimated direct photolysis losses > 50% of an initial concentration over 30 days/nights of sunlight 

exposure by substituting kmax(solar) for k(solar) in equation 9.
ENV/JM/TG/RD(2006)1/REV 

91

91. If estimated direct photolysis losses (assuming a maximum direct photolysis rate constant) are 

less than 50% of the initial concentration over a 30-day period direct photolysis is considered to be an 

insignificant process. In such a case, no further direct photolysis work is performed. If required, go to 

Indirect Photolysis Tier 1 (experimental/theoretical screen).

92. If estimated direct photolysis losses (assuming a maximum direct photolysis rate constant) are 

greater than or equal to 50% of the initial concentration over a 30-day period, go to Direct Photolysis Tier 

2 (experimental screen and range finding) or Direct Photolysis Tier 3 (determination of the quantum yield). 

Tier 2 - Experimental screen and range finding test for higher tiers

93. Dissolve the test chemical in sterilized, buffered pure water and place an optically dilute solution 

in photolysis test vessels. Expose at least half of the test vessels to a filtered xenon arc lamp or sunlight and 

allow an equal number or fewer of the test vessels to serve as dark controls. 

94. Determine the concentrations of the test chemical in the irradiated test vessels and in the dark 

control test vessels at adequate enough sampling intervals to approximately determine the direct photolysis 

rate constant of the test chemical.

95. At each sampling interval, sacrifice an irradiated photolysis cell for analysis. The number of 

sampling intervals for which a dark control is also sacrificed for analysis should depend upon a prior 

knowledge of the rate of hydrolysis of the test chemical at the experimental pH. For example, if the test 

chemical is susceptible to appreciable rates of hydrolysis at the experimental pH, a dark control cell should 

be sacrificed for analysis each time an irradiated cell is sacrificed. However, if the test chemical is stable 

to hydrolysis at the experimental pH, a minimum of 3 sampling intervals for the dark control will be 

adequate.

96. If the data follow first-order kinetics, compute irradiated and dark control rate constants for the 

test chemical by using linear regression on equation 3 or non-linear regression on equation 2.

97. Once the irradiated and dark control rate constants for the test chemical have been determined, 

compute the approximate laboratory direct photolysis rate constant from the following equation:

kkk d screen irradiated dark ( ) = − [eq. 13]

Estimation of direct photolysis rate constants in near surface clear natural water from laboratory values

98. Estimate a direct photolysis rate constant for the test chemical in near surface clear natural water 

exposed to average daily solar photon fluxes L_ from the direct photolysis rate constant in a photolysis cell 

exposed to continuous photon fluxes from a filtered xenon arc lamp using the following equation 

(assuming Dcell the pathlength l; see Annex 4):

k j D

l

k L

I

d solar

cell

d xenon

xenon

( )

( )

( )

. =

∑

∑ 23

295

800

0

ε

ε

λ λ

λ λ

[eq. 14]
ENV/JM/TG/RD(2006)1/REV 

92

where:

kd(solar) = average daily direct photolysis rate constant (1/day) for the test chemical in the near 

surface of clear natural water exposed to average daily solar irradiation fluxes Lλ

kd(xenon) = direct photolysis rate constant (1/sec) for the test chemical in buffered pure water in 

photolysis cells exposed to constant filtered xenon arc lamp fluxes I0λ

ελ(chem) = molar absorptivity of the test chemical (L/mol•cm; M-1 cm-1)

Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area (cm)

lxenon = light pathlength in cell exposed to a filtered xenon arc lamp

I0_(xenon) = incident filtered xenon arc lamp photon flux (photons/cm2•day) over a 1 nm interval 

centered at wavelength λ

Lλ = 24 hour average daily solar photon flux over wavelength interval λ centered at 

wavelength λ for the weakly absorbing near surface of clear natural water bodies in 

(mmol photons/cm2•day) 

j = conversion factor = 6.02 x 1020 (L/cm3)(photons/mol)

99. To use equation 14, the photon flux from the filtered xenon arc lamp that is incident on the 

photolysis cells (I0λ) must be determined as a function of wavelength using actinometers and (if available) 

a spectral radiometer. The solar photon fluxes as a function of wavelength for the near surface of clear 

natural waters (Lλ) for various mid-seasons and latitudes are readily available (2)(12). 

Criteria for deciding whether to go to Direct Photolysis Tiers 3 and/or 4

100. Determine if the screen direct photolysis rate constant for the test chemical in the near surface of 

a clear natural water exposed to sunlight would result in estimated direct photolysis losses > 20% of an 

initial concentration over 30 days/nights of sunlight exposure by substituting kd(screen)(solar) for k(solar) in 

equation 9. 

101. If estimated direct photolysis losses (assuming the screen direct photolysis rate constant) are less 

than 20% of the initial concentration over a 30-day light/dark sunlight exposure period, direct photolysis is 

considered to be an insignificant process. In such a case, no further direct photolysis work is performed. If 

required, proceed to the Indirect Photolysis Tier 1 screen.

102. If estimated direct photolysis losses (assuming the approximate direct photolysis rate constant) 

are greater than or equal to 20% of the initial concentration over a 30-day light/dark sunlight exposure 

period, proceed to Direct Photolysis Tiers 3 and/or 4. If determining the quantum yield of the test chemical

and using it to estimate direct photolysis rate constants for types of water bodies, seasons, and latitudes is 

required, proceed to Direct Photolysis Tier 3. Otherwise, proceed to direct photolysis Tier 4 to perform a 

more definitive kinetics study. 

Tier 3 - Determination of the quantum yield and estimations of direct photolysis rate constants

103. Determine the quantum yield of the test chemical in buffered solutions using monochromatic 

irradiation (12), filtered xenon-arc lamp polychromatic irradiation (8) or sunlight polychromatic irradiation 

(2). Each has advantages and disadvantages compared to the others.

104. The main advantage in using monochromatic irradiation to determine the test chemical quantum 

yield of the test chemical is that (as will be shown later by equations 19 through 21) it eliminates the need 

to directly measure the incident monochromatic irradiation. However, a disadvantage is that it will take 

longer to determine the quantum yield with monochromatic irradiation than with the use of polychromatic 
ENV/JM/TG/RD(2006)1/REV 

93

irradiation because the actinometer and the test chemical are absorbing photon fluxes at just one 

wavelength instead of over multiple wavelengths. 

105. The main advantage in using polychromatic irradiation (filtered xenon arc lamp or sunlight) 

instead of monochromatic irradiation to determine the test chemical quantum yield is that the time required 

to determine the quantum yield may be substantially shorter (8). The reason is due to the absorption of

photon fluxes by the actinometer and test chemical over multiple wavelengths instead of just one 

wavelength. The main disadvantage in using polychromatic irradiation to determine the test chemical

quantum yield is that unlike with monochromatic irradiation (as will be shown later by equations 20 and 

21), its use requires the determination of the photon flux.

106. The advantage in using sunlight instead of a filtered xenon arc lamp, if polychromatic irradiation 

is used to determine the quantum yield, is that tables of average solar intensities as a function of season and 

latitude are readily available. Also, if sunlight is used, it eliminates the necessity of having a 

monochromatic or polychromatic artificial light source. However, if sunlight is used, an actinometer with 

an adjustable quantum yield is needed so the direct photolysis rate of the actinometer can be adjusted to be 

comparable to that of the test chemical under the same irradiation conditions (2)(12). Parallel exposure of 

the test chemical and actinometer to sunlight over comparable time frames minimizes errors due to changes 

in solar flux caused by fog, varying cloud cover, etc. (12)(51).

107. An example of an adjustable quantum yield actinometer is the p-nitroacetophenone - pyridine 

(PNAP-PYR) actinometer (52). For a 1.0 X 10-5 M p-nitroacetophenone solution, the quantum yield of the 

actinometer in experiments carried out with monochromatic irradiation at 313 nm can be adjusted by 

varying the concentration of pyridine in the solution according to the following equation (2)(12):

φ act= • 0 0169 .[ ] pyridine [eq. 15]

where:

φact = quantum yield of the 1.0 X 10-5 M p-nitroacetophenone

[pyridine] = molar concentration of pyridine

108. Suitable chemical actinometers used in aqueous photochemical studies include the following: 

Actinometer System Literature Citation

(a) p-nitroacetophenone/pyridine 52

(b) p-nitroanisole/pyridine 26, 52

(c) valerophenone 53

(d) ferrioxalate 19, 23, 37, 54, 55

(e) o-nitrobenzaldehyde 37, 52

(f) Reinecke’s salt 12

(g) aberchrome 540 56, 57

109. The first two actinometers listed in the above table have adjustable quantum yields and are 

therefore particularly useful when solar irradiation is used (12). The first three actinometers listed in the 

table are low optical density actinometers and the last four are high optical density actinometers. The 

characteristics of those actinometers including the optimal wavelength range for their use are discussed in 

references 12 and 58. Other chemical actinometers are described in the IUPAC document on chemical 

actinometers (58).
ENV/JM/TG/RD(2006)1/REV 

94

Determination of the quantum yield using monochromatic irradiation 

110. Place solutions of the test chemical and the selected optically low density actinometer in separate 

but identical photolysis cells. Irradiate at least half of the photolysis cells with monochromatic irradiation 

of wavelength λ and let an equal or fewer number (depending upon the hydrolytic stability of the test 

chemical) serve as dark controls. Determine the concentration of the test substance in the irradiated cells 

and in the dark controls at adequate enough sampling intervals to determine the monochromatic direct 

photolysis rate constant of the test chemical (kd_(chem)) and of the actinometer (kdλ(act)) at the wavelength λ of 

irradiation. At each sampling interval, sacrifice an irradiated photolysis cell for analysis. The number of 

sampling intervals for which a dark control is also sacrificed for analysis should depend upon a prior 

knowledge of the rate of hydrolysis of the test chemical at the experimental pH.

11. The decline of a test chemical or actinometer concentrations at low optical density should follow 

pseudo first-order kinetics so that the monochromatic irradiated and the dark control rate constants can be 

determined by using linear regression on equation 3 or non-linear regression on equation 2. The decline of 

a test chemical or actinometer at high optical density should follow zero-order kinetics so that the 

monochromatic irradiated and the dark control rate constants can be determined by using linear regression 

on the following zero-order kinetics equation:

C C kt = − 0 [eq. 16]

112. Once the monochromatic irradiated and dark control rate constants for the test chemical and the 

actinometer have been determined, the monochromatic direct photolysis rate constant of the test chemical

(kd_(chem)) and of the actinometer kdλ(act) at wavelength λ are given by:

kk k d chem or irradiated chem or act dark chem or act λλ λ ( ) ( )( ) ( )( ) act = −

[eq. 17]

Note: In a sterilized aqueous buffer, the dark control rate constant (kdark) should be equal to the 

hydrolysis rate constant at the pH of the buffer.

113. Determine the molar absorption coefficients of the test chemical (ελ(chem)) and (if a low optical 

density actinometer is being used) the actinometer (ελ(act)) at the wavelength λ of the monochromatic 

irradiation using the Beer-Lambert law assuming that ελC is >> than the attenuation coefficient αλ of the 

test medium:

ε λ

λ

(

( )

( ) [ ] chem

chem act

chem act

A

l C or act)

 or 

 or 

=

[eq. 18]

where:

ελ (chem) = molar absorption coefficient of the test chemical or actinometer 

L/mol•cm; M-1cm-1)

Aλ(chem or act) = absorbance of test chemical or actinometer solution at wavelength λ

L = light pathlength (cm) generally equivalent to Dcell

[C(chem or act)] = concentration of test chemical or actinometer solution (mol/L)

114. The quantum yield of an optically dilute test chemical determined using monochromatic 
ENV/JM/TG/RD(2006)1/REV 

95

irradiation and a low optical density (low absorbance) actinometer is given by (12)(47) (See Annex 4):

φ φ

ε

ε

λ λ

λ λ

chem act

d chem act

d act chem

k

k = ( )()

()( ) [eq. 19]

where:

φchem = the quantum yield of the test chemical

φact = the quantum yield of the actinometer

kdλ(chem) = monochromatic direct photolysis rate constant (1/day, pseudo first-order) of a test 

chemical exposed to monochromatic irradiation at wavelength λ

kdλ(act) = monochromatic direct photolysis rate constant (1/day, pseudo first-order) of an 

actinometer exposed to monochromatic irradiation at wavelength λ

ελ(chem) = molar absorption coefficient (L/mol•cm; M-1 cm-1) of the test chemical at 

wavelength λ 

ελ (act) = molar absorption coefficient (L/mol•cm; M-1 cm-1) of the actinometer at wavelength λ 

115. The quantum yield of an optically dilute test chemical determined using monochromatic 

irradiation and a high optical density (high absorbance) actinometer is given by (12)(47). See Annex 4:

φ φ

ε λ

λ

λ

chem

act

chem cell

d chem

D d act zero

k

k = 2 3. ( )

( )

( )( order) [eq. 20]

where:

φchem = the quantum yield of the test chemical

φact = the quantum yield of the actinometer

kd_(chem) = monochromatic direct photolysis rate constant (1/day, pseudo first-order) of a test 

chemical exposed to monochromatic irradiation at wavelength λ

kd_(act)(zero) = monochromatic direct photolysis rate constant (mol/L•day, zero-order) of an 

actinometer exposed to monochromatic irradiation at wavelength λ

Dcell = depth of irradiated cell (cm) = volume of irradiated cell/incident area (cm)

116. The quantum yield of a test chemical at high optical density (high absorbance) determined using 

monochromatic irradiation and a high optical density (high absorbance) actinometer is given by (12)(47). 

See Annex 4):

φ φ λ

λ

chem act

d chem zero

d act zero

k

k = ( )( )

( )( ) [eq. 21]

where:

φchem = the quantum yield of the test chemical

φact = the quantum yield of the actinometer

kd_(chem)(zero) = monochromatic direct photolysis rate constant (mol/L•day, zero-order) of a test 

chemical exposed to monochromatic irradiation at wavelength λ
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96

kd_(act)(zero) = monochromatic direct photolysis rate constant (mol/L•day, zero-order) of an 

actinometer exposed to monochromatic irradiation at wavelength λ

117. As can be seen from equations 19, 20, and 21, it is not necessary to determine the photon flux if 

monochromatic irradiation is used to determine the test chemical quantum yield.

Determination of the quantum yield using polychromatic irradiation (lamp or sunlight)

118. The quantum yield of an optically dilute test chemical using a low optical density (low 

absorbance) actinometer and sunlight or a polychromatic filtered xenon arc lamp can be determined from 

equations 22 and 23, respectively (2)(6)(12)(See Annex 4):

φ φ

ε

ε

λ λ

λ

λ

λ λ

λ

chem act λ

d chem

d act

act

nm

nm

chem

nm

nm

k

k

L

L

= 



 





























=

=

=

=

∑

∑

( )

( )

( )

( )

295

800

295

800

[eq. 22]

φ φ

ε

ε

λ λ

λ λ

chem act

d chem

d act

act xenon

chem xenon

k

k

I

I

= 



 





























∑

∑

( )

( )

() ( )

()( )

0

295

800

0

295

800

[eq. 23]

where:

kd(chem) = direct photolysis rate constant (1/day, pseudo first-order) of the test chemical

kd(act) = direct photolysis rate constant (1/day, pseudo first-order) of the actinometer

φchem = quantum yield of the test chemical (dimensionless ratio) 

φact = quantum yield of the actinometer (dimensionless ratio)

ελ(chem) = molar absorption coefficient (L/mol•cm; M-1 cm-1) of the test chemical at 

wavelength λ

ελ(act) = molar absorption coefficient (L/mol•cm; M-1 cm-1) of the actinometer at 

wavelength λ

Lλ = average daily solar photon flux (mmol photons/cm2•day) over wavelength interval 

∆λ centered at wavelength λ

I0λ(xenon) = incident filtered xenon arc lamp photon flux (photons/cm2•day) over a 1 nm 

interval centered at wavelength λ

119. As can be seen from equations 22 and 23, it is necessary to independently and accurately 

determine the incident photon flux as a function of wavelength when polychromatic irradiation is used to 

determine the test chemical quantum yield.

120 The determination of the test chemical quantum yield using polychromatic irradiation and an 

actinometer are discussed in detail in (2) for the use of sunlight, and in (8) for the use of a filtered xenon 

arc lamp. 

Dependence of the quantum yield on wavelength
ENV/JM/TG/RD(2006)1/REV 

97

121. Most of the direct photolysis equations involving polychromatic irradiation in this document and 

in the literature are presented with the quantum yield taken outside of the integral or summation over 

different wavelengths. The reason is that in aqueous solutions, the quantum yields of test chemicals are (in 

general) approximately independent of wavelength, even across different absorption bands. In aqueous 

solutions, molecules in higher excited electronic states tend to undergo radiationless decay to their first 

excited state before any photoreaction occurs (12). If photoreaction does occur, it generally occurs in 

competition with other deactivation processes while the molecule is in its first (not higher) excited 

electronic state. 

122. However, the quantum yield may sometimes differ between absorption bands (6)(14). Therefore, 

it may sometimes be necessary to determine the quantum yield at least one wavelength for each adsorption 

band within the 295-800 nm range using monochromatic irradiation. The wavelength chosen for each 

absorption band should correspond to the maximum absorption. In cases where the quantum yield varies 

with wavelength, the quantum yield should be brought inside any integrals or summations over 

wavelengths.

123. In cases where the quantum yield remains constant within the same absorption band, but varies 

across different absorption bands, equations 5 and 6 for the direct photolysis rate constant can be modified 

to give:

k I d i ii

i begin

i end

i

i n

= ∑ ∑ =

=

φ ε λ λ

λ

λ

0

1 ()

( )

[eq. 24]

k L d i

i

i n

i i

i begin

i end

= =

=

∑ ∑ φ ε λ λ

λ

λ

1 ()

( )

[eq. 25]

where:

φI = quantum yield for absorption band i

εiλ = molar absorption coefficient (L/mol•cm; M-1 cm-1) at wavelength λ within 

absorption band i

I0iλ = photon flux per nm (photons/cm2 •sec/nm) at wavelength λ within absorption 

band i

λI = wavelength interval in nm generally = 1 nm

Liλ = average daily solar photon flux irradiance (mmol photons/cm2•day) over 

wavelength interval ∆λ centered at wavelength λ within absorption band i

I = indice for different absorption bands up to the maximum of n bands

iλ(begin) = wavelength at the beginning of absorption band i

iλ(end) = wavelength at the end of absorption band i

Use of the quantum yield to estimate direct photolysis rate constants

124. Once the quantum yield of a test chemical has been determined, it should be used as input into 

computer programs such as GCSOLAR (18) or ABIWAS (35)(36) to estimate direct photolysis rate 

constants for seasons, latitudes, and water body depths and light attenuations of interest. GCSOLAR and 

ABIWAS can be used to estimate direct photolysis rate constants at any given time in near surface water or 

at any specified depth. The programs can also average rate constants over any given time and/or depth 
ENV/JM/TG/RD(2006)1/REV 

98

interval of interest.

125. Although programs such as GCSOLAR or ABIWAS are readily available, equation 5 is often 

used to estimate the direct photolysis rate constant for optically dilute solutions of test chemicals in pure 

water in photolysis cells or in the near surface of clear natural water exposed to solar irradiation (2)(12).

Criteria for deciding whether to go to direct photolysis Tier 4

126. A Direct Photolysis Tier 4 kinetics study should be performed if identification/kinetics data on 

the major transformation products are applicable and if the direct photolysis rate constant(s) for the parent 

test chemical (estimated from its quantum yield) indicate that 

direct photolysis is a potentially significant process.

127. Determine if the direct photolysis rate constant (estimated from the quantum yield) for the test 

chemical in shallow natural water exposed to sunlight would result in estimated direct photolysis losses 

> 20% of an initial concentration over 30 days/nights of sunlight exposure by substituting kd(solar) for k(solar) 

in equation 9.

128. If estimated direct photolysis losses are less than 20% of the initial concentration over a 30 day 

light/dark sunlight exposure period, direct photolysis is considered to be an insignificant process. In such a 

case, no further direct photolysis work is performed. If of interest, proceed to indirect photolysis Tier 1. 

Proceed to direct photolysis Tier 4 if:

•Estimated direct photolysis losses are greater than or equal to 20% of the initial concentration over a 30 

day light/dark sunlight exposure period, and

•The identification of major transformation products and/or the determination of their rates of formation 

and decline are required. 

Tier 4 - Kinetics study on the test chemical and (if applicable) major transformation products

Experimental determinations of direct photolysis rate constants in the laboratory

129. In direct photolysis Tier 4, the decrease in the concentrations of the test chemical is determined in 

buffered pure water exposed to a filtered xenon arc lamp or sunlight. If applicable, the rates of formation 

and decline of major transformation products are also determined. For the purposes of this guideline, a 

major transformation product is one accounting for > 10% of the applied radioactivity (or molar basis) at 

any time during the study.

130. If a flow-through system is used to trap volatile test chemicals and/or their major transformation 

products, the following considerations should be made. The air should be passed through microbial filters 

to keep the samples sterile. Moist air should be used to prevent evaporation. The system should be 

checked for sterility at the study termination.

131. Place the test solution in photolysis test vessels. At least half the test vessels should be exposed 

to a filtered xenon arc lamp or sunlight and an equal or fewer number of the test vessels should serve as 

dark controls. 

132. Determine the concentrations in the irradiated test vessels and in the dark control test vessels at 

adequate enough sampling intervals to determine the direct photolysis rate constant of the test chemical. 

133. At each sampling interval, sacrifice an irradiated photolysis cell for analysis. The number of 
ENV/JM/TG/RD(2006)1/REV 

99

sampling intervals for which a dark control is also sacrificed for analysis should depend upon the rate of 

hydrolysis of the test chemical at the experimental pH.

134. f the data follow first-order kinetics, compute irradiated and dark control rate constants for the 

test chemical by using linear regression on equation 3 or non- linear regression on equation 2.

135. Once the irradiated and dark control rate constants for the test chemical have been determined, 

compute the laboratory direct photolysis rate constant from the following equation:

kk k d irradiated dark = − [eq. 26]

136. If a filtered xenon arc lamp is used for irradiation, estimate an approximate average daily direct 

photolysis rate constant for the test chemical in shallow natural water exposed to average daily solar 

photon fluxes Lλ from equation 14.

137. In direct photolysis Tier 4, the decrease in the concentrations of the test chemical is determined in 

buffered pure water exposed to a filtered xenon arc lamp or sunlight. If applicable, the rates of formation 

and decline of major transformation products are also determined. For the purposes of this guideline, a 

major transformation product is one accounting for > 10% of the applied radioactivity at any time during 

the study.

138. If a flow-through system is used to trap volatile test chemicals and/or their major transformation 

products, the following considerations should be made. The air should be passed through microbial filters 

to keep the samples sterile. Moist air should be used to prevent evaporation. The system should be checked 

for sterility at the study termination. 

139. The following equation can be used to estimate the direct photolysis rate constant for the test 

chemical in shallow natural water exposed to one or more different sunlight conditions from those in the 

study (see Annex 3 - Equation Derivations):

k k

L

L

sun shallow sun shallow water

nm

nm

nm

( ) nm

( )

( )

 water)2 = =

=

=

=

∑

∑ 1

2

295

800

1

295

800

ε

ε

λ λ

λ

λ

λ λ

λ

λ

[eq. 27]

where:

kd(sun shallow water)2 = estimated direct photolysis rate constant in the near surface of clear natural 

water exposed to solar irradiation fluxes (Lλ(2)) different from solar irradiation 

fluxes (Lλ(1)) used in the experiment.

kd(sun shallow water)1 = estimated direct photolysis rate constant in the near surface of clear natural 

water exposed to the experimental solar irradiation fluxes.

Lλ(2) = average daily solar photon fluxes different from those closest 

(Lλ(1)) to the experimental conditions.

Lλ(1) = average daily solar photon fluxes closest to the experimental ones 

based on the season and latitude at which the study was performed.

140. Compute direct photolysis half-lives by substituting the estimated direct photolysis rate constants 

into equation 4.
ENV/JM/TG/RD(2006)1/REV 

100

Major transformation products (if required)

141. Identify and quantify major phototransformation products using analytical methods and criteria 

indicated in paragraphs 39 through 43.

142. In cases where data follow first-order kinetics and the parent test chemicalsubstance rapidly 

transforms, use the same first-order methods applied to the test chemical to also estimate first-order direct 

photolysis rate constants and corresponding half-lives for any major primary transformation products 

formed from the parent which are still at substantial concentrations (e.g. at > 10% of applied) after the 

parent has degraded to negligible levels.

143. Equation 14 for estimating sunlight rate constants from filtered xenon arc lamp rate constants and 

equation 27 for estimating a sunlight rate constant for a given season and latitude from a sunlight rate 

constant for a different season and/or latitude are also applicable to any first-order photolysis rate constants 

determined for any major transformation products. 

144. In cases where primary or secondary transformation products are being substantially formed over 

the same time period they are being substantially lost, their direct photolysis rate constants and 

corresponding half-lives cannot be readily estimated as they were for the parent. However, as an option, 

attempts can be made to estimate first-order rate constants for the formation and decline of major 

transformation products using non-linear regression to fit concentration versus time series data to various 

equations derived from assumed transformation pathways.

Performance of indirect photolysis tiers 

Tier 1 for indirect photolysis - Experimental and/or theoretical screen

62.145. Perform a screen in simulated water (containing humics as a surrogate for CDOM and when 

applicable, nitrate) or natural water to determine if indirect photolysis represents a potentially significant 

transformation pathway. 

63.146. Place solutions of the test chemicaltest substance in simulated natural water (SNW) and/or 

simulated natural water with nitrate (SHW-N) and/or natural water (NW).) and in buffered pure water 

(PW) into equal numbers of photolysis cells. Expose at half of the photolysis cells containing each type of 

solution to a Xenon arc lamp or sunlight and let an equal or smaller number of the photolysis cells 

containing each type of solution serve as dark controls. 

147. The use of a filtered xenon arc lamp for indirect photolysis studies is not recommended at this 

time. The complexity of indirect photolysis processes does not allow straightforward extrapolation of 

results from filtered xenon arc lamps to sunlight equivalents and may not be feasible (2)(12). 

64.148. Determine the concentrations in the irradiated photolysis cells and in the dark control photolysis 

cells at adequate enough sampling intervals to determine the irradiated and dark control rate constants of 

the test chemicaltest substance in SNW and/or SNW-N and/or in NW. and in PW. At each sampling time, 

sacrifice an irradiated photolysis cell. The number of sampling intervals for which a dark control is also 

sacrificed for analysis should depend upon the stability of the test chemicaltest substance in the dark 

control at the experimental pH. Assuming the decline of the test chemicaltest substance follows first-order 

kinetics, determine the irradiated and dark control rate constants in the irradiated and dark control solutions 

by using linear regression on equation 3 or non-linear regression on equation 2. 

65.149. Once the irradiated and dark control rate constants for the test chemicaltest substance in SNW 
ENV/JM/TG/RD(2006)1/REV 

101

and/or SNW-N and/or NW and in PW have been determined, the net photolysis rate constants, the direct 

photolysis rate in PW, and the indirect photolysis rates constants can be respectively determined from the 

following equations: 

kk k pX X irrad X dark = − ( ) () [eq. 928] 

kk k d PW PW irrad PW dark () ( ) ( ) = − [eq. 29]

k kk iX approx pX d PW ( ) () = − [eq. 30]

 where: 

 X = SNW, SNW-N, or NW 

Note: Equation 30 is only approximate because the direct photolysis rate constant in PW (kd(PW)) will be 

greater than the direct photolysis rate constant in SNW, SNW-N or NW (kdX) due to light 

attenuation (12). However, the approximation is adequate for a screen. Also, uUnlike indirect 

photolysis Tier 2, this indirect photolysis Tier 1 screen does not take into account the 

photobleaching of humic solutions in photolysis cells by sunlight as reported by Winterle and 

Mill (6059). 

66.150. Determine if the approximate indirect photolysis rate constant for the test chemicaltest substance

in shallow natural surface water would result in estimated indirect photolysis losses > 20% of an initial 

concentration over 30 days/nights of sunlight exposure by substituting kpX kiX(approx) for k(solar) in equation 9.

When a Xenon arc lamp is used its intensity of irradiation has to be related to natural sunlight (see Annex 

&).

67.151. If estimated indirect photolysis losses (assuming an approximate indirect photolysis rate 

constant) are less than 20% of the initial concentration over a 30 day/night sunlight exposure period, 

indirect photolysis is considered to be an insignificant process. In such a case, no further indirect 

photolysis work is performed. 

68.152. If estimated indirect photolysis losses (assuming an approximate indirect photolysis rate 

constant) are greater than or equal to 20% of the initial concentration over a 30-day light/dark sunlight 

exposure period, indirect photolysis is considered to be a potentially significant process. Proceed to 

Indirect Photolysis Tier 2 to perform a more definitive kinetics study. 

153. Mill (21) has summarized structural activity relations (SARs) for estimating second-order rate 

constants for the reaction of organics with photooxidants in water. He also summarized typical 

concentrations of various photooxidants generally found in the top layers of surface water exposed to 

sunlight. In some cases, it may be possible to supplement or even replace the experimental screen with a 

theoretical screen based on using structural activity relationships (SARs). These SARs estimate rate 

constants and information on the typical concentrations of photoxidants in surface water. Although a 

theoretical screen involving one or two potential photooxidants may indicate that going to Tier 2 is 

justified, it is doubtful that one can indicate that going to Tier 2 is not justified. The reason is that a 

theoretical screen will generally cover only one or two of the large number of possible types of indirect 

photolysis reactions. Further details on using structure activity relations for estimating rate constants for 

the reaction of test chemicals with photoxidants in water such as the hydroxyl radical and singlet oxygen 

are provided in reference 51.
ENV/JM/TG/RD(2006)1/REV 

102

Tier 2 - Experimental determination of rate constants (and identification/quantification of major 

transformation products, when required)

69.154. In the indirect photolysis Tier 2, the decrease in the concentration of the test chemicaltest 

substance is determined in simulated natural water (SNW) and/or simulated natural water with nitrate 

(SNW-N) and/or one or more natural surface waters (NW) and in buffered pure water (PW) exposed to 

sunlight or a Xenon arc lamp.. If applicable, the identities and rates of formation and decline of major 

transformation products are also determined. For the purposes of this guideline, a major transformation 

product is one accounting for > 10% of applied at any time during the study.

70.155. The performance of Tier 2 differs from that in Tier 1 in several ways including: 

• The number of sampling times is greater. 

•The direct photolysis constant in humic water is estimated by inputing the chemical’s quantum yield and 

the attenuation constants of the humic solution into GCSOLAR or ABIWAS. 

•If applicable, the rates of formation and decline of major transformation products are determined.

• If a study lasts for several days or more such that “photobleaching” of the humic solutions in 

photolysis cells may be substantial in some cases, it can (as an option) be accounted for as indicated in 

Leifer (12) and OPPTS 835.5270(2). 

71.156 Dissolve the test chemicaltest substance in SNW and/or SNW-N and/or NW and in PW and place 

the solutions in equal numbers of photolysis cells. Expose at least half of the photolysis cells containing 

each type of solution to sunlight or a Xenon arc lamp and let an equal or lesser number of the photolysis 

cells containing each type of solution serve as dark controls. 

72.157. Determine the concentrations in the irradiated cells and in the dark controls at adequate enough 

sampling intervals to determine the irradiated and dark control rate constants of the test chemicaltest 

substance in simulated natural water (SNW), and/or simulated natural water with nitrate (SNW-N), and/or 

natural surface water (NW)., and in buffered pure water (PW). At each sampling interval, sacrifice an 

irradiated test vessel for analysis. Sacrifice dark controls for analysis at an equal or fewer number of 

sampling intervals 

73.158. Determine the irradiated and dark control rate constants of the test chemicaltest substance in 

SNW and/or SNW-N and/or NW and in PW by using linear regression on equation 3 or non-linear 

regression on equation 2. 

159. Once the irradiated and dark control rate constants for the test chemical in SNW and/or SNW-N 

and/or NW and in PW have been determined, the net photolysis rate constants in those waters and the 

direct photolysis rate constants in PW can be determined from equations 28 and 29.

160. If the same light source and photon flux, test vessels, and experimental set-up are used for the 

definitive determination of the direct photolysis rate constant in PW in the Tier 4 Direct Photolysis study, a 

separate determination of the direct photolysis rate constant in PW in the Tier 2 Indirect Photolysis study 

as just described is not necessary. 

Determination of direct photolysis rate constant in SNW, and/or SNWN, and/or NW from one in PW

161. Determine attenuation constants as a function of wavelength for all experimental waters (SNW, 

SNW-N, and/or NW and PW) from the following equation:
ENV/JM/TG/RD(2006)1/REV 

103

α λ λ( ) ( ) = A

l [eq. 31]

where:

α(λ) = light attenuation constant as a function of wavelength λ (1/cm)

A(λ) = absorbance of test medium as a function of wavelength λ

l = light pathlength in cell (cm) 

162. Estimate average daily direct photolysis rate constant for the test chemical in near surface PW 

and in SNW and/or SNW-N, and/or NW by inputting its quantum yield and attenuation constants for PW 

and for SNW, and/or SNW-N, and/or NW into GCSOLAR or ABIWAS. If the quantum yield of the test 

chemical was determined in Direct Photolysis Tier 3, use it as input. Otherwise, estimate the quantum yield 

by substituting the experimental direct photolysis rate constant in PW and ελLλ into equation 6 and 

rearranging to solve for the quantum yield. 

163. Estimate the direct photolysis rate constant in SNW and/or SNWN and/or NW in a photolysis 

cell from the following equation:

k

k

k

k dX

dX GCSOLARorABIWAS

d PW GCSOLARorABIWAS

d PW = ( )

( ) ( )

( )(exp)

[eq. 31]

where: 

X = SNW, SNW-N, or NW

kdX = direct photolysis rate constant in SNW, SNWN, or NW in a photolysis cell

kdX(GCSOLAR) = GCSOLAR or ABIWAS estimated average daily direct photolysis rate constant 

in near surface SNW, SNW-N, or NW

kd(PW)(GCSOLAR) = GCSOLAR or ABIWAS estimated average daily direct photolysis rate constant 

in PW

kd(PW)(exp) = experimentally determined direct photolysis rate constant in PW in a photolysis 

cell

74.164. Once the irradiated and dark control net and direct photolysis rate constants in SNW and/or 

SNW-N and/or NW have been determined, the indirect photolysis rate constant in those waters can be 

estimated from the following equation 9.:

kkk iX pX dX = − [eq. 32]

where X = SNW, SNW-N, or NW

75. Determine if the indirect photolysis rate constant for the test substance in shallow natural surface 

water would result in estimated indirect photolysis losses > 20% of an initial concentration over 30 days of 

sunlight exposure by substituting kpX for k(solar) in equation 9. When a Xenon arc lamp is used its intensity 

of irradiation has to be related to natural sunlight (see Annex 6).

76. If estimated indirect photolysis losses (assuming an indirect photolysis rate constant) are less than 

20% of the initial concentration over a 30 day sunlight exposure period, indirect photolysis is considered to 

be an insignificant process. In such a case, no further indirect photolysis work is performed.
ENV/JM/TG/RD(2006)1/REV 

104

77. If estimated indirect photolysis losses (assuming an indirect photolysis rate constant) are greater 

than or equal to 20% of the initial concentration over a 30-day sunlight exposure period, indirect photolysis 

is considered to be a potentially significant process. Proceed to Indirect Photolysis Tier 3. 

165. The methods used in the identification and kinetics analysis of transformation products formed 

during indirect photolysis studies are comparable to those formed during direct photolysis studies.

Method of accounting for “photobleaching”

78.166. If a study lasts for several days or more, the decline of the test chemicaltest substance in 

irradiated SNW, SNW-N, or NW may not be first-order due to the “photobleaching” of humic solutions. 

Photobleaching in photolysis cells that has been studied and reported by Winterle and Mill (6054). 

79.167. Photobleaching does not appear to be a significant process in the actual environment because 

CDOM appears to be generated at much faster rates than the rates of its decline through photobleaching 

(581). However in the laboratory, the photobleaching of humic solutions in photolysis cells by sunlight 

somewhat complicates a more definitive determination of the indirect photolysis rate. Because of 

photobleaching, the indirect photolysis rate constant for a test chemicaltest substance in SNW, SNW-N, or 

NW solutions in photolysis cells exposed to sunlight decreases over time. If that occurs, there are two 

options: 

80.168. When photobleaching begins to become substantial, the study can be terminated. In such cases, 

only data obtained from the study prior to substantial photobleaching should be used in the analysis. The 

study can be continued by accounting for photobleaching. 

Extrapolation from cell or near surface to the same water over a deeper depth, and from 

experimental to other solar photon fluxes 

81.169. Unfortunately, methods of extrapolation to greater depths and light attenuation and to other solar 

conditions are not as straightforward for indirect photolysis as they are for direct photolysis, even when 

irradiation is restricted to sunlight. The reason is that indirect photolysis may involve the absorption of 

solar flux by many different types of sensitizers with different absorption spectra and by nitrate. 

Therefore, the results of an indirect photolysis study conducted in cells exposed to natural sunlight are only 

applicable to the near surface and the same set of solar conditions as the experimental ones. 

Tier 3 Identification of major phototransformation products

82. In indirect photolysis Tier 3, the test substance in SNW and/or SNW-N and/or one or more 

natural waters is exposed to a filtered xenon arc lamp or sunlight. The rates of formation and decline of 

transformation products are determined, if possible. Major phototransformation products are isolated and 

identified. For the purposes of this guideline, a major transformation product is one accounting for > 10% 

of the applied radioactivity (or molar basis) in an individual sample at any time during the study.

83. If a flow-through system is used to trap volatile test substances and/or their transformation 

products, the following considerations should be made. The air should be passed through microbial filters 

to keep the samples sterile. Moist air should be used to prevent evaporation.

84. Place the test solution in photolysis test vessels. At least half the test vessels should be exposed to 

a filtered xenon arc lamp or sunlight and an equal or fewer number of the test vessels should serve as dark 

controls. 
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105

85. Determine the concentrations of the transformation products in the irradiated test vessels and in 

the dark control test vessels at adequate enough sampling intervals. 

86. At each sampling interval, sacrifice an irradiated photolysis cell for analysis. The number of 

sampling intervals for which a dark control is also sacrificed for analysis should depend upon the rate of 

hydrolysis of the test substance at the experimental pH.

87. Identify and quantify major phototransformation products using analytical methods and criteria 

indicated in paragraph 26.

88. In cases where data follow first-order kinetics and the parent test substance rapidly transforms, 

use the same first-order methods applied to the test substance to also estimate first-order photolysis rate 

constants and corresponding half-lives for any major transformation products formed from the parent 

which are still at substantial concentrations (e.g. at > 10% of applied) after the parent has degraded to 

negligible levels.

89. In cases where primary or secondary transformation products are being substantially formed over 

the same time period they are being substantially lost, their photolysis rate constants and corresponding 

half-lives cannot be readily estimated as they were for the parent. However, as an option, attempts can be 

made to estimate first-order rate constants for the formation and decline of transformation products using 

non-linear regression to fit concentration versus time series data to various equations derived from 

assumed transformation pathways.

REPORTING OF DATA AND RESULTS

90.170. The degree of presentation of results depends on the complexity and purpose of the study, that is, 

the number of tiers. Reporting should follow the IUPAC recommendations (61).

Direct photolysis data and results

Tier 1 

•Provide the UV-VISIBLE spectrum or spectra of the test chemical as the molar absorption coefficients 

(L/mol•cm or M-1cm-1) on the y-axis versus wavelengths (nm) on the x-axis.

•Report the estimated maximum photolysis rate constant and associated direct photolysis losses over a 30 

days/nights sunlight exposure period. Provide the equations and calculations used to estimate them. 

Provide the rationale for selecting the season and latitude of the average daily solar photon fluxes (Lλ) 

used in the calculations.Report whether or not the results of the calculations trigger Direct Photolysis 

Tiers 2 and/or 3.

Tier 2 

•If a filtered xenon arc lamp was used, provide the incident photon flux in a graph as the photon flux 

(photons/cm2•sec) on the y-axis versus the wavelength (nm) on the x-axis.

•If sunlight was used, provide tabular values of solar photon fluxes for the season and latitude closest to 

those of the study.
ENV/JM/TG/RD(2006)1/REV 

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•Provide table(s) and graph(s) which show the concentrations of the test chemical in irradiated and dark 

control solutions versus time. Show the best fit regression lines on the graph(s) along with the data 

points.

•Report the irradiated, dark control, and approximate direct photolysis rate constant of the test chemical

along with their corresponding half-lives. Provide the equations, calculations, and regression 

coefficients used to compute them together with regression n and r2 values, and standard errors of 

coefficients (for example, slope, intercept).

•Based upon the approximate direct photolysis rate constant, report the associated estimated direct 

photolysis losses over a 30 days/nights sunlight exposure period. Provide the equations and 

calculations used to estimate them. Provide the rationale for selecting the season and latitude of the 

average daily solar photon fluxes (Lλ) used in the calculations. Report whether or not the results of the 

calculations trigger Direct Photolysis Tiers 3 and/or 4.

Tier 3 

•For the determination of the test chemical quantum yield with the use of an actinometer, provide table(s) 

and graph(s) showing the concentrations of the test chemical and actinometer in irradiated and dark 

control solutions versus time. Show the best fit regression lines on the graph(s) along with the data 

points.

•Report the irradiated, dark control, and direct photolysis rate constants of the test chemical and 

actinometer. Provide the equations, calculations, and regression coefficients used to compute them 

together with regression n and r2 values, and standard errors of coefficients (for example, slope, 

intercept).

•Report the quantum yield of the test chemical. Provide the equation and calculation used to compute it.

•Report estimated direct photolysis rate constants for the test chemical in natural water that are estimated 

from the test chemical quantum yield for seasons, latitudes, and water body types of interest. Provide 

and/or describe the equations and calculations or computer program (e.g. GCSOLAR or ABIWAS) 

and inputs to estimate them.

Tier 4

•If a filtered xenon arc lamp was used, provide the incident photon flux in a graph as the photon flux 

(photons/cm2•sec) on the y-axis versus the wavelength (nm) on the x-axis.

•If sunlight was used, provide tabular values of solar photon fluxes for the season and latitude closest to 

those of the study.

•Provide table(s) and graph(s) showing the concentrations of the test chemical and (if applicable) major 

phototransformation product concentrations and mass balance determinations in irradiated and dark 

control solutions versus time. Show the best fit regression lines on the graphs along with the data 

points.

•Report the irradiated, dark control, and direct photolysis rate constants of the test chemical along with 

their corresponding half-lives. Provide the equations, calculations, and regression coefficients used to 

compute them together with regression n and r2 values, and standard errors of coefficients.
ENV/JM/TG/RD(2006)1/REV 

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•If applicable, report the rate constants for the formation and decline of the major transformation products. 

Provide the transformation pathway model (see next paragraph), equations, calculations, and 

regression coefficients used to compute them together with regression n and r2 values, and standard 

errors of coefficients.

•Provide chemical structures and names of the test chemical and (if applicable) major transformation

products in both irradiated and dark control solutions. Provide graphical presentations of postulated 

transformation pathways in both the irradiated and dark control solutions.

Indirect photolysis data and results

Tier 1

• When Assuming sunlight was used for indirect photolysis as recommended, provide tabular values of 

solar photon fluxes for the season and latitude closest to those of the study. When a Xenon arc lamp 

was used report intensities of irradiation of the lamp and relate it to sunlight (for details, see Annex 6).

• Provide important characteristics of all simulated and/or natural waters used including DOC (= 

Dissolved Organic Carbon), humic acid content (if simulated), pH, dissolved oxygen, nitrate. Include 

the UV-VISIBLE spectrum of the test media from 295290 to 800 nm. For natural water, include site 

of collection. 

• Provide table(s) and graph(s) showing the concentrations of the test chemical test substance in 

irradiated and dark control solutions versus time. Show the best fit regression lines on the graph(s) 

along with the data points. 

• Report the irradiated, dark control, and approximate net, direct, and indirect photolysis rate constants 

of the test chemical test substance along with their corresponding half-lives.. Provide the equations, 

calculations, and regression coefficients used to compute them together with regression n and r2 values, 

and standard errors of coefficients. 

• Report the approximate indirect (or indirect plus hydroxyl) photolysis rate constant and associated 

indirect photolysis losses over a 30 days/nights sunlight exposure period. Provide the equations and 

calculations used to estimate them. Provide the rationale for selecting the season and latitude of the 

average daily solar photon fluxes (Lλ) used in the calculations. Report whether or not the results of the 

calculations trigger Tier 2 Indirect Photolysis. 

Tier 2

• Provide tabular values of solar photon fluxes for the season and latitude closest to those of the study. If 

a Xenon arc lamp was used, report photon fluxes and/or intensities of irradiation of the lamp and relate 

it to sunlight (for details, see Annex 6).

• Provide important characteristics of all simulated and/or natural waters used including DOC, humic 

content (if simulated), pH, dissolved oxygen, and nitrate. The UV-VISIBLE spectrum (295290 to 800 

nm) of the test media should be included. For natural water, include the site of collection. 

• Provide table(s) and graph(s) showing the concentrations of the test chemicaltest substance and (if 

applicable) major transformation product concentrations and mass balance determinations in irradiated 
ENV/JM/TG/RD(2006)1/REV 

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and dark control solutions versus time. Show the best fit regression lines on the graphs along with the 

data points. 

•Report the irradiated, dark control, and net, direct, and indirect photolysis rate constants of the test 

chemical test substance along with their corresponding half-lives. Provide the equations, calculations, 

and regression coefficients used to compute them together with regression n and r2 values, and 

standard errors of coefficients. If applicable and possible, supply the same information for major 

transformation products. 

• Report the indirect photolysis rate constant and associated indirect photolysis losses over a 30 days 

sunlight exposure period. Provide the equations and calculations used to estimate them. Provide the 

rationale for selecting the season and latitude of the average daily solar photon fluxes (Lλ) used in the 

calculations. Report whether or not the results of the calculations trigger Tier 3 Indirect Photolysis.

•If applicable, report the rate constants for the formation and decline of the major transformation products. 

Provide the transformation pathway model (see next paragraph), equations, calculations, and 

regression coefficients used to compute them together with regression n and r2 values, and standard 

errors of coefficients.

•Provide chemical structures and names of the test chemical and (if applicable) major transformation 

products in both irradiated and dark control solutions. Provide graphical presentations of postulated 

transformation pathways in both the irradiated and dark control solutions.

Tier 3

• Provide table(s) and graph(s) showing the concentrations of the phototransformation product 

concentrations and mass balance determinations in irradiated and dark control solutions versus time. 

Show the best fit regression lines on the graphs along with the data points.

• If applicable, report the rate constants for the formation and decline of the transformation products. 

Provide the transformation pathway model, equations, calculations, and regression coefficients used to 

compute them together with regression n and r2 values, and standard errors of coefficients.

• Provide chemical structures and names of the major transformation products in both irradiated and 

dark control solutions. Provide graphical presentations of postulated transformation pathways in both 

the irradiated and dark control solutions.

Additional reporting requirements

General information

• Title of the study, author(s), performing laboratory (name and address), laboratory report identification 

number, date of completion. In some instances, it may be necessary to identify who is submitting the 

study, their address and any other identification number. 

• Name(s) of the test substance, chemical nomenclature, Chemical Abstracts Registry Number (CAS 

Reg. No.). A tabular format may be used to present this infomation, but it is optional. 

• Physical and chemical properties applicable to the test substance. As an option, these physical and 

chemical properties may be listed in a table. 
ENV/JM/TG/RD(2006)1/REV 

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• Report if the test substance is the primary chemical or any transformation product resulting from 

abiotic hydrolysis and/or biotransformation. Report if the test substance is an industrial chemical, a 

pharmaceutical chemical, a pesticide, etc. 

• For radiolabelled test substances, the radiolabelled element and isotope, radiolabelling position(s), 

chemical and radiolabel purity, and specific activity should be reported. 

Experimental information 

• Standard laboratory equipment and reagents. 

• Analytical method(s) used for identifying and quantifying the test substance. The report should include 

a brief description of the method, reference standards, instrumentation, accuracy and precision, LOD 

and LOQ. Results of independent laboratory validation (if required). 

• Quality Control (QC) results such as analytical recoveries and relative percent standard deviations for 

replicate analyses. 

Specific experimental information related to photolysis (60) 

• Experimental conditions, reagents, reaction vessels and other information relevant to each of the tiers 

comprising the study. This may include: 

 - Temperature 

 - Solvent and/or buffers used for preparing stock and test solutions for direct photolysis, 

including dark control samples. 

 - Co-solvent and percent by volume used, if any. 

 - Nominal and actual concentration of test substance in the test media 

• Absorption Spectroscopy: 

 - Description of instrumentation 

 - Pathlength and material of optical cells 

 - Spectral resolution 

 - Concentration of test chemical test substance in solution 

 - Co-solvent and percent by volume used, if any 

 - Temperature and pH 

• Specific information on reaction vessels: 

 - Material (type of glass, quartz, etc) 

 - Geometry of the reaction vessels and positioning of the reaction vessels with respect to the 

light source. 

 - Any significant optical properties of the vessel material (wavelength cut-off, etc.) 

 - Illustration of the reaction vessel(s) arrangement and/or a photograph is encouraged to be 

included in the report. 

• Xenon-light source: 

 - Manufacturer, model 
ENV/JM/TG/RD(2006)1/REV 

110

 - Filters and their purpose 

 - Measured spectrum of the light source between 295290 and 800 nm at at least the beginning 

and end of the study. 

 - Comparison of the spectrum of the light with that of natural sunlight for the conditions 

required for specific tiers. 

 - Intensity (radiant flux, fluence rate) at sample (J m-2s

-1; photons m-2s-1) and area irradiated. 

• Irradiation dose (fluence) (J m-2): 

 - State if continuous or intermittent irradiation was used 

 - If continuous irradiation, state radiant flux (intensity) and time 

 - If intermittent irradiation, state radiant flux, cycle duration and number of cycles 

•Quantum Yields:

- Experimental conditions (temperature, test media, concentration of test substance in test 

media

- Description of reaction vessels (diagram and/or photograph)

- Specify if monochromatic or polychromatic irradiation was used and source

 - Monochromatic wavelength and bandwidth used.

- Intensity (radiance flux)

- Photometric methods, if any

- Chemical actinometer used (criteria for selection, concentration of actinometer in test media, 

absorption spectrum of the actinometer in the test media).

- Positioning of the actinometer with respect to the test substance

• Computational methods for: 

 - Calculation of molar absorption coefficients (ελ) utilizing Beer-Lambert’s relationship 

 - Maximum photolysis reaction rate assuming φ = 1 (direct and/or indirect photolysis) 

 - Experimental photolysis reaction rate (direct and/or indirect photolysis) 

 - Quantum yield calculations (quantum yield should be expressed as a fraction and not as a 

percent)

 - GCSOLAR or ABIWAS calculation of reaction rates at different latitudes, seasons, time of 

the day, water bodies (when applicable). 

 - Half-lives or DT50, DT75, and DT90 values and appropriate associated confidence intervals. 

Discussion and interpretation of results

171. The purpose of conducting the study should be discussed in terms of the potential for the test 

chemical to be present in surface water. For example, chemicals can be transported to surface water in 

numerous ways including by direct point discharge into water bodies, runoff, spray drift or atmospheric 

deposition.

172. The depth of the discussion depends on the complexity of the study. Regardless of the 

complexity of the study, the discussion should be focused on the effect sunlight may have on the rate and 

pathway of transformation of the test chemical in aqueous environments.

173. The potential for the test chemical to undergo direct and/or indirect photolysis should be 

discussed. When applicable, the rate constants for irradiated and dark control test chemical should be 

compared to distinguish between hydrolytic and photolytic transformations. When possible, discuss the 
ENV/JM/TG/RD(2006)1/REV 

111

relative contribution(s) of direct and indirect photolysis to the overall net photolysis rate.

174. When quantum yields are determined, the calculated rate constants at different latitudes, seasons 

and for water bodies with depths and light attenuation coefficients of interest should be used to discuss 

how sunlight would affect the transformation rate of the test chemical at the time when the concentration of 

the chemical in surface water is expected to be the maximum.

175. When transformation products are identified and quantified in direct and indirect photolysis 

studies, photolytic products should be distinguished from hydrolytic products. When feasible, potential 

hydrolytic and/or photolytic stability of primary transformation products should be included in the results. 

When available, the rates of formation and decline of primary transformation products should be discussed. 

In turn, transformation products from direct and indirect photolysis studies should be compared to 

distinguish products unique to each. 

176. When studies are conducted with both SNW and SNW-N, the potential contribution of hydroxyl 

radical should be assessed by comparing the results in the two types of water. 

177. Sources of error and uncertainty such as losses of the test chemical and/or transformation 

products due to volatilization and/or adsorption to glassware should be discussed.

LITERATURE

1. OECD. 1998. OECD Working Group for the Development of a Test Guideline on 

Phototransformation of Chemicals in Water. Rome, 21- 22 September 1998. 

2. U.S. EPA. 1998. OPPTS 835.2210 - Direct Photolysis Rate in Water by Sunlight. United States 

Environmental Protection Agency. Washington, DC. 

http://www.epa.gov/docs/OPPTS_Harmonized/835_Fate_Transport_Transformation_Test_Guideli

nes/Series/ 

3. U.S. EPA. 1998. OPPTS 835.5270 - Indirect Photolysis Screening Test. United States 

Environmental Protection Agency. Washington, DC.

4. ASTM. 1987. E 896-87: Standard Test Method for Conducting Aqueous Direct Photolysis Tests. 

American Society for Testing and Materials. Philadelphia PA. 

5. U.K. DOE. 1993. Determination of the Stability of Substance in Simulated Sunlight- A proposal 

for a Test Guideline. Submitted by the Department of the Environment, United Kingdom. 

6. UBA. 1992. Direct Phototransformation of Chemicals in Water - A proposal for a Test Guideline. 

Submitted by UBA, Germany. Berlin. 

7. SETAC. 1995. Procedures for Assessing the Environmental Fate and Ecotoxicity of Pesticides. 

Section 10 in Aqueous Photolysis (page 28- 30). Mark Lynch, Editor. SETAC - Europe 

Publication. 

8. ECETOC. 1984. The Phototransformation of Chemicals in Water: Results of a Ring Test. 

ECETOC Technical Report No

 12. Brussels, Belgium 
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9. Van Henegouven B. 1990. Sunlight induced Degradation of Chemicals in Surface Water. Report 

of Centre for Bio-Pharmaceutical Sciences. Leiden, The Netherlands. 

10. OECD. 1997. Guidance Document on Direct Phototransformation of Chemicals in Water. OECD 

Environmental Health and Safety Publication. Series on Testing and Assessment No.7. Paris, 

France. 

21.19. IUPAC. 1996. Glossary of terms used in photochemistry (IUPAC Recommendations 1996) Pure 

& Appl. Chem. 68: pp 2223 - 2286. 

22.20. Leifer A. 1988. The Kinetics of Environmental Aquatic Photochemistry. Theory and Practice. 

ACS Professional Reference Book. Published by the American Chemical Society. Washington, 

D.C. 

23.21. Mill T and Mabey W. 1985. Photodegradation in water. Chapter 8 in Environmental Exposure 

from Chemicals. Neely W and Blau G, editors. CRC Press. Boca Raton FL. pp. 175-216 

24.22. Harris J. 1982. Rates of Direct Aqueous Photolysis. Chapter — in Handbook of Chemical 

Property Estimation Methods. Environmental Behavior of Organic Compounds. Lyman W, Reehl 

W, and Rosenblatt D, editors. McGraw-Hill Publishers. New York, NY. 

25.23. Mill T, Mabey W, Bomberger D, Chou T, Henry D, and Smith JH. 1982. Laboratory Protocols 

for Evaluation of the Fate of Organic Chemicals in Air and Water. U.S. Environmental Protection 

Agency. EPA 600/3-82-002. Washington, D.C. 

26.24. Mill T, Davenport J, Dulin D, Mabey W, and Bawol R. 1981. Evaluation and Optimization of 

Photolysis Screens. U.S. Environmental Protection Agency. EPA 560/5-81-003. Washington, 

D.C. 

27.25. Zepp R. 1978. Quantum yields for reaction of pollutants in dilute aqueous solutions. Environ. 

Sci. & Technol. 12: pp. 327-329. 

28.26. Zepp R, and Cline D. 1977. Rate of direct photolysis in aquatic environment. Environ. Sci. & 

Technol. 11(4): pp. 359-366. 

19. Boule P., Guyon C. and Lemaire J. 1982. Photochemistry and Environment IV : Photochemical 

behaviour of monochlorophenols in aqueous dilute solution. Chemosphere 11, 1179-1188.

20. Boule P., Guyon C., Tissot A. and Lemaire J. 1987. Specific phototransformation of xenobiotic 

compounds : chlorobenzene and chlorophenols. Photochemistry of Environmental Aquatic Systems (R.G. 

Zit % W: J. Cooper, eds.), Miami, 10-26.

29.21. Zepp R, Schlotzhauer P, and Sink R. 1985. Photosensitized transformations involving electronic 

energy transfer in natural waters: Role of humic substances. Environ. Sci. & Technol. 19: pp. 74-81. 

30.22. Zepp R, Baughman G, and Scholtzhauer P. 1981. Comparison of photochemical behavior of 

various humic substances in Water. I. Sunlight induced reactions of aquatic pollutants photosensitized by 

humic substances. Chemosphere 10: pp. 109-117. 

31.23. Mill T. 1999. Predicting photoreaction rates in surface water. Chemosphere 38(6): pp. 1379-

1390. 
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32.24. Brezonik P and Fulkerson-Brekken J. 1998. Nitrate-induced photolysis in natural waters: 

Controls on concentrations of hydroxy radical photo-intermediates by natural scavenging agents. Environ. 

Sci. & Technol. 32: 3004-3010. 

33.25. Vaughan P and Bough N. 1998. Photochemical formation of hydroxy radical by constituents of 

natural waters. Environ. Sci. Technol. 32: 2947-2952903. 

34.26. Blough N and Zepp R. 1995. Reactive oxygen species in natural waters. Chapter in Active 

Oxygen in Chemistry. Foote C, Valentine G, Greenberg A, and Liebman J, editors. Published by 

Chapman and Hall. New York, NY. pp. 280-333. 

35.27. Mabury S and Crosby D. 1994. The relationship of the hydroxyl reactivity to pesticide 

persistence. Chapter 10 in Aquatic and Surface Photochemistry. G Helz, R Zepp, and D Crosby, editors. 

Lewis Publishers. Boca Raton, Florida, USA. pp. 149- 161. 

36.28. Zepp, R.G., Hoignée, J. and Bader, H. 1987. Nitrate-induced photooxidation of trace organics in 

water. Environ. Sci. Technol. 21: 443-450. 

37.29. Haag W and Hoignée’ J. 1986. Singlet oxygen in surface waters. III. Steady state 

concentrations in various types of waters. Environ. Sci. & Technol. 20: 341-348. 

38.30. Haag W, Hoignée’ J, Gassmann E, and Braun A. 1984. Singlet oxygen in surface waters. II. 

Quantum yields of its production by some natural humic materials as a function of wavelength. 

Chemosphere 13: pp. 641-650. 

39.31. Mill T, Hendry D, and Richardson H. 1980. Free radical oxidants in natural waters. Science 

207: pp. 886-889. 

40.32. Draper W and Crosby D. 1981. Hydrogen peroxide and hydroxyl radical: Intermediates in 

indirect photolysis reactions in water. J. Agric. Food Chem. 29: pp. 699. 

41.33. Wolff C, Halmans M and Van der Heijde H. 1981. The formation of singlet oxygen in surface 

waters. Chemosphere 10: pp. 59. 

42.34. Zepp R, Wolfe N, Baughman G, and Hollis R. 1977. Singlet oxygen in natural waters. Nature 

267: pp. 421-423. 

43.35. Zhou, X. and Mopper, R. 1990. Determination of photochemically produced hydroxyl 

radicals in seawater and fresh water. Mar. Chem. 30: 71-88. 

44.36. Mopper, R. and Zhou, X. 1990. Hydroxyl radical photoproduction in the sea and its potential 

impact in marine processes. Science 250: 661-664. 

37. Aguer J.P., Richard C. and Andreux F. 1997. Comparison of the photoinductive properties of 

commercial, synthetic and soil extracted humic substances. J. Photochem. Photobiol. A 103, 163-168.

38. Vialaton D., Richard C., Baglio D. and Paya-Perez A. 1998. Phototransformation of 4-chloro-2-

methylphenol in water; influence of humic substances on the reaction. J. Photochem. Photobiol. A 119, 39-

45.
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39. Niessen R., Lenoir D. and Boule P. 1988. Phototransformation of phenol induced by excitation of 

nitrate ions. Chemosphere 17, 1977-1984.

40. Mailhot G., Astruc M. and Bolte M. 1999. Degradation of tributyltin chloride in water 

photoinduced by iron(III). Applied Organometallic Chem. 13, 53-61.

45.41. Frank R and Klöpffer W. 1988. Spectral solar photo irradiance in Central Europe and the 

adjacent North Sea. Chemosphere 10: p. 985. 

46.42. Frank R and Klöpffer W. 1988. A convenient model and program for the assessment of abiotic 

degradation of chemicals in natural waters. Ecotox. Environ. Safety 17: pp. 323-332. 

47.58. Calvert, J.G. and Pitts, J.N., Jr. 1966. Photochemistry. Wiley, New York, USA. 

35.Turro N. 1991. Modern Molecular Photochemistry. Published by University Science Books. Sausalito, 

California, USA.

49.43. Zepp R. 1999. Personal communication (December 30, 1999). 

50.44. Faust B. 1994. A review of the photochemical redox reactions of iron(III) species in 

atmospheric, oceanic, and surface waters: Influence on geochemical cycles and oxidant formation. Chapter 

1 in Aquatic and Surface Photochemistry. G Helz, R Zepp, and D Crosby, editors. Lewis Publishers. 

Boca Raton, Florida, USA. pp. 3- 35 

51.45. Sulizberger B, Laubscher H, and Karametaxas G. 1994. Photoredox reactions at the surface of 

iron (III) hydr(oxides). Chapter 3 in Aquatic and Surface Photochemistry. G Helz, R Zepp, and D Crosby, 

editors. Lewis Publishers. Boca Raton, Florida, USA. pp. 53- 73. 

52.46. U.S. EPA. 1985. Hazard Evaluation Division. Standard Evaluation Procedure: Aqueous 

Photolysis Studies. Office of Pesticide Programs, U.S. Environmental Protection Agency. EPA-540/9-85-

014 

47. OECD Test Guideline 111. 2003 Hydrolysis as a Function of pH.

53.48. Ruzo L, Shepler K, and Toia R. 1995. Pesticide hydrolysis and photolysis studies: Guideline 

challenges and experimental approaches. Chapter 8 in Agrochemical Environmental Fate - State of the Art. 

M Leng, E Leovey, and P Zubkoff, editors. Lewis Publishers. Boca Raton, Florida, USA. pp 77- 91. 

49. European Union (EU). 1995. Commission Directive 95/36/EC amending Council Directive 

91/414/EEC concerning the placing of plant protection products on the market. Annex I: Fate and 

behaviour in the Environment.

50. Méallier, P. 1999. The handbook of Environmental Chemistry (C. Richard and G. Grabner, eds.), 

240-260.

54.51. Gao H and Zepp R. 1998. Factors influencing photoreactions of dissolved organic matter in a 

coastal area of the Southeastern United States. Environ. Sci. Technol. 32: 2940-2946. 

55.52. IHSS. 1999. Typical Humic Acid Data Packet. International Humic Substances Society (IHSS). 

Available from website http://ihss.gatech.edu/dataha.html. 

56.53. Zepp R and Schlotzhauer P. 1981. Chemosphere 10: pp. 479 
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57.54. Zepp, R.G. 1982. Experimental Approaches to Environmental Photochemistry. Chapter in the 

Handbook of Environmental Photochemistry, edited by Hutzinger, O.; Springer-Verlag, New York 2, Part 

B: pp 19-40. 

58.55. Faust, B.C. 1993. Generation and use of simulated sunlight in photochemical studies of liquid 

solutions. Rev. Sci. Instrum. 64:(2) pp. 577- 578. 

59.56. (a) The Book of Photon Tools. 1999. Drozdowicz, Z., Editor. Oriel Instruments, Stratford, 

Connecticut 006615, USA. (b) Atlas Electric Devices Company, 114 Ravenswood Avenue, Chicago, 

Illinois 60613 USA.Oriel Instruments. 1999. 

60.57. Lemarie, J., Guth, J. A., Klais, D., Leahey, J., Merz, W., Philp, J. Wilmes, R., and Wolff C.J.M. 

1985. Chemosphere, 14: pp 53- 77. 

61.58. Mill T. 2000. Documents on Calibration of Lamp and Indirect Photoreaction Estimation 

Methods, prepared for and submitted to the U.S. Environmental Protection Agency, Office of Pesticide 

Programs under PR Number 9W-2901-NASA, February 22, 2000. 

59. Calvert, J.G. and Pitts, J.N., Jr. 1966. Photochemistry. Wiley, New York, USA.

36.Dulin D. and Mill T. 1982. Development and evaluation of sunlight actinometers. Environ. Sci. 

Technol. 16(11): pp. 815-820.

37.Zepp, R.G., Gumz, M.M., Miller, W., and Gao, H. 1998. Photoreaction of Valerophenone in Aqueous 

Solutions. J. Phys. Chem. A, 102: pp. 5716- 5722.

38.Murov et al. 1993. Chemical Actinometry. In Handbook of Photochemistry. 2nd ed. Marcel Decker Inc., 

New York. pp. 298-305.

39.Millet et al. 1998. Abiotic Degradation of Halobenzonitriles: Investigations of the Photolysis in 

Solution. Ecotoxicol. Environ. Safety. 41: pp. 44-50.

40.Heller, H. and Langan J. 1981. A new reusable chemical actinometer. EPA Newsletter 71.

41.Bahnemann, D. 1993. Ultra-small metal oxide particles: preparation, photophysical characterization and 

photocatalytic properties. Isr. J. Chem. 33: pp. 115.

42.IUPAC. 1989. Chemical Actinometry. Prepared for publication by H.J. Kuhn, S.E. Braslavsky, and R. 

Schmidt. Pure & Appl. Chem., Vol. 61, No. 2, (1989) pp. 187- 210.

69.60. Winterle J and Mill T. 1985. Toxic substances process data generation and protocol 

development. Work assignment 18: Indirect Photoreaction Protocol. Submitted to Office of Toxic 

Substances, U.S. Environmental Protection Agency. EPA Contract No. 68-03-2981.

70.61. IUPAC. 1984. Recommended Standards for Reporting Photochemical Data (1984). Pure & 

Appl. Chem. 56: pp 939- 934. 

71.62. AWWA. 1991. Standard Methods for the Analysis of Water and Wastewater. American Water 

Works Association, 6666 West Quincy Avenue, Denver, Colorado 80295290, USA. 

http://www.awwa.org/stanmeth/contents.htm 
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72.Mill T, Mabey W, Winterle J, Davenport J, Barich V, Dulin D, Tse D, and Lee G. 1982. Design and 

validation of screening and detailed methods for environmental processes. Final Report. Submitted to 

Office of Toxic Substances, U.S. Environmental Protection Agency. EPA Contract No. 68-01-6325.
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ANNEX 1

Definitions and Units

Test substance: any substance, whether the parent compound or relevant transformation products

Transformation products: all substances resulting from biotic or abiotic transformation reactions of the 

test substance including CO2.

Half-Life, t0.5 is the time taken for 50% transformation of a test substance when the transformation can be 

described by first-order kinetics; it is independent of the concentration.

DT50 (Disappearance Time 50) is the time within which the concentration of the test substance is reduced 

by 50%; it is different from the half-life t0.5 when transformation does not follow first-order kinetics.

DT75 (Disappearance Time 75) is the time within which the concentration of the test substance is reduced 

by 75%.

DT90 (Disappearance Time 90) is the time within which the concentration of the test substance is reduced 

by 90%.

The limit of detection (LOD) is the concentration of a substance below which the identity of the substance 

cannot be distinguished from analytical artefacts.

The limit of quantification (LOQ) is the concentration of a substance below which the concentration 

cannot be determined with an acceptable accuracy.

 The definitions and units are taken from the Glossary of terms used in photochemistry (IUPAC 

Recommendations 1996) published in Pure and Applied Chemistry, 68, 2223-2286, 1996. Commonly used 

and SI units are provided. Cited with permission from the International Union of Pure and Applied 

Chemistry 

Absorbance (A), [Dimensionless] - The logarithm to the base 10 of the ratio of the spectral radiant power 

of incident, essentially monochromatic radiation (I= ∫λ I λ dλ) to the radiant power of transmitted radiation 

(Pλ): 

A = log(Pλ

o

 /Pλ) = -log T 

In practice, absorbance is the logarithm to the base 10 of the ratio of the special radiant power of light 

transmitted through the reference sample to that of the light transmitted through the solution, both observed 

in identical cells. T is the (internal) transmittance. 

The terms “absorbancy”, “extinction” and “optical density” should no longer be used. 

Absorption Coefficient (a), [m-1 or cm-1] - Absorbance divided by the optical pathlength, l. The term 

“absorptivity” is not recommended

Actinometer - A chemical system or physical device which determines the number of photons in a beam 

integrally or per unit time and it is commonly applied to devices used in the ultraviolet and visible 

wavelength ranges. Examples of a chemical actinometers are iron(III) oxalate solutions. Examples of 

physical devices are bolometers, thermopiles and photodiodes. Actinometers give a reading that can be 

correlated to the number of photons detected. 
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Beer-Lambert Law - The absorbance of a beam of collimated monochromatic radiation in a homogeneous 

isotropic medium is proportional to the absorption pathlength, l, and to the concentration, c. This law holds 

only under the limitations of the Lambert Law and for absorbing species exhibiting no concentration 

dependent aggregation. The law can be expressed as, 

A = log(Pλ

o

 /Pλ) = ε c l

or 

Pλ = Pλ

o

 10 -εcl

where the proportionality constant, ε, is called the molar (decadic) absorption coefficient.

Units: For l in cm and c in mol dm3

 or M, ε will result in dm3mol-1cm-1 or M-1cm-1. These are the 

commonly used units. The SI units for ε is m2mol-1. 

Chromophore- That part of a molecular entity consisting of an atom or group of atoms in which the 

electronic transition responsible for a given spectral band is approximately localized. 

Depth of Penetration (of light), [m] - The inverse of the absorption coefficient. If the decadic absorption 

coefficient, a, is used, the depth of penetration (1/a) is the distance at which the spectral radiant power, Pl

decreases to one tenth of its incident value, Pλ

o

. 

Einstein - One mole of photons. It is not an IUPAC-sanctioned unit. It is sometimes defined as the energy 

of one mole of photons. 

Energy Transfer - From a phenomenological point of view, the term is used to described the process by 

which a molecular entity absorbs light and a phenomenon originates from the excited state of another 

molecule. In mechanistic photochemistry, the term has been reserved for the photophysical process in 

which an excited state of a molecular entity (the donor) is deactivated to a lower-lying state by transferring 

energy to a second molecular entity (the acceptor), which is thereby raised to a higher energy state. The 

excitation may be electronic, vibrational, rotational or translational. The donor and acceptor may be two 

parts of the same molecule, in which case the process is called intramolecular energy transfer. 

Excited State - A state of higher energy than the ground state of a chemical entity. In photochemistry an 

electronically excited state is usually meant. 

Frequency (v or ω), [sec-1 or rad*sec-1] - The number of waveperiods per unit time. The linear frequency, v, 

is the number of cycles per unit time. For the angular frequency, the symbol ω (= 2 πv) is used. 

Ground State - The lowest energy state of a chemical entity. In photochemistry, ground electronic state is 

usually meant. 

Half-width (of a band) - The full width of a spectral band at a height equal to half of the height at the band 

maximum. Also known as full width at half maximum (FWHM). The dimensions of band width should be 

either inverse length (wavenumbers, ) or inverse time (frequencies) so that the values give an indication of 

the energies. Note the hyphen in half-width. Half bandwidth has the meaning of half-width at half 

maximum. 

Hyperchromic Effect - Increase in the intensity of a band due to substituents or interactions with the 

molecular environment. Hypochromic effect is the opposite to hyperchromic effect. 

Intensity- Traditional term for photon flux, irradiance or radiant power (radiant flux). In terms of an object 
ENV/JM/TG/RD(2006)1/REV 

119

exposed to radiation, the term should now be used only for qualitative descriptions. 

Intensity (l) (of a light source)- Same as radiant intensity. 

Intensity (of a spectral feature)- Describes the magnitude of the particular feature in the spectrum. 

Irradiance (E), [W m-2] - The radiant flux or radiant power, P, of all wavelengths incident on an 

infinitesimal element of surface containing the point under consideration divided by the area of the element 

(dP/dS,, simplified expression: E= P/S when the radiant power is constant over the surface area 

considered). E = ∫Eλ dλ, where Eλ is the spectral irradiance at wavelength λ. For a parallel and 

perpendicular incident beam not scattered or reflected by the target or its surrounding fluence rate (Eo) is 

an equivalent term. 

Lambert Law - The fraction of light absorbed by a system is independent of the spectral radiant power ( 

Pλ

o

 ). This law holds only if Pλ

o

 is small, scattering is negligible, and multiphoton processes, excited state 

populations, and photochemical reactions are negligible. 

Lamp - A source of incoherent radiation. 

Molar Absorption Coefficient, Molar Decadic Absorption Coefficient - 

[dm3

mol-1cm-1, M-1cm-1 or dm3mol-1mol-1] 

Absorbance divided by the absorption pathlength, l, and the concentration, 

.ε = [1 / c l] log(Pλ

o

 /Pλ) = A / c l: 

The term molar absorptivity for molar absorption coefficient should be avoided. 

Photochemical Reaction - This term is generally used to describe a chemical reaction caused by absorption 

of ultraviolet, visible, or infrared radiation. 

Photochemistry - The branch of chemistry concerned with the chemical effects of light (far UV to IR). 

Photodegradation - The photochemical transformation of a molecule into lower molecular weight 

fragments, usually in an oxidation process. This term is widely used in the destruction (oxidation) of 

pollutants by UV-based processes. 

Photolysis - A light-induced bond cleavage. 

Photon - The quantum of electromagnetic energy at a given frequency. This energy, E = hv is the product 

of the Planck (h) constant and the frequency of the radiation (v). 

Photooxidation - Reactions induced by light. Common process are: 

• The loss of one or more electrons from a chemical species as a result of photexcitation of that species; 

• The reaction of a substance with oxygen under the influence of light. When oxygen remains in the 

product this latter process is also called photooxygenation. Reactions in which neither the substrate nor 

the oxygen are electronically excited are sometimes called “photoinitiated oxidations”.

Photoreductions - Reduction reactions induced by light. Common process are: 

• Addition of one or more electrons to a photoexcited species; 

• The photochemical hydrogenation of a substance. Reactions in which the substrate is not electronically 

excited are sometimes called “photinitiated reductions”. 
ENV/JM/TG/RD(2006)1/REV 

120

Photosensitization - The process by which a photochemical or photophysical alteration occurs in one 

molecular entity as a result of initial absorption of radiation by another molecular entity called a 

photosensitizer. In mechanistic photochemistry the term is limited to cases in which the photosensitizer is 

not consumed in the reaction. 

Primary Photochemical Process (Primary Photoreaction)- Any elementary chemical process undergone by 

an electronically excited molecular entity yielding a primary photoproduct.

Primary (Photo)product - The first observable chemical entity which is produced in the primary 

photochemical process and which is chemically different from the reactant. 

Quantum (of radiation) - An elementary particle of electromagnetic energy in the sense of the waveparticle duality. 

Quantum Yield (φ) - The number of defined events which occur per photon absorbed by the system. The 

integral quantum yield is: 

φ = number of events/ number of photons absorbed

For a photochemical reaction,

φ = amount of reactant consumed or product formed/amount of photons absorbed 

The differential quantum yield is Φ = d[x]/dt /n, where d[x]/dt is the rate of change of a measureable 

quantity, and n is the amount of photos (mol or its equivalent einstein) absorbed per unit time. Φ can be 

used for photophysical processes or photochemical reactions. 

Radiant Energy (Q), [J] - The total energy emitted, transferred or received as radiation of all wavelengths 

in a defined period of time (Q= ∫ Qλ dλ ). It is the product of radiant power, and time, t:Q= P t when the 

radiant power is constant over the time considered. 

Radiant Exposure (H), [J m-2] - The irradiance, E, integrated over time of irradiation (∫Edt, simplified 

expression H= E t when the irradiance is constant over the time considered). 

Radiant Intensity (I), [W sr -1] - Radiant (energy) flux or radiation power, P, at all wavelengths per unit 

solid angle, ω. The radiant power emitted in a given direction by a source or an element of the source in an 

infinitestimal cone containing the given direction divided by the solid angle of the cone (dP/dω, simplified 

expression: I = P/S when the radiant power is constant over the surface area considered). 

Radiant Power (P), [J s-1, or W] - Power emitted, transferred or received as radiation. 

Singlet Molecular Oxygen - The oxygen molecule (dioxygen), O2 , in an excited singlet state. The ground 

state of oxygen is a triplet 3

Σg

-

). The two metastable singlet states derived from the ground state 

configuration are 1

∆g and 1Σg

-

. 

The IUPAC discourages the use of the term “singlet oxygen” alone, without mentioning the oxygen 

species (in this case, dioxygen) to avoid confusion with 1S and 1D excited states of the oxygen atom. 

Singlet State - A state having a total electron spin quantum number equal to 0. 
ENV/JM/TG/RD(2006)1/REV 

121

Solvent Shift - A shift in the frequency of a spectral band of a chemical species arising from its interaction 

with its solvent environment. Bathochromic shift, also known to as “red shift”, refers to the shift of a 

spectral band to lower frequencies (longer wavelenghts) as result of interaction with the solvent

Hypsochromic shift, also known as “blue shift”, refers to the shift of a spectral band to higher frequencies 

(shorter wavelengths) as a result of interaction with the solvent.. 

Spectral Irradiance (Eλ), [W m-3, or W m-2 nm-1] - Irradiance, E, at wavelength λ per unit wavelength. 

Triplet State - A state having a total soin quantum number of 1. 

Wavelength (λ )- The distance, measured along the line of propagation, between two corresponding points 

on adjacent waves. The wavelength depends on the medium in which the wave propagates 

Wavenumber (σ ,υ), [m-1 or cm-1]- The reciprocal of the wavelength, λ, or the number of waves per unit 

length along the direction of propagation. 

Xenon Lamp - An intense source of ultraviolet, visible and near-infrared light produced by electrical 

discharge in xenon under high pressure.
ENV/JM/TG/RD(2006)1/REV 

122

ANNEX 2 

Symbols and Units Related to Photochemistry 

Symbol Name SI Units Common Units

A Absorbance Dimensionless Dimensionless 

a Decadic absorption 

coefficient 

m-1 cm-1

D Attenuance Dimensionless Dimensionless 

H0 Fluence J m-2 Same 

E0 Fluence rate W m-2

E Irradiance W m-2

ε

Molar (decadic) 

absorption coefficient 

m-2 mol-1 cm-1dm3

mol-1 or

cm2

mmol-1 

Sometimes L mol-1cm-1

 φ Quantum yield Dimensionlessa Dimensionlessa

Q Radiant energy J 

H Radiant exposure J m-2

I Radiant intensity W sr-1

P W 

I Spectral radiance 

intensity 

W sr-1m-1 W sr-1 nm-1

λ Wavelength m nm 

σ; υ

Wavenumber m-1 cm-1
ENV/JM/TG/RD(2006)1/REV 

123

ANNEX 3

Proposed test scheme for indirect photolysis

Tier 1: Experimental Screen

Estimation of Indirect Photolysis Rate

in SNW and/or SNW-N and/or NWs

and PW

Decision Criteria:

Photolysis Loss ≥20% in 30

Days Sunlight Exposure?

no

yes

Tier 2 Kinetics Study

Determination of Indirect Photolysis 

Rate in SNW and/or SNW-N and/or 

NWs and PW

Decision criteria:

Photolysis Loss ≥20% in 30

Days Sunlight Exposure?

no

yes

Tier 3: Isolation/Identification

Of Major Phototransformation Products

(≥ 10% of dose applied)

No Further

Photolysis Work
ENV/JM/TG/RD(2006)1/REV 

124

Figure 1: Proposed test scheme for direct photolysis

Tier 1 - Theoritical Screen

3.If the test chemical absorbs solar irridiation > 295 nm, estimate a maximum direct photolysis rate 

constant Kd(max) in shallow natural water by assuming the quantum yield φ = 1; Kd(max) = Σ ελLλ

4.Determine if the maximum direct photolysis rate constant would result in estimated direct photolysis 

losses of ≥20% of an initial concentration over a 30-day/night sunlight exposure period using the 

following equation: C/C0 = exp [ - Kd(max) (30-days) ]

 Yes↓→No → Quit or go to Tier 1 Indirect Photolysis

Tier 2 - Experimental Screen

4.Measure the decline of the test chemical in sterilised buffer exposed to a xenon lamp or sunlight. If data 

fit a first order kinetics model, determine the apporximate direct photolysis rate.

5.Determine if approximate direct photolysis rate constant would result in estimated direct photolysis losses 

of ≥10% of an initial concentration from the following equation: C/C0 = exp [-Kd(max) (30 days)] If yes, 

go to next step; if no, quit or go to Tier 1 Indirect Photolysis.

6.Is there interest in determining the quantum yield and in estimating theoretical direct photolysis rate 

constant from the quantum yield?

 Yes↓→No → Go to Tier 4 Direct Photolysis

Tier 3 - Quantum Yield and Estimation of Theoretical Rate constant

3.Determine the quantum yield and use it to estimate theoretical direct photolysis rate constant for 

seasons, latitudes, elevations and types of water bodies of interest.

4.Is there interest in comparing experimental direct photolysis rate constants with theoretical ones and/or 

identifying/quantifying major transformation products?

 Yes↓→No → Quit or go to Tier 1 Indirect Photolysis

Tier 4 - Determine the Direct Photolysis Rate Constant

4.Measure the decline of the test chemical (and if of interest) the formation and decline of major 

transformation products in sterilised buffers exposed to a xenon lamp or sunlight.

5.If data fit a first order kinetics model, determine the direct photolysis rate constants and corresponding 

half-life. Otherwise, determine the DT50, DT75 and DT90 values from fitting data to a non-first order 

model. If practical and of interest, determine the same parameters for major transformation products.

6.Is there interest in indirect photolysis? If yes, go to Tier 1 Indirect Photolysis; if no, quit.
ENV/JM/TG/RD(2006)1/REV 

125

 Figure 2: Proposed Test Scheme for Indirect Photolysis

Tier 1 - Screen

4.Measure the decline of the test chemical in synthetic water containing humics (and if applicable, nitrate) 

in buffered distilled water and estimate indirect photolysis rate constant Ki(approx).

5.Determine if the approximate indirect photolysis rate constant would result in estimated indirect 

photolysis losses of ≥10% of an initial concentration over a 30-day/night sunlight exposure period 

using the following equation: C/C0 = exp [- Ki(approx) (30 days)]

6.In some cases, it may be possible to supplement or even replace the experimental screen with a 

theoretical screen based on using structural activity relationships (SARs) to estimate second order rate 

constants and on information on the typical concentrations of photooxidation in surface water.

Yes↓→No → Quit 

Tier 2 - Definitive Indirect Photolysis Test

4.Measure the decline of the test chemical (and if of interest) the formation and decline of major 

transformation products in synthetic and/or natural waters and in buffered distilled water exposure to a 

xenon lamp or sunlight.

5.If data fit a first order kinetics model, estimate net, indirect, and direct photolysis rate constants and 

corresponding half-lives for the test chemical. Correct for photobleaching and attenuation effects.

6.If data fit a non-first order model, determine the DT50, DT75 and DT90 values for fitting data to a nonfirst order model. If practical and of interest, determine the same parameters for major transformation 

products.
ENV/JM/TG/RD(2006)1/REV 

126

ANNEX 4

Derivations of Selected Equations

Background equations from Leifer (12):

d C

dt j

I

jD

IF F

sys

 [ ]



 



 =− =−

λ

λ λλ λ

φ φ

(abs by chem) (abs by sys) (abs by chem) 0

[eq. 3-1]

d C

dt j

I

jD

IF F

sys

[ ]

=− =− ∑ ∑ φ φ

λ λλ λ

295

800

295

800

(abs by chem) (abs by sys) (abs by chem) 0

[eq. 3-2]

where:

φ = quantum yield

j = conversion factor to make units consistent with the L/mol•cm unit of the molar absorption 

coefficient and the mol/L unit of concentration = 

6.02 x 1020 (L/cm3)(photons/mol)

Iλ(abs by chem) = photon absorption rate by the chemical per unit volume over a 1 nm interval centered at 

wavelength λ (photons/cm3•day)

Dsys = depth of irradiated system (cm) = volume of irradiated system/incident area

I0λ = incident photon flux over a 1 nm interval centered at wavelength λ 

(photons/cm2 •day)

Fλ (abs by sys) = fraction of incident photon flux absorbed by the system

Fλ (abs by chem) = fraction of incident photon flux absorbed by the chemical

[ ] F

I I

I

I

I

t

C l

C l

λ

λ λ

λ

λ

α ε

λ

α ε

λ λ

λ λ

(abs by sys) = − = − = −

− +

0 − +

0

0

0

1 10

1 10

( [ ])

( [ ])

 [eq. 3-3]

F

C

λ C

λ

λ λ

ε

α ε (abs by chem) = +

[ ]

[ ]

[eq. 3-4]

where:

αλ = attenuation coefficient of the system at wavelength λ (1/cm)
ENV/JM/TG/RD(2006)1/REV 

127

ελ = molar absorption coefficient of the chemical at wavelength λ (L/mol•cm) 

l = light pathlength (cm)

Inserting equations 3-3 and 3-4 into equation 3-1 gives:

[ ] d C

dt jD

I

C

sys C

C l [ ] [ ]

[ ]

 ( []



 



 =− − •

+





 



 − +

λ

λ

α ε λ

λ λ

φ ε

α ε

λ λ

0 1 10

[eq. 3-5]

Summing equation 3-5 over all wavelengths from 295 nm to 800 nm gives:

[ ] d C

dt jD

I

C

sys C

C l [ ] [ ]

[ ]

( [] =− − • +





 



 − +

=

=

∑ φ ε

α ε λ

α ε λ

λ λ λ

λ

λ λ

0

295

800

1 10

[eq. 3-6]

For solar irradiation on an aqueous system (12),

I t It It 0λ λλ ( ) solar d s () () () = +

[eq. 3-7]

lD t d sys ≅ sec ( ) θ

[eq. 3-8]

l D s sys = 12.

[eq. 3-9]

where:

I0λ(solar) = total incident solar photon flux as a function of time

Idλ (solar) = direct incident solar photon flux as a function of time

I0λ (solar) = sky radiation incident solar photon flux as a function of time

ld = average light pathlength for direct irradiation

ls = average light pathlength for sky irradiation

{ } [ ] [ ] dC

dt jD

I I

C

sys C d

C D

s

sys C D =− − + −

+





 



 − + − +

=

=

∑ φ ε

α ε λ

αε θ

λ

α ε λ

λ λ λ

λ

λ λ λ λ 1 10 1 10 1 2

295

800

( [ ]) sec ( [ ]) . [ ]

[ ]

[eq. 3-10]

Derivation of equation 5 in the main text (12):

10-y = exp(-2.3y) [eq. 3-11]

Therefore,
ENV/JM/TG/RD(2006)1/REV 

128

1 10 1 2 3 1 − =− − + =− − { } − + ( [ ]) exp . ( [ ]) exp( ) α ε

λ λ

λ λ α ε C l Cl x

[eq. 3-12]

where:

x Cl A =+ = 23 23 . [] . ( ) α ε λ λ

[eq. 3-13]

A = absorbance of the solution

A series expansion of the exponential in equation 3-12 gives:

1

2 3 4 5

2345

− − = + − + − + ⋅⋅⋅⋅⋅ exp( ) ! ! ! !x x

xxxx

 [eq. 3-14] 

For an optically dilute solution of a test chemical in pure water or in shallow water or in the near surface of 

natural water such that the absorbance of the system is < 0.02, all of the higher terms in equation 3-14 are 

small compared to x such that equation 3-14 reduces to:

1− −≅ exp( x) x

[eq. 3-15]

Combining equations 3-13 and 3-15 with equation 3-12 gives for an optically dilute solution of a test 

chemical in pure water or clear shallow water or in the near surface of natural water:

1 10 2 3 − ≅+ ( ) − + ( [ ]) . [] α ε

λ λ λ λ α ε C l C l

[eq. 3-16]

Substituting equation 3-16 into equation 3-6 and canceling out the resulting identical terms in the 

numerator and denominator gives for an optically dilute solution of a test chemical in pure water or in clear 

shallow water or in the near surface of natural water:

d C

dt j

l

D

I C sys

[] . = − [ ]















 =

=

∑ 2 3 0

295

800 φ ε λ λ

λ

λ

[eq. 3-17]

By comparing equation 3-17 to equation 1 in the main text, it can be seen that for a test chemical in an 

optically dilute solution in pure water or in clear shallow water or in the near surface of natural water, the 

direct photolysis rate constant is given by:

k

j

l

D

I d

sys

= ∑ 2 3 0

295

800 . φ ε λ λ

[eq. 3-18]

where equation 3-18 is identical to equation 5 of the main text
ENV/JM/TG/RD(2006)1/REV 

129

Derivation of equation 6 in the main text (12)

By analogy to equation 3-16, it can be seen that for a solar irradiated, optically dilute solution of a test 

chemical in pure water or in clear shallow water or in the near surface of natural water such that the 

absorbance of the system is < 0.02,

1 10 2 3 − ≅+ − + ( [ ]) sec . ( [ ]) sec αε θ

λ λ

λ λ αε θ C D

sys

sys C D

[eq. 3-19]

 

1 10 2 3 12 1 2 − ≅+ − + ( [ ])( . ) . ( [ ])( . ) α ε

λ λ

λ λ α ε C D

sys

sys C D

[eq. 3-20]

Substituting equations 3-19 and 3-20 into equation 3-10 and canceling out the resulting identical terms in 

the numerator and denominator gives:

d C

dt j Zt C [] . = − ()[ ]





 

 ∑  2 3φ ε λ λ

[eq. 3-21]

where:

Zt I t I t λλ λ d s ( ) ( ) sec . ( ) = + θ 12

[eq. 3-22]

Equation 3-21 gives the direct photolysis rate as a function of time because the solar photon flux at any 

given wavelength varies over time as represented by Z(t) for an optically dilute solution.

Z

Z t dt

hours λ

λ

=

∫ ( ) 0

24

24

[eq. 3-23]

Mill et al (1985) defined 24 hour average solar irradience values as: 

L

j λ λ = Z

2 3.

[eq. 3-24]

Substituting equation 3-24 into 3-21 gives the average direct photolysis rate over a 24-hour period:

d C

dt L C [ ] [ ] 



 



 = − 



 

 φ ε ∑  λ λ

[eq. 3-25]

By comparing equation 3-25 to equation 1 in the main text, it can be seen that for an optically dilute 

solution of a test chemical in pure water or clear shallow water or in the near surface of natural water 

exposed to sunlight, the 24 hour average direct photolysis rate constant is given by:

k L d solar ( ) = φ ε ∑ λ λ

295

800

[eq. 3-26]
ENV/JM/TG/RD(2006)1/REV 

130

where equation 3-26 is identical to equation 6 in the main text.

Derivation of equation 8 in the main text:

Rearranging equation 3-2 gives the total photon absorption rate per unit volume by the test chemical:

 

I I jdC

dt () () abs by chem abs by chem

[ ] = =− ∑ λ

295 φ

800

[eq. 3-27]

where:

Iλ(abs by chem) = photon absorption rate by the chemical per unit volume over a 1 nm interval centered at 

wavelength λ (photons/cm3•day)

I(abs by chem) = total photon absorption rate by the chemical per unit volume (photons/cm3•day)

Multiplying equations 3-18 and 3-25 by (-j / .) gives the total photon absorption rate per unit volume by the 

test chemical in an optically dilute solution exposed to xenon arc lamp or solar irradiation, respectively:

I

l

D

I C sys

( xenon abs by chem) xenon xenon

=295

= 















=

2 3 ∑ 0

800

. [] ( ) ε λ λ

λ

λ

[eq. 3-28]

I j LC (solar abs by chem) = solar





 

 ∑ ε  λ λ [ ]

[eq. 3-29]

The total number of photons absorbed per unit volume by a test chemical in a low optical density solution 

over x days of xenon arc lamp irradiation or over 30 day/nights of solar irradiation are given respectively 

by:

Total # photons abs by chem / cm days xenon ( ) 3 = X ∑ l

D

I C sys

xenon xenon ( . )[ ] ( ) 2 3 0 ε λ λ

[eq. 3-30]

Total # photons abs by chem / cm days / nights solar ( ) 3 = 



 

 30 j LC ∑  solar ε λ λ [ ]

[eq. 3-31]

The number of days (X) of constant irradiation by a filtered xenon arc lamp that is equivalent to 30 

days/nights of solar irradiation of a low absorbance water can be obtained by equating equations 3-30 and 

3-31, assuming that the average concentration over 30 days/nights of solar irradiation is equal to the 

average concentration over X days of constant irradiation by a xenon arc lamp, and solving for X:

X days xenon = 



 





























= 



 





























∑

∑

∑

∑

30

2 3 13

0 0

j D

l

L

I

j

D

l

L

I

sys

xenon

sys

xenon

.

() ()

ε

ε

ε

ε

λ λ

λ λ

λ λ

λ λ

[eq. 3-32]

where equation 3-32 is identical to equation 8 in the main text.
ENV/JM/TG/RD(2006)1/REV 

131

Derivation of equation 710 in the main text:

Substituting equation 34-16 (see Part 1)for an optically dilute solution (applicable to a low optical density 

actinometer solution) into equation 34-5 (see Part 1), rearranging and incorporating subscripts to designate 

the use of monochromatic irradiation from a xenon arc lamp (obtained with a monochromator) gives: 

d C

dt j

l

D

I C act

sys

act xenon

[ ] . [ ] () ( )





 



 = − λ

λ λ

φ ε 2 3

0

 [eq. 34-133] 

By comparing equation 34-133 to equation 1 in the main text, it can be seen that the monochromatic 

pseudo first order rate direct photolysis rate constant of a low optical density actinometer exposed to 

monochromatic irradiation from a xenon arc lamp is given by: 

k

j

l

D

I d act

sys

λ λλ act act xenon φ ε () () ( )

. = − 2 3 0

 [eq. 4-23-34] 

Rearranging equation 4.234 to solve for I0_(xenon) gives: 

I j D

l

k

xenon

sys d act

act act

0 2 3 λ

λ

λ φ ε ( )

( )

( ) . =

 [eq. 4-3-35] 

where equation 43-35 is identical to equation 710 in the text. 

Derivation of equation 811 in the main text

For a high optical density actinometer solution, 

10 0 − + ≅ ( [ ]) α ε λ λ C l [eq. 43-436] 

and 

ε λ λ [ ] C > > α [eq. 34-537)] 

such that 

ε

α ε

ε

ε

λ

λ λ

λ

λ

[ ]

[ ]

[ ]

[ ]

C

C

C

+ C ≅ = 1

 [eq. 34-638] 

Substituting equations 34-436 and 34-638 for a high optical density actinometer solution into equation 3-6

(see Part 1), summing only over the absorption spectrum of the actinometer above 295290 nm, and using 

subscripts to designate the use of a filtered xenon arc lamp gives: 

d C

dt jD

I act

sys

xenon

initial

final [ ] ( ) = − ==

∑ φ λ

λ

λ

0

 [eq. 34-739] 

Equation 34-739 is a zero order equation such that the zero order direct photolysis rate constant of the high 

optical density actinometer is given by: 

k

jD

I d act zero order

act

cell

xenon

initial

final

( )( ) ( ) = =

=

∑ φ λ

λ

λ

0

 [eq. 34-840) 
ENV/JM/TG/RD(2006)1/REV 

132

Rearranging equation 34-840 gives: 

I jD k xenon

cell

act

d act

initial

final

0λ

λ

λ

φ ( ) ( )( = =

=

∑ zero order)

 [eq. 34-941] 

where equation 4-93-41 is identical to equation 118 of the main text 

Derivation of equation 14 in the main text:

Dividing equation 3-25 (for the pseudo first order rate constant of a test chemical in an optically dilute 

solution exposed to sunlight) by equation 3-18 (for the pseudo first order rate constant of a test chemical in 

an optically dilute solution exposed to a filtered xenon arc lamp) and rearranging gives:

k j D

l

k L

I

d solar

cell

d xenon

xenon

( )

( )

( )

. =

∑

∑ 2 3

295

800

0

ε

ε

λ λ

λ λ

[eq. 3-42]

where equation 3-42 is identical to equation 14 in the main text

Derivation of equation 19 in the main text:

By analogy to equation 3-34 for a low optical density actinonmeter, the monochromatic pseudo first order 

direct photolysis rate constant of a test chemical in an optically dilute solution exposed to monochromatic 

irradiation from a xenon arc lamp is given by:

k

j

l

D

I d chem

sys

λ λλ chem chem xenon φ ε ( ) ( )( )

. = − 2 3 0

[eq. 3-43]

Dividing equation 3-34 by equation 3-43, and rearranging gives:

φ φ ε

ε

λ λ

λ λ

chem act

d chem act

d act chem

k

k = ( )()

()( ) [eq. 3-44]

where equation 3-44 is identical to equation 19 of the main text

Derivation of equation 20 in the main text:

Substituting equations 3-36 and 3-38 (for a high optical density actinometer solution exposed to 

monochromatic irradiation from a xenon arc lamp) into equation 3-5 gives:

d C

dt jD

I act

cell

xenon

[ ] ( )





 



 = −

λ

λ

φ 0

[eq. 3-45]

Equation 3-45 is a zero order equation such that the monochromatic zero order direct photolysis rate 

constant of a high optical density actinometer is given by:

k

jD

I act zero order

act

cell

λ λ xenon

φ ( )( ) ( ) = 0

[eq. 3-46]
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133

Dividing equation 3-43 (for the monochromatic pseudo first order direct photolysis rate constant of a test 

chemical in an optically dilute solution exposed to monochromatic irradiation from a xenon arc lamp) by 

equation 3-46 (for the monochromatic zero order direct photolysis rate constant of a high optical density 

actinometer exposed to monochromatic irradiation from a xenon arc lamp), and rearranging gives:

φ φ

ε λ

λ

λ

chem

act

chem cell

d chem

D d act zero

k

k = 2 3. ( )

( )

( )( order) [eq. 3-47]

where 3-47 is identical to equation 20 in the main text

Derivation of equation 21 in the main text:

By analogy to equation 3-46 for a high optical density actinometer, the monochromatic zero order direct 

photolysis rate constant of a test chemical at high optical density in a solution exposed to monochromatic 

irradiation from a xenon arc lamp is given by:

k

jD

I chem zero order

chem

cell

λ λ xenon

φ

( )( ) ( ) = 0

[eq. 3-48]

Dividing equation 3-48 by equation 3-46 and rearranging gives:

φ φ λ

λ

chem act

d chem zero

d act zero

k

k = ( )( )

( )( ) [eq. 3-49]

where equation 3-49 is identical to equation 21 in the main text

Derivation of equation 22 in the main text:

Dividing equation 3-25 (for the pseudo first order rate constant of a test chemical in an optically dilute 

solution exposed to sunlight) by equation 3-25 (for the pseudo first order rate constant of a low optical 

density actinometer solution exposed to sunlight), and rearranging gives:

φ φ

ε

ε

λ λ

λ λ

chem act

d chem

d act

act

chem

k

k

L

L

=

∑

∑

( )

( )

( )

( )

295

800

295

800

[eq. 3-50]

where equation 3-50 is identical to equation 22 in the main text

Derivation of equation 23 in the main text:

Dividing equation 3-18 (for the pseudo first order rate constant of a test chemical in an optically dilute 

solution exposed to a filtered xenon arc lamp) by equation 3-18 (for the pseudo first order rate constant of a 

low optical density actinometer solution exposed to a filtered xenon arc lamp), and rearranging gives: 

φ φ

ε

ε

λ λ

λ λ

chem act

d chem

d act

act xenon

chem xenon

k

k

I

I

=

∑

∑

( )

( )

() ( )

( )( )

0

295

800

0

295

800

[eq. 3-51]

where equation 3-51 is identical to equation 23 in the main text
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Derivation of equation 27 In the main text:

Dividing equation 3-25 (for the pseudo first order rate constant for a test chemical in an optically dilute 

solution in shallow or near to the surface water exposed to sunlight conditions 2 by equation 3-25 (for 

sunlight conditions 1), and rearranging gives:

k k

L

L

d shallow solar d shallow solar

chem

chem

( )( ) ( )( )

( )

( )

2 1

2

295

800

1

295

= 800

∑

∑

ε

ε

λ λ

λ λ

[eq. 3-52]

where equation 3-52 is identical to equation 27 in the main text
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ANNEX 5

Preparation of Direct Photolysis Test Media

Water meeting ASTM Type IIA standards is recommended to prepare buffer solutions for direct photolysis 

studies. This water is described in ASTM D 1193-99- Standard Specification for Reagent Water. This 

document can be obtained from ASTM. In the USA, this document can be obtained from:

American Society for Testing and Materials (ASTM)

1961 Race Street

Philadelphia, Pennsylvania 19103

For outside the USA, see http://www.astm.org/dist.htm 

A recommended procedure for preparing buffers for direct photolysis studies is given in USEPA OPPTS 

835.2210 Direct Photolysis Rate in Water by Sunlight (2). This guideline recommends:

Preparing all buffer solutions at 25o C

Using reagent grade chemicals free of impurities that could behave as photosensitizers

For pHs in the range of 3 to 6, use NaH2PO4/HCl 

For pHs in the range of 6 to 8, use KH2PO4/NaOH

Using procedures such as those described in the Handbook of Chemistry and Physics or by any 

well establish method.

Always checking the pH with a well calibrated pH meter 

Using the minimum concentration of buffer

To prepare the test solutions for the direct photolysis studies, add the buffer solution to the test chemical to 

attain the desired pH. If necessary, adjust the pH with 1 M HCl or NaOH.

It is highly recommended that the actual concentration of the test chemical in the test solution be 

determined prior to starting the study.

Preparation of Indirect Photolysis Study Test Media

1. SNW (Simulated Natural Water):

(a) Prepare an initial stock solution of humic acid, followed by filtration, preaging and dilution. The 

stock solution should have about 60 ppm DOC of humic acid and absorbance of approximately 

1.7 at 313 nm and 0.7 at 370 nm. Aging can be accomplished by exposing the stock humic acid 

solution in a 2-L flask for 3-4 days to direct sunlight (2). The absorbance of the solution is 

checked and used in estimating a dilution factor for preparing the test solution. 

(b) Buffer the SHW at pH 7 with 0.010 M phosphate solution. 

(c) The SNW test media should have an absorbance of 0.05 at 370 nm and a dissolved organic 

carbon concentration of about 5 mg/L or ppm. 

(d) The appropriate amount of the test chemical test substance is introduced to the test media 

immediately prior to irradiation. 

2. SNW-N (Simulated Natural Water with Added Nitrate):

(a) Prepare test media as for SNW, Steps a through c; 
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136

(b) Add nitrate as NaNO3 or KNO3. In freshwater streams of the United States, average nitrate 

concentrations range from less than 0.1 mg/L to approximately 4 mg/L with the highest 

concentrations in agricultural followed by urban areas (USGS 1993). The mean nitrate 

concentration in some lakes may range up to 9 ppm (2422). Nitrate concentrations in the 

agricultural and urban areas of Europe often exceed 9 mg/L (German personal communication). 

(c) The test chemicaltest substance is introduced immediately prior to irradiation. 

3. NW (Natural Water): 

Natural surface water may be taken from lakes, rivers, ponds, streams, creeks or coastal water 

bodies where the test chemicaltest substance is expected to be present. Proper water sample 

collection and characterization is required. The geographical location (including longitude and 

latitude), site characteristics and date of collection should be recorded. 

 Collection and Sampling

 (a) Sample containers such as polyethylene bottles should be soaked in 10% HNO3 overnight, 

thoroughly washed, and then rinsed with distilled water at least three times. 

(b) Samples are collected from the surface (top few centimeters) of the selected water body. 

(c) Water samples that are not used shortly after collection should be stored at 4o

 C. 

 

 Characterization

In the laboratory: The dissolved organic carbon (DOC), pH, and nitrate/nitrite need to be 

determined (621). As an option, additional factors that influence indirect photolysis can be 

determined including diisolved oxygen, redox potential, alkalinity, turbidity, other major major 

anions (HCO3

-

, Cl -, SO4

2-), major cations (Na+

, K+, Ca2+, Mg2+) and other metal ions such as iron 

and manganese (621). 

All waters (SNW, SNW-N, and NW) prepared for the indirect photolysis test should be filter-sterilized 

with a 0.2 μm filter. The absorptivity should be computed at 350 nm (absorptivity = 

2.303 x absorbance/pathlength). Computing the absortivity at 350 nm in SHW, SHW-N, and NSW 

provides an estimate of CDOM. CDOM exhibits a much stronger correlation with indirect photolysis rates 

than DOC. 
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ANNEX 6

Example for Measurement of the Intensity of Radiation

E.g. by a global sensor in the special sample platform during the measurement 

Date/time Intensity of irradiation

mW/cm2 Ws/cm2·min

Start of test

End of test

Average during test e.g. 968

Calculation of Environmental Days based on Light Intensity of an Artificial Light Source

(Example: Suntest unit used as artificial light source)

Light Intensity in the Suntest Unit

Light intensity of a Suntest unit measured at 300-800 nm (radiometer): 968 W/m2

Light intensity of an artificial light source (calc. for 300-3000 nm):

58% of the light intensity 300-3000 nm falls in the range of 300-800 nm3

(968 W·m-2/58%·100%) 1669 W/m3

Radiation (300-3000 nm) of a Suntest unit during 1 hour (1h = 3600 s): 6008276 Ws/m2

(Ws = J) 6008276 J/m2

(M = 1000000) 6.01 MJ/m2

Examples for Global Radiation (300-3000 nm)2:

Location Latitude Month Global radiation

1 day (MJ/m2)

Phoenix, Arizona (USA) 33.26 N June 31

Athens, Greece (EU) 38.03 June 20

Edmonton, Alberta (CAN) 53.33 June 22

Tokyo, Japan 35.11 April 15

Irradiation Time (Suntest Unit) Transferred to Environmental Days

Location Suntest

(hours)

Environment

(days)

Suntest

(hours)

Suntest

(days)

Environment

(days)

Phoenix, Arizona, USA 5.16 1 154.8 6.45 30

Athens, Greece, (EU) 3.33 1 99.9 4.16 30

Edmonton, Alberta (CAN) 3.66 1 109.8 4.58 30

Tokyo,Japan 2.50 1 74.9 3.12 30

 

3

 Solar radiation data:

Handbook of Material Weathering; 2nd edition, Chemtec Publishing, Ontario, Canada (1995).
ENV/JM/TG/RD(2006)1/REV 

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ANNEX 3 

November 2004 

Compilation of comments on the proposal for a new Guideline on Phototransformation of Chemicals in Water – Direct and Indirect 

Photolysis and Secretariat’s responses.

The following document comprises all comments received from Member Countries and industry on the proposal (August 2000) for a new OECD 

Test Guideline: Phototransformation of Chemicalsin Water – Direct and Indirect Photolysis. This proposal was circulated for comments to OECD Member Countries in September 2000. Comments were received from seven countries (Canada, France, Germany, Japan, Switzerland, UK and USA), the European Commission (EU) and industry (CropLifeInternational/ECPA). In this annotated guideline document, the comments are identified by their origin (see above). Responses from the Secretariat are provided for all 

Member Countries comments explaining (i) which points were adopted and how the subsequent changes were made, and (ii) which points could not be adopted. 

Country Comment Secretariat’s Response 

General Comments 

Canada In general, the current guideline is well presented and has detailed information on the 

subject. 

Agreed. 

France In a general point of view, French experts welcome the draft proposal. They think that the 

document is complete, useful and the most important points are mentioned. They underline, in particular, the recommendations made for the choice of the labelled carbon. 

Agreed. 

Germany The document as a whole would profit a lot from a better structuring and substantially 

shortening some chapters (table of content). The actual information for the user of the guideline on what to do and how to do it is embedded in a lot of background information which can also be found in the abundant list of references. Alternatively, the “practical part” should be more clearly separated from the theoretical background. The restriction of the test results should be noted at a very prominent place; just to say what the whole exercise is good for: to get a rough estimate of something like an overallphototransformation rate for natural water bodies, while accepting a considerable degree 

of “scientific fuzziness”. 

The draft document is split into two separate guidelines for direct and indirect photolysis, respectively. It is also restructured and shortened and thus should cover these points. Scope and approach are sufficiently described in paragraph 9. 

Germany To be scientifically correct one would have to study the transformation process(es) much 

more in detail even for pure water systems (e.g. wavelength dependence, side reaction with dissolved oxygen, etc.). Using “natural” water as a solvent, things get even more complicated under kinetic aspects since several reactions may proceed in parallel or in 

The main purpose of this guideline is to estimate rates of photolysis and environmental life-times for compounds in pure and natural water. It is not a guidance how “scientific research” should be conducted. 
ENV/JM/TG/RD(2006)1/REV 

139

Country Comment Secretariat’s Response 

General Comments 

competition to each other (2nd order rate constants for singlet oxygen, peroxy-radicals, 

OH-radicals, H O2 2). 

Japan First of all, Japan highly appreciates the Secretariat’s effort for developing this draft new 

test guideline. Although the main parts of the proposed draft TG would be acceptable, we would herewith send our comments. Furthermore, the test guideline of “Photolysis Studies” and “Fate of Photolysis Studies” of MAFF would be sent in the near future for the Secretariat’s reference and we would be grateful that the Secretariat would take into account our guidelines as well. 

We are looking forward to receive the MAFF guidelines to take these into account. 

Switzerland Our experts have reviewed the draft document “Phototransformation of Chemicals in 

Water – Direct and Indirect Photolysis (August 2000). We feel that the publication of such a guideline is timely and welcome. Below we give you general remarks followed by more specific comments and recommendations. They are structured in the order of the paragraphs, as they appear in the document. 

Agree. 

Switzerland According to our experience, the direct and indirect photolysis is important for the 

assessment of the fate of chemicals and pesticides. The tier-wise approach is useful for both direct and indirect phototransformation processes. We have, however, realised that trivial equations (such as first order degradation) are presented in the text whereas more complex equations which are needed are just referenced. In many cases, the reproduction 

of the equation is desirable, especially in cases where the reference is not in the open literature (see below). 

Agree. With regard to the reproduction of more complex equations, please indicate which equations should be reproduced in the text. 

Switzerland The subject of photochemistry is quite a complex one, especially where photosensitisers 

and radical quenchers are involved. Reading through the draft, we see that especially in the field of indirect photolysis the experimenter is confronted with a wide choice of conditions to be applied with respect to nature and concentration of sensitizer and quencher. A more precise guidance how to select appropriate conditions would be desirable. Furthermore, the document does not provide sufficient information how to extrapolate the results found under the various conditions to natural environmental conditions. At least, this should be a major goal of the guideline. We therefore feel that the present draft needs a careful revision before it can be considered for adoption as an OECD test guideline. 

Photochemical reactions in the presence of photosensitisers and radical quenchers are certainly more complex than direct photolysis reactions in pure water. Some choice of conditions is needed to evaluate the influence of sensitisers and quenchers. The document provides the information which is available based on the present state of the art. 

UK We commend the efforts of the authors of this very complex guideline. Keeping the 

mathematical derivations out of the main body text makes the document much more readable, whilst not loosing the detail. In order to make these studies easier to perform we would like to suggest the inclusion of worked examples (in a new Annex). Also a diagram of typical experimental systems would aid the reader not familiar with these types of study.

Worked examples are available for Tier 1, 2 and 3 in references (2) and (12) as “Illustrative Examples”. Typical experimental set-ups will be included in an Annex to the guideline. 
ENV/JM/TG/RD(2006)1/REV 

140

Country Comment Secretariat’s Response 

General Comments 

USA(1) Direct and indirect photolysis studies should be addressed in separate guidelines. 

US position: Agree. Separation of the two types of studies would result in much simpler, 

clearer guidance for each type. Separate documents or chapters are needed. 

The draft guideline will be split in two separate guideline parts as proposed. 

USA(1) The proposed guidance does not make efficient use of laboratory resources. The tiered 

approach to direct photolysis studies may require the same experiment to be set up and run four times for different purposes. This will increase the cost of the study beyond the value of the added information obtained. US position: Agree. The number of tiers for direct photolysis should be reduced to two: a 

screening step to determine if a laboratory study is needed (like tier 1) and full study (like tier 4). Tier 2 should be eliminated. Tier 3 (determination of quantum yield) should be optional. 

This point should be covered by the changes made. 

USA(1) The proposed guideline gives the laboratory the discretion to run a full study, or not 

depending on intermediate results from the tiered decision tree. The laboratory should not have this discretion. US position: Agree. The instructions on when to proceed to the next tier must be explicit. 

It is exactly the goal of the tiered system to have the discretion to conduct only those studies which are triggered by the decision criteria / trigger values given in the text and in Annex 3. 

USA (1) The proposed guideline requires the laboratory to use computer models (GCSOLAR or 

ABIWAS) to determine estimated photolysis rate constants, as part of the decision tree to determine the need for a full study. We disagree with this requirement. US position: Agree. Modelling studies to estimate rate constants should not be part of the 

laboratory protocol. The result of the study should be a measured rate constant.

Computer models such as GCSOLAR and ABIWAS may be used as additional possibility to estimate environmentally relevant photolysis rate constants. Their results do not trigger a full kinetic study in the revised tiered system. 

USA(1) The proposed guidance is overly vague in many places. For example, the choice of 

simulated or natural water in paragraph 26, “fairly substantial volatilisation” in paragraph 31, the choice of natural or commercial humic and fulvic substances in paragraph 55, “substantial decline” in paragraph 57, “somewhat comparable to that of sunlight” in paragraph 74 and the choice of actinometers and radiometers in paragraphs 80-85. US position: Agree. See also commenter 2, paragraph 31, for proposed benchmark for 

“fairly substantial volatilisation”. The vagueness of the guideline on the points will make it difficult to write a study protocol and to interpret the study results. 

A test guideline always also needs some flexibility in the description of the procedures. It is not possible to exactly define every single detail like in cook-book. 

USA(2) In general, the document could be improved by reformatting all of the equations to a 

similar size and font, and using a font size that is easily read. It is difficult to read equations in paragraph 27-30 (pp. 26 and 28) due to the small font size. US position: Agree. 

Agree. Equations will be re-formatted to the same readable font size. 

USA(2) A review of the OPPTS 835.2210 guidelines for direct photolysis in water and the current 

draft of the OECD guidelines reveals that there are a few points on which the two 

This draft guideline was written under consideration of a variety of national and international guidelines and the 
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Country Comment Secretariat’s Response 

General Comments 

documents are not in agreement. While these are minor points (such as the acceptability of the use of the term molar absorptivity or the range of wavelengths that should be taken into account), it is a shame that the two documents, which represent the state- of- the- art guidance on the conduct of aqueous photolysis studies, are not in agreement. US position: Agree. 

recommendations of the experts attending the Rome meeting in September 1998. So, it is not surprising that it is not in agreement with OPPTS 835.2210 in every aspect. 

USA(2) The text contains multiple references to a “30 days / nights” test period. It would be 

helpful to clarify the intended length of irradiation exposure per day (such as 12 hours light / 12 hours dark) unless multiple calculations (for different seasons and locations) are intended. Additionally, it would be helpful to discuss at some point whether the use of continuous irradiation (24 hours per day for 15 days) is preferable or whether a dark “recovery” period should be used in most cases. While this is more applicable to photodegradation in soil, it should at least be mentioned in this guideline. A suitable place might be paragraph 73 on page 14. US position: Agree. The proposed mechanism for selecting the irradiation period is too 

complex. Either 15 days continuous, or 30 days with 12 hours light / 12 hours dark should be required. This will help to ensure the comparability of studies on different chemicals. 

The irradiation of the samples should be done at the minimum until 75% transformation of the test substance or for a period equivalent to 30 days sunlight exposure whichever comes first. Whether continuous 15 days or 30 days with light and dark cycles are used, seems mainly a question of which light source is used (Xenon lamp or sunlight). 

USA(3) The proposed guideline incorporates many of the developments in environmental 

photochemistry of the past 20 years. The principle of the test guideline is straightforward and clear, but the guideline is very complicates and lacks regulatory tightness. We would suggest that at least 2 or 3 documents need to be produced: A Photolysis Guidance Document would incorporate the principles, experiments, etc. 

from the draft guideline document. Separate Direct and Indirect Photolysis Guidelines 

would spell out the regulatory requirements in an ambiguous fashion, including the proposed tiered approach to decision making which we see as useful. It might be possible to combine some of the processes within the tiers and satisfy requirements of multiple tiers. The Photolysis Guidance Document has been greatly improved by moving many of the equations to Annex 4, but there is still a lot of background information woven into the main body of the guideline. Unfortunately, that features makes it difficult to understand how to conduct the required experiments. We suggest further separation of the experimental procedures from the supporting information. US position: Agree. However, such an OECD Guidance Document already exists (see 

Ref. 10). The proposed new document would supersede that 1997 reference. The number of tiers for direct photolysis should be reduced to two: a screening step to determine if a laboratory study is needed (like tier 1) and a full study (like tier 4). Tier 2 should be eliminated. Tier 3 (determination of quamtum yield) should be optional. 

The draft document will be split into two separate guidelines for direct and indirect photolysis. An OECD Guidance Document (No. 7) already exists, at least for Direct Phototransformation. Also, the draft was shortened and restructured based on the numerous comments and thus the revised version should be clearer and less complicated. 

USA(3) The primary goal of aqueous photolysis guidelines should be to produce a photolysis rate These points should be covered by the changes made in the 
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Country Comment Secretariat’s Response 

General Comments 

that can be used in risk assessments (i.e. Tier 1 model inputs) and to define the significant photolysis products (10% of initial dose) that may be important to analyze for in dissipation studies. It is important that the data generated be as real world as possible. As written, this guideline would generate a variety of direct and indirect photolysis rate constants (under buffered systems, with artificial or natural humic substances and with nitrate, also theoretical rates determined by models based on quantum yields and tables of sunlight irradiance). A study conducted in accordance with the guideline would generate many more rate constants than are needed for risk assessment. US position: Agree. The rational for collection of each type of rate constant needs to be 

better explained. 

tiered testing schemes for direct (part 1) and indirect photolysis (part 2), respectively. 

USA(3) Currently, when Pesticide Concentrations in Drinking Water are estimated with EXAMS 

the aqueous photolysis input is set at 1/124th of the aquatic photolysis rate constant. 

Thus all the nuances of the photolytic rate constant determined in this guideline are completely swamped by the 1/124th factor and will not affect the risk assessment. 

Therefore, this guideline should examine the photolytic behaviour of test compounds under the most important scenarios. These important scenarios are probably buffered pure water (PW) and representative natural water (NW) from the proposed usage area. Expansion of the requirements to additional scenarios (simulated natural water (SNW)), nitrate (SNW-N), multiple pH’s, etc. is academically interesting, but it may be argued that at times other situations could be just as important (algae blooms, oxidants such as superoxide, hydrogen peroxide or singlet oxygen). Given the current regulatory use of aqueous photolysis outputs, these studies should be limited to buffered pure water (PW) and representative natural water (NW). US position: Agree in part. The comment regarding EXAMS is EPA program-specific, 

and should have no effect on the guideline. The option of using synthetic natural water should be preserved, but better guidance on the formulation of SNW is needed. Data will be more comparable between chemicals if SNW is used. 

 As the draft document is written it seems to give enough flexibility to the Study Director to use SNW and/or SNW-N and /or natural water (NW) for indirect photolysis studies. The formulation/composition of SNW seems to be sufficiently well described in Annex 5. 

USA(3) Recent work, such as that of Scott Mabury suggests that photolysis data generated with a 

typical sterile buffer, containing no photosensitisers, is not relevant to the real world. Thus, the use of natural waters for photolysis should be elevated in importance when the results are to be used in risk assessment. This approach would effectively cover the situation most relevant to indirect photolysis, photolysis in shallow water bodies such as rice paddies. US position: Disagree in part. The option of using natural water should be preserved, but 

better guidance on the formulation of SNW is needed. Data will be more comparable 

It may well be that the use of natural water for photolysis studies seems to better reflect real environmental situations and thus be more important. However, the principles laid down in this draft document were agreed upon at the experts’ meeting in Rome in September 1998. 
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Country Comment Secretariat’s Response 

General Comments 

between chemicals if SNW is used. 

USA(3) Regarding calculation of rate constants, we believe that pseudo-first order kinetics are 

acceptable, but second order should also be expected. Equations and compartment kinetics provided for that option should be included in the guideline. It would be good to specify what output is wanted, half-life, DT50 or dissipation (if volatilization occurs). US position: Agree. 

This point should be covered by the changes made. 

USA(3) There is some confusion over the duration of the study. Photolysis studies using 

continuous irradiation with simulated light should be acceptable since the objective of the guideline is the determination of the photolytic fate of the test chemical. The study should be conducted for a sufficient time to allow determination of the DT75 of the test chemical. US position: Agree. The proposed mechanism for selecting the irradiation period is too 

complex. Either 15 days continuous, or 30 days with 12 hours light/12 hours dark should be required. This will help to ensure the comparability of studies on different chemicals. 

As we understand the text in paragraph 72, the irradiation of the samples should be done at minimum until 75% transformation of the test substance or for a period equivalent to 30 days sunlight exposure whichever comes first. Whether continuous 15 days or 30 days with light and dark cycles are used, seems mainly a question of which light source is used (Xenon lamp or natural sunlight). 

EU This guideline is a very comprehensive document on photochemical methods. Indeed, our 

impression is that it might be difficult to use, because of its complexity. 

The separation in one version for direct photolysis and another one for indirect photolysis and the other changes should make this guideline simpler and easier to use. 

CropLife/ECPA We greatly appreciate the efforts of the authors for preparing this very complex test 

guideline. It is a comprehensive document covering many important aspects of phototransformation in aqueous medium and is certainly a good start for developing a corresponding OECD Test Guideline. Transferring most of the mathematical equations from the main body text to an Appendix makes the document much more readable, whilst not loosing the detail. Nevertheless, there are still many equations left in paragraphs which describe experimental procedures; these equations should be transferred to a separate 

chapter “Treatment of results and calculations”. 

The paragraphs containing these equations are often also giving explanations needed for the understanding of the experimental procedures. Thus, this point is not adopted. 

CropLife/ECPA However, the guideline is not straightforward, is still rather bulky and contains too much 

textbook knowledge. Due to its bulky structure and its voluminous contents the document is rather difficult to read, even for readers familiar with these types of photolysis studies. In order to increase readability, the guideline should be more structured (titles and subtitles) and a table of contents should be given. We would like to suggest the inclusion of worked examples (in a new Annex) to make these studies easier to perform. Another proposal for improving readability and applicability of this guideline is to split it into separate guidelines for direct and indirect photolysis, respectively. In this case, the direct photolysis guideline could include a statement on when an indirect photolysis study should be conducted. 

These points should be covered by the changes made. With regard to the inclusion of worked examples, see previous response on UK comment. 

CropLife/ECPA The test schemes for direct and indirect photolysis shown in Annex 3 are not in agreement 

with those proposed by the photolysis experts attending the Rome meeting in September 

Annex 3 is revised accordingly and should now be in line with the text in the corresponding paragraphs. 
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Country Comment Secretariat’s Response 

General Comments 

1998. As experimental steps and decision steps were mixed together the schemes have also lost their transparency. These flowcharts for decision-making should be simplified to contain critical information only, because detailed reasoning is already given in the main text. There are also differences between the text and the schemes with regard to the trigger percentages. In addition, the percentage triggers given in the text in paragraphs 17-19, 26, 90-92, 100-102, 128 and 150-152 differ from those agreed upon by the experts attending the Rome meeting (i.e. 10% transformation in 5 days); we would greatly appreciate if the authors of this guideline could explain their reasons for these changes. In the text explaining the schemes, sentences like “if required, proceed to next tier” (e.g. paragraphs 91 and 102 are found) but nothing is said under which circumstances tests at these Tiers are required. Also double work is often recommended (e.g. Tier 2 and 4 in direct photolysis, Tier 1 and 2 in indirect photolysis). The reporting requirements for the pre-tests should be specified and the possibility to omit lower Tiers and to directly to a higher Tier should be given. 

CropLife/ECPA As this is a guideline which describes how to conduct aqueous photolysis experiments the 

reader can expect some illustrations of useful experimental set-ups. Therefore, examples of appropriate photolysis apparatus must be given in additional Figures. Otherwise, such an experimental guideline is of limited value. 

Examples of useful experimental set-ups are included in Annex 6. 

CropLife/ECPA Different letter sizes are used in the various equations given in the main text and also in 

Annex 4. Some letters are so small that they can hardly be read without a magnifying glass. The same letter size should be used in all equations through the whole document. 

This point is adopted. 

CropLife/ECPA When comparing this guideline with other OECD E-Fate Test Guidelines it is obvious that 

additional definitions, such as test substance, transformation products, DT50, DT75, halflife, etc. have to be included in the chapter DEFINITIONS AND UNITS. 

This point is adopted. 

ECPA member 

company 

Maybe the guideline contains too much details about different methods or possibilities for calculating half-lives, quantum yields, etc. Many of those details could be deleted because they can be found in the high quality list of references. 

Readers may prefer to find these details in the guideline instead of looking for it in a various references. 

ECPA member 

company 

The major issue I would like to address here is a statement on Section 9 (PRINCIPLE OF THE STUDY, Scope and approach). This is my personal experience and that of all my colleagues I have talked to: At the moment (and even much more when following this new tiered draft), we waste a lot of resources for investigating the direct photolysis kinetics and product formation in pure water or buffer because that studies are required for the registration data packages. In most cases, the outcome of those studies is totally irrelevant for the environment. I have often performed those studies and I have got scientifically interesting results, but they can only be used for storage in a library. Thus, the direct photolysis in pure water or buffer should 

The principles of this guideline were established by the experts attending the Rome meeting in September 1998 based on the available national and international guidelines. 
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only be further investigated, if it is definitely a very fast reaction (e.g. in case of an E-fate DT50 <1 day). Indeed, we will get that information by Tier 1 (theoretical screen, section17) and then, when relevant, by Tier 3 (determination of the quantum yield and its use to estimate direct photolysis rate constants, section 19). The outcome of both Tiers results in kinetic data, which clearly belong to the physical-chemical data of a test chemical and, therefore, that data should be a basic information in the physical-chemical data packages of a chemical. Further, they can be used for calculating maximum E-fate half-lives via modelling, then. 

ECPA member 

company 

However, corresponding requests on what amounts and kind of metabolites are formed under natural sunlight conditions in highly pure water or buffer (i.e. during e.g. 30 solar days) make not any sense , because there is not any highly pure water present in any environmental aqueous solution! However, then performing a well-designed photolysis study in natural water we will get all the information together: the net (combined direct and indirect) photolysis kinetics as well as the degradation products relevant for environmental assessment and effects testing. Even in highly pure water (or buffer) the indirect processes can hardly be excluded totally. This is the more the fact the longer the irradiation must be performed, namely in that cases where the direct photodegradation is a comparatively slow process (i.e. for most of the 

pesticides we have to investigate).I would like to suggest changing the title of the Guideline to “Pure and Natural Water Photolysis”. From the scientific point of view, the earlier title is more accurate, but in practice we can hardly differentiate the individual processes. The “pure and natural water photolysis” of a chemical is what we actually investigate in the tests for getting data on environmental assessment. Comparing the situation to e.g. an aerobic soil metabolism study, nobody would have any interest to set up a guideline for creating detailed data, what individual processes (either biotic degradation by fungi, bacteria, algae, etc., or by different abiotic catalysed reactions in soil solution) are responsible for the observed degradation. Thus, we always determine the sum of processes leading to the respective degradation kinetics and pathway in a simulated ecosystem. The same should consequently be followed with the phototransformation in water, too. 

To perform a well-designed photolysis study in natural waters only could certainly be an interesting alternative to the proposed draft document. This might be a topic for discussion for the National Coordinators. 

ECPA member 

company 

There is not any doubt that the draft guideline is an excellent review and guide through the complex issue of phototransformation in water. However, it should be clearly highlighted in Section 9 (page 2), what data and / or information are definitely needed for environmental risk assessment purposes in order to create a complete data package, e.g. for registration of a pesticide. The major needs are: 

We think that paragraph 9 clearly highlights what data and information are provided by the draft test guideline. 
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- The quantum yield of direct photodegradation in pure water or buffer (as a basic physical-chemical parameter), and resulting theoretical direct photolysis rate constants for the chemical for various types of water bodies, seasons, and latitudes of interest. - The combined (direct and indirect) photolysis rate constant of the test chemical and the transformation pathway, identities and rates of formation and decline of major phototransformation products in two selected natural surface waters (e.g. pond, reservoir, river, etc) probably (intended use area) or in fact contaminated by the test chemical in the environment (indicated by monitoring data). The selected natural waters should differ in their characteristics, i.e. in pH, CDOM and nitrate content. 

Paragraph 1 

Switzerland The role and relative importance of photosensitisers NO3 ̄, NO2 ̄, Fe-III-complexes) is not 

clearly described. Nitrite is a strong sensitiser but also readily oxidised in water, therefore a fast decrease in concentration is expected. Comments on choice of sensitisers, concentrations are required. 

The role of sensitisers is mentioned, for example, in paragraphs 4 to 6. 

CropLife/ECPA Typing error in last line: environment should read “environmental”. This point is adopted. 

Paragraph 6 

Germany CDOM in first line; this abbreviation is used without definition throughout the text. CDOM is explained in the last line of paragraph 4 on page 1. 

Paragraph 8 

USA(3) This section is too general. It should specifically state the desired endpoints of the study 

and briefly explain how these results may be used in a risk or environmental fate assessment. For example, it is mentioned that the results from an aqueous photolysis experiment are used as inputs for environmental fate computer models. Clarification may be necessary to specify if these values are net photolysis, direct photolysis, indirect photolysis, etc. As mentioned in the general comments, a photolysis rate constant, the significant metabolites formed at >10% of the initial dose, and quantum yields for estimating rate constants under various scenarios are the desired endpoints of the study. The section should document the need to generate multiple rate constants. A balance must be struck between practicality and the desire to develop theoretical information. The opening sections in general are well written but need to discuss the environmental significance of aqueous photolysis as it relates to regulatory decisions. US position: Agree. The rationale for the collection of data should be better explained. 

The right to request generation of more data must be reserved, if the situation warrants it. 

This draft is a guideline which should instruct a user how to conduct photolysis experiments; it is not the goal of an experimental guideline to explain how the results can be used in a risk assessment. The desired endpoints are listed in paragraph 9 and are further explained in paragraphs 17-20 for direct photolysis and in paragraphs 26-27 for indirect photolysis. 

Paragraph 9 

USA(3) There is a great amount of potential information to be generated in this guideline. The 

data requests must satisfy the requirements of the regulatory community. How crucial to the risk assessment are photolysis determinations in the presence of nitrate or at multiple 

As this draft is based on existing national and international guidelines, it should satisfy also the requirements of the regulatory community. The revised version asks in case of 
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General Comments 

pHs? Also, the guideline seems to be asking for quantum yield determination (to determine estimated direct photolysis rate constants) and also asks for laboratory determined rate constants. Are both needed in the regulatory assessment? US position: Agree in part. The rationale and advantages for collection of each type of 

rate constant needs to be better explained. 

direct photolysis for quantum yield determination (Tier 3) which allows the calculation of half-lives and environmental life-times of chemicals for a variety of conditions. Laboratory determined rate constants in Tier 2 are optional. Tier 4 only asks for the isolation, quantification and identification of phototransformation products as recommended by the experts attending the Rome meeting in September 1998. 

CropLife/ECPA In point 2, the short wave cut-off value is given here and in many other sections of this 

guideline as 295 nm. It should be noted that in section 2.9.1 of the EU Commission Directive 94/37/EC, in the OECD Guidance Document No. 7 on Direct Phototransformation of Chemicals in Water (reference 10) and also in the SETAC guidelines (reference 7) 290 nm is stipulated as a cut-off value. This comment also refers 

to paragraphs 12, 17, 43, 50, 52, 53, 56, 60, 83, 85, 87, 88, 122, 170, Annex 3 and Annex 4. 

This point is adopted. 

Canada Delete “major” before phototransformation products. This change is needed because of 

the following reasons: - it is necessary to analyse all the possible phototransformation products at different 

time intervals, to identify the major and minor phototransformation products - information on major and minor phototransformation products is needed to determine the phototransformation pathway - information on minor phototransformation products is important, particularly if they are highly toxic - other OECD Guidelines on chemical transformation do not specify the analysis of only the major transformation products (e.g. Aerobic and anaerobic transformation in aquatic sediment systems – Guideline No. 308). Therefore, the above change is necessary to have consistency between the OECD Guidelines on chemical transformation. 

This point is adopted. 

Paragraph 10 

UK The triggers in Annex 3 are not consistent with the body text. This Annex requires 

correction. 

This point should be covered by the changes made in Annex 3. 

CropLife/ECPA The triggers in Annex 3 are not consistent with the body text (in paragraphs 17-19 and 

26) and thus this Annex requires correction. 

This point should be covered by the changes made in Annex 3. 

Paragraph 11 

Canada Insert “at time t” before “(mol/L)”. This change is necessary to specify that, c = test 

chemical concentration at time t (mol/L). 

This point is adopted. 
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Germany The recommended symbol of the amount of substance concentration is c not C. The 

substance itself can be specified in brackets or as a subscript, e.g. c(HCl) = 0.1 mol/L or cHCl = 0.1 mol/L. 

This point is adopted. 

UK Half-life does not appear in eq. 1-3. Move to eq. 4. This point is adopted. 

USA(3) The guideline should allow for kinetics other than pseudo first order. Section 16 

mentions that pseudo first order assumptions are valid for studies conducted for one to several weeks or for 2-4 hours mid-day, but it may not be valid for studies lasting four hours to several days or for those conducted beyond 30 days. This is confusing and needs further clarification. US position: Agree. Further clarification is needed. 

This point should be covered by inclusion of a corresponding sentence at the end of this paragraph. 

CropLife/ECPA This paragraph should be rather part of a chapter “Treatment of results and calculations”. 

This comment also applies to paragraphs 14, 15, 25, 73, 78, 81, 83, 88, 98, 107, 112-118, 123, 135, 139, 149, 161, 163 and 164. Also half-life does not appear in equations 1-3 and should thus be moved to equation 4. 

In principle we agree with this comment but adopting the first point would require major restructuring of the guideline and may make it more difficult to understand. The second point is adopted. 

Paragraph 12 

Germany Delete “of light” at the end of sentence 1. This point is adopted.. 

USA(3) This section specifies wave lengths 295-800 nm. The Suntest instrument is supposed to 

generate energy within the distribution but we are aware of instruments that only measure up to 700 nm. We are not sure what the availability is for an instrument in this range. US position: Disagree. The guideline provides references for the 295 to 800 nm range.. 

To be in line with OECD Guidance Document No. 7 the wavelength range is changed in 290-800 nm. 

Paragraph 13 

UK In line 6 „e.g.“ should read “i.e.”. This point is adopted. 

CropLife/ECPA In sentence 4 e.g. should read i.e. This point is adopted. 

Paragraph 14 

UK It is not clear if A < 0.02 refers to a single wave length or 295-800 nm. Suggest the text 

from para 69 is used or reference is made to this para. 

A < 0.02 refers to the absorption maximum of a chemical and thus to a single wavelength. 

USA(3) The symbol defining light path length in eq. 5 and in the definition and everywhere else 

should be consistent. US position: Agree. 

Agree. 

CropLife/ECPA It is not clear if A<0.02 refers to a single wave length or to the whole range of 290-800 

nm. Suggest the text from paragraph 69 is used or reference is made to this paragraph. 

A < 0.02 refers to the absorption maximum of a chemical and thus to a single wavelength. 

Paragraph 17 

Germany The expression “near surface” means actually not any attenuation due to ingredients in 

water. In fact one can expect that absorbance of light by the test chemical is much less 

Paragraph 17 describes the theoretical screen proposed as Tier 1 test for direct photolysis reactions and thus attenuation 
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than attenuation caused by e.g. humics. This is just a poor trigger for higher tier investigations. To get a more realistic idea about what goes on in natural waters we should take into account this effect (ref. to document of R. Frank /equ. 9). Beside attenuation from cloud cover the ABIWAS programme allows to calculate rate constants for any depths and attenuation from natural water ingredients. We would prefer to use a trigger based on a more realistic surface water body; e.g. 100 cm depth, standardised optical properties of the natural water and test chemical concentration (e.g. 10-6 – 10-10

mol /L). 

due to water ingredients is not of interest at this stage. 

Japan In this guideline, the general situation for evaluation is “in summer at lat. 40 ̊ or 50 ̊. But 

the general situation is different among countries, therefore, the description of the situation should be as examples. For your information, Japan is located at lat. 20 ̊ ~ 40 ̊ N., and the situation for evaluation is determination to lat. 35 ̊ N. and in April to June, taking into account our general application pattern of pesticides. 

This paragraphs describes the estimation of a maximum photolysis rate assuming a quantum yield of one, so the relevance of the latitude is rather low 

UK These tabular data should be included in the guideline, along with a worked example. In 

addition, Annex 3 does not reflect the text. 

Agree. Worked examples are available in refs. (2) and (12). Annex 3 is revised accordingly. 

USA(3) Triggers for proceeding to higher tiers are not consistent throughout the document. For 

example, compare Section 17 and Annex 3. Also, it is not clear what regulation the statement “if required” would refer to. This type of wording needs clarification in order to understand whether indirect photolysis is required. The wording “if required” also appears in Section 18 & 20 and maybe other places. US position: Agree. If the tiered approach is kept the triggers should be made consistent. 

The words “if required” should be deleted if a higher-tiered study is meant to be triggered automatically. 

Annex 3 is revised to be in line with the text in this and also in the other related paragraphs. The words “if required” in Tier system are meaningless and are thus generally deleted. 

USA(3) Tables of daily sunlight irradiance should be included. For instance, Section 17 indicates 

that tabular irradiance data should be used to determine a maximum direct photolysis rate. US position: Disagree. The needed references are provided. 

Agree with US position. 

CropLIfe/ECPA The tabular solar irradiance data should be included in the guideline, along with a 

worked example. The triggers given here are different from those given in Annex 3 (this also refers to paragraphs 18, 19, 26, 90, 100 and 150). 

The needed references are provided. Annex 3 will be revised to reflect the text in this and other related paragraphs. 

ECPA member 

company 

Change last sentence in: If the maximum possible losses are ≥50, proceed to Pure Photolysis Tier 3 (determination of the quantum yield). All the other options are nice to have, but not needed. However, the quantum yield determination should be obligatory in that cases, in order to get this physical-chemical parameter of the test chemical. Compare section 90-92, too. 

The wording in this paragraph leaves enough flexibility for the Study Director; he may go to Tier 2 or directly to Tier 3 as an option. 

Paragraph 18 
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UK With regard to the triggers, Annex 3 does not reflect the text. Guidance on when Direct 

photolysis Tier 4 is required should be added. 

Annex 3 will be revised accordingly. The choice to go also directly to Tier 4 from Tier 2 is not in line with the recommendations of the experts attending the Rome meeting in September 1998 and thus is removed here. 

USA(3) It is difficult to understand when one would go to higher tiers or conduct additional 

experiments. Statements such as “...proceed to o Indirect Photolysis Tier 1, if required” 

leave open the question of when the indirect photolysis tier 1 is required. US position: Agree. If the tiered approach is kept, the triggers should be made consistent 

The words “if required” should be deleted if a higher-tiered study is meant to be triggered automatically. 

This point should be covered by the changes made. 

CropLife/ECPA With regard to 30 days/nights, does this mean the number of 24-hour days or the 

equivalent days of summer sunlight received? Also, guidance should be given when to proceed directly to Tier 4. However, a kinetic study at Tier 4 was not proposed by the experts attending the Rome meeting in September 1998; Tier 4 was only recommended for Identification of Photoproducts (>10% of degraded parent chemical). This comment also applies to paragraph 20. 

The first point should be covered by the changes made. The choice to also go directly to Tier 4 is removed as this was not recommended at the Rome meeting. 

Paragraph 19 

Germany Same comment as for paragraph 17. See response on comment for paragraph 17. 

UK These programs (GCSOLAR or ABIWAS) do not easily run on modern PCs without a 

great deal of work by computer experts. These programs are difficult to use. Also, have the programs been GLP validated? These programs should be updated / improved before being recommended for general use. With regard to the triggers, Annex 3 does not reflect the text. 

The first point should be covered by the changes made. Annex 3 will be revised accordingly. 

USA(3) For some reason GCSOLAR does not run well on a PC. A revised version should be 

prepared if the program will be used to meet data requirements. US position: Agree. Appropriate modifications to any required computer program 

should be made. 

This point should be covered by the changes made. 

Croplife/ECPA These programs do not easily run on modern PCs without a great deal of work by 

computer experts. Therefore, these programs are difficult to use. It is also not clear whether these programs have been validated. The programs should be updated / improved before being recommended for general use. This comment also applies to paragraphs 89, 124, 125, 155, 163, 163 and 170. 

This point should be covered by the changes made. 

ECPA member 

company 

Delete the sentences after Ref. (2) and add: Further work on Pure Water Photolysis (i.e. the Tier 4) is only required, if the estimated losses are greater than 50% during one day under relevant environmental irradiation 

This point should be covered by the changes made. 
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conditions. Otherwise proceed to Natural Water Photolysis (Tier 1 or 2). Compare section 126.128, too. 

Paragraph 20 

UK Guidance is needed how to do this, since in some circumstances the transformation 

product may also be susceptible to photodegradation. 

The triggers in Tier 3 define when to conduct Tier 4 studies. 

USA(2) In this paragraph, under the Section “Summary of the proposed Tiered Approach for 

Direct Photolysis”, and in paragraph 129 (page 25), it would be appropriate to expand the definition of major transformation products by indicating that the 10% guideline value applies to individual replicate data rather than mean data, particularly when a pattern of increase with time is evident from a review of the replicate data. In numerous fate studies submitted for registration / re-registration, I have observed registrants’ claims that certain compounds were not major degradates because they were not present at greater than or equal to 10% of the applied; however, a review of replicate data often showed that the compounds were present as major degradates in many of the replicates, but the mean values were lowered by the inclusion of outlier data points. US position: Agree. 

This point should be covered by the changes made. 

USA(3) When would it be necessary to estimate the half-life of a transformation product? What 

is the best method for doing so? Log-transformed data with a best-fit line through all the formation and declining concentration points is not adequate modelling. Appropriate kinetic modelling should be conducted to determine the rate constants for decline of photolysis products, if possible. US position: Agree. Clear instructions for measuring the kinetics of transformation 

products should be given.

Some explanations / instructions are given in the experimental section (paragraph 144). 

Croplife/ECPA Change title before paragraph 20 in “Identification of Major Transformation Products”; 

for reasons see comments on paragraph 18. 

This point is adopted. 

Paragraph 21-25 

USA(3) Sections 21-25 are good examples of background information that could be separated 

from the experimental procedure. US position: Agree. 

Paragraphs 21-25 are not part of the TEST METHODOLOGY. They belong to main chapter PRINCIPLE OF THE STUDY and thus are separated from the experimental procedure. 

Paragraph 26 

UK It should be made clear that this tier is not required if comparable direct photolysis data 

is available, as mentioned in paragraph 160 (this paragraph should be moved closer to the start of the indirect photolysis section to highlight this information). The text should be referred to at each mention of PW, or mention of PW should be removed or moved to an appropriate section. 

Indirect photolysis studies are only carried out when direct photolysis of a chemical is not a relevant process (see revised Annex 3). Therefore, the inclusion of a test in buffered pure water is not reasonable. 
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The statement that “the use of a xenon lamp light source for indirect photolysis studies is not recommended at this time” should be explained further as it is not obvious why this should be the case. Annex 3 Figure 2 states a xenon lamp may be used. With regard to the triggers, Annex 3 does not reflect the text. 

We agree that a Xenon lamp should be allowed as alternative to natural sunlight. Annex 3 is revised accordingly. 

USA(3) A xenon arc lamp should be allowed for indirect photolysis. In an indirect photolysis 

screen, we can expect to determine whether the chemical can be degraded by an indirect process and whether the process will be significant in natural systems. Since indirect photolysis is a second order process, the rate constant will be dependent on the concentration of radicals (and their precursors) more than on the intensity and wave length distribution of the light source. Hence, a light source simulating sunlight should be acceptable as long as the source’s emission spectrum overlaps the action spectrum for 

radical production. Is it possible to extrapolate results from the use of simulated sunlight to evaluate indirect photolysis? Most registrants have xenon-arc lamp equipment and the investigation of indirect photolysis will require them to have natural sunlight equipment under this guidance. US position: Agree. The rationale for exclusion of xenon-arc lamps from indirect 

photolysis studies should be provided. 

The use of a Xenon arc lamp will be included as alternative to natural sunlight. 

USA(3) This section does not clearly define the criteria for when SNW, SNW-N or NW is the 

preferred experimental conditions. Without clear guidance, registrants could do a SNW test and only find out years later that the regulatory agency desired SNW-N. It would appear that there is an intent to discern photooxidative mechanisms, which goes beyond what is useful for a regulatory decision. The indirect photolysis study should be conducted with natural water (NW). US position: Agree in part. The option of using synthetic natural water should be 

preserved, but better guidance on the formulation of SNW is needed. Data will be more comparable between chemicals if SNW is used. 

As the draft document is written it seems to give enough flexibility to the Study Director to use SNW and/or SNW-N and /or natural water (NW) for indirect photolysis studies. The formulation/composition of SNW seems to be sufficiently well described in Annex 5. 

CropLife/ECPA It should be made clear that this is not required when comparable direct photolysis data 

are available as mentioned in paragraph 160; perhaps this paragraph should be moved closer to the start of the indirect photolysis section to highlight this information. As indirect photolysis studies are only conducted when direct photolysis does not occur to a significant extent under the conditions of this model experiment it does not make sense to include tests in buffered pure water. Why should a Xenon lamp not be used for indirect photolysis studies? Annex 3 states in Fig. 2 that a Xenon lamp may be used. Also the tiered approach for indirect photolysis consists of only 2 Tiers although at the Rome meeting the experts agreed upon 3 Tiers; we would appreciate if the authors of this 

Indirect photolysis studies are only carried out when direct photolysis of a chemical is not a relevant process (see revised Annex 3). Therefore, the inclusion of a test in buffered pure water is not reasonable. We agree that a Xenon lamp should be allowed as alternative to natural sunlight. The last point is adopted (see revised Annex 3). 
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guideline could explain their reasons for this. 

ECPA member 

company 

Paragraphs 26 and 27 are nice to have but not needed. Thus, directly proceed to Tier 2. This point is not adopted. 

Paragraph 27 

USA(3) Criteria for substitution of SAR to estimate rate constants is not clear – examples are 

needed. US position: Agree. SAR studies may be beyond the capabilities of some laboratories, 

and may not be properly included in a guidance for a laboratory protocol. Better guidance is needed. 

Agree with US position. This paragraph is deleted. 

Paragraph 28 

UK Annex 3 Figure 2 states a xenon lamp may be used. This point is adopted. 

CropLife/ECPA Change title in “Identification of Major transformation Products” because kinetic 

information was not recommended for Tier 3 at the Rome meeting. With regard to light source see comment on paragraph 26. 

This point should be covered by the changes made. 

ECPA member 

company 

Change text as follows: Determine the rate of decline in a realistic environmental concentration of the test chemical and its combined (direct and indirect) degradation rate constants in at least two NWs irradiated with sunlight or simulated sunlight. Identify / quantify the major transformation products (set up the degradation pathway) and determine their rates of formation and decline. The selected NWs should represent a surface water (e.g. pond, reservoir, river, etc.) and originate from an area probably (or in fact) contaminated with the chemical. They should differ in their characteristics, i.e. pH, CDOM and nitrate content. Compare section 154.160, too. 

This point is not adopted. 

Paragraph 30 

Japan The pHs of the test solution (pH 5, 7, 9) are not consistent to those in OECD TG 111 

(pH 4, 7, 9). So the pHs should be consistent to those in OECD TG 111. 

This point is adopted. 

UK Reference to environmental pH values (as elsewhere) should be added, i.e. pH 5 - 9. This point is adopted. 

USA(3) EU data requirements specify pH 4-9. Harmonisation would be desirable. The test 

should always be conducted at environmentally relevant pHs. Section 30 contains the statement that if the half-life of the parent molecule is < 1 day in the hydrolysis study then photolysis should be conducted on each major hydrolytic product (defined as all compounds seen at > 10% in hydrolysis study). We have a problem with this because it clearly introduces a new or change in the data requirements. This study is concerned with photoproducts, not the photolysis of hydrolysis products. If the photolysis of significant hydrolysis products is proposed it must be done based on a weight of the evidence approach to determine what is significant. As written, this guideline could have 

Reference will be made to OECD TG 111 which recommends pH range 4-9 as environmentally relevant. If a test substance is readily hydrolysed at a relevant environmental pH at 20-25 ̊C, it seems reasonable to us to conduct a photolysis study with the major hydrolysis product(s). 
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photolysis studies conducted on environmentally insignificant compounds. This change in data requirements would require further discussion. At this time we believe that the photolysis of transformation products from parent study should be sufficient. US position: Disagree as to pH range. It is well recognised that pH 5, 7, and 9 represent 

environmentally relevant pH. The comment on new or changed data requirements is EPA program-specific. Flexibility in the design of the experiment must be retained, on a case-by-case basis. 

CropLife/ECPA Reference to environmental pH values (as elsewhere) should be added to sentence one 

(i.e. pH 5-9). With regard to additional photolysis studies with all relevant hydrolysis products, it should be rather recommended to conduct the photolysis studies with the parent test substance and see if hydrolytic transformation products are built up there. 

The first point is adopted. With regard to the second point, see response above on USA(3) comment. 

ECPA member 

company 

Change 2nd sentence in: However, pure water photolysis tests should be conducted at a 

pH ff. 

This point is not adopted. 

Paragraph 31 

UK Further guidance when photolysis studies are not required would be welcome, e.g. 

trigger values. 

We propose not to include trigger values here to avoid long discussions on what values are appropriate. 

USA(2) In this paragraph it would be helpful to give a reference value to help define or clarify 

what is meant by the term “fairly substantial volatilisation”. I suggest adding 10-3atm-cu 

m/mole as a benchmark order of magnitude for Henry’s Law constant, above which you would expect sufficient volatilisation from water as to make the test non-applicable or indicate the need for the precautions noted in the text. Also in that paragraph, I would like to indicate the need for confirmatory sterility checks if an aliquot sampling method is utilised. US position: Agree that better guidance is needed. The decision to run a study or not 

should be made on a case-by-case basis. 

With regard to a reference value for volatilisation see previous response on UK comment. An appropriate place for sterility checks is in the experimental sections. 

USA(3) There seems to be a contradiction in this section. If a compound has a Henry’s Law 

Constant or VP that indicates substantial volatilisation will occur then an aqueous photolysis study may not be appropriate. The physical properties of the compound (high escaping tendency, low water solubility) may indicate that the compound will never reach a significant concentration in water. US position: Agree (Note that commenter 2 provided a potential benchmark for this 

factor). The decision to run a study or not should be made on a case-by-case basis. 

This paragraph mentions possible problems with volatile compounds and recommends some precautions how to minimise volatility losses but the decision to run a study or not has to be made by the Study Director on a case-by-case basis. 

Croplife/ECPA Further guidance when photolysis studies would not be required for volatile chemicals 

would be welcome (e.g. by inclusion of appropriate trigger values). 

See above responses on related comments. 

Paragraph 32 

Germany Supplement list by: The UV-vis spectrum is measured in Tier 1 for direct 
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(m) UV-vis spectrum (n) Information about biodegradability. 

photolysis. The other point seems of little relevance for aqueous photolysis experiments. 

CropLife/ECPA The draft guideline contains two chapters dealing with information on the test substance 

(paragraphs 32-34 and 44-48, respectively). These should be combined in a similar way as in other OECD E-Fate Test Guidelines (e.g. 307 and 308). 

This point is adopted. 

Croplife/ECPA The information in points (g) to (l) is normally required for the test substance in section 

“Test report” and should be deleted here. The test temperatures required for the determination of physical-chemical properties in the corresponding OECD Guidelines (i.e. 104, 105, 107, 111 and 112) are different from the range given in the “Note”. Therefore, change this “Note” as follows: The temperatures at which the physical-chemical properties listed above were determined should be reported. 

This point is adopted. 

Paragraph 33 

CropLife(ECPA “Solubility in organic solvents” should be added to the items in paragraph 32 and deleted 

here. The other two points of information are of little relevance for aqueous photolysis studies and should be deleted here. 

These points are adopted. 

Paragraph 35-42 

CropLifeECPA This chapter contains too much unnecessary detail. Paragraphs 36 and 37 overlap with 

paragraph 62 and thus can be deleted here. We propose to create separate chapters for REFERENCE SUSTANCES and for QUALITY CRITERIA with a similar wording as in other OECD E-Fate test Guidelines (e.g. 307 and 308). The analytical methods should be included as additional paragraph in the chapter INFORMATION ON THE TEST SUBSTANCE. 

These points are adopted. 

Paragraph 36 

Switzerland This paragraph does not contain information; delete.. This point is adopted. 

CropLife/ECPA TLC should be included as standard analytical technique because the concerns raised can 

also be applied to the other analytical techniques quoted.? 

This point is adopted. 

Paragraph 38 

Switzerland This paragraph does not contain information, delete. This point is adopted. 

CropLife/ECPA The use of radio-labelled reference standards is not necessary; unlabelled material is 

suitable fort his purpose. 

This point is covered by the changes made. 

Paragraph 40 

Croplife/ECPA The statement “if samples are stored prior to analysis, storage stability test should be 

performed” is too rigid. The necessity to conduct stability tests depends on: - the time period of storing the samples under usual storage conditions (-20 ̊C in the 

This point is adopted. 
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dark) and - the formation of photoproducts susceptible to hydrolysis. But if samples are deep-frozen, hydrolysis is very unlikely to happen. Therefore, such samples should then be stable for several weeks, even in the presence of photosensitisers and this paragraph should be deleted. 

Paragraph 43 

Japan The dry heat sterilising method should be added to sterilising method. This method is 

usual and there is no problem to date. 

This point should be covered by the changes made. 

USA(3) Test Materials: should provide more specification as to type of material and shape here. 

It is buried in other parts of the document, but this is an obvious place for it. 

US position: Agree. Paragraphs 43-64 are about the Test Materials (vessels, media, test 

substances, et.). Paragraph 43 is only about test vessels. Thus this comment appears unclear. 

Agree with US position. 

ECPA member 

company 

In case of pure water photolysis tests all glassware ff. This point is not adopted. 

Paragraph 44-48 

CropLifeECPA See previous comment for chapter INFORMATION ON THE TEST SUBSTANCE 

(page 8). The whole chapter should be shortened by using a similar wording as other OECD E-Fate Test Guidelines (e.g. 307 and 308). Unlabelled test substance should also be allowed for all kinetic measurements. 

Considering this and previous comments on paragraphs 35-42, these paragraphs are renamed “Test substance application” (new paragraphs 34-36). They now contain the contents of paragraph 53 and 68-71. 

Paragraph 44 

Switzerland Chemicals possessing more complex structures are often difficult to judge where the 

“stable portion” is. Stability may also be dependent on actual conditions. Be more precise. 

This point should be covered by the changes made. 

CropLife/ECPA The position of the radiolabels should allow the identification of transformation products 

and the elucidation of the transformation pathway. The use of other isotopes (other than 

14C) should be limited to those which would aid the analysis. Typing error in line 5 

“pathways”. 

This point should be covered by the changes made. 

ECPA member 

company 

If radio-labelling is used, test chemicals (i.e. that used for transformation pathway studies) should be labelled at one or more atoms within a stable or a relatively stable portion of the molecule (usually in rings). Experiments with more than one radiolabel should be performed, if the pattern of decline cannot be evaluated with one label (e.g. for chemicals containing more rings or if the molecule is thought to be readily susceptible to cleavage to two parts). ff 

This point should be covered by the changes made. 

Paragraph 47 
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USA(2) It would be helpful to state some of the special criteria that may apply if a formulated 

product is used in place of the active ingredient. For example, will it be a requirement that the carriers in the formulated product be tested separately to determine if they are photosensitisers? Or, will separate photolysis data be needed for each formulation type if this option is taken? It may be helpful to address these issues as well as to give an example.of when it may be preferable to use the formulated product. US position: Agree. If formulations contain photosensitisers, testing should be done on 

the pure test compound also, to separate direct and indirect photolysis effects. 

Aqueous photolysis studies are normally conducted with the pure test substance; therefore, the use of formulated products is not recommended and this paragraph is deleted. 

Paragraph 49 

UK Annex 5 does not recommend pH 9 buffer details – these should be added. This point is adopted. 

Paragraph 50 

Switzerland The inclusion of a list of suitable buffer systems which are adequate would be desirable. A reference is made to OECD TG 111 in which suitable 

buffer systems are extensively described. 

CropLife/ECPA It may be helpful to specify the sterilisation method for the buffer solutions /autoclaving 

or 0.2 μm filtration?). 

Sterilisation methods are mentioned in paragraph 33. A reference to this paragraph is included. 

Paragraph 51 

CropLife/ECPA For non-ionisable compounds, the test should be conducted at a pH at which the test 

substance is hydrolytically most stable. The recommendation to use buffers of pH 5, 7,and 9 as in the SETAC guidelines, however, is in contradiction to section 2.9.1 of the EU Commission Directive 94/37/EC in which pH 4, 7 and 9 are recommended. 

These points are adopted. 

Paragraph 52 

USA(3) EU data requirements specify pH 4-9. Harmonisation would be desirable. This section 

seems to indicate that photolysis experiments at up to 3 pHs could be required. The guide lines should recommend that photolysis be conducted at the single pH exhibiting the highest UV/VIS absorbance. Could rate constants for the other pHs be calculated based on extrapolation from the UV/VIS absorbance data? For an ionic compound where 3 radiolabels are used, one would normally do 3 experiments at the most stable pH (6 

experiments if corresponding SNW or NW experiments are done). Practical limits on the number of experiments to be conducted need to be set, balancing the use of the data in risk assessments against the cost. Given the significance of photolysis in risk assessment (see general comment), the large number of possible separate experiments does not appear warranted. US position: Disagree as to pH range (also see US position on section 300 above). It is 

well recognised that pH 5, 7, and 9 represent environmentally relevant pH. Flexibility in the design of the experiment must be retained, on a case-by-case basis. 

This point is adopted. 

Paragraph 53 
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USA(3) There are cases where low water solubility and the desire to have enough material in 

solution for reliable analysis, requires the use of 10% co-solvent. US position: Disagree. Co-solvent should be used only when absolutely necessary and 

limited to 1% in order to reduce potential buffer effects. 

The use of up to 10% co-solvent is not excluded in paragraph 53. 

Croplife/ECPA Lists of permissible co-solvents and of known sensitisers would be helpful. Acetonitrile is recommended as appropriate solvent. 

Paragraph 54 

USA(2) In this paragraph, there are several references listed. I suggest also adding as a reference 

the chapter “Phototransformation of Pesticides in Aqueous Solution” by P. Méallier, pp. 240-260 in The Handbook of Environmental Chemistry (1999), eds. C. Richard and G. Grabner. This chapter is an excellent source of information on indirect photolysis in aqueous solution. It also includes a discussion of direct photolysis, quantum yield, and typical reaction types, for which examples (with initial and modified chemical structures) are given. US position: Agree. 

This point is adopted. 

USA(3) The triggers for SNW-N experiments are not clear. A clear criteria to trigger SNW-N 

studies needs to be set if this requirement is decided to be significant for risk assessment purposes. As mentioned previously, there are a variety of water conditions where photosensitisers, suspended sediments, algae may play a significant role in effecting photolysis rates in NW systems. The natural water systems that the pesticide will be exposed to can be highly complex and understanding all the effects of one of these factors (SNW-N production of OH radicals) may not be possible. An indirect photolysis in NW representative of the proposed use pattern seems more useful. The guidelines should specify taking UV/VIS of test media for the indirect photolysis without test substances in it, as well as when the test substance is in it. US position: Agree in part. The option of using synthetic natural water should be 

preserved, but better guidance on the formulation of SNW-N is needed. Data will be more comparable between chemicals if SNW-N is used. 

A Study Director may first conduct studies in SNW and later also in SNW-N and NW to confirm that indirect photolysis is a relevant process for the chemical tested. The requirement for measuring UV-Vis spectra of the test media is mentioned in paragraph 55. The UV-Vis spectrum of the test substance will be available from Direct Photolysis tier 1. 

Paragraph 55 

US(3) If the use of SNW is to be required, specific guidance on the humic acid concentration 

needs to be stated. US position: Agree. 

In formation on humic acid concentration in SNW is given in Annex 5. 

Paragraph 56 

UK Should both methods be required since they serve the same function? Suggest 

“actinometers or a spectral analyser” would be sufficient. 

This point is adopted. 

USA(2) The UV-Visible wavelength of interest is stated as “295-800 nm”. As the visible light 

region of the spectrum ends at 750 nm, this statement (which appears in multiple places 

Several references in this document mention the wavelength range of 290-800 nm, for example references (2)(3)(8)(10) 
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throughout the document) should be amended to improve its accuracy. Additionally, the choice of 295 nm as the initial value in the range is interesting; often it is presented as 290 nm in the literature, perhaps because of the different indirect photolysis reactions that occur at on either side of the 290 nm cut-off. I suggest considering the use of 290 nm as the initial value in the range for that reason. US position: Disagree. The guideline provides references for the 295 to 800 nm range. 

etc. Therefore, this range will be used in the revised version. 

USA(3) The statement that xenon lamps are not recommended needs to be supported. Need to 

re-examine the recommendation that sunlight is the only option at this time. Natural sunlight is not necessarily the best choice. It limits locations and times of the year for facilities that are not located in “sunny” climates to conduct studies, and the natural sunlight in Madison, Wisconsin, is not a model for use of the pesticide in San Diego, or Houston. In particular these studies are meant to be guidance for field expectations at a number of different locations within the US or across Europe / around the world (if a global registration data package is being developed). There are going to be too many variables between these simulated waters and even natural waters selected for these studies to be more than guidance. US position: Agree in part. The rationale for exclusion of xenon-arc lamps from indirect 

photolysis studies should be provided. The option of using synthetic natural water should be preserved, but better guidance on the formulation of SNW is needed. Data will be more comparable between chemicals if SNW is used. 

In the revised versions, Xenon lamps are allowed for both direct and indirect photolysis studies. 

CropLife/ECPA Irradiation with the xenon arc lamp should be continuous and should also be allowed for 

indirect photolysis studies. With regard to sentence 3, should both methods be required since they serve the same function. We suggest that “actinometers or a spectral analyser” 

would be sufficient. Because comparative measurements with a new and used Xenon lamp (1600 operating hours) have shown the long-term stability of the lamp spectrum of a Suntest apparatus, it should be sufficient to measure the integral photoflux at the 

beginning and at the end of an irradiation experiment. 

These comments should be covered by the changes made. 

ECPA member 

company 

Change 2nd sentence in: For natural water photolysis studies, the recommended light 

source is sunlight or filtered xenon arc lamp light. 

This point is not adopted. 

Paragraph 58 

Switzerland For pesticides, the application season may not be equal to highest concentration in the 

aquatic compartment. Give more information on possible time lags. 

We do not see a need to go beyond these very general recommendations. 

UK More guidance is needed for chemicals with wide dispersive use, on season, latitude, etc. 

May not be known or may vary significantly. It is suggested that conditions under which the study was conducted are reported. 

Agree that these general recommendations may not be very helpful and that the conditions under which the study was conducted should be reported. 
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Paragraph 60 

France In the first line, (generally quatz) has to be removed by (generally quartz). This point is adopted. 

CropLife/ECPA If it can be demonstrated that no significant differences in the light intensity spectrum 

transmitted is detected between quartz and borosilicate, the latter material should also be allowed. The “merry-go-round” equipment is good for static systems when no trapping of volatiles is needed but is not suitable to trap volatiles in flow-through systems. As this supposed to be an experimental guideline, illustrations of appropriate apparatus have to be included as examples in corresponding Figures (see also General Comment). 

It may be simpler to use optical cells made of quartz glass to avoid long discussions about the acceptability of borosilicate glass. Illustrations of appropriate experimental set-ups will be included in an Annex. 

Paragraph 61 

UK Phosphate buffers will not cover the full pH range 5-9. The text here and in Annex 5 are 

not consistent. Either use the same buffers as for hydrolysis (pH 5, 7 and 9) or phosphate buffer and test pH 5-8. 

These points should be covered by the changes made. 

USA(3) A cautionary note on the use of phosphate buffers in cases where they might promote a 

catalytic effect could be included (see current EPA guidelines). US position: Agree. 

This point is adopted. 

Croplife/ECPA Phosphate buffers will not cover the full pH-range 5 to 9. The text here and in Annex 5 

are not consistent; either use the same buffers as for hydrolysis (pH 5, 7 and 9) or 

phosphate buffer and test at pH 5-8. 

These points should be covered by the changes made. 

Paragraph 65 

Canada Replace this section with the following: The study should be conducted at a temperature 

chosen for other laboratory transformation studies, and should be maintained within ± 2 ̊C. The temperature should be monitored periodically. 

As rates of photolytic reactions are almost independent of the temperature, this temperature range was proposed by the experts attending the Rome meeting. Agree that temperature should be monitored periodically. 

USA(2) Under “Test Methodology –Test Conditions” (paragraphs 65 and 67) I suggest adding a 

comment which notes that the dark controls should be kept at the same temperature as the irradiated samples. A 10 degree temperature variation allowance seems a bit wide in range, so, at the very least, it should be specified that the dark controls are to be subjected to a similar temperature pattern (inclusive of range of temperatures and duration of exposure to various temperatures within that range). US position: Agree. A temperature of 25 ± 1 ̊C should be maintained to minimise 

volatilisation losses and to allow for better comparison to other studies. 

The first point is adopted by adding a corresponding sentence to paragraph 67. With regard to the second point, see above response on Canadian comment. 

Paragraph 66 

Canada Replace “± 0.5” before pH with “± 0.1”. This point is adopted. 

Paragraph 67 
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USA(2) See comment on paragraph 65. This point is adopted. 

Paragraph 68 

CropLife/ECPA Paragraphs 68-71 are related to the test substance application and should be combined 

with paragraph 53 in a separate chapter Test substance application.

This point is adopted (see paragraphs 34-36). 

Paragraph 70 

USA(3) Guidance should be given on the appropriate concentration of SNW and SNW-N to use 

in these studies. If NW systems are to be used some general parameters that fit the proposed use pattern of the pesticide should be outlined. US position: Agree. 

Information on these aspects is given in Annex 5. 

Paragraph 71 

USA(3) There should be more discussion of the requirement to maintain oxygen content prior to 

completion of the guideline. It is not clear that changes in oxygen content during an aqueous experiment will significantly change the photolysis of most chemicals. Requiring air-saturated solutions may result in a de-facto requirement for aliquot sampling from large systems in which a continuous flow of air can be maintained. If this requirement is retained, then we suggest making it very clear that the solution should be saturated with air only at the beginning of the experiment. Also, “saturated” is imprecise in its use here. US position: Agree in part. Because sacrificial sampling is preferred, oxygenation will 

be difficult. 

It seems that the authors of this guideline were fully aware of the significant influence of oxygen on the rate of photolysis in water. They also made it very clear that the aqueous solutions should be saturated with air at the beginning of the study. We also think that “saturated” is precise enough for the purpose of the studies. 

CropLife/ECPA The guideline stipulates that the water should be fully oxygenated. Could the authors of 

this guideline please recommend a method of saturation which would maintain the sterility of the aqueous solution? 

Saturation can be achieved, for example, by bubbling filtersterilised air/oxygen through the aqueous solutions. 

Paragraph 73 

Switzerland Equation 8 contains a factor j which is not commented in the caption. This point is adopted. 

UK Note. This is true for any “box” cell, it is not only true of rectangular cells. Disagree. 

USA(1) It is not clear what irradiation period is required (15 days continuous, or 30 days dark / 

light). The proposed guideline calls for the laboratory to calculate an irradiation period from equation8, based on “equating photons absorbed by the test chemical per unit volume...” US position: Agree. The proposed mechanism for selecting the irradiation period is too 

complex. Either 15 days continuous, or 30 days with 12 hours light / 12hours dark should be required. This will help to ensure the comparability of studies on different chemicals. 

The guideline calls for a duration equivalent to 30 days sunlight exposure which corresponds to approx. 15 days of continuous exposure to a Xenon arc lamp. 

USA(3) If the study should be performed for the length of time necessary to simulate 30 days of The only place where this point might be a problem is in Tier 
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exposure to sunlight, then of course the question is – What is the intensity of sunlight? The US industry has more or less standardised on irradiance of summer sunlight at noon at 40 N latitude, and we recommend that this standard should be adopted. The only way to avoid arguments over sunlight irradiance is to require determination of the quantum efficiency and calculation of rates at different latitudes and times of the year using GCSOLAR, ABIWAS or EXAMS. The identity of photoproducts would be required under controlled conditions simulating the fastest rate of photolysis in the geographical region where the parent chemical is likely to be found. US position: Agree. The adjustment for season or latitude should be done after the 

laboratory work in most cases. Flexibility in experimental design should be preserved for special cases. 

2 which is an optional step. With the inclusion of the determination of the quantum yield such arguments can be avoided. 

CropLife/ECPA Note. This is true for any “box” cell, not only for rectangular cells. Disagree. 

Paragraph 74 

USA(2) This paragraph should be re-written for clarity and accuracy. In general, the document 

contains numerous and repetitive errors in grammar, syntax and punctuation. In some cases, these interfere with sentence meanings. I find that the OPPTS 835.2210 guidelines are more easily understood. US position: Agree. 

We would highly appreciate if the commenter could supply a clearer and more accurate version of this paragraph and also detailed information on the numerous and repetitive errors in grammar, syntax and punctuation. 

USA(3) Section 74 introduces a new data requirement to the US and EU, namely the 

determination of the photolytic fate of hydrolysis products. The existing guidance in the US specifies conduct of the photolysis study at the pH where the parent is most stable. Rather than complicate the photolysis study guideline, we suggest a change in the data requirements if photolysis of hydrolysis products is important for environmental fate. In most regulatory assessment schemes, photolysis is not considered an important factor for determination of Predicted or Estimated Environmental Concentrations. We suggest that expanding the data requirements is inappropriate when the current data requirements have minimal value in risk assessments. US position: Disagree. This is a proposed test method and not a new or changed data 

requirement. Data requirements are determined by the appropriate regulatory authority. Flexibility in the design of the experiment must be retained, on a case-by-case basis. The right to request generation of more data must be reserved, if the situation warrants it. 

Agree with US position. 

Paragraph 75 

Switzerland If the analytical methodology produces data with a precision of ± 15%, the number of 

data points suggested in the text (less than five) is not sufficient to establish a reliable regression function. 

The number of sampling points also depends on other factors, such as the rate of photolysis, duration of the experiment, etc. It is finally in the competence of the Study 
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Director to establish the appropriate number of samples. 

Paragraph 76 

USA(3) Agitation is most common in natural systems. What about allowing for stirring in 

samples? This would be particularly useful in situations where larger “bulk” samples are sub-sampled. US position: Disagree. Agitation should not be allowed, as it may cause volatilisation of 

the test substance. 

Agree with US position. 

Paragraph 77 

USA(3) The requirement for three replicates is not warranted. Generally, two replicates have 

been found to be adequate when 5-6 samples are taken over the course of the experiment. US position: Disagree. At least three replicates should be required for a statistically 

robust experiment. 

This point should be covered by the changes made. 

CropLife/ECPA A minimum of 3 replicates is excessive, in particular for all experiments conducted 

under controlled laboratory conditions. It should be sufficient to use duplicates. 

This point should be covered by the changes made. 

Paragraph 79 

UK More guidance should be given. When are they required and if so, how should they be 

conducted? 

As these calculations are optional, the given guidance seems to be sufficient. 

CropLife/ECPA More guidance should be given when this is required and if so, how this should be 

carried out. 

See above response. 

Paragraph 82 

USA(3) Sections 82, 83, 84 (missing) and 85 offer guidance rather than requirements for conduct 

of the experiments. This is the kind of information that should be included in a Photolysis guidance Document. The use of a single actinometer solution should be proposed to give a reasonable estimate of the photon flux. Again, the use of a single standard actinometer solution should be adequate since the current data requirements have minimal value in risk assessments. US position: Agree as to sections 82-85. Better guidance on the appropriate use of 

actinometer solutions is needed. 

This guidance how to determine the photon flux of a Xenon arc lamp is certainly needed as an important aspect of the TEST METHODOLOGY. Guidance on the use of actinometers is given later in the experimental sections of Tier 3. 

Paragraph 83 

Germany An explanation would be desirable on why for an optically dense solution a zero-order 

kinetics is employed instead of a pseudo-first-order one. 

Agree. Please supply explanation. 

Paragraph 86 

USA(3) Compare recommendation for pH to section 74. 

US position: See response to section 74 comment. 

Agree with US position. 
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Paragraph 87 

USA(3) Ordinarily, one would not use the UV spectrum obtained in a neat organic solvent unless 

there was no other way to obtain the UV spectrum of the test chemical. We suggest omitting this recommendation unless there is a clear use of the information. US position: Disagree. For example, this is useful for ionisable chemicals. 

Agree with US position. 

CropLife/ECPA A trigger value for the molar extinction coefficient should be set (e.g. >10). In last 

sentence, change “neat” in pure. 

This trigger is not needed here. The second point is adopted. 

Paragraph 89 

Switzerland Include Table 2 of Ref. 12 in the text. This point is not adopted. 

UK With regard to GCSOLAR and ABIWAS, see previous comments on paragraph 19 These programs are mentioned here only as alternative 

methods. 

Paragraph 90 

UK Annex 3 does not reflect the text. Annex 3 will be revised accordingly. 

USA(3) In the guidance section, the rationale behind the selection of the cut-off of 50% 

photolysis might be explained. We are accustomed to waiving the study if the molar absorptivity is less than 10. The criterion in section 90 makes sense, but needs greater explanation. US position: Agree. Better explanation of rationale is needed. 

The trigger of 50 % is a rather conservative value when considering that the maximum photolysis rate is estimated for a quantum yield of one. 

Paragraph 91 

USA(3) The statement “if required” needs to be clarified. The criteria for going to Indirect 

Photolysis Tier I is unclear. US position: Agree. The words “if required” should be deleted if a higher-tiered study is 

meant to be triggered automatically. 

The words “if required” are generally deleted. 

Paragraph 93 

Canada It should be noted that the test chemical and some of the transformation products formed 

during the study may be volatile. Therefore, it is necessary to detect and quantify the formation of organic volatiles and CO2. Consequently, both the direct and indirect 

photolysis test should be carried out in test vessels attached to suitable traps for the collection of organic volatiles and CO2. This is also necessary to establish the mass balance in the study and to maintain consistency among the OECD Guidelines on chemical transformation. Sections 146 and 156 should also be modified based on this comment. 

This point should be covered by the changes made. 

USA(3) Sections 93-97: If Tier 1 says photolysis is possible, then a logical next step is a 

determination of the quantum efficiency. Tier 2, in effect, is a rough determination of q.e., and one might as well go straight to Tier 3 and do a reasonably good determination. 

According to the recommendations of the experts attending the Rome meeting in September 1998, the tiered system is revised in the text and also in Annex 3. As Tier 2 is optional, 
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What is the uncertainty associated with screening data? If using “screening” data always results in unacceptable uncertainty, then registrants might always have to go to the next tier. Would a preliminary screening test on the test compound be needed to estimate a rough half-life? Or is the half-life estimated from the theoretical screen sufficient to determine the appropriate composition of the actinometer. US position: Agree in part. The rationale and advantages for collection of each type of 

rate constant needs to be better explained. The number of tiers should be reduced to two: a screening step to determine if a laboratory study is needed (like Tier 1) and a full study (like Tier 4). Tier 2 should be eliminated. Tier 3 (determination of quantum yield) should be optional. 

it practically consists of three Tiers: • Tier 1 – Theoretical Screen • Tier 2 - Quantum Yield determination • Tier 3 - Identification of Photolysis Products. 

Paragraph 95 

USA(3) A significant number of dark samples should be required to calculate a “net” rate 

constant. The registrant should choose the appropriate number of dark controls. US position: Disagree. A dark control sample for each sampling interval is needed to 

account for other loss processes. 

This paragraph leaves sufficient flexibility to the Study Director to select the appropriate number of dark control samples. 

Paragraph 98 

Switzerland Equation 14 describing the attenuation of natural sunlight (seasonal variation, clouds...) 

has to be included. 

Equation 14 is already included in this paragraph. 

UK L_ should be Lλ. This point is adopted. 

CropLife/ECPA It was not proposed at the Rome meeting to determine a direct photolysis rate constant 

for the test substance in near surface clear natural water; therefore, delete this and the following paragraph. Typing error: L_ should be Lλ. 

The first point is not adopted; this Tier 2 is optional. The second point is adopted. 

Paragraph 99 

UK Should both methods be required since they serve the same function? Suggest 

“actinometers or a spectral analyser” would be sufficient. 

This point is adopted. 

CropLife/ECPA Same comment as for paragraph 56 (page 11). Same response as for paragraph 56. 

Paragraph 100 

UK Annex 3 does not reflect the text. Annex 3 is revised accordingly. 

Paragraph 102 

CropLife/ECPA The experts attending the Rome meeting in September 1998 agreed upon that at Tier 4 

identification of major phototransformation products (>10% of applied dose) should be carried out; There is also no need to do additional kinetic measurement when quantum yield is already determined at Tier 3. Therefore, delete “and/or 4” in the first sentence and also in title of this sub-chapter and also delete the last sentence. 

These points are adopted. 

Paragraph 104-109 
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USA(3) These sections contain good discussions but do not provide experimental procedures. 

Also, the length of time it takes to do the q.e. experiment will depend on the intensity of the light source more than the number of wavelengths in the emission spectrum. US position: Agree. Sections 104-109 should be moved to the background document, or 

to an appendix. Clarification of the quantum efficiency experiment issue is needed. 

Paragraphs 104-109 are transferred to Annex 7. 

Paragraph 108 

Germany The azobenzene actinometer should be included. This is a very convenient actinometer 

as it affords no exact initial concentrations, is insensitive to oxygen, solutions can be handled at daylight, it affords no additional analytical procedures, it has been calibrated at various wavelengths and it affords no sophisticated data analysis (Reference: G. Gauglitz and S. Hubig, Photokinetische Grundlagen moderner Aktinometer, Z. Phys. Chem. N. F. 139 (1984) 237-246. // G. Gauglitz and S. Hubig, Chemical actinometry in the UV by azobezene in concentrated solutions: A convenient method, J. Photochem. 30 (1985) 12-125. 

This point is adopted by adding the azobenzene actinometer and the references to the Table in Annex 7. 

CropLife/ECPA The suggested actinometers do not include the oxalic acid which appeared in the 

ECETOC report No. 12 (reference 12). 

This point is adopted. 

Paragraph 111 

CropLife/ECPA Typing error; paragraph number should be 111. This point is adopted. 

Paragraph 116 

France On our paper version, equation 21 could not be read clearly (web transmission 

distortion?). 

This point is adopted. 

Paragraph 121-123 

USA(3) Sections 121-123 introduce the possible, but unlikely, need to determine q.e. in different 

absorption bands for a general guideline, it should be adequate to specify how the determinations should be made for most cases. Exceptional cases will always require specialised knowledge and expert judgement to determine the need for additional experiments. US position: Agree. Special cases can be handled individually. 

This point is adopted by deleting these paragraphs. 

Paragraph 122 

CropLife/ECPA In second line, insert at between yield and at least. This paragraph is deleted. 

Paragraph 126 

CropLife/ECPA As a consequence of comments on paragraph 102, change first sentence in : 

A direct photolysis Tier 4 study should be performed if the direct photolysis rate 

constant(s) for the parent test substance (estimated from its quantum yield) indicate(s) 

This point is adopted. 
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that direct photolysis is a significant process.

Paragraph 128 

CropLife/ECPA Delete last point/sentence of this paragraph. For reasons see previous comments on 

paragraphs 102 and 126. 

This point is adopted. 

Title Tier 4 

CropLife/ECPA AS a consequence of previous comments on paragraphs 102, 126 and 128, the title 

should be changed in: Identification and quantification of major phototransformation 

products. Also, the two subtitles should consequently be deleted and the paragraphs 129-

144 should be re-phrased accordingly. 

This point is adopted. 

Paragraph 129 

USA(2) With the exception of the three-word phrase in parentheses, paragraphs 129 and 137 are 

identical. Likewise, paragraph 138 is a repeat of paragraph 130. US position: Agree. Paragraphs 137 and 138 should be deleted. 

This point is adopted. 

Paragraph 130 

Canada Delete “/or their major” before transformation. All transformation products including 

organic volatiles and co2 should be measured. 

This point is adopted. 

UK No recommended methods are stated. Suggest this text is deleted as it is very difficult to 

prove the solutions were sterile. 

This point is adopted. 

CropLife/ECPA No methods for checking sterility are stated here. We suggest to delete this text as it is 

rather difficult to prove the solutions were sterile. 

This point is adopted. 

Paragraph 134 

Japan “f the date...” should be changed to “If the data...” This paragraph is deleted. 

UK “f” should read “If”. This paragraph is deleted. 

CropLife/ECPA Typing error: f should be if. This paragraph is deleted. 

Paragraph 137 

UK More guidance should be given. When are they required and if so, how should they be 

conducted? 

This paragraph is deleted. 

Paragraph 138 

Switzerland 138 duplicates 130, omit. This point is adopted. 

Paragraph 139 

France All equations up to number 25 are written with “big” character size and the size is 

reduced for the following. It would be better to harmonise all equation sizes. 

This paragraph is deleted. 

UK Text should refer to Annex 4. This paragraph is deleted. 

Performance of indirect photolysis tiers 

CropLife/ECPA A 3-Tier system was proposed by the experts attending the Rome meeting, we would Annex is revised to be in line with the recommendations of 
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General Comments 

greatly appreciate if the authors of this guideline could explain the reasons for deleting one of these Tiers. 

the Rome meeting. 

Paragraph 146 

CropLife/ECPA Should the guideline specify that the water be sterilised in this paragraph? Sterilisation may be problematic as some sterilisation 

methods might alter the properties of the water ingredients. 

Paragraph 147 

USA(3) The recommendation to use sunlight for indirect photolysis experiments needs 

elaboration and discussion. As long as the intensity of the source and the emission spectrum overlap the action spectrum of the photosensitiser or radical precursor similarly to sunlight, the source should be acceptable. Is this recommendation consistent with the comments in section 160? US position: Agree. The rationale for exclusion of xenon-arc lamps from indirect 

photolysis studies should be provided. 

The Xenon lamp is included as alternative irradiation source for indirect photolysis studies. 

CropLife/ECPA The use of a Xenon lamp should also be allowed for indirect photolysis studies. This 

comment also applies to paragraphs 154 and 156. 

The Xenon lamp is included as alternative irradiation source for indirect photolysis studies. 

Paragraph 150 

UK Annex three does not reflect the text. Annex 3 is revised accordingly. 

Paragraph 153 

USA(1) The proposed guideline calls in paragraph 153 for the laboratory to conduct StructureActivity Relationship 8SAR) studies to determine the second-order rate constants for the 

disappearance of degradates. We disagree with this requirement. US position: Agree. SAR studies may be beyond the capabilities of some laboratories, 

and may not be properly included in a guidance for a laboratory protocol. Better guidance is needed. 

Agree with US position. The paragraph is deleted. 

Tier 2(page 29) 

CropLife/ECPA Identification/quantification of major phototransformation products should be treated as 

a separate Tier (see previous comment on performance of indirect photolysis study on page 27). 

This point should be covered by the changes made. 

Paragraph 154 

CropLife/ECPA Experiments in buffered pure water (PW) should be deleted; for reasons see comment on 

paragraph 26 (page 7). This comment also applies to paragraphs 156-159. 

This point is adopted. 

Paragraph 155 

UK Should the recommended method be detailed in this guideline, along with guidance on 

when this effect should be accounted for? 

We think that some guidance is given in paragraphs 166-168. 

CropLife/ECPA Last point. Should the recommended method be detailed in this guideline, along with We think that some guidance is given in paragraphs 166-168. 
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General Comments 

some guidance on when this effect should be accounted for? 

Paragraph 160 

UK This text should be moved further forward in the guideline. Due to the changes made, this paragraph is not needed and 

thus deleted. 

CropLife/ECPA This paragraph should be deleted (see comments on paragraphs 26 and 154). This point is adopted. 

Paragraph 161-165 

CropLife/ECPA Direct photolysis rates are determined in direct photolysis studies and should be deleted 

here; for reasons see comments on paragraph 26. 

This point is adopted. 

Paragraph 162 

UK As these programs are difficult to use, worked examples would be helpful. Worked examples can be found in reference 12. 

Paragraph 166-168 

USA(3) Is the decrease in the determined rate constant due to photobleaching significant enough 

to warrant this level of refinement in the experimental test given the significance of photolysis in risk assessment? US position: Disagree. The guideline directs the laboratory to end the study if 

photobleaching occurs, or to continue the study and then correct for photobleaching. 

Agree with US position. 

Paragraph 169 

USA(3) Yes. Consequently, restricting indirect photolysis experiments to irradiation with 

sunlight reflects an unrealistic expectation for use of the data. At most, the experiment indicates the possibility for degradation of the test chemical by indirect photolysis. Using rate constants from such an experiment may be justified if the field experiments clearly demonstrate that direct photolysis alone cannot account for the observed degradation rates or products. Using the rate constants from the laboratory experiments to determine an environmental concentration requires assumptions not only about the light source, but also about the concentrations of photosensitisers, suspended sediments, radical precursors and radical scavengers. Those assumptions may be valid and conclusions about the significance of indirect photolysis may be valid, but we must accept that our ability to model degradation by indirect photolysis is at a very early stage. US position: Agree. The rationale for exclusion of xenon-arc lamps from indirect 

photolysis studies should be provided. 

The xenon lamp is included as alternative irradiation source for indirect photolysis studies. 

CropLife/ECPA In line 3, insert space between is and that. This point is adopted. 

Paragraph 170 

Germany Under computational methods for, on page 36: “Quantum yield calculations (...fractions 

and not as a percent)”. What is the difference? The following symbols have the same 

This point is adopted. 
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General Comments 

meaning: 5 = 10-2 = 0.01. So the sentence can be deleted. 

Japan Second point under Tier 1 on page 33: According to this paragraph, the site of collection 

and the preservation method are also important factors. So, they should be described well. The full term of “DOC” (Dissolved organic Carbon) is described in only Annex 5: It should be described in the text of this draft. 

Instructions on collection and sampling of NW are given in Annex 5. The second point is adopted. 

CropLife/ECPA The reporting requirements seem to be unnecessarily detailed and should be shortened. 

A corresponding title for reporting test substance data should be added. 

This point should be covered by the changes made. 

Paragraph 171-177 

CropLife/ECPA Discussion greatly depends on the individual results obtained in each study and thus is 

normally done in each test report. We have some doubts whether these paragraphs 171-177 are o great help for the reporting study directors and therefore propose to just include it as a separate point Interpretation and discussion of results. 

This point is adopted. 

Literature 

EU Typing errors in the following references: 

- 26, 27 and 28: Hoigné instead of Hoigne or Hoigne’ -...41: Sulzberger instead of Suizberger - 50: Lemaire instead of Lemarie. 

These points are adopted. 

EU It would be useful to consider adding some of the following references: 

Paragraph 4, Direct photolysis: 

Boule P., Guyon C. and Lemaire J. 1982. Photochemistry and Environment IV : photochemical behaviour of monochlorophenols in aqueous dilute solution. Chemosphere 11, 1179-1188. Boule P., Guyon C., Tissot A. and Lemaire J. 1987. Specific phototransformation of xenobiotic compounds : chlorobenzene and chlorophenols. Photochemistry of Environmental Aquatic Systems, R.G. Zit and W.J. Cooper (eds) Miami, 10-26. Influence of Humic Substances:

Aguer J.P., Richard C. and Andreux F. (1997). Comparison of the photoinductive properties of commercial, synthetic and soil extracted humic substances. J. Photochem. Photobiol. A 103, 163-168. Vialaton D., Richard C., Baglio D. and Paya-Perez A. 1998. Phototransformation of 4-chloro-2-methylphenol in water; influence of humic substances on the reaction. J. Photochem. Photobiol. A 119, 39-45. Nitrate ions:

Niessen R., Lenoir D. and Boule P. 1988. Phototransformation of phenol induced by excitation of nitrate ions. Chemosphere 17, 1977-1984. 

These references are included. 
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General Comments 

Hydroxyl radicals:

Mailhot G., Astruc M. and Bolte M. 1999. Degradation of tributyltin chloride in water photoinduced by iron(III). Applied Organometallic Chem. 13, 53-61. 

Annex 1 

France Page 45: For the wavelength and wave number definitions, it seems that some symbols 

are forgotten in the brackets. 

All definitions are in line with those given in the IUPAC reference on page 42. 

Germany Page 43: Energy transfer: The first sentence is a single statement without any relation to 

the rest of the text. It is interesting but it can be cancelled. Page 44: Photodegradation: Replace “molecular weight” by molar mass. 

Photolysis: In the text of the guideline “photolysis” is often used. In most cases this does 

not fit to the definition of photolysis given in Annex 1. So, either “photoreaction or Photochemical reaction or phototransformation or a similar word” should be used instead of “photolysis” or the definition of photolysis in Annex should be altered. The latter surely would cause a lot of additional misunderstandings. 

These points are not adopted. These definitions are taken from the IUPAC document mentioned on page 42. 

Germany Photon flux: This quantity is frequently used in the text but is not given in Annex 1. 

Obviously, it is used in the sense of irradiance or spectral irradiance. If photon flux or irradiance is defined as the amount of photons incident on a given surface per time, its unit becomes mol m-2 s-1 or (more usual in photochemistry) mol cm-2 s-1.Thus the “units” 

photons cm-2 s-1 or photons cm-2 d-1 frequently used in the text as the unit of the photon 

flux are not correct. In some cases, “mol photons” or “Einstein” is used, this also has to be replaced by “mol”. Expressions like these obviously stem from those times before the SI quantity “amount of substance” with the unit of mol was introduced. To clarify what is meant: Photon is another word for expressions like “quantum of light” or “light particle” but it is no unit. We should clearly distinguish quantities, units and numbers and not mix them up to entities like “number of meters per second” when we are speaking about the velocity. The still wide spread “number of mols” is another example of such an obscure mixture of numbers and units. Quantum yield: The quantity “quantum yield (Φ) should carefully be defined. A straight 

forward definition could be: Φ = amount of transformed chemical / amount of photons absorbed by chemical 

The rest of the text in this paragraph is unclear, in part this is due to missing text and symbols. Spectral irradiance: The quantity power and the corresponding units W m-3 or 

W m-2 nm-1 are without use in photokinetics. In photokinetics we need to know the 

We assume that all the definitions in the IUPAC paper were discussed and agreed upon by photochemistry experts. It is therefore surprising and disturbing that they should not be applicable to this photolysis test guideline. 
ENV/JM/TG/RD(2006)1/REV 

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General Comments 

incident amount of photons per time and irradiated area, so the corresponding unit becomes mol m-2 s-1 or mol cm-2 s-1 nm-1. The analogue holds for all other quantities 

which are related to the irradiance. Irradiation: Irradiation is not defined in Annex 1. So irradiation should be replaced by 

irradiance throughout the text. Solar photon flux spectrum: This quantity is not defined in Annex 1. It should be 

replaced by spectral solar irradiance. 

Annex 3, Figure 1 

Japan In figure 1, the trigger from tier 1 to Tier 2/3 and from Tier 2 to Tier 3/4 are different 

from the text of this draft TG. They should be the same. 

Annex 3 is revised accordingly. 

USA(2) The term “Kd” is used in reference to the rate of direct photolysis. This term (with a 

capitalised K) is generally used in reference to a coefficient of adsorption in soil, and the use of it here seems inappropriate. Elsewhere in the document (and as generally done in scientific documents), a small “k” is used to represent the rate constant. Figure 1 should be amended to conform with the standard usages of scientific parameter notation. Similar corrections should be applied to Figure 2 in Annex 3, as the same type of mistake occurs under Tier 1. 

US position. Agree. 

This point should be covered by the changes made. 

USA(2) Figure 1 is incomplete with regard to the decision criteria/items listed under Tier 3. The 

trigger value and other decision criteria information provided on page 6 of the main text should be added so that the figure more completely describes the decision process. 

US position: Agree. If the tiered approach is kept, the triggers should be made 

consistent. 

This point should be covered by the changes made. 

CropLife/ECPA Typing error in Tier 2 box, line 2: approximate. This point is covered by the changes made. 

Annex 3, Figure 2 

Japan In Figure 2, the trigger from Tier 1 to Tier 2 is different from the text of this draft TG. 

They should be the same. 

This Figure will be revised accordingly. 

USA(2) Figure 2 in Annex 3 is incomplete with regard to decision criteria/items listed under Tier 

1. A definitive question that may be answered with either “yes” or “no” is needed so that the reader is guided to proceed to tier 2 or to quit. US position: Agree. If the tiered approach is kept, the triggers should be made 

consistent. 

This Figure will be revised accordingly. 

Annex 5 

Japan Under point 3 NW (Natural Water): Add “paddy fields” in the collection places. This point is adopted. 

UK These buffers do not cover the full range of environmental pH values (i.e. 5-9); see This point is adopted. 
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General Comments 

earlier comment on pH. 

USA(2) In the discussion of the preparation of test media in Annex 5, it states that the test water 

should be “filter-sterilised with a 0.2μm filter”. The unit appears to be incorrect as given, and should likely be a smaller unit such as “_m”. US position: Agree.

The proposed unit is not readable. 

CropLife/ECPA The buffers recommended do not cover the full range of environmental pH values (i.e. 

5-9); see earlier comments on paragraph 61 (page 12). Is filtration through 0.2 μm filters the only acceptable sterilisation method or can autoclaving and gamma-irradiation be used instead? 

The first point is adopted. We would not recommend autoclaving because this might change the nature of the water ingredients. 

The comments from the USA came from the following three sources: 

- (1) U.S. Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and Effects Division, Environmental Fate 

Tech Team, Washington, D.C. 20460 

- (2) Syracuse Research Corporation, 1215 Jefferson Davis Highway, Arlington, VA 22202 

- (3) American Crop Protection Association, 1156 15th St, NW, Washington, DC20005 
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ANNEX 4 

DRAFT DOCUMENT 

February 2004 

OECD GUIDELINES FOR THE TESTING OF CHEMICALS

PROPOSAL FOR A NEW GUIDELINE 

Phototransformation of Chemicals on Soil Surfaces

INTRODUCTION

1. 1. Man-made chemicals may reach soil directly via deliberate application (e.g. 

agrochemicals) and/or via indirect routes (e.g. via waste water → sewage sludge → soil or air → wet/dry 

deposition). For risk assessment of these chemicals, it is important to estimate their potential for 

transformation in soil and for distribution in and between the various environmental compartments (soil, 

water, air). 

2. 2. Chemicals can be transformed in soil by microbial, chemical and/or photochemical 

processes. The method described in this Test Guideline deals with the photochemical transformation (rate 

and route) of chemical substances on soil surfaces. It is based on existing guidelines (1) (2) (3) (4) (5). 

SCOPE AND RELEVANCE

3. When irradiating a test substance on a soil surface under the experimental conditions described in 

this Test Guideline, besides photochemical transformations (direct and indirect photolysis) also chemical

(hydrolysis, oxidation) and microbial transformations can occur, the latter in particular when “viable” 

(microbially active) soils are used. To assess the relevance of the latter two processes, non-irradiated dark 

controls are included in this test. The difference in transformation rates and routes observed between 

irradiated samples and dark controls can then be attributed to phototransformation processes on the soil 

surface. The methodology is neither designed to determine the quantum yield of a test substance on a solid 

surface nor to distinguish between direct and indirect photolytic reactions, but should allow to estimate

photolysis half-lives likely to occur on soil surfaces under outdoor conditions and also to isolate and 

identify phototransformation products.

PRINCIPLE OF THE TEST

3.4. Thin-layers of soil are treated with the test substance and irradiated by simulated sunlight (Xenon 

lamp) under controlled laboratory conditions or, as an option in sunny climatic regions, by natural sunlight. 

Simultaneously, not-irradiated dark controls are kept under identical conditions to distinguish between 

photochemical and other reactions. After appropriate time intervals, the soil thin-layers are extracted and 

analysed for the parent test substance and/or for major phototransformation products. Volatile products are 

collected by appropriate absorption devices and are also analysed for at time intervals. Using 14C-labelled 

material, phototransformation products can be identified and a mass balance, including the formation of 

soil bound (non-extractable) residues, can be established. 
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APPLICABILITY OF THE TEST

4.5. The method is applicable to all test substances (unlabelled and radiolabelled) for which an 

analytical method with sufficient accuracy and sensitivity is available. It is applicable to slightly volatile 

or non-volatile, water-soluble or water-insoluble compounds. The test should not be applied to chemicals 

which are highly volatile from soil (e.g. fumigants, organic solvents) and thus cannot be kept in soil under 

the experimental conditions of this test. 

INFORMATION ON THE TEST SUBSTANCE

6. Unlabelled or labelled test substances can be used to measure the rate of phototransformation. 

Labelled material is required for studying the pathway of phototransformation and for establishing a mass 

balance. 14C-labelling is recommended but other isotopes, such as 13C, 15N, 3

H, 32P, may also be useful. As 

far as possible, the label should be positioned in the most stable part(s) of the molecule4. The chemical and 

radiochemical purity of the test substance should be at least 95 %. 

7. 6. Before carrying out a phototransformation test on soil surfaces, the following information 

on the test substance should be available: 

(a) UV-VIS Absorption Spectra [OECD Guideline 105] (6);

 (a) (a) solubility in water [OECD Guideline 105] (6); 

 (b) solubility in organic solvents; 

 (c) vapour pressure [OECD Guideline 104] (6) and Henry’s Law constant; 

 (d) n-octanol/water partition coefficient [OECD Guidelines 107 and/or 117] (6); 

 (e) adsorption coefficient (kd, kf or KOC) [OECD Guideline 106 and/or Guideline 121] 

(6); 

(f) chemical stability in the dark (hydrolysis) [OECD Guideline 111] (6); 

(g) aerobic and anaerobic transformation in soil [OECD Guideline 307] (6); 

(h) phototransformation in water [OECD Guideline, in preparation](7). 

(i) dissociation constant (pKa) [OECD Guideline 112] (6).

The temperature at which these measurements were made should be reported. 

7.8. An appropriate analytical method of known accuracy, precision and sensitivity for the 

quantification and identification of the test substance and its phototransformation products in soil should be 

available. The analytical detection limit for the test substance and its phototransformation products (>10 % 

of the applied dose) should also be known (see paragraph 13 12). 

REFERENCE SUBSTANCES

8.9. Reference substances should be used for the characterisation and/or identification of 

phototransformation products by spectroscopic and chromatographic methods. 

 

4

 For example, if the test substance contains one ring, labelling on this ring is required; if the test substance contains 

two or more rings, separate studies may be needed to evaluate the fate of each labelled ring and to obtain suitable 

information on formation of phototransformation products. 
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DEFINITIONS AND UNITS

10. 9. See Annex 1. 

QUALITY CRITERIA

Recovery 

10.11. Extraction and analysis of, at least, duplicate soil samples immediately after the addition of the 

test substance gives a first indication of the repeatability of the analytical method and of the initial 

recoveries uniformity of the application procedure for the test substance. Recoveries for later stages of the 

experiments are given by the respective mass balances obtained with labelled material. Recoveries should 

range from 90 % to 110 % for labelled chemicals (5) and from 70 % to 110 % for non-labelled chemicals 

(3). 

Repeatability and sensitivity of analytical method

11.12. Repeatability of the analytical method (excluding the initial extraction efficiency) to quantify test 

substance and phototransformation products can be checked by duplicate analysis of the same extract of 

the soil, incubated long enough for formation of phototransformation products. 

13. 12. The limit of detection (LOD) of the analytical method for the test substance and for the 

phototransformation products should be at least 0.01 mg · kg-1 soil (as test substance) or 5 1 % of the 

applied dose which ever is lower. The limit of quantification (LOQ) should also be specified equal or less 

than 10% of the applied concentration.

Accuracy of transformation data

14. 13. Regression analysis of the concentrations of the test substance as a function of time gives 

the appropriate information on the reliability of the phototransformation curve and allows the calculation 

of the confidence limits for half-lives (in case of pseudo first order kinetics) or DT50 values and, if 

appropriate, DT90 values. 

DESCRIPTION OF THE TEST METHOD

Light source

15. 14. Natural sunlight or Simulation of artificial sunlight simulated with solar simulators under 

well-controlled laboratory conditions are is considered the optimal light sources. A characteristic of 

spectral power distribution for sunlight is shown in Annex 2 given elsewhere (8); it can also be obtained 

from ISO 10977 (8) or from GCSOLAR (9) as function of latitude and season. The cut-off of radiation of 

the light source used has to be 290 295 nm (10). Xenon lamps equipped with appropriate filters, such as 

the Hanau Suntest apparatus (see Annex 2) are recommended as irradiation source (5)(7)(8) because they 

simulate very closely natural sunlight in the 2905-800 nm region. (see Annex 2). Whereas mercury-metal 

halide arc lamps or equivalents can be used for other tests requiring lower levels of energy than xenon 

lamps, such lamps are less appropriate not suitable to evaluate the phototransformation of chemicals 

because they have less power than xenon lamps. For the assessment of phototransformation of chemicals, 
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177

generally higher level of irradiation power is required which can only be provided by xenon lamps for the 

time being. 

16. Since light in the wavelength range of 2905 to 400 nm is most relevant for phototransformation 

of chemicals in the environment (11)(13), the spectral energy distribution of the Xenon light source (i.e. 

the incident light available at the soil surface level) in this range is recorded using, for example, a portable 

spectroradiometer5, consisting of a holographic grating monochromator and a Teflon cosine diffuser in 

combination with glass fibre optics. Alternatively, the light intensity during a study performed with a 

Xenon lamp/filter system (e.g. Hanau Suntest) can be measured by a calibrated radiometer with sufficient 

accuracy or can be obtained from the manufacturers data. Typical light intensities (irradiance) between 300 

and 400 nm at different exposure positions are given in Annex 3. 

Conversion of laboratory irradiation time into days of natural summer sunlight

17. 16. In order to relate the rate of phototransformation caused by the Xenon light to the rate 

likely to occur in sunlight, it is necessary to compare the irradiance of the Xenon lamp with sunlight (13).

(9). For this purpose, the integral of light intensity between 290 5 to 400 nm is either measured at the 

various positions of the soil thin-layers in the test systems described in paragraphs 21-24 16 - 18 before 

and after the test or can be obtained from the data supplied by the manufacturer of the lamp. These values 

are compared to energy values of natural summer sunlight (midday) at latitude 30 to 50° N6 which can be 

obtained from reference (12) (10) or which can also be measured with the spectroradiometer (see Annex 

3). 

18. 17. Furthermore, it is assumed that the average daily sun radiation intensity is approx. 75 % of 

the maximum intensity over a 12-hour period (13) (10), whereas irradiation intensity of the Xenon lamp is 

constant over time. The equivalent days d of natural summer sunlight can be calculated by the following 

equation: 

0.75 12

h r d ⋅

⋅ =

where d = days of summer sunlight 

 h = hours of irradiation by the Xenon lamp 

 r = ratio of intensity (irradiance) of the Xenon radiation to that of summer sunlight 

 0.75 = correction for diurnal variation of natural sunlight 

 12 = conversion factor of hours to days.

 The above calculations are just one possible way for transferring the experimental light intensity to 

environmental irradiation conditions. Light intensity can also be calculated based on the following 

equations:

Light intensity (300-400 nm) (W/m2) · 6.07% / 100% = global light intensity (300-3000 nm) (W/m2)

Global light intensity (300-3000 nm) (W/m2) · 3600 sec = global light intensity in one hour experiment 

(300-3000 nm) (MJ/m2).

 

5

 For example, Model LI-1800, LICOR Ltd., Lincoln, NE 68504, USA can be used. This information is given for the 

convenience of readers. Equivalent instruments may be used if they can be shown to lead to the same result. 62 Intensities of sunlight depend on the geographical latitude in spring, autumn and winter, but vary little by latitude

during the are practically not influenced in summer. 
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178

This experimental light intensity can then be related to environmental light intensity per day which are 

available for different locations and seasons, e.g. in reference (14).

Another way is to compare the intensity of the Xenon lamp with tables of GCSOLAR instead of measured 

intensity. In this way, the days of irradiation are calculated to a “standard” and the results obtained at 

different times and locations can be compared. The tables of GCSOLAR give average daily irradiancies 

L(λ) in millieinstein per day per cm2 which can be compared to irradiance by using the following formula:

L(λ)[millieinstein/cm2·day] · 10-3 · 6.023 · 1023 [photons/einstein] · 1/86400 [day/sec] · 104 [cm2/m2] · h ·

c/λ · 10-9 [Watt/m2]

where h = Planck’s constant (6.63 · 10-34 J/Hz) and

 c = velocity of light (3.00 · 108 m/sec).

This calculation should be performed for each wavelength λ (in nm). After that, the values obtained 

can be summated over the region 300 - 400 nm and the result compared to the measured value of the

Xenon light source.

Incubation Test system

19. 18. The treated soil thin-layers (for preparation see paragraphs 27 – 28) are incubated and 

irradiated under temperature-controlled conditions in static or a flow-through systems which allow to trap 

any volatile products formed during the experiments. An example of a useful flow-through system is 

shown in Annex 4A (13). (9). Other test systems are described in references (15)(16)(17). (11) (12) (13).

20. 19. This test system consists of a stainless steel cooling tank containing the soil plates. This 

tank is sealed with a quartz lid, using teflon tape between and bound around the lid/tank interface, and 

placed beneath the Xenon lamp or exposed to sunlight. Cooling water at a pre-set temperature is pumped 

through the base of the tank to control the temperature of the irradiated soil-coated plates. The temperature 

is monitored in the soil during the experiments using an electronic thermologger7

. 

21. 20. In order to trap any volatile products formed during irradiation, air is drawn sucked 

through the tank at a certain flow rate (e.g. 10 – 12 ml/min) using a peristaltic pump. The incoming air is 

bubbled through traps containing 2 M potassium hydroxide solution and then through traps containing 

water to ensure that moist, carbon dioxide free air enters the system. The effluent air is bubbled through a 

series of traps containing, for example, 1 M sulphuric acid solution (to trap volatile basic compounds), 

methoxyethanol (to trap volatile organosoluble compounds) and ethanolamine (to trap 14CO2 or other 

acidic volatile compounds).

22. Alternatively, static systems consisting, for example, of individual quartz vessels (e.g. inner 

surface approx 10-12 cm2) equipped with traps containing soda lime and polyurethane foam may be used

(see Annex 4B).

 

7 For example, Grant Instruments Cambridge Ltd., Barrington, Cambridge, UK. This information is given for the 

convenience of readers. Equivalent instruments may be used if they can be shown to lead to the same result.
ENV/JM/TG/RD(2006)1/REV 

179

Laboratory equipment and chemicals

21.23. Standard laboratory equipment is required, in particular the following: 

- analytical instruments such as GLC, HPLC and TLC equipment, including the appropriate 

detection systems for analysing labelled or unlabelled substances or inverse isotope dilution 

method; 

- instruments for identification purposes (e.g. MS, GC-MS, HPLC-MS, NMR, etc.); 

- liquid scintillation counter for radiolabelled test substance; 

- oxidiser for combustion of labelled material; 

- extraction apparatus (for example, centrifuge tubes for cold extraction and Soxhlet apparatus 

for continuous extraction under reflux); 

- instrumentation for concentrating solutions and extracts (e.g. rotating evaporator) 

- water bath 

24. 22. Chemicals used include: organic solvents, analytical grade such as acetone, methanol, etc.; 

scintillation liquid; various absorption solutions (see paragraph 230). 

Soil

Soil selection 

25. 23. For phototransformation studies on soil surfaces, the use of one two soil types is

recommended. sufficient. One soil should be the same type as used in the aerobic soil transformation study. 

As these experiments require glass or metal plates to be covered with thin-layers of soil, only those soils 

can be used which can form thin and solid layers on a glass or metal surface. Generally, a silty loam or 

clay loam soil rather than a sandy soil should be selected (5). 

26. 24. The selected soil should be characterised, at least, for texture [according to FAO and 

USDA classification systems (18) (14)], pH, organic carbon content and water holding capacity. For 

determination of soil characteristics the methods recommended in references (19)(20)(21)(22)(23)(24) (15) 

(16) (17) (18) (19) can be used. 

Collection and storage of soil 

1. 25. The soil should be taken from the top layer (A-horizon) to a maximum depth of 20 cm. 

Remains of vegetation, macrofauna and stones should be removed. The soils are air-dried at room 

temperature (preferably between 20 - 25°C). Desaggregation should be performed with minimal force, so 

that the original texture of the soil will be changed as little as possible. The soils are sieved through a < 2 

mm sieve. Before use the soils can be stored at ambient temperature and kept air-dried as microbial 

activity is of little relevance when studying physico-chemical processes on the soil surface. No limit on 

storage time is recommended but soils stored for more than 3 years should be re-analysed prior to use with 

respect to their organic carbon content and pH.

2. When “viable” (microbially active) soils are required for conducting the experiments, the soils 

should be collected freshly from the field and should be handled cautiously as outlined in the OECD Test 

Guideline 307 (6) and also in references (25)(26)(27).

27. 26. Detailed information on the history of the field sites from where the test soil is collected 

should be available. Details include exact location [exactly defined by UTM (Universal Transversal 

Mercator-Projection/European Horizontal Datum) or geographical co-ordinates], vegetation cover, 
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180

treatments with crop protection chemicals, treatments with organic and inorganic fertilisers, additions of 

biological materials or accidental contaminations (25). (20).

Preparation of soil thin-layer plates

28. 27. Soil thin-layers are prepared by applying an aqueous slurry of soil to a glass or metal plate3 

8 in such a way that a soil layer of about 2 mm thickness is formed. Thereafter, the soil layers are allowed 

to air-dry to reach a soil moisture corresponding to a pF of 2.0 to 2.5. This is the same moisture level as 

also required in the aerobic soil transformation study. of about 75 % of field capacity (about 24 hours). 

24.

29. 28. An additional set of soil thin-layers is allowed to completely dry-out before test substance 

application. Theses samples are used to measure phototransformation on irradiated dry soil surfaces. 

Test substance application

30. 29. To apply the test substance to the soil thin-layers it should be dissolved in water (deionised 

or distilled) or in a suitable organic solvent. Each soil thin-layer is then evenly treated with the test 

substance by distributing small droplets (e.g. 2-3 μl) of the test substance solution (e.g. 40-50 μl total 

volume) over the whole soil surface by means of an appropriate syringe (e.g. Hamilton microliter syringe

or by a Hamilton dispenser unit). In case an organic solvent is used, it should not be a photosensitiser and 

should be evaporated from the soil thin-layers prior to the start of the irradiation process. As light 

penetration in soils is generally lower than 200 μm (28) it is important to apply the test substance in a very 

low volume of solvent to minimise its penetration into deeper soil layers.

31. 30. The amount of test substance applied to the soil thin-layers should correspond, for general 

chemicals, to the estimated amount normally reaching the soil, or for crop protection chemicals, to the 

maximum recommended use rate, and should be related to the surface area of the soil thin-layers used49

.

Test conditions

Test temperature 

32. 31. During the test, the treated soil thin-layers are kept in the incubation test system described 

in paragraphs 21-24 18 - 20 at a constant temperature of 20 ± 2°C. The temperature is monitored during 

the test by a thermocouple which is inserted in the slurry of one soil thin-layer before the slurry dries out.

Adequate temperature control is necessary to help maintain also soil moisture and comparable microbial 

activities within the light-exposed and dark controls.

 

3 8 An appropriate plate size is 5.5 cm × 3.5 cm. Parts of the soil thin-layers are scraped off at the edges of the plate, 

so that a soil area of 5 cm × 2.5 cm is left which corresponds to approx. 12.5 cm2. 4 9 The amount to be applied to the soil thin-layers can be calculated by the following formula: 

[ ] [ ][ ] [ ]

10 [ ] cm / ha

A kg / ha 10 g / kg F cm M g 8 2

9 2 ⋅ μ ⋅ μ =

where: M = amount applied on soil thin-layer [μg] 

 A = rate of application [kg · ha-1] 

 F = surface area of soil thin-layer [cm2

] 
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Moisture content 

33. 32. Soil thin-layers are kept either moist (at a pF of 2.0-2.5, see paragraph 325

) 75 % of the 

field capacity10) or air-dried during the irradiation period. Soil moisture is adjusted in the moist samples 

each time when samples are removed for analysis or at least every 48 hours by adding water to the soil 

thin-layers when necessary. Soil moisture should be measured and recorded. .

Dark controls 

34. 33. Additional moist and air-dried soil thin-layers are kept under identical conditions in the 

dark but not irradiated to distinguish between photochemical and other reactions (5). 

Test duration 

35. 34. Duration of exposure to sunlight (natural or artificial) should not exceed 30 environmental 

solar days or two half-lives of the test substance whichever is shorter. The soil layers should be incubated

in appropriate static or flow-through systems for time periods corresponding to the above limits. For 

example, All soil thin-layers are continuously incubated in the described test system for a period of 240 

hours (=10 days) which corresponds to approx. 30 days of natural midsummer sunlight (calculated by the 

equation given in paragraph 18 7 using intensity values of Annex 3). 

Performance of the test

36. After treatment with the test substance, the soil thin-layers are placed in an incubation 

system such as the ones illustrated in Annex 4 the stainless steel cooling tank illustrated in Figure 3. The

system tank is then sealed with the quartz lid as described in paragraph 17 and placed beneath the Xenon 

light source or exposed outside to natural sunlight. The soil thin-layers are then irradiated for appropriate 

time intervals under the test conditions described above (see paragraphs 36-39). 31 – 34). Irradiation in the 

laboratory may be carried out continuously or discontinuously simulating day and night cycles (e.g. 12 

hours each). The latter procedure would allow for compensation of soil moisture during the dark period 

and thus would not interfere with the irradiation process. 

36.37. At least, D duplicate soil thin-layers are removed at appropriate time intervals and the soil 

samples are extracted with appropriate solvents of different polarity and analysed for the test substance 

and/or phototransformation products. Also, absorption solutions are removed at the same time intervals 

and analysed for volatile products. Besides a soil sample taken directly after application (0-hour sample) at 

least 6 5 additional sampling points should be included. Time intervals should be chosen in such a way 

that pattern of decline of the test substance and patterns of formation and decline of phototransformation 

products can be established (e.g. 0, 17, 24, 41, 65, 137 and 240 hours). 

37.38. When using 14C-labelled test substance, non-extractable residues will be quantified by 

combustion and a mass balance will be calculated for each sampling interval. 

 

5 10 For determination of water holding capacities, see OECD Guideline 307. 
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DATA AND REPORTING

Treatment of results

38.39. The amounts of test substance, phototransformation products, volatile products and nonextractables should be given as % of applied initial dose for each sampling interval. Also a mass balance 

should be given in percentage of the applied initial dose for each sampling interval. A graphical 

presentation of the test substance percentages against time will allow an estimation of its 

phototransformation half-life or DT50. Major phototransformation products should be identified and their 

percentages should also be plotted against time to show their rates of formation and decline. A major 

phototransformation product is any product representing > 10 % of the applied dose at any time during the 

study. 

40. More accurate determinations of half-lives or DT50 values and, if appropriate, DT90 values should 

be obtained by applying appropriate kinetic model calculations. The half-life and DT50 values should be 

reported together with the description of the model used, the order of kinetics and the correlation 

coefficient (r2

). If appropriate, the calculations should also be applied to the major phototransformation 

products. Examples of appropriate models are described in references (29) (21) to (34). (24). 

34.

35.

Test report

40.41. The test report must include: 

Test substance: 

 - common name, chemical name, CAS number, chemical structure (indicating position of label 

when radiolabelled material is used) and relevant physical-chemical properties (see paragraph 

76); 

 - purity (impurities) of test substance; 

 - radiochemical purity of labelled chemical and specific activity (where appropriate). 

 Test soil: 

 - details of collection site; 

- date and procedure of soil sampling; 

- properties of soil, such as pH, organic carbon content, texture and water retention 

 characteristics; 

- length of soil storage and storage conditions 

 Xenon light source: 

- manufacturer and model; 

- filters and their purpose; 

- measured spectrum of the light source between 290 5 and 800 nm given by the manufacturer 

or measured at the beginning and the end of the test; 

 of the test;

-comparison of the spectrum of the light with that of natural sunlight; 

- intensity at sample level (Watt • m-2) and area irradiated. 
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183

 Test conditions: 

 - dates of the performance of the studies; 

 - amount of test substance applied; 

 - solvents used and method of application for the test substance; 

- area of soil thin-layers treated; 

- description of the incubation system used; 

- air flow rates (if applicable); 

- temperature of experimental set-up; 

- soil moisture content during incubation; 

- method(s) of extraction; 

- methods for quantification and identification of the test substance and major 

phototransformation products in soil and absorption solutions including LOD and LOQ; 

- number of replicates. 

 Test results: 

- tables of results expressed as % of applied dose for the soil thin-layers; 

- mass balance during and at the end of the studies; 

- quantification characterisation of non-extractable (bound) radioactivity or residues in soil; 

- quantification of volatile products; 

- plots of percentages in soil versus time for the test substance and, where appropriate, for the 

major phototransformation products; 

- half-life or DT50 and DT90 for the test substance and, where appropriate, for major 

phototransformation products including confidence limits; 

- an assessment of phototransformation kinetics for the test substance and, where appropriate, 

for major phototransformation products; 

- proposed pathway of phototransformation, where appropriate; 

- discussion and interpretation of results. 

LITERATURE

(1) US-Environmental Protection Agency (1982). Pesticide Assessment Guidelines, Subdivision N. 

Chemistry: Environmental Fate. 

(2) Agriculture Canada (1987). Environmental Chemistry and Fate. Guidelines for 

 registration of pesticides in Canada. 

(3) European Union (EU) (1995). Comission Directive 95/36/EC amending Council Directive 

91/414/EEC concerning the placing of plant protection products on the market. Annex I: Fate and 

Behaviour in the Environment. 

(4) Dutch Commission for Registration of Pesticides (1995). Application for registration 

of a pesticide. Section G: Behaviour of the product and its metabolites in soil, water and air. 

(5) SETAC (1995). Procedures for Assessing the Environmental Fate and Ecotoxicity of 

 Pesticides. Mark R. Lynch, Ed. 

(6) OECD (1993). Guidelines for the Testing of Chemicals. Paris. OECD (1994 – 2000): Addenda 

 6-11 to Guidelines for the Testing of Chemicals. 
ENV/JM/TG/RD(2006)1/REV 

184

(7) OECD (2000). Draft Guideline “Phototransformation of Chemicals in Water” – Direct and 

Indirect Photolysis. 

(8) ISO 10977 (1993). Photography – Processed photographic colour films and paper prints – 

Methods for measuring image stability.

(9) Zepp, R.G. and Cline, D.M. (1977). Rates of direct photolysis in aquatic environment. 

Environm.Science and Technology, 11, 359-366.

(10) OECD (1997). Guidance Document on Direct Phototransformation of Chemicals in Water.

 Series on Testing and Assessment No. 7.

(11) Miller, G.C. and Zepp, R.G. (1983). Extrapolating photolysis rates from the laboratory to the 

environment. Residue Reviews, 85, 89-110.

(129) Mill,T., Mabey, W.R., Bomberger, D.C., Chou, T.-W., Hendry, D.G. and Smith, J.H. 

(1982).Laboratory Protocols for Evaluation of the Fate of Organic Chemicals in Air and Water. 

U.S. Environmental Protection Agency; EPA 600/3-82-002. 

 

(130) Parker, S. and Leahey, J.P. (1988): Development of a method to investigate the photodegradation 

of pesticides. Brighton Crop Protection Conference – Pests and Diseases – 1988, 663 – 668.

(14) Wypych, G. (19..?). Handbook of Material Weathering. Second Edition, Chapter 4, 61-94.

(151) Burkhard, N. and Guth, J.A. (1979). Photolysis of organophosphorus insecticides on soil 

surfaces. Pestic. Sci. 10, 313 – 319. 

(162) Guth, J.A. (1980). The study of transformations. In Interactions between Herbicides and the Soil. 

R.J. Hance, Ed., Academic Press, 123 – 157. 

(173) Guth, J.A. (1981). Experimental approaches to studying the fate of pesticides in soil. Progress in 

Pesticide Biochemistry. D.H. Hutson, T.R. Roberts, Eds. J. Wiley & Sons. Vol. 1, 85 – 114. 

(184) Soil Texture Classification (US and FAO systems). Weed Science, 33, Suppl. I (1985) and Soil 

Sci. Soc. Amer. Proc. 26, 305 (1962). 

(195) Methods of Soil Analysis (1986). Part I, Physical and Mineralogical Methods (A. Klute, Ed.). 

Agronomy Series No. 9, 2nd Edition. 

(2016) Methods of Soil Analysis (1982). Part 2, Chemical and Microbiological Properties (A.L. Page, 

R.H. Miller and D.R. Kelney, Eds.). Agronomy Series No. 9, 2nd Edition. 

(2117) ISO Standard Compendium Environment (1994). Soil Quality – General aspects; chemical and 

physical methods of analysis; biological methods of analysis. First Edition. 

(2218) Mückenhausen, E. (1975). Die Bodenkunde und ihre geologischen, geomorphologischen, 

mineralogischen und petrologischen Grundlagen. DLG-Verlag, Frankfurt/Main. 
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185

(2319) Scheffer, F. and Schachtschabel, P. (1998). Lehrbuch der Bodenkunde. F. Enke Verlag, 

Stuttgart.

(24) Schlichting, E., Blume, H.-P. and Stahr, K. (1995). Bodenkundliches Praktikum: eine

 Einführung in pedologisches Arbeiten für Oekologen. 2nd Edition. Blackwell, Berlin-Oxford.

(250) OECD (1995). Final Report of the OECD Workshop on Selection of Soils/Sediments. Belgirate, 

Italy, 18-20 January 1995.

(26) ISO 10381-6 (1993). Soil Quality – Sampling – Part 6: Guidance on the collection, handling and 

storage of soil for the assessment of aerobic microbial processes in the laboratory.

(27) Anderson, J.P.E. (1987). Handling and storage of soils for pesticide experiments. In Pesticide 

Effects on Soil Microflora. L. Somerville, M.P. Greaves, Eds. Taylor & Francis, 45-60.

(28) Ciani, A. (2003). Photolysis processes in soils and other porous media. Diss. ETH No. 15306.

(291) Hamaker, J.W. (1976), The application of mathematical modelling to the soil persistence and 

accumulation of pesticides. Proc. BCPC Symposium: Persistence of Insecticides and Herbicides, 

181 – 199. 

(3022) Goring, C.A.I., Laskowski, D.A., Hamaker, J.W. and Meikle, R.W. (1975). Principles of 

pesticide degradation in soil. In Environmental Dynamics of Pesticides. R. Haque and V.H. 

Freed, Eds., 135 – 172. 

(3123) Timme, G. and Frehse, M. (1980). Statistical interpretation and graphic representation of the 

degradational behaviour of pesticide residues. I. Pflanzenschutz – Nachrichten Bayer 33, 47 – 

60. 

(3224) Timme, G., Frehse, H. and Laska, V. (1986). Statistical interpretation and graphic representation 

of the degradational behaviour of pesticide residues. II. Pflanzenschutz – Nachrichten Bayer 39, 

188 – 204.

(33) Hill, B.D. and Schaalje, G.B. (1985). A two compartment model for the dissipation of 

deltamethrin on soil. J. Agri. Food Chem., 33,1001-1006.

(34) Gustafson, D.I. and Holden, R.L. (1990). Nonlinear pesticide dissipation in soil: A new model 

based on spatial variability. Environm. Science and Technology, 24, 1032-1038.
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ANNEX 1

DEFINITIONS AND UNITS

Test substance: any substance, whether the parent compound or relevant transformation products. 

Transformation products: all substances resulting from biotic or abiotic transformation reactions of the 

test substance including CO2 and products that are in bound residues. 

Bound residues: “Bound residues” represent compounds in soil, plant or animal, which persist in the 

matrix in the form of the parent substance or its metabolite(s)/transformation product(s) after extraction. 

The extraction method must not substantially change the compounds themselves or the structure of the 

matrix. The nature of the bond can be clarified in part by matrix-altering extraction methods and 

sophisticated analytical techniques. To date, for example, covalent ionic and sorptive bonds, as well as 

entrapments, have been identified in this way. In general, the formation of bound residues reduces the 

bioaccessibility and the bioavailability significantly (1) [modified from IUPAC 1984 (2)]. 

Phototransformation: reactions occurring on soil thin-layers under the influence of light (> 290 nm). 

Soil is a mixture of mineral and organic chemical constituents, the latter containing compounds of high 

carbon and nitrogen content and of high molecular weights, animated by small (mostly micro-) organisms. 

Soil may be handled in two states: 

(a) undisturbed, as it has developed with time, in characteristic layers of a variety of soil types; 

(b) disturbed, as it is usually found in arable fields or as occurs when samples are taken by 

digging and used in this guideline (3). 

Xenon lamp: An intensive source of ultraviolet, visible and near-infrared light produced by electrical 

discharge in Xenon under high pressure. 

Intensity: Traditional term for photon flux, irradiance or radiant power (radiant flux). In terms of an 

object exposed to radiation. The term should now be used only for qualitative descriptions. 

Irradiance (E) [W • m-2]: The radiant flux or radiant power P of a all wavelengths incident on an 

infinitesimal element of surface containing the point under consideration divided by the area of the element 

(dP/dS; simplified expression: E = P/S when the radiant power is constant over the surface area 

considered). 

Radiant Power (P), [J • s-1 or W]: Power emitted, transferred or received as radiation. 

Half-Life, t0.5 is the time taken for 50 % transformation of a test substance when the transformation can be 

described by first-order kinetics; it is independent of the concentration. 

DT50 (Disappearance Time 50) is the time within which the concentration of the test substance is reduced 

by 50 %; it is different from the half-life t0.5 when transformation does not follow first order kinetics. 

DT90 (Disappearance Time 90) is the time within which the concentration of the test substance is reduced 

by 90 %.
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The limit of detection (LOD) is the concentration of a substance below which the identity of the substance 

cannot be distinguished from analytical artefacts.

The limit of quantification (LOQ) is the concentration of a substance below which the concentration cannot 

be determined with an acceptable accuracy.

LITERATURE REFFERRED IN THE ANNEX 1

(1) DFG: Pesticide Bound Residues in Soil. Wiley – VCH (1998). 

(2) T.R. Roberts: Non-extractable pesticide residues in soils and plants. Pure Appl. Chem. 56, 945 – 

956 (IUPAC 1984). 

(3) OECD Test Guideline 304 A: Inherent Biodegradability in Soil (adopted 12 May 1981). 
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ANNEX 2

LIGHT SOURCE

A. SCHEME OF THE HANAU SUNTEST APPARATUS (159) 
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189

ANNEX 2 (continued)

B. SPECTRUM OF UNFILTERED XENON ARC COMPARED TO SUNLIGHT 

 
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190

ANNEX 3

LIGHT INTENSITIES OF A XENON LAMP AND OF NATURAL SUMMER SUNLIGHT

A. XENON LAMP1

Position 

No. 

Irradiance E (300 – 400 nm) 

[W/m2

] 

r* 

 Start End Mean 

1 + 69.2 69.7 69.4 1.036 

2 78.0 78.9 78.4 1.171 

3 79.6 80.7 80.1 1.196 

4 ++ 79.2 80.8 80.0 1.194 

5 78.5 79.9 79.2 1.182 

6 75.3 76.4 75.9 1.132 

7 + 63.3 65.0 64.1 0.957 

Mean 74.7 ± 6.2 75.9 ±6.2 75.3 1.124 ± 0.09 

 75.3 ± 6.0 

(1) U. Morgenroth, personal communication.

+ edge positions 

++ central position 

* r = ratio between artificial and natural light 

B. NATURAL SUMMER SUNLIGHT

[Values taken from references (127) and (138)] 

Region latitude Irradiance E (300 – 400 nm) 

[W/m2

] 

Florida, USA 30°N 66.98 

Greensboro, NC, USA 40°N 68.98 

Basel, Switzerland 50°N 65.32 

Mean 30 – 50°N 67 ±3 % 
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ANNEX 4A

EXAMPLE OF A TEST SYSTEM FOR PHOTOTRANSFORMATION EXPERIMENTS ON SOIL 

SURFACES (138)

A. INCUBATION AND COOLING TANK 

B. SOIL PHOTOLYSIS PLATE 
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ANNEX 4B
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ANNEX 5 

February 2004 

Compilation of comments on the proposal for a new Guideline on Phototransformation of Chemicals on Soil Surfaces

and Secretariat’s responses. 

The following document comprises all comments received from Member Countries and industry on the proposal (January 2002) for a new OECD 

Test Guideline: Phototransformation of Chemicals on Soil Surfaces. This proposal was circulated for comments to OECD Member Countries until 26th April 2002. Comments were received from eight countries (Australia, Austria, France, Germany, Japan, The Netherlands, Switzerland and 

USA) and industry (CropLifeInternational/ECPA). In this annotated guideline document, the comments are identified by their origin (see above). Responses from the Secretariat are provided for all 

Member Countries comments explaining (i) which points were adopted and how the subsequent changes were made, and (ii) which points could not 

be adopted. 

Country Comment Secretariat’s Response 

General Comments 

Australia Only suggestion is that there may be more recent degradation model references that could 

be added (pursuant to para 39/refs 21-24), eg.: "Gustafson DI and Holden LR (1990). Nonlinear pesticide dissipation in soil: a new model based on spatial variability. Environmental Science and Technology 24(7): 1032-1038." "Hill BD and Schaalje GB (1985). A two-compartment model for the dissipation of deltamethrin on soil. Journal of Agricultural and Food Chemistry 33(5): 1001-1006." 

I note that the refs given on this are the classical ones - but there may be a preference to 

look for more real world models than the Timme ones (eg I think Hamaker/Goring & Hill/Schaalje are two compartment ones ... but I also note the latter seems to have an error in the presentation of the model [missing ( )'s] ). Maybe there is a better more recent review or book? Just as a thought, if not, maybe there is room for a review article with a mathematician? 

This point is adopted by including the proposed references. 

France In a general point of view, this proposal is very close to the SETAC guideline currently 

used for registration of Plant Protection Product according to the European directive 

71/414/EC. However, it gives rise to a few remarks (see specific comments). 

Agreed. 

The Netherlands In general, the documents of phototransformation of chemicals on soil surfaces and 

phototransformation in water should be uniform in the way of measuring light irradiances in a lab test and comparing the values obtained with “actual sunlight”. 

Agreed. 

Germany The German position reflects the view of several involved experts, expressing We certainly understand these concerns. We may not 
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Country Comment Secretariat’s Response 

General Comments 

considerable concern with regard to e.g. - lacking scientific understanding of underlying processes; - excessive influence of soil composition; - transferability to “real” out-door conditions; - degree of validation. At the present state, the guideline gives just a rough estimate of phototransformation potential at optimal conditions. 

completely understand all the processes occurring on/in soil under the influence of sunlight. On the other hand, such experiments required since many years for the registration of crop protection products can give useful information whether a chemical can be transformed on soil surfaces and which products can be formed under the influence of sunlight. 

Germany It has to be recognized that the processes on soil causing “phototransformation” are 

scientifically not very well understood, yet. When irradiating a test substance adsorbed to soil surface several processes occur in parallel: (I) direct photolysis, (ii) photosensitisation of the soil matrix and (iii) light induced radical formation (OH, NO3, O, Cl, 

etc.). The relative contribution to an overall breakdown of a test substance depends to a large extent on the chosen type(s) of soil(s). Given the broad variety of soil compositions, it is worthwhile to deliberate the idea of an artificial well-defined standard soil in order to enhance comparability. The measured degradation rates from this test are just relative values, depending not only on chosen type of soil but also on temperature, geometry of test system, etc. It is much doubted by some experts that different laboratories would yield rather identical results (i.e. within an acceptable range). For more confidentiality in the outcome of such test a laboratory inter-comparison exercise should be performed. It should also be considered to include a substance of well known photochemical behaviour as an internal standard for some sort of calibration. The method also suffers from another severe shortcoming: that of transferability to “real” outdoor conditions. From this aspect it would be much more desirable to determine quantum efficiencies (as for phototransformation in water and air). Clearly, such an approach is more difficult in the adsorbed state, e.g. requiring remission (diffuse reflectance-) spectroscopy instead of simple absorption spectroscopy for measuring the optical density and, hence, the number of photons adsorbed. However, it allows for calculation of transformation rates at any season or geographical latitude. The present guideline is suitable for summer only. The reason for this deficiency is that many chemicals absorb only weakly at wavelengths greater than 295 nm (often just a “tailing” from a maximum at shorter wavelengths). But it is just for that region around 300 nm that there are great differences in irradiance between winter and summer [Frank, R.; Klöpffer, W.: Spectral solar photon irradiance in Central Europe and the adjacent North Sea. 

Chemosphere 17 (1988) 985-994]. Calculations based on summer-time irradiance, including a relatively strong fraction of UV near 300 nm substantially overestimate 

We agree that the processes occurring on soil surfaces under the influence of light are scientifically not well understood. This is the reason why a group of specialists at the Swiss Federal Institute for Environmental Science and Technology (EAWAG) in Dübendorf has started a corresponding research program; their basic findings might be appropriate for inclusion in the present or later versions of this guideline or lead to the development of additional OECD Test Guidelines. For the time being we have to accept some lack of knowledge in this area. Nevertheless, experiments such as the one described in this draft guideline are carried out in a variety of laboratories since many years for registra-tion purposes and have given reproducible results. The requirement to use “viable” soils does not allow to work with an artificial well-defined standard soil. In addition, it is not the purpose of this guideline to measure quantum yields in the adsorbed state or to distinguish between direct and indirect photolysis reactions. The above mentioned research group has developed a procedure to determine quantum yields for chemicals on solid surfaces which could become the basis for a corresponding OECD Test Guideline in the future. We will include an additional paragraph explaining the SCOPE AND RELEVANCE of the test procedure. 
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Country Comment Secretariat’s Response 

General Comments 

phototransformation rates. As it is now, the guideline gives just a rough estimate of phototransformation potential at optimal conditions. This should be stated, e.g. in a chapter on scope and relevance. 

Switzerland One goal of the test is to determine the rate of phototransformation on soil surfaces under 

controlled conditions so that it can be related to environmental conditions. We believe that this goal cannot be achieved because: - many factors that will have a crucial impact on the outcome of the experiments are not sufficiently controlled  - in many cases the measured rate will be influenced or even dominated by diffusion kinetics and will not represent the pure phototransformation kinetic. 

It may well be that not all the factors which have a crucial impact on the outcome of this studies are sufficiently controlled but past experience over many years has shown that the procedure described can give useful and satisfactory results. Diffusion may be a relevant factor depending on the chemical’s properties but it is also a process occurring under out-door conditions. 

Switzerland Spectrum and intensity of the light 

The document contains detailed information about the light source that should be used so that the light spectrum and intensity can be compared to natural sun light. However, there is no discussion about how light penetrates into a soil and how this process is affected by the soil properties. The light intensity in the soil depends on the absorption and scattering coefficients (k and s, both are wavelength dependent) of the soil and can vary largely as a function of particle size, type of mineral, humidity and organic carbon content. Results of a specific phototransformation experiment are only comparable to others or to natural conditions if the k and s values of the used soil are reported. (Application of Kubelka-Munk equation will be required!). Up to now, k and s values for soils in the range 300 – 400 nm cannot be found in the literature so that the range of values that must be expected for natural soils is unknown. We plan to measure these values in our lab for a set of standard soils in the near future. 

As described in this draft guideline the test substance is not incorporated into the whole soil layers but is applied directly to the surface of the soil thin-layers in a very small volume of solvent to avoid significant penetration of the chemical into deeper zones of the layers by which light penetration could become a factor influencing the results. According to your results with 19 different soils, light penetration seems not to be significantly affected by the soil type in the most relevant wavelength range o 300-400 nm. 

Switzerland Sorption in organic material 

According to the guidelines the organic carbon content of the soil needs to be reported but there is no statement about the preferable organic carbon content and there is also no advice on how organic carbon content could affect the results and how this should be considered in the evaluation of the results. It seems likely that chemicals absorbed in organic matter are not susceptible to direct phototransformation because light probably 

cannot enter into the organic matter to a significant degree (apparently, experimental results concerning this topic do not yet exist.) 

There is certainly some experimental evidence that it is the dissolved (non-adsorbed) portion of a chemical in a soil which is susceptible for transformation. On the other hand, we know from experience with crop protection chemicals that strongly sorbed, not-extractable material can also be degraded in soil. This may also occur in a phototransformation experiment. 

Switzerland Thickness of the soil layer and diffusion kinetics 

The penetration depth of light in soils is in the order of 0.1 – 0.5 mm(1, 2). Hence, most of the 2 mm soil layer that is used as test system remains in the dark. Obviously, direct 

As already outlined above, the test substance is directly applied to the surface of the soil thin-layers to minimise problems with light penetration and with other interfering 
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General Comments 

phototransformation can only occur to the small fraction of the chemical that resides in the irradiated part of the soil. Most of the chemical applied to the soil must first diffuse from the deeper, dark parts of the soil to the upper, irradiated part of the soil before it can be degraded. This has to effects: 

1) if diffusive transport of the compound from the dark to the irradiated part of the 

soil is fast compared to the photolytic transformation then the dark part of the soil acts as an instantaneous reservoir. As a consequence the observed transformation rate for the compound in the complete soil layer is directly proportional to the thickness of the soil layer (3). 

2) If the diffusion kinetic that is responsible for the transport of the compound from 

the dark parts of the soil into the irradiated parts of the soil cannot supply the compound faster than it is transformed then the experimentally determined rate of phototransformation truly is a combination of both kinetics: diffusion and phototransformation. In many cases the overall kinetic can even be dominated by the diffusion process. The type of transformation curve that results from a diffusion controlled process under the given boundary conditions does not deviate significantly from a first order kinetic as it is expected for phototransformation. Hence, the form of the degradation curve will not allow to decide whether the observed process was diffusion controlled or not (3). 

The diffusion kinetics strongly depend on parameters that are not (or not sufficiently) standardized in the experimental procedure: surface area of the soil, type of minerals, organic carbon content and humidity. The kinetics also depends on the thickness of the soil layer. The guidelines do not suggest any method on how to gain a soil layer that would exactly match the desired 2 mm. This is not a trivial task and it must be expected that the true thickness of the used soil layer would rather range from 1 to 3 mm or more. All these factors have nothing to do with phototransfomation itself but can have a significant effect on the observed ‘transformation rate’ as we have shown in a detailed paper by experiments and theoretical considerations (3).

processes such as diffusion which could influence the results. Considering this, the true thickness of the complete layer may not be so relevant. 

Switzerland Humidity of the soil layer 

Soil humidity may exhibit a considerable influence on the results. However, this influence is not recognised in the document and the suggested control of soil humidity is insufficient. Humidity exhibits the following effects which have a direct or indirect impact on the observed experimental results: 

- Light penetration is very different in soils at field capacity or in air-dry condition. 

(Everybody knows that moist soils look darker than the same soils in air-dry condition). 

It is one of the goals of this test guideline to determine the influence of soil moisture on the transformation rates in the irradiated samples and dark controls. But it is certainly not a trivial task to maintain a constant soil moisture during the study. The procedure described in this draft guideline tries to minimise moisture losses from the soil thin-layers in two ways: 

- by ventilation of the incubation system by moistened 
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General Comments 

- At field-capacity the chemical will mostly be sorbed inside the organic matter 

(and therefore not susceptible to the light). At air-dry conditions the chemical will partly or mostly be adsorbed to the mineral surfaces where it is susceptible to light (4). 

- The diffusion kinetic (which is part of the observed degradation rate, see above) 

of a chemical in the soil is inversely proportional to its sorbed fraction and therefore also depends on humidity. Diffusion also depends on whether the pore space is filled with water or air.

air and 

- by adding water to the layers, whenever necessary. 

Switzerland Control of Soil Humidity. 

Concerning the humidity of the soil the guideline suggests that the experiments are conducted at “about 75 % of field capacity”. It is further suggested that this condition is reached if the 2 mm soil layer are allowed to air-dry for about 24 hours. 75 % of field capacity correspond to a soil that is in equilibrium with 99% relative humidity in air. 

However, a soil layer of 2 mm that is allowed to air-dry for 24 hours will usually reach an equilibrium with the laboratory air which most likely is in the range of 30 – 80% rh. This difference has a huge impact on the sorbed state of the chemical and on its diffusion kinetics as well as on light penetration (see above). 

In point 32. One reads: “Soil moisture is adjusted in the moist samples each time when samples are removed for analysis.” However, there is no advice given how this can be performed so that the desired 75% of field capacity will be reached. From our experience we conclude that this goal is impossible to reach. Large variations in the experimental conditions during the course of the experiment and between different laboratories will be the consequence. Most importantly, rewetting of the soil can often lead to an instantaneous loss of major amounts of the chemical because molecules that are adsorbed on the mineral surface in the air-dry soil are quantitatively displaced by water when the soil layer becomes wetted. We have observed complete loss of compound in our experiments with kaolinite layers (3). Similar effects have been observed in the field: No suggestions are supplied in the guidelines about how to handle the air-dry soil layers. However, adsorption of all organic compounds to air dry minerals depends exponentially on relative humidity (6) (see Fig.2). Hence, it matters whether these air-dry soil layers are kept at 40 or at 80 % rh. The guideline suggests that the soil moisture content during the incubation is to be reported in the test report. However, it will not be possible to correctly record the soil 

It is common practice in many laboratories to prepare soil samples with a certain level of soil moisture, either as pFvalue, as % of field capacity or as a percentage of the maximum water holding capacity, for conducting aerobic transformation studies. For details how to determine these moisture the guideline refers to OECD Test Guideline 307. See also previous response. 
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General Comments 

moisture content during the course of the experiment. Even if this could be done it would be of little value for the evaluation of the results because the possible ways of how humidity can affect the results are much too complex to be easily understood. 

Switzerland CONCLUSION

Within the wide limits given by the guideline the possible experimental conditions can vary so much that one can find degradation rates for a given chemical that might vary largely. Such results are impossible to evaluate and therefore must no be taken as a basis for a risk assessment.

See previous responses on the above German comments. 

Figures provided by the Swiss expert 

Time (h) 

Evaporative flux from the soil surface 
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0 10 0

2 10 -54 10 -56 10 -58 10 -51 10 -4

1.2 10 -4

0 20 40 60 80 100

% relative humidity

 Kads= (m

3/m2

)

quartz

Fe

2

O

3

CaCO

3

Al

2

O

3

adsorbed concentration

concentration in the gas phase

Fig. 2: Air/surface-adsorption constants of 1,2-dichlorobenzene on various minerals as a function of relative humidity (6). 

References referred to the Swiss comments 1. Miller, G.C., et al. Chemosphere 1989, 18, 1265-1274. 

2. Helmig, D., J. Müller, and W. Klein Chemosphere 1989, 19, 1399-1412. 3. Balmer, M., K.-U. Goss, and R.P. Schwarzenbach Environ. Sci. Technol. 2000, 34, 1240-1245. 

4. Goss, K.-U. and R.P. Schwarzenbach Environ. Sci. Technol. 1999, 33, 4073-4078. 

5. Majewski, M., et al. Environ. Sci. Technol. 1993, 27, 121-128. 

6. Goss, K.-U. and S.J. Eisenreich Environ. Sci. Technol. 1996, 30, 2135-2142.
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General Comments 

Croplife/ECPA In general, we support the guideline as proposed with the additions of the comments 

listed below. We would greatly appreciate if these comments could be introduced and supported in future discussions of this draft guideline.

Croplife/ECPA The methodology described in this draft guideline is typical of what is currently viewed 

as standard procedure for conducting phototransformation tests on soil surfaces as it considers all available national and international test guidelines. 

Agreed. 

ECPA member 

company 

Laboratories may have different technical equipment. Therefore it is necessary, that common guidelines should be more general in points, where different technical equipment can be used. This should be considered especially according to requirements, 

which do not have any influence on the essential outcomes of the study (examples: flowthrough versus biometer flask test systems, measurement of light intensity, etc.). Guidelines should not introduce further requirements which do not provide new information needed for risk assessment of fate and behaviour of a test substance in the environment, and which are not pointed out in the valid existing guidelines (example: data on photolysis on a dry soil surface may be nice to have, but there the reactions are much more slowly and, therefore, are not at all further relevant for the environment). 

This point should be covered by the changes made in paragraphs 15 and 18. As the surface of a field soil quickly becomes dry during sunny periods, the rate on a dry soil surface certainly has some environmental relevance. Together with the rates obtained on moist soil, it gives to some extent the range for the photolysis rate of a particular chemical on the soil surface. 

Austria “...may reach soil directly via deliberate application (e.g. agrochemicals) and/or via 

indirect routes...” Man-made chemicals (agrochemicals as well as biocides) may reach soil by direct application. However, in addition indirect routes can also contribute to the overall exposure. 

This point is adopted. 

Germany Paragraph 1 is from introduction to adsorption on soil. It should be replaced by a more 

specific justification on why it is useful to do such a test and what kind of information one can yield beyond from tests in aquatic solution. (photocatalytic effects, semiconductors in soil) 

This point should be covered in the new paragraph SCOPE AND RELEVANCE. 

Paragraph 2 

Germany Emphasis should be laid not just on the rate of degradation but also on the route of 

degradation. Products may be different from those of bio-degradation. 

This point is adopted. 

Paragraph 3 

Germany Insert: As far as possible, the label should be positioned either in the most stable part(s) 

of the molecule or in the effective functional moiety(es), e.g. for bioactive substances. 

The purity of the test substance. 

This point is sufficiently covered in paragraph 5 and footnote 1. 

US(1) Agree to the use of Xenon lamps to replace natural sunlight. Spectrum and intensity Natural sunlight is included as an optional irradiation source 
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source should be submitted before and after experiment at least. This will allow assessment of difference from natural sunlight. Spectrum of chemical should also be submitted to determine what wavelengths it absorbs. US position: The U.S. partially agrees. If a Xenon lamp is used, the irradiation intensity 

to which the soil thin layers are exposed should be measured at least before and after the study. However, this comment is unnecessary because the determination of the irradiation intensity of the Xenon lamp is adequately addressed later in paragraphs 15 and 16 of the guideline. The U.S. does not agree that the use of a xenon lamp (instead of sunlight) be mandated. At the very least, the use of sunlight should remain as an option. (See reviewer #2: para 14 comment on Xenon lamp versus sunlight.)

for certain climatic regions. 

Paragraph 4 Paragraph 5 

US(1) (EFED noted that 3H was among isotopes identified in the guideline as potentially useful 

for radiolabeling.) Use of tritium should not be allowed for radiolabeling because of potential proton exchange with media. US position: The U.S. agrees. 

This point is adopted. 

US(2) There should be a distinction made between chemical and radiochemical purity. For 

labeled compounds, indicate the minimal acceptable chemical and radiochemical purity. 

For unlabeled compounds, indicate the minimal acceptable chemical purity. US position: The U.S. agrees. There is a major difference between chemical and 

radiochemical purity. 

This point is adopted. 

Croplife/ECPA This paragraph states that “as far as possible, the label should be positioned in the most 

stable part(s) of the molecule. For example, if the test substance contains one ring, labelling on this ring is required; if the test substance contains two or more rings, separate studies may be needed to evaluate the fate of each labelled ring and to obtain suitable information on formation of phototransformation products.” We think that the location of the label should be left to the discretion of the study director. While a ring is often the choice location in most cases, there may be occasions when equally valuable data can be generated by the use of labels in locations other than rings. Of course, the degradation of the molecule would need to demonstrate that the location of the label has indeed provided all potential degradates of concern. Therefore, the wording “labelling on this ring is required” should be changed and the choice of the label location should not be restricted to rings only. 

It is always left to the discretion of the study director and/or the synthesis chemist to decide where the label can be positioned in a particular molecule by the available synthesis methods. It is considered as recommendation to position the label, whenever possible, in the most stable part(s) of a molecule. The text in footnote 1 is given as additional explanation and just mentions ring-labelling as example but not as a mandatory requirement. 

Paragraph 6 

Austria Addition suggested: “(I) pKa if a molecule is liable ionisation (e.g. protonation) [OECD 

Guideline 112]” 

This point is adopted. 
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Reason: Degradation in moist samples is also investigated. 

Germany Replace: Before carrying out a phototransformation test on soil surfaces, the following 

information on the test substance should be availablemay be helpful.

 Insert a foodnote for (a): Note: absorption behaviour of test substances in 

aqueous solution (homogenous phase) may differ from adsorption behaviour on surfaces (heterogenous phase) 

The formulation used in the first sentence of this paragraph is in agreement with that used in other environmental fate test guidelines (e.g. 307, 308, 309). The proposed footnote is not needed because Point (a) is deleted based on a comment from the Netherlands. 

The Netherlands (h) already required UV VIS absorbence spectra. Extra UV VIS spectra therefore do not 

provide extra information. Anyway, the use of UV-VIS spectra can be disputed for phototransformation of chemicals on soil surfaces, since indirect photolytic degradation 

cannot be excluded. 

This point is adopted. 

US(2) Indicating that the temperatures at which each study was conducted should be reported 

implies that the results of each study should be reported. Therefore in the first sentence, change “should be available” to “should be available and reported”. 

US position: The U.S. agrees. 

The information listed in paragraph 6 may be available and useful before being finally reported. This point is therefore not adopted. 

US(3) This seems like a long list of physical/fate properties that “should” (I assume that means 

must be) available before the test can be run. If this is followed, the soil phototransformation test could only be run after all of these other tests. Some of the list make sense (e.g., phototransformation in water) but others don’t (e.g., anaerobic transformation in soil). Also, most of the physical properties could be estimated accurately enough to run the test. I don’t see why this test has to be run as the last in a sequence of tests. Many of these could be run concurrently. US position: The U.S. agrees. 

All this information may be helpful before starting a test on phototransformation on soil surfaces but it is not required that all this must be available at that time. None of these tests has to be conducted in a certain sequence; they all can be carried out concurrently as far as possible. The physical-chemical data (a-d and i) will certainly be available first. 

Croplife/ECPA Only the solubility in water and solvents and the vapour pressure are truly needed to 

conduct this test. Soil metabolism information may be nice additional information, but this study can be conducted concurrently with the soil metabolism study. Requiring all this other information prior to initiation of the soil photolysis study can delay the initiation of the soil photolysis study while this other information is being generated. 

See response above. 

ECPA member 

company 

the following information on the test substance should be available: 

this sentence should be “The following information may be helpful (but is not essential and may be not available before commencement of the study):” 

See response above. 

Paragraph 7 

Austria Insertion: “The analytical detection limit for the test substance and its 

phototransformation products (≥10 % of the applied dose or lower where relevant) ...” 

In the current discussion of the risk assessment of metabolites in connection with Directive 91/414/EEC the 10 % trigger for characterisation and identification of degradation products is not applied automatically. Degradation products, which fall within the definition of “relevant metabolites” have to be evaluated as the active 

This point should be covered by the changes made. 
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substance and therefore all relevant endpoints have to be obtained, even if their concentration in degradation studies is below 10%. 

Germany Replace and Delete: The analytical detection determination limit for the test substance 

and its phototransformation products (>10 % of the applied dose) should also be known 

Comment: This is no information on the test substance itself (cf. heading). This item is also mentioned in paragraph 11 and 12. 

This information is required under the same heading also in other environmental fate test guidelines (e.g. 307, 308). 

US(2) As currently written, the last sentence of paragraph 7 could be misinterpreted to mean 

that the detection limit for the test substance and for its phototransformation products is > 10% of the applied dose. Therefore, the last sentence should be replaced by “The 

analytical detection limit(s) for the parent and any phototransformation products that could possibly represent greater than 10% of applied at any time during the study should be known (see paragraph 12).” US position: The U.S. agrees. 

This point should be covered by the changes made. 

ECPA member 

company 

This paragraph provides no information on the test substance itself. This item is also mentioned in paragraph 11 and 12. 

See response above on German comment. 

Paragraph 8 Paragraph 9 

Paragraph 10 

Japan(1) When the amount of the test substance applied to soil is small, in general, it is difficult to 

measure the transformation rate of the test substance accurately and the mass balance is low because the recovery of the test substance from soil is difficult. Therefore, the 

recovery should be taken into account when the amount of the application is nominated. 

It may be difficult to always recover the test substance when small amounts are applied to soil. However, the recovery ranges given here agree with those defined also in other guidelines for similar low rates (e.g. 307, 308). 

US(2) A clearer distinction should be made between recovery (accuracy) of the soil analytical 

method for the parent and degradates, and total recovery of radiolabeled material (mass balance). That can be done by deleting the 2nd sentence (which refers to mass balance) 

and inserting an additional paragraph (with a new #) that separately addresses the total recovery of radiolabeled material (mass balance). US position: The U.S. agrees. 

This point should be covered by the changes made. 

Paragraph 11 

Germany Duplicate analysis is not sufficient for repeatability of an analytical method. This 

involves also accuracy and sensitivity of the method. 

Duplicate analysis was found sufficient for other fate test guidelines (e.g. 307, 308). 

US(2) Give an example of a quantitative measure of repeatability (precision) of the soil 

analytical method such as the relative percent deviation. US position: The U.S. agrees. 

As this strongly depends on the analysis method available for a chemical it seems rather difficult to define a generally acceptable relative deviation. 
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Paragraph 12 

Germany Replace: The limit of detection determination (LOD) of the analytical method for the 

test substance and for the phototransformation products should be at least 0.01 mg · kg-1

soil (as test substance) or 1 % of the applied dose which ever is lower. The limit of detection quantification should also be specified. 

This point should be covered by the changes made. 

The Netherlands LOD<0.01 mg/kg soil. How to specify the limit of quantification (LOQ)? Please give 

definitions of LOD and LOQ, since different definitions are used. 

This point is adopted by including the definitions used in guideline 309. 

Croplife/ECPA Why is a LOD of 1% required? 5% of applied would be more reasonable. This point is adopted. 

Paragraph 13 

Austria Deletion of the bracket term or correction suggested: 

1st alternative: “... the confidence limits for half-lives (in case of pseudo-first-order 

kinetics)” 2nd alternative: “...(in case of first-order and pseudo-first-order kinetics) ...” 

In Annex I of the Guideline the term half-life is defined as time taken for 50 % transformation of a test substance when the transformation can be described by firstorder kinetics. Therefore the definition should be precise and consistent throughout the document. So either “pseudo” or add “first-order” should be deleted in the text and updated with the definition of half-life in the Annex. Another possibility would be to delete the bracket term, which refers to our understanding to the definition of half-life. 

This point is adopted. 

Germany linear regression of log concentration versus time => R2 is the common term. It may also be non-linear regression. 

US (EFED noted that the guideline “allows for” the determination of confidence levels.) 

Require calculation of confidence limits on half-life, to determine the statistical validity of the results. US position: The U.S. agrees. Although this is a somewhat subtle point, the current 

statement in the guideline does (to some degree) imply that the determination of confidence limits is optional. The statement should be strengthened to indicate that the determination of confidence limits should be performed. 

Confidence limits are mentioned in paragraph 40 under test results and thus are clearly required. 

US Replace “Accuracy of transformation data” with “Confidence limits for regression 

derived parameters”. US position: The U.S. agrees that the latter title more accurately reflects the contents of 

the paragraph than the former.

We would like to keep the present sub-title because it covers more than just” confidence limits”. 

Croplife/ECPA While fitting half-lives, confidence intervals are not generally calculated. The goodness 

of fit is described by r2. There are some EU guidance documents where it is stated that r2 should be >0.7 for an acceptable DT50. As our current tools which we use for fitting do not deliver confidence intervals, this requirement should not be obligatory or removed from the guideline. 

This requirement for confidence limits is in line with other fate test guidelines (e.g. 307, 308). 
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ECPA member 

company 

Linear regression of log concentration versus time => R2 is the common term Non-linear regression may also be used. 

Paragraph 14 

France Paragraphs 14, 15 & 16. The lower cut-off value for light wavelength is proposed to be 

295 nm but a value of 290 nm is stated in the scheme of the Hanau Suntest apparatus (Annex 2). The SETAC guideline requires a filter removing wavelengths below 290 nm. We support this last recommandation because it takes account of presence of UVB even if they are in low proportion. 

This point is adopted. 

Germany Delete:...(5)(8) because they simulate very closely natural sunlight in the 295-800 nm 

region (see Annex 2). Comment: Annex 2 (B) shows that the spectra are not identical is the sensitive region around 290 nm. It may be that the Xenon-arc spectrum in B is measured without filter, but in that case this should be mentioned or, better, the Xespectrum with filter should be presented. If the spectrum in Annex 2, B is correct, i.e. measured with filter, the method is bound to produce much too high degradation rates.  Replace: Whereas mercury-metal halide arc lamps or equivalents can be used for other tests requiring lower level of energy than xenon lamps, such lamps are less 

appropriate not suitable to evaluate the phototransformation of chemicals because they 

have less power than xenon lamps. Comment: This type of source should not be strictly excluded just for reasons of power.  Comment: Reference (5) is deemed not helpful here. 

These points should be covered by the changes made. 

Japan(2) The cut-off wavelength of a light source should be harmonized between Para14 and 

ANNEX2. 

This point is adopted. 

The Netherlands ISO 10977 (1993) is used as characteristic of spectral power distribution for sunlight. In 

the OECD and US EPA guidelines for phototransformation in water, tables generated by a computer program (GC SOLAR) are used as characteristic of spectra powers (as function of latitude and season). Why not use these tables? They give a daily average of the irradiance reaching the surface of the earth. 

This point is adopted. 

US(2) The option of using sunlight (or some rationale for not using sunlight) should be 

provided. In fact, arguments have also been made for preferring the use of sunlight in cases (such as phototransformations on soil) where indirect photolysis generally predominates. The argument is based on the difficulty in accurately converting artificial irradiation durations to sunlight irradiation duration equivalents for indirect photolysis processes. For indirect photolysis, the conversion has to be based (as in this proposed guideline) on equating total photon/energy flux across some wavelengths that may not even be involved in the process. The conversion is more accurate for direct photolysis because it can be based on equating total photons/energy absorbed by the test substance. 

This point is adopted. 
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US position: The U.S. partially agrees. Although the use of sunlight should be provided 

as an option, its use should not be mandated over the use of the xenon lamp (as possibly implied by the comment). There are pros and cons for both light sources. 

US(3) “Whereas mercury-metal halide arc lamps or equivalent can be used for other tests 

requiring lower level of energy than xenon lamps [shouldn’t this be lower wavelength – 

mercury arc lamps are at 254 nm which is higher energy not lower], such lamps are not suitable to evaluate the photransformation [should be phototransformation] of chemicals 

because they have less power [same as above – lower wavelength or higher energy] than 

xenon lamps.” US position: The U.S. agrees. The guideline should refer to a mercury lamp as a lower 

wavelength/higher energy lamp than a xenon lamp. In addition, the wording should be changed to more explicitly emphasize that while a xenon lamp with appropriate filters can simulate multiple wavelength solar irradiation above the 295 nm solar irradiation cutoff at the earth’s surface, the 254 nm mercury lamp cannot. 

This paragraph is discussing the use of lamps equipped with appropriate filters to exclude short-wave UV light below 290 nm. Thus this point seems to be of little relevance. 

Paragraph 15 

Austria Footnote: Deletion of footnote 2 is strongly suggested for the following reason: The 

citation of a distinct spectroradiometer model of a company should be avoided since this Guideline should not contribute to the advertisement of a specific instrument. Nevertheless a description of such an instrument as the current standard which has to be met when following this test procedure should be given without referring to a specific instrument model including the producer company. Addition of a reference suggested: “Since light in the wavelength range of 295 to 400 nm is most relevant for phototransformation of chemicals in the environment, ...” We would appreciate, if this statement is connected to one or more references. 

These points are adopted. 

Germany Light intensities for a too broad wavelength range are not appropriate. For most test 

substances and organic matter of soil the region around 300 nm is of particular importance (cf. General Comments). For typical Xenon light sources this narrow section is just a small portion of intensity and relative variations in this region are not adequately reflected by total intensity. At least an optical filter of measured absorption characteristics should be employed. So, complement text: In case of Xenon lamp and filter (system ®Suntest, Hanau) a 

characteristic spectrum is available by the manufacturer. Additionally, it is generally accepted that the quantum yield of phototransformation is not dependent on the wavelength within one electronic band. Therefore, it is not necessary to measure the energy distribution by a monochromator within this study. The light intensity during a study performed in a Xenon lamp/filter system (e.g. ®Suntest, Hanau) can be measured 

This point should be covered by the changes made. 
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by a calibrated radiometer with sufficient accuracy. 

Japan(2) “using ... glass fiber optics.” Should be deleted, because it is not necessary to limit an 

optical spectrum distribution measurement equipment to the specific equipment. 

This point should be covered by the changes made. 

ECPA member 

company 

In case of Xenon lamp and filter (system ®Suntest, Hanau) a characteristic spectrum is 

available by the manufacturer. Additionally, it is generally accepted that the quantum yield of phototransformation is not depended on the wavelength within one electronic band. Therefore, it is not necessary to measure the energy distribution by a monochromator within this study. The light intensity during a study performed in a Xenon lamp/filter system (e.g. ®Suntest, Hanau) can be measured by a calibrated 

radiometer with sufficient accuracy. 

See above response on German comment. 

Paragraph 16 

Germany Replace in 4th line: ... described in paragraph 16 – 18 18 – 20 before and after ... . 

Comment: the calculations given here apply to summer midday conditions. This is not a worst case or even an annual average !!

The first point is adopted. With regard to the second point, such calculations can be done also for other periods of the year (e. g. spring, autmn,etc.). 

The Netherlands The tables mentioned under (3) can be used to compare the irradiance of the Xenon lamp 

with sunlight. It can be done by simply summating the values given in these tables (at appropriate latitude and season) over 300 (or 295 nm?) until 400 nm. In this way, it is not necessary to have a spectroradiometer measuring the irradiance. The irradiance measured by the Xenocal 300-400 nm light irradiance meter should be sufficient, under the constraints that the Xenon radiant power as function of wavelength is given by the manufacturer. 

These points should be covered by the changes made. 

US(3) footnote #3: “Intensities of sunlight depend on the geographical latitude in spring, 

autumn and winter, but are practically not influenced in summer.” Suggest different wording – “, but the intensities vary little by latitude during the summer.” 

US position: The U.S. agrees with the suggested change in wording.

This point is adopted. 

Paragraph 17 

Germany The given calculations given here are just one way for transferring the experimental light 

intensity to environmental irradiation conditions. There are others, too. Light intensity can also be calculated based on the following equation: Light intensity (W/m2) (300-400 nm) * 6.07% /100% = Global light intensity (300- 3000 

nm) (W/m2) 

Global light intensity (300- 3000 nm) (W/m2) 3600 sec = Global light intensity in one hour in experiment (300- 3000 nm) (MJ/m2) This experimental light intensity can then be related to environmental light intensities per day, which are available for different locations and different seasons e.g. in George Wypych, 2nd edition of HANDBOOK of MATERIAL WEATHERING, Chapter 

This point is adopted in an additional paragraph 19. For the inclusion of the reference, we would greatly appreciate if the commenter could supply number and year of the edition. 
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4, Pages 61 –94 ISBN 1-895198-12-7 ChemTech Publishing / Canada 

The Netherlands It is better to compare the intensity of the Xenon lamp with tables of GC SOLAR instead 

of a measured intensity. In this way, the days of irradiation are calculated to “a standard” and the results obtained at different times or from other labs can be compared. In a “self measured” intensity is used, it is more difficult to compare the results of different studies. Further, the formula gives other results than the tables of GC SOLAR. Maybe this is due to the assumption that 75 % of the daily maximum intensity gives the average intensity over a 12-hour period. The formula of 17 underestimates the incubation period, compared to tabulated values of GC SOLAR. The tables of GC SOLAR give average daily irradiances (m Einstein per day per cm2), which can be compared to irradiance in 

Watt/m2 by using of the following formula: 

  × ×  ×

  ×  × ×  ×

−

−

2 9 2

2

23 3

2

Watt

m 10 *

cm

m

10000

day

sec 86400

1 photons

einstein

10 * 023 . 6 10

day

mEinsteins

cm

) (

λ

λ

c h

L

h = constant of Planck (6.63*10-34 J/Hz) 

c = speed of light (3.00*108 m/s). (Values of constants are rounded; non-rounded values 

should be used) This calculation should be performed for each wavelength (λ, in nm). After that, the 

values obtained can be summated over the region 300 – 400 nm and the result obtained compared to the measured value of Xenon light source. A spreadsheet is very useful to perform these calculations and the comparison with the irradiance of the Xenon light source. 

This point is adopted in an additional paragraph 20. 

US(2) The equation needs to be slightly modified in one of two alternate ways to be more 

accurate. To keep 12 (hrs/day) in the denominator, the r in the numerator needs to reflect the ratio of the Xenon lamp to early Spring (rather than mid-Summer) solar irradiation. Alternately, to keep r as the ratio of the Xenon lamp to mid-Summer solar irradiation, the 12 in the denominator needs to be increased to reflect longer than 12 hours of sunlight/day in mid-Summer. In addition to modifying the equation as recommended in the previous comment, it 

The equation (including the various factors) given in this paragraph is just an example how such estimations can be carried out. They can always be adapted to other environmental conditions. Also the equation can always be rearranged to cover other needs. 
ENV/JM/TG/RD(2006)1/REV 

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should be rearranged to give what the equation is being used to calculate (the number of hours a sample needs to be continuously irradiated with a xenon lamp to give a total irradiation approximately equivalent to what the sample would receive if it was exposed to the sun by being placed outside for 30 days) US position: The U.S. agrees. 

ECPA member 

company 

The given calculations are just one example how to transfer the experimental light intensity to environmental irradiation conditions. Light intensity can also be calculated based on the following equation: Light intensity (W/m2) (300-400 nm) * 6.07% /100% = Global light intensity (300- 3000 

nm) (W/m2) 

Global light intensity (300- 3000 nm) (W/m2) 3600 sec =  Global light intensity in one hour in experiment (300- 3000 nm) (MJ/m2) This experimental light intensity can then be related to environmental light intensities per day, which are available for different locations and different seasons e.g. in WYPYCH, HANDBOOK of MATERIAL WEATHERING. 

See response above on German comment. 

Paragraph 18 

Japan(2) Paragraph 18,19,20 and 31. It is appropriate that a title and an applicable term of “Test 

System” should be replaced with “Incubation System” etc., because the word “Test system” is used to define incubation tank and examination equipment in this document and it would lead to misunderstanding. 

This point is adopted. 

The Netherlands Annex is not optimal for comparison of irradiated soil with soil under dark treatment: 

the temperatures in the exposed soil may be different from the covered soil as the cover absorbs the light and converts it into heat. The set-up of Annex 4 is much better, in this respect. 

Agree. Therefore, the system shown in Annex 4 is given as a useful test system. Annex 2 is referenced in paragraph 14 only to show and explain the Hanau Sun Test apparatus. 

ECPA member 

company 

(paragraphs 18 to 20) As alternative a static system consistent of individual quartz glass vessels (e.g. inner surface about 10-12 cm2) equipped with trap attachments with soda lime and PU foam may be used (biometer flask systems) 

This point should be covered by the addition of a new paragraph 24. 

Paragraph 19 

Austria Deletion of footnote 4 strongly suggested. See comment above. This point is adopted. 

Croplife/ECPA The description of the test system is nice, but it should be stated that this is an example, 

not a requirement. There is no reason that the cooling tank has to be made of stainless steel and sealed with Teflon tape. It could just as easily be made of another material with a gasket system to seal it. The test guideline should allow some flexibility in study design. 

Paragraph 18 clearly says that the described system is shown as a useful example and thus does not exclude other study designs and materials. Static systems will be included in a separate paragraph. 

Paragraph 20 
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Germany Laboratories may have different technical equipment. Therefore guidelines should be 

more general, where different technical equipment can be used. This should be considered especially according to requirements which do not have any influence on the essential outcomes of the study (examples: flow-through versus biometer flask test systems, measurement of light intensity, etc.) so Add: As alternative static systems consistent of individual quartz glass vessels (e.g. inner surface about 10-12 cm2) equipped with trap attachments with soda lime and PU foam may be used (e.g. biometer flask systems) 

This point should be covered by the addition of a new paragraph 24. 

US(2) 1st sentence: Replace “air is sucked through” with “air is drawn through” 

US position: The U.S. agrees. 

This point is adopted. 

Croplife/ECPA Although this is a guideline, we think that this paragraph is excessively detailed. We see 

no reason to ensure a CO2 free air inlet as only radiolabelled CO2 is detectable. Also, no suggestion other than a peristaltic pump for the flow-through system. 

The system described here is just given as an example. Static systems will be included as alternative. The use of CO2-free air prevents rapid saturation of the sodium hydroxide traps. 

Paragraph 21 

Japan(1) “isotop” should be corrected to “isotope”. This point is adopted. 

The Netherlands (also includes the comment of paragraph 18). Temperature registration (and control) See response above. 

Paragraph 22 Paragraph 23 

Austria One of the experts is of the opinion that it might be advisable to use 2 soil types for the 

determination of the rate of phototransformation because the matrix is of relevance for photochemical processes. Soils of different organic composition absorb differently, which has an influence on the reaction kinetics. 

This point is adopted. 

Germany The use of one soil is not believed to be sufficient. Is type of soil of minor importance? 

Is there any evidence behind? 

Two soil types are recommended in the revised version. 

US(1) (EFED noted that the guideline indicates “the use of one soil is sufficient”.) At least one 

soil must be the same type as used in an acceptable aerobic soil metabolism study submission. This facilitates the modelling of combined routes of dissipation. US position: The U.S. partially agrees. The U.S. agrees whenever it is experimentally 

practical. However, in cases where only one or more sandy soils were used in the aerobic soil metabolism, it may not always be practical to successfully generate and/or maintain soil thin layers for such sandy soils as indicated in paragraph 23. 

This point is adopted. 

Paragraph 24 

Austria Insertion: "...The selected soil should be characterised at least for texture [according to 

FAO, USDA or DIN classification systems (14)]..." 

The first point is adopted. With regard to the second point we think that in an international OECD Test Guideline we should 
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Proposal for the reference: "German Standardisation: DIN 4220, Standard Committee; Berlin, Köln (Beuth 

rather refer to international than to national Classification Systems when those are available. 

Germany Replace content of reference 19 with Schlichting E., Blume H.-P., Stahr K. (1995) 

Bodenkundliches Praktikum: eine Einführung in pedologisches Arbeiten für Oekologen, 2nd edition, Blackwell Berlin-Oford. 

This reference is added to the literature list. 

Paragraph 25 

Austria Reference to the ISO Guideline 10381-6 Soil Quality - Sampling "Guidance on the 

collection, handling and storage of soil for the assessment of aerobic microbial processes 

in the laboratory" should be made in the relevant place of this paragraph. 

This point is adopted by adding an additional paragraph outlining how soils should be handled when “viable” (microbially active) soils are required for conducting these studies. 

Germany Conditions during experiment (temperature, moisture, 30-d-period) favour bioactivity. 

Biodegradation may cause formation of other metabolite, altering CO2 balance or partition equilibria in soil, etc. So add text: In case of test substances susceptible to biodegradation, soil samples (or whole test system) should be pretreated prior to starting the test, e. g. by irradiation with short wavelength UV light, in order to minimise such impact. 

This pre-treatment cannot be recommended because some guidelines (e.g. SETAC, USEPA) require the use of “viable” soil at the start of the study. However, due to the irradiation during the study (in some cases up 30 day equivalents) microbial activity of the test soils may decrease. 

Croplife/ECPA The sentence “Desaggregation should be performed at minimal force ...” claims not to 

change the original texture of the soil. The relevance of this statement is not clear. The sieving process should not change the particle size distribution of the soil, but later on a slurry will be made of the soil, so the original “texture” is heavily affected. 

Suspended slurries of soil are used for the determination of the particle size of soils and thus slurry preparation should not affect the texture of a soil. 

Paragraph 26 Paragraph 27 

Austria Please add a reference where the assumption, that 24 hours after the application of the 

soil slurry to a plate the field capacity is about 75 % is based on. 

This value of approx. 24 hours is based on experience with such experiments and was thought as help for people conducting such experiments. But as this value may vary from soil to soil it is deleted. 

Germany Moisture of the soil should be in the range used in soil degradation studies (75% of 1/3 

bar moisture or 40-60 % of max. water holding capacity). A moisture of 75% of field capacity is regarded as too wet and not representative for usual outdoor conditions. 

Footnote 5 refers to a very specific test device. 

We agree that soil moisture should be in the same range as used in the degradation studies. Thus to harmonise with OECD guideline 307 this paragraph is revised accordingly. The footnote is thought as help for experimenters to give them some idea of the dimensions of plates and soil thin-layers which can be used together with the experimental set-up shown as example in Annex 4. 

France (Paragraphs 27 and 28) Photodegradation is proposed to be investigated with both wet Perhaps the SETAC guideline is not very clear in this respect. 
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(75% FC) and air-dried soil. Because it is difficult to maintain constant soil moisture under light conditions and thus differences in soil moisture content may occur between light and dark conditions, we believe that photodegradation should be preferably determined on air-dried soil as stated in the SETAC guideline. 

We think that both experiments should be conducted to show the different rates in dry and moist soil as quasi 2 extreme situations. We agree that it may be different to maintain soil moisture during irradiation but know from practice that this can be done by regularly controlling total weights of the soil plates. 

Japan(2) Paragraph 27 and 28. Although the studies under two conditions (using moist soil of 

about 75% of field capacity and completely dry-out soil) are mentioned in this document, the study using dry-out soil should be treated as a supplementary or optional study. And it is necessary to be defined specific test conditions for the study using dryout soil. (i.e, soil thin layer or ventilation conditions, etc.) in this document. 

See responses above on French comment. 

The Netherlands Could you explain the necessity studying both moist soil layers and dry soil surfaces? Is 

paragraph 28 an extra requirement? It is hardly possible to keep the 2 mm soil layers 

moist under high radiant power source. 

As the surface of a field soil can become quickly dry during sunny periods, the rate on a dry soil surface has certainly some environmental relevance. Together with the rates obtained on moist soil, it gives to some extent the range of photolysis rates which can be expected for a particular chemical on soil surfaces. 

US(3) Paragraph[s] 27 [& 32]: "Therefore, the soil layers are allowed to air-dry to reach a soil 

moisture of about 75% of field capacity (about 24 hours).” This implies that at 24 hours after coating, the soil moisture will be 75% of field capacity. US position: The U.S. agrees. Guidance on how to maintain soil moisture should be 

included. 

See response above on Austrian comment. Guidance on how soil moisture is maintained is described in paragraph “Moisture content”. 

Croplife/ECPA (Paragraph 27 & 32 including footnote 7) OECD guideline 307 describes determination 

of the water holding capacity. In these paragraphs, the test guideline refers to field capacity. This is not the same. To avoid confusion, we suggest to use the term ”water holding capacity” instead of field capacity. 

This point should be covered by the changes made. 

ECPA member 

company 

Moisture of the soil should be in the range used in soil degradation studies ( 75% of 1/3 bar moisture or 40 – 60 % of max. water holding capacity). A moisture of 75 % of field capacity is regarded as too wet and not representative for usual outdoor conditions. 

This point should be covered by the changes made. 

Paragraph 28 

Germany Only in special cases it may be interesting to investigate phototransformation on dry soil 

surface. In general, no additional information with respect to the above mentioned experiment (paragraph 27) is to be expected. 

See above response on Netherlands comment on paragraph 27. 

US(2) Because soil moisture may substantially affect phototransformation rates and/or 

transformation pathways on soil, the use of dry as well as moist soil in the study can certainly be justified from a scientific standpoint. However, from a resource as well as scientific standpoint, it could also be argued that the additional resources could be better 

Agree with US position. See also above response on Netherlands comment on paragraph 27. 
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spent on replicate experiments and/or additional sampling intervals using moist soil. That is particularly true of agricultural soils in the environment for which some moisture will presumably be maintained to some degree. US position: The U.S. agrees. However, the reviewer should also have indicated that in 

paragraph 32, an option is given to use either moist or dry soil. While it is OK to use dry as well as moist soil in separate experiments, the use of dry soil should never replace the use of moist soil. 

Croplife/ECPA It is not clear if the conduct of an additional test on dry soil is obligatory. We 

recommend that such tests are only required in special cases. 

See responses on this point made above on paragraph 27 and on this paragraph. 

ECPA member 

company 

Only in special cases it may be interesting to investigate phototransformation on dry soil surface. In general, no additional information with respect to the above mentioned experiment (paragraph 27) is to be expected. In addition, this is a new requirement with respect to the valid guidelines. 

See responses on this point made above on paragraph 27 and on this paragraph. 

Paragraph 29 

France Suitable organic solvent is proposed to be used for application of the test compound. It 

should be specified that solvent should not be a photosensitiser. 

This point is adopted. 

Germany Ad text. “by means of an appropriate syringe, e.g. Hamilton microlitre syringe. 

Other techniques of application should also be allowed, e.g. spray application. Such techniques can help to reduce non-uniform penetration depths of droplets in 2mm layer thus reducing uneven shelter from irradiation. 

The first point is adopted. We would not recommend spray applications for reasons of substance losses and cross contamination. 

Japan(1) A method to estimate the amount of the test substance distributed to soil in the 

environment should be shown. In addition, the standard amount of the application of the test substance should be shown. 

As amounts reaching the soil differ from chemical to chemical, a standard amount cannot be given. 

Japan(2) It would be appropriate to mention clearly that photosensitized solvents (acetone etc) 

should not used in this study. 

This point is adopted. 

US(1) (EFED noted that the guideline only recommends test chemical application via syringe to 

soil thin layers.) EFED recommends that both methods of test material application, mixing test substance with soil before plating, and application with syringe to prepared thin layer, be allowed. A single method may not be applicable to all chemicals. US position: The U.S. partially agrees. Mixing the test substance with soil before plating 

should also be included as an option, but only for chemicals that remain stable (degradation and/or volatilization are negligible) during the pre-test drying period (up to 24 hours). 

We should not recommend incorporation because the goal of this test is to measure phototransformation on the soil surface where the chemical is available for the irradiation. 

Paragraph 30 

Japan(1) A method to estimate the amount of the test substance distributed to soil in the 

environment should be shown. In addition, the standard amount of the application of the test substance should be shown. 

See response above for paragraph 29. 
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The Netherlands In combination with paragraph 12: Is it not better to relate the LOD and LOQ to the 

maximum recommended use rate instead of fixing the LOD at a value of 0.01 mg/kg(or at 10% of activity applied whichever is lowest? 

The limits given in paragraph 12 of the revised version are in agreement with those of other fate test guidelines (e.g.307). 

US(1) (EFED noted that the guideline calls for a pesticide test substance application rate 

equivalent to the maximum recommended use rate.) EFED recommends that exaggerated application rates (be used) to increase the amount of degradates formed to concentrations 

above the analytical method limit of quantification, be allowed. US position: The U.S. partially agrees. The use of exaggerated application rates may be 

necessary as a supplemental study to adequately characterize degradates. However, unless phototransformation is following pseudo-first order kinetics, such exaggerated application rates should only be used in a study that is supplemental to a study on the phototransformation rate of the parent. The reason is that unless the phototransformation rate of the parent is following pseudo-first order kinetics, its DT50 will depend (in part) upon its initial concentration in the soil. 

Agree with US position. 

US(2) Footnote equation: The equation is correct. 

US position: The U.S. agrees. 

Agreed. 

Paragraph 31 

Japan(1) The test temperature should be in the range from 20 to 25 °C, kept a constant ±2 °C. A temperature of 20°C is recommended, for example, by the 

SETAC guideline. See also US position below. 

Japan(2) Considering the Japanese weather condition, the test temperature defined in this 

document (20±2 °C) is too low, therefore it is desirable that the test temperature include 25°C or description such as "although 20°C is recommended as test temperature, it can also be set up, taking into account the weather conditions of each country" be added to the text. Also it seems to be difficult to keep the temperature within the limit of ±2 °C in the halfseal container under irradiation, “2°C” should be defined as a recommended margin as the accuracy of soil temperature. 

As photochemical rates are relatively insensitive to temperature changes within the environmental range (see US position below), it seems not relevant whether these experiments are carried out at 20 or 25°C. However, whenever possible the lower temperature is recommended because substance losses and particularly soil moisture losses should be lower. 

US(1) Temperature range should be between 18-30 + 1 °C (rather than 20 + 2 °C) as this 

reflects the range of growing season temperature for most crops. US position: The U.S. partially agrees in that the proposed temperature range is more 

reflective of temperature differences among the member countries of the OECD. However, the use of 20°CC is also acceptable. The reason is that photochemical kinetics are relatively insensitive to temperature changes within the environmental range. Temperature control is needed in a phototransformation on soil study more to ensure comparable temperatures between irradiated and dark control samples (because of the sensitivity of non-photochemical kinetics such as hydrolysis and biotransformation to temperature) than to ensure that the photochemical kinetics occurs at any particular 

Agree with US position. 
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temperature. Also, although maintaining a + 1°C tolerance is desirable, it is not critical 

for photochemical kinetics for the reasons stated above. In addition, maintaining a +

1°C tolerance may not be as practical in a photochemical on soil study as maintaining a + 2°C tolerance. 

US(2) It should be pointed out that while direct temperature effects on photochemical 

processes are generally small, adequate temperature control is necessary to help maintain soil moisture and comparable microbiological activities within the light exposed and dark controls. US position: The U.S. agrees. 

This point is adopted. 

Paragraph 32 

Germany Moisture content affects extractability of test substances from soil, thus apparent 

degradation. Intervals and methods to determine and adjust the soil moisture depend on the study design and should not be committed in detail; just a maximum interval for correction of moisture should be prescribed. So add text: ...are removed for analysis or at least every 48 hours. Soil moisture should be determined and recorded. It is unclear why air-dried soils should be tested. Phototransformation will be at least less effective. For reasons of comparability of results this option should not be allowed for. 

The first point is adopted. With regard to the second point see response made above on French comment on paragraph 27. 

US(1) Paragraph 32 and 33: (EFED noted that the guideline allows for the use of either moist 

or dry soil thin-layers, and the use of additional moist soil thin layers for dark controls). EFED recommends that moisture conditions be identical for both irradiated and control test samples. This facilitates comparison of control to irradiated samples for “dark correction”. US position: The U.S. agrees that “moisture conditions be identical for both irradiated and control test samples.” [Although the U.S. thinks that it is OK to conduct a study on dry soil in addition to conducting one on moist soil, a study on dry soil should not be 

used to replace a study on moist soil.] 

This point is adopted by including “dry soil thin-layers” in paragraph 33. 

US(3) "Soil thin-layers are kept either moist (75% of field capacity) or air-dried during the 

irradiation period." [But the] Protocol does not indicate how to maintain the moisture content and the irradiation apparatus does not have any provisions for maintaining moisture content. US position: The U.S. agrees. Guidance on how to maintain soil moisture should be 

included. 

As described in paragraph 20, moist air is pumped through the system to saturate the tank atmosphere with water and thus minimise moisture losses from soil thin-layers. In addition, moisture losses of individual soil thin-layers are measured and water is added to the “moist” samples, when necessary, at the time intervals given in paragraph 32. 

US(4) It will be extremely hard to maintain a constant moist soil (i.e. 75% of field capacity) 

during the study period. The air flow and heat generated by the Xenon lamps will dry the soil. It would be impractical to add water to soil thin layers during the test. 

It is certainly not easy to maintain a constant moist soil during the study period. The procedure described in this draft document tries to minimise moisture losses from soil thin-
ENV/JM/TG/RD(2006)1/REV 

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US position: The U.S. partially agrees. It is difficult to maintain a constant soil moisture 

of 75% field capacity because of air flow and heat generation by the lamps. The document should include guidance on how to maintain soil moisture. However, it should be practical to approximately maintain at least some soil moisture through various methods such as the use of cooling tanks for temperature control and moist air flow as discussed in the guidance document.

layers in two ways: 

- by ventilation of the incubation system with moist air 

and 

- by adding water to the layers at the time intervals 

mentioned in paragraph 32, when necessary. 

ECPA member 

company 

Intervals and methods to determine and adjust the soil moisture depend on the study design and should not be committed in detail. 

Nevertheless, some guidance has to be given in a test guideline on how to adjust soil moisture during such a test. 

Paragraph 33 

Austria Following change suggested: 

"Additional moist and air-dried soil thin layers are kept under identical conditions..." 

In order to distinguish between photochemical and other reactions irradiated and dark soil thin-layers should be kept under identical conditions. Therefore also air-dried soilthin layer plates treated with test substance should be treated as dark controls. 

This point is adopted. 

Germany Replace...but not irradiated by “in the dark”. This point is adopted. 

US(2) If dry as well as moist soil is used, dry as well as moist soil dark controls need to be 

used. Therefore change “Additional moist soil” to “Additional moist and dry soil”. 

US position: The U.S. agrees. 

This point is adopted. 

Paragraph 34 

Germany Add. Duration of exposure to light should not exceed 30 environmental solar days or two 

half-lives of the test substance, whatever is longer. 

The sentence is included but modified by the phrase “whichever is shorter”. If substances are susceptible to photolysis they react rather quickly and certainly within 30 days; thus it seems not reasonable to prolong the test beyond this period. 

Japan(1) The incubation period should depend on the irradiance energy used because the 

irradiance energy is not necessarily the same as ANNEX 3. 

This point should be covered by the changes made. 

Croplife/ECPA (Paragraph 34 & 35) It also not clear whether irradiation is required continuously or in 

day and night cycles. Both options should be allowed and discussed. 

The first point is adopted. The second point should be covered by the changes made in paragraph 35. 

ECPA member 

company 

Duration of exposure to light should not exceed 30 environmental solar days or two half lives of the test substance, whatever is longer. 

See above response on German comment. 

Paragraph 35 

Germany Replace Figure 3 by “Annex 4” and solvents of different polarity by “appropriate 

extraction solvents”. 

This point is adopted. 

Japan(1) “Figure 3” should be corrected to “ANNEX4”. This point is adopted. Japan(2) 1) "Cooling tank illustrated in Figure 3" should be corrected to "cooling tank illustrated 

in ANNEX 4." 

The first point is adopted. The second point should be covered by the changes made. 
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 2) Cooling tank does not have to be limited to those which are made of stainless steel. 

ECPA member 

company 

Details of “stainless steel cooling tank” and “the quartz lid” will depend on the specific 

test design. 

This point should be covered by the changes made. 

Paragraph 36 

US(2) 3rd sentence: Change “at least 5" to “at least 6". 

US position: The U.S. agrees, particularly if the kinetics does not follow pseudo-first 

order and/or the rates of formation and decline of major degradates needs to be determined. Note that the example given in para 36 of the guidance includes 6 rather 

than 5 non-zero hour sampling points. 

This point is adopted. 

US(2) Some clarification is needed with respect to the purpose of removing duplicate soil thin 

layers at each sampling interval for extraction and analysis. Such a protocol falls short of fully duplicating the experiment, but is somewhat of an overkill for determining the precision of the analytical method. Replicate experiments (preferably a minimum of 3) are recommended (when practical) to determine variability and reduce uncertainty in the determination of kinetic parameters. However, the uncertainty in the determination of kinetic parameters can also be somewhat reduced by increasing the number of sampling intervals within each experiment. That is particularly true when rates of transformation are not first order and/or when the rates of formation and decline of major transformation products are being determined. Therefore, with generally limited resources, the researcher should be given the flexibility of weighing the pros and cons of increased experimental replication versus increased numbers of sampling times per experiment in designing the study. US position: The U.S. agrees that some additional clarification is needed. The U.S. also 

agrees that in a kinetics study, some flexibility and practical balance needs to be maintained in allocating resources between additional experimental replication and additional sampling intervals. 

This point should be covered by the changes made. 

Croplife/ECPA This paragraph states that “Duplicate soil thin-layers are removed at appropriate time 

intervals and ...”. Can the duplicate sampling requirement be met when two different labelled versions of the test substance are used and a single sample from each label is sampled and analysed? The guideline should clarify if in such a case duplicate sampling for each label is still required. 

We think that when two different labels of a test substance are used, two separate experiments should be conducted 

ECPA member 

company 

“solvents of different polarity” should be replace by: ... with appropriate extraction 

solvents and ..... 

This point is not adopted. 

Paragraph 37 

Germany Replace sentence by: “When using 14C-labelled test substances, mineralization, volatile, 

extractable and non-extractable residues are determined as to set up detailed mass 

balances for each sampling interval. 

Analysis of extractable and volatile materials is already mentioned in paragraph 36 and thus there is no need to repeat this in this paragraph. 
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The Netherlands Should a mass balance also include duplo non-extractable residues or is the soil of one 

plate per sample time enough (under the requirement that the mass balance is between 90 and 110%)? It does not provide extra information if non-extractables are analyzed in duplo if the mass balance is between 90 and 110% for the soil of one plate. 

The amounts of soil per plate are normally rather small in these experiments; in the order of a few grams depending on the dimensions of the plates. So duplicate analysis may not always be possible and thus single analysis should be sufficient. 

US(2) Paragraphs 37 and/or 38: Mention again (as earlier in paragraph 13) that confidence 

limits should be computed and provided for rate constants, half-lives, DT50s, DT90s, etc. US position: The U.S. agrees. 

This point should be covered by the changes made in paragraph 39. 

Paragraph 38 

Austria see comments to p.2, para. 7, Addition: 

“... during the study or a phototransformation product below 10%, where relevant.” 

The proposed changes were adopted in paragraph 7 with regard to the analytical method. This paragraph deals with the identification of major phototransformation products and their plots against time. It is not considered necessary to produce such plots also for minor products. 

Germany We suggest to add: ...10% of the applied dose at any time during the study “or > 5% 

toward the end of the test without trend to further decline in concentration. 

This definition of major phototransformation products is in agreement with that used in other OECD fate test guidelines (e.g. 307, 308, 312). 

Japan(1) When the amount of the test substance applied to soil is small, in general, it is difficult to 

measure the transformation rate of the test substance accurately and the mass balance is low because the recovery of the test substance from soil is difficult. Therefore, the 

recovery should be taken into account when the amount of the application is nominated. 

A prerequisite to conduct such studies is the availability of a sufficiently sensitive and reproducible analytical method to analyse the small concentrations applied to the soil thinlayers. If the available analysis method is not sensitive enough, higher application rates may be used. 

The Netherlands Why should the results be graphically plotted? A tabular representation is sufficient. This requirement for graphical plots is in line with that in 

other OECD fate test guidelines (e.g. 307,308). 

Paragraph 39 

Austria “If appropriate, the calculations should also be applied to the major all relevant

phototransformation products." 

The term “major phototransformation products” is used in agreement with other OECD fate test guidelines (e.g. 307, 308, 309, 312) and thus should not be exchanged by another term. 

Austria For the calculations of half-lives and DTx values more updated references should be 

given. If the degradation behaviour of a chemical is complex and influenced by a number of processes, then simulation tools should be used. E.g. - TOPFIT, Pharmacokinetic and Pharmacodynamic Data Analysis System, Heinzel, G., Woloszczak, R. and Thomann, P. Gustav Fischer, Stuttgart, Jena, New York. - ModelMaker, Cherwell Scientific Publishing Limited, Oxford, UK. 

These references will be included in the literature list. But please supply additional information such as year of publication, authors (for ModelMaker), etc. 

France First order kinetic should be recommended because it can be easily handled. However, Always that model should be used which best fits the data. 
ENV/JM/TG/RD(2006)1/REV 

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other models might be used if data do not fit first order. Therefore, first order kinetics cannot be generally 

recommended. 

Germany References given here are not up-to-date. Models currently in use for such purposes are 

MODELMAKER (Heinzel, G. Woloszczak, R. Thomann, P. (1993): TopFit 2.0: Pharmacokinetic and PharmacodynamicData Analysis System for PC; VCH Publishers Inc., NY), TOPFIT (ModelMaker version 4.0, FamilyGenetics Ltd, The Magden Centre, Oxford, GB) or ACSL (ACSL Optimize version 1.2 (Oct. 1996) MGA software, Concord, Massachusetts, USA). 

These references will be added to the literature list. But please supply additional information such as authors, year of publication, meaning of abbreviation (ACSL), etc. From the above Austrian comment it seems that TOPFIT and MODELMAKER are two different models. In your comment you use both names for the same model; this is confusing. Could you please clarify this? 

The Netherlands r2 is not the correlation coefficient but the determination coefficient. The determination 

coefficient compares the actual Y-values with estimated y-values (from the model used). the correlation coefficient (ρXY) gives the relation between (an array of) x and y values. 

This point is adopted. 

Paragraph 40 

Austria Test results: 

"- plots of percentages in soil versus time for the test substance and, where appropriate, for the major all relevant phototransformation products. 

- half life DT50 and DT90 for the test substance and, where appropriate, for the major all 

relevant phototransformation products. 

- ... kinetics for the test substance and, where appropriate, for the major all relevant

phototransformation products." See previous comment to p.2, para. 7. As the recommendation for identification of degradation products in environmental compartments is under discussion in the framework of 91/414/EWG the wording of the Test Guideline should take into account this discussion by lowering the trigger value of 10% for phototransformation products for cases, where relevant. 

The term “major transformation product” was used in OECD fate test guidelines so far (e.g. 307, 308, 309, 312). Thus it seems not appropriate to introduce another term because this may be confusing for the users of these guidelines. 

Germany Please add under: 

- properties of soil: “type and rates of clay minerals (if available), 

- xenon light source: measured spectrum ... “or radiometer results and 

spectrometer of the manufacturer”, 

- Test conditions: “air flow rates (if applicable)” and “limit of detection, limit of 

determination, data on method validation” under methods for quantification and identification. 

As active sites of clay minerals are, in general, covered by organic matter in natural soils, their potential influence on reactivity of a chemical can be neglected and thus this point is not adopted. The second point is adopted. The third point is partially adopted; data on method validation are not part of this test. 

Japan(2) Light source intensity should be described together with wavelength-range(e.g. 

OOW/m2 (OO-OOnm)). 

The required wavelength range is already given. Light intensity at sample level depends not only on the light source but also on the experimental set-up used and thus cannot be specified. 
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Country Comments Secretariat’s Response 

The Netherlands A spectrum given by the manufacture from which the values are calculated to the actual 

values seems sufficient, especially since in the light calculations the spectral information is not used. 

This point is adopted. 

Croplife/ECPA With regard to “test soils”, different people may understand different things under 

“water retention characteristics”. For the scope of this test guideline, the water holding capacity according to OECD Test Guideline 307 should be sufficient and consistent (we think it is not appropriate to be asked to measure pF-curves for this test). 

This general term is used also in other OECD fate test guidelines (e.g. 307, 312). 

Croplife/ECPA With regard to “test results”, why is characterisation of bound residues required? If we 

are asked here for fractionation of the bound residue, this information does not really add anything to the study. We think what is really meant here is quantification of the bound residues. The requirement for the confidence limits should be removed (see also comment on paragraph 13). 

This point is adopted. 

ECPA member 

company 

- measured spectrum of the light source between 295 and 800 nm at the beginning and 

the end of the test (or radiometer results and spectrum of the manufacturer);

- air flow rates (if applicable); 

These points are adopted. 

Annex 1 

Germany Please correct under Phototransformation: reactions occurring under the influence of 

light. Delete “on soil thin-layers” and “(>290 nm). 

Comment on DT5o: DT50 is identical with t1/2 in the case of 1st- order kinetics. The 

shape of the curve may be different, not the time at which 50% reduction is observed. 

Please correct under intensity: Photon flux. 

The definition refers to this test and therefore “soil thinlayers” are included. 290 nm is the cut-off value recommended by many references and is in agreement with that of paragraph 15 of the revised version. Agree with your comment. The last point is adopted. 

Annex 2 

Germany Please put in line: the cut-off of 290 nm here is not inline with the cut-off in paragraph 

14 which is 295 nm. 

The cut-off is in line with that of the revised version. 

Croplife/ECPA We would like to propose to change the title of the Figure as follows: “Spectrum of 

Xenon Arc (without Cut-off Filter) compared to Sunlight”. 

This point is adopted. 

Annex 3 

Germany A reference is missing for Table A. 

Integral light intensities (300-400nm) are only relevant for substances which strongly absorb in the whole range and provided that quantum yield is not dependent on wavelength (different absorption bands may occur in this broad range!)cf. comments on para 15. Even for summertime it is surprising that Basel has the same irradiance as Florida. Typical values for Central Europe are 20-30 W/sqm at 300-400 nm (=0.2-0.3 mol/sqm/h 

A reference will be included in the revised version. Agree with your comment which describes the situation in aqueous medium. In this test, we do not distinguish between direct and indirect photolysis and also cannot measure the quantum yield. The irradiance values listed in Table B are those reported by Mill et al. (1982). Other sources may show different 
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Country Comments Secretariat’s Response 

photons). And again, such irradiance is not representative for many regions, e.g. Central Europe (cf.reference Frank/Klöpffer given in General Comments). 

irradiance values (e.g. A. Leifer, Zepp & Cline, Frank and Klöpffer). 

Croplife/ECPA One would expect a higher irradiance in Florida (30°N) than in Greensboro (40°N), but 

we would like to propose to delete the names of regions and just mention the latitudes. 

The names of the regions can give the reader some information for which geographical regions these values may be applied. 

The comments from Japan came from the following two sources: 

- (1) Yamaguchi-Takashi@meti.go.jp

- (2) Takemoto Akio (takemoto@deljp-ocde.fr) 

The comments from the USA came from the following four sources: 

- (1) U.S. Environmental Protection Agency, Office of Prevention, Pesticides, and Toxic Substances, Office of Pesticide Programs, 

Environmental Fate and Effects Division, 1200 Pennsylvania Ave., N.W., Washington, D.C. 20460 

- (2) U.S. EPA, Office of Prevention, Pesticides, and Toxic Substances, Office of Science Coordination and Policy, Exposure Division, 

1200 Pennsylvania Ave., N.W., Washington, D.C. 20460 

- (3) Syracuse Research Corporation, Environmental Science Center, 301 Plainfield Rd., Syracuse, NY 13212 

- (4) Rohm & Haas Company, 727 Norristown Road, P.O. Box 904, Spring House, PA 19477 
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ANNEX 6 

DRAFT DOCUMENT 

July 2005 

OECD GUIDELINES FOR THE TESTING OF CHEMICALS

PROPOSAL FOR A NEW GUIDELINE 

Phototransformation of Chemicals on Soil Surfaces

INTRODUCTION

1.42. Man-made chemicals may reach soil directly via deliberate application (e.g. agrochemicals) 

and/or via indirect routes (e.g. via waste water → sewage sludge → soil or air → wet/dry deposition). For 

risk assessment of these chemicals, it is important to estimate their potential for transformation in soil and 

for distribution in and between the various environmental compartments (soil, water, air). 

2.43. Chemicals can be transformed in soil by microbial, chemical and/or photochemical processes. 

The method described in this Test Guideline deals with the photochemical transformation (rate and route) 

of chemical substances on soil surfaces. It is based on existing guidelines (1) (2) (3) (4) (5). 

SCOPE AND RELEVANCE

3.44. When irradiating a test item on a soil surface under the experimental conditions described in this 

Test Guideline, besides photochemical transformations (direct and indirect photolysis) also chemical 

(hydrolysis, oxidation) and microbial transformations can occur, the latter in particular when “viable” 

(microbial active) soils are used. To assess the relevance of the latter two processes, non-irradiated dark 

controls are included in this test. The difference in transformation rates and routes observed between 

irradiated samples and dark controls can then be attributed to phototransformation processes on the soil 

surface. The methodology is neither designed to determine the quantum yield of a test item on a solid 

surface nor to distinguish between direct and indirect photolytic reactions. However, it should allow 

estimating minimum photolysis half-lives under optimum outdoor irradiation conditions and showing 

maximum of phototransformation products of test item likely to occur on soil surfaces, which have to be 

isolated and identified in order to get a complete risk assessment for the environment, then. Simulating a 

higher degree of translocation or diffusion of test item into dark zones of soil and/or a stronger binding of 

test item to soil matter (i.e. whenever the soil is rather dry) would not lead to a risk assessment differing 

from that under exclusion of sunlight as a degradation process relevant for the environment. 

PRINCIPLE OF THE TEST

4.45. Thin-layers of soil are treated with the test item and irradiated by simulated sunlight (xenon 

lamp) under controlled laboratory conditions or, as an option in sunny climatic regions, by natural sunlight. 

Simultaneously, not-irradiated dark controls are kept under identical conditions to distinguish between 

photochemical and other reactions. After appropriate time intervals, the soil thin-layers are extracted and 
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analyzed for the parent test item and/or for major phototransformation products. Volatile products are 

collected by appropriate absorption devices and are also analyzed for at time intervals. Using 14C-labeled 

material, phototransformation products can be identified and a mass balance, including the formation of 

soil bound (non-extractable) residues, can be established. 

APPLICABILITY OF THE TEST

5.46. The method is applicable to all test items (unlabeled and radiolabel) for which an analytical 

method with sufficient accuracy and sensitivity is available. It is applicable to slightly volatile or nonvolatile, water-soluble or water-insoluble compounds. The test should not be applied to chemicals which 

are highly volatile from soil (e.g. fumigants, organic solvents) and thus cannot be kept in soil under the 

experimental conditions of this test. 

INFORMATION ON THE TEST ITEM

6.47. Unlabeled or labeled test items can be used to measure the rate of phototransformation. Labeled 

material is required for studying the pathway of phototransformation and for establishing a mass balance. 

14C-labeling is recommended but other isotopes, such as 13C, 15N, 32P, may also be useful. As far as 

possible, the label should be positioned in the most stable part(s) of the molecule11. The chemical and 

radiochemical purity of the test item should be at least 95 %. 

7.48. Before carrying out a phototransformation test on soil surfaces, the following information on the 

test item may be helpful in order to optimize test design and procedures of treatment and processing: 

(a)(j) solubility in water [OECD TG 105] (6); 

(b)(k) solubility in organic solvents; 

(c)(l) vapor pressure [OECD TG 104] (6) and Henry’s Law constant; 

(d)(m) dissociation constant (pKa) [OECD TG 112] (6). 

(e)(n) chemical stability in the dark (hydrolysis) [OECD TG 111] (6); 

(f)(o) aerobic and anaerobic transformation in soil [OECD TG 307] (6); 

(g)(p) phototransformation in water [OECD TG in preparation](7). 

8.49. An appropriate analytical method of known accuracy, precision and sensitivity for the 

quantification and identification of the test item and its phototransformation products in soil should be 

available. The analytical detection limit for the test item and its phototransformation products should also 

be known (see paragraph 13). 

REFERENCE SUBSTANCES

9.50. Reference substances should be used for the characterization and/or identification of 

phototransformation products by spectroscopic and chromatographic methods. 

 

11 For example, if the test item contains one ring, labeling on this ring is required; if the test item contains two or more 

rings, separate studies may be needed to evaluate the fate of each labeled ring and to obtain suitable information on 

formation of phototransformation products. 
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DEFINITIONS AND UNITS

10.51. See Annex 1. 

QUALITY CRITERIA

Recovery 

11.52. Extraction and analysis of, at least, duplicate soil samples immediately after the addition of the 

test item gives a first indication of the repeatability of the analytical method and of the initial recoveries for 

the test item. Recoveries for later stages of the experiments are given by the respective mass balances 

obtained with labeled material. Recoveries should range from 90 % to 110 % for labeled chemicals (5) and 

from 70 % to 110 % for non-labeled chemicals (3). 

Repeatability and sensitivity of analytical method

12.53. Repeatability of the analytical method (excluding the initial extraction efficiency) to quantify test 

item and phototransformation products can be checked by duplicate analysis of the same extract of the soil, 

incubated long enough for formation of phototransformation products. 

13.54. The limit of detection (LOD) of the analytical method for the test item and for the 

phototransformation products should be at least 0.01 mg kg-1 soil (as test item) or 1 % of the applied dose 

whichever is lower. The limit of quantification (LOQ) should also be specified. 

Accuracy of transformation data

14.55. Regression analysis of the concentrations of the test item as a function of time gives the 

appropriate information on the reliability of the phototransformation curve and allows the calculation of 

the confidence limits for half-lives (in case of pseudo first order kinetics) or DT50 values and, if 

appropriate, DT75 and DT90 values. 

DESCRIPTION OF THE TEST METHOD

Light source

15.56. Natural sunlight or artificial sunlight simulated with solar simulators under well-controlled 

laboratory conditions is considered the optimal light sources. A characteristic of spectral power distribution 

for sunlight is shown in Annex 2; it can also be obtained from ISO 10977 (8) or from GCSOLAR (9) as 

function of latitude and season. The cut-off of radiation of the light source used has to be 290 nm (10). 

Xenon lamps equipped with appropriate filters, such as that of Suntest12 apparatus (see Annex 2) are 

recommended as irradiation source (5)(7)(8) because they simulate very closely natural sunlight in the 290-

800 nm region. Whereas mercury-metal halide arc lamps or equivalents can be used for other tests 

requiring lower levels of energy than xenon lamps, such lamps are less appropriate to evaluate the 

phototransformation of chemicals because they have less power than xenon lamps. For the assessment of 

phototransformation of chemicals, generally higher level of irradiation power is required which can only be 

provided by xenon lamps for the time being. 

 

12 Atlas MTT GmbH, Germany
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16.57. Since light in the wavelength range of 290 to 400 nm is most relevant for phototransformation of 

chemicals in the environment (11)(13), the spectral energy distribution of the xenon light source (i.e. the 

incident light available at the soil surface level) in this range is recorded using, for example, a portable 

spectroradiometer, consisting of a holographic grating monochromator and a Teflon cosine diffuser in 

combination with glass fiber optics. Alternatively, the light intensity during a study performed with a 

xenon lamp/filter system (e.g. Suntest) can be measured by a calibrated radiometer with sufficient accuracy 

(e.g. irradiance monitor XenoCal WB 300-800 nm)2

 or obtained from the manufacturer’s data. Typical 

light intensity (irradiance) between 300 and 400 nm at different exposure positions is given in Annex 3. 

Conversion of laboratory irradiation time into days of natural summer sunlight

17.58. In order to relate the rate of phototransformation caused by the xenon light to the rate likely to 

occur in sunlight, it is necessary to compare the irradiance of the xenon lamp with sunlight (13). For this 

purpose, the integral of light intensity between 290 to 400 nm or between 290-800 nm (dependent on the 

irradiance monitor used) is either measured at the various positions of the soil thin-layers in the test 

systems described in paragraphs 21-24 before and after the test or can be obtained from the data supplied 

by the manufacturer of the apparatus. These values are compared to energy values of natural summer 

sunlight (midday) at latitude 30 to 50° N13 which can be obtained from reference (12) or which can also be 

measured with the spectroradiometer (see Annex 3) or the irradiance monitor used. 

18.59. Furthermore, it is assumed that the average daily sun radiation intensity is approx. 75 % of the 

maximum intensity over a 12-hour period (13), whereas irradiation intensity of the Xenon lamp is constant 

over time. The equivalent days d of natural summer sunlight can be calculated by the following equation: 

0.75 12

h r d ⋅

⋅ =

 where d = days of summer sunlight 

 h = hours of irradiation by the xenon lamp 

 r = ratio of intensity (irradiance) of the xenon radiation to that of summer sunlight 

 0.75 = correction for diurnal variation of natural sunlight 

 12 = conversion factor of hours to days. 

19.60. The above calculations are just one possible way for transferring the experimental light intensity 

to environmental irradiation conditions. Light intensity can also be calculated based on the following 

equations (compare table in Annex 2): 

Measured light intensity at 300-400 nm (W/m2) / (6.07 x 100) = global light intensity at 300-3000 

nm (W/m2

) 

or 

Measured light intensity at 300-800 nm (W/m2) / (57.9 x 100) = global light intensity (300-3000 nm) 

(W/m2

) 

The calculated global light intensity (W/m2) x 3600 sec = global light intensity in one hour 

experiment at 300-3000 nm (MJ/m2

). 

 

3 Intensities of sunlight depend on the geographical latitude in spring, autumn and winter, but vary little by latitude during the summer. 
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This experimental global light intensity per one hour experiment can then be related to 

environmental light intensity per day (expressed as MJ/m2) which are available for different 

locations and seasons, e.g. in reference (14). 

20.61. Another way is to compare the intensity of the xenon lamp with tables of GCSOLAR instead of 

measured intensity. In this way, the days of irradiation are calculated to a “standard” and the results 

obtained at different times and locations can be compared. The tables of GCSOLAR give average daily 

irradiancies L(λ) in millieinstein per day per cm2

 which can be compared to irradiance. The calculation 

should be performed for each wavelength λ (in nm). After that, the values obtained can be summated over 

the region 300 - 400 nm and the result compared to the measured value of the xenon light source. 

 

Incubation system

21.62. The treated soil thin-layers (for preparation see paragraphs 27–33) are incubated and irradiated 

under temperature-controlled conditions in static or flow-through systems which allow to trap any volatile 

products formed during the experiments. An example of a useful flow-through system is shown in Annex 

4B (13). Other test systems are described in references (15)(16)(17). 

22.63. This test system consists of a stainless steel cooling tank containing the soil plates. This tank is 

sealed with a quartz lid, using Teflon tape between and bound around the lid/tank interface, and placed 

beneath the Xenon lamp or exposed to sunlight. Cooling water at a pre-set temperature is pumped through 

the base of the tank to control the temperature of the irradiated soil-coated plates. The temperature is 

monitored in the soil during the experiments using a thermocouple connected to a data logger. 

23.64. In order to trap any volatile products formed during irradiation, air is drawn through the tank at a 

certain flow rate (e.g. 10 – 12 ml/min) using a peristaltic pump. The incoming air is bubbled through traps 

containing 2 M potassium hydroxide solution and then through traps containing water to ensure that moist, 

carbon dioxide free air enters the system. The effluent air is bubbled through a series of traps containing, 

for example, 1 M sulphuric acid solution (to trap volatile basic compounds), methoxyethanol (to trap 

volatile organosoluble compounds) and ethanolamine (to trap 14CO2 or other acidic volatile compounds). 

24.65. Alternatively, static systems consisting, for example, of individual quartz vessels (e.g. inner 

surface approx 10-12 cm2) equipped with traps containing soda lime and polyurethane foam may be used 

(see Annex 4C). In such a case para. 32 is not applicable since the thin soil layer is prepared and handled 

cautiously as outlined for larger portions of soil in the OECD Test Guideline 307 (6). 

Laboratory equipment and chemicals

25.66. Standard laboratory equipment is required, in particular the following: 

- Analytical instruments such as GLC, HPLC and TLC equipment, including the appropriate 

detection systems for analyzing labeled or unlabeled substances or inverse isotope dilution 

method; 

- instruments for identification purposes (e.g. MS, GC-MS, HPLC-MS, NMR, etc.); 

- liquid scintillation counter for radiolabel led test item; 

- oxidizer for combustion of labeled material; 

- extraction apparatus (for example, centrifuge tubes for cold extraction and Soxhlet apparatus 

for continuous extraction under reflux); 
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- instrumentation for concentrating solutions and extracts (e.g. rotating evaporator) 

- water bath. 

26.67. Chemicals used include: organic solvents, analytical grade such as acetone, methanol, etc.; 

scintillation liquid; various absorption solutions (see paragraph 23). 

Soil

Soil selection 

27.68. For phototransformation studies on soil surfaces, the use of two soil types is recommended. One 

soil should be the same type as used in the aerobic soil transformation study. As these experiments require 

glass or metal plates to be covered with thin-layers of soil, only those soils can be used which can form 

thin and solid layers on a glass or metal surface. 

28.69. The selected soil should be characterized, at least, for texture [according to FAO and USDA 

classification systems (18))], pH, organic carbon content and water holding capacity. For determination of 

soil characteristics the methods recommended in references (19)(20)(21)(22)(23)(24) can be used. 

Collection and storage of soil 

29.70. The soil should be taken from the top layer (A-horizon) to a maximum depth of 20 cm. Remains 

of vegetation, macro fauna and stones should be removed. The soils are air-dried at room temperature 

(preferably between 20 - 25°C). Crushing of aggregates should be performed with minimal force, so that 

the original texture of the soil will be changed as little as possible. The soils are sieved through a < 2 mm 

sieve. Before use the soils can be stored at ambient temperature and kept air-dried as microbial activity is 

of little relevance when studying physico-chemical processes on the soil surface. No limit on storage time 

is recommended but soils stored for more than 3 years should be re-analyzed prior to use with respect to 

their organic carbon content and pH. 

30.71. When “viable” (microbial active) soils are required for conducting experiments closely 

simulating field conditions, the soils should be collected freshly from the field and should be handled 

cautiously as outlined in the OECD Test Guideline 307 (6) and also in references (25)(26)(27). 

31.72. Detailed information on the history of the field sites from where the test soil is collected should 

be available. Details include exact location [exactly defined by UTM (Universal Transversal MercatorProjection/European Horizontal Datum) or geographical co-ordinates], vegetation cover, treatments with 

crop protection chemicals, with organic and inorganic fertilizers, and additions of biological materials or 

accidental contaminations (25). 

Preparation of soil thin-layer plates

32.73. Soil thin-layers are prepared by applying aqueous slurry of soil to a glass or metal plate14 in such 

a way that a soil layer of about 2 mm thickness is formed. Thereafter, the soil layers are allowed to air-dry 

to reach soil moisture corresponding to a pF of 2.0 to 2.5. This is the same moisture level as also required 

in the aerobic soil transformation study. 

 

14 An appropriate plate size is 5.5 cm x 3.5 cm. Parts of the soil thin layers are scrapped off the edges of the plate, so 

that a soil area of 5 cm x 2.5 cm is left which corresponds to approx. 12.5 cm2. 
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33.74. If further information is needed, an additional set of soil thin-layers is allowed to completely dryout before test item application. Theses samples may be used to measure phototransformation on dry soil 

surfaces for determination of minimum phototransformation rates. 

Test item application

34.75. To apply the test item to the soil thin-layers it should be dissolved in water (deionized or 

distilled) or in a suitable organic solvent. Each soil thin-layer is then evenly treated with the test item by 

distributing small droplets (e.g. 2-3 μl) of the test item solution (e.g. 40-50 μl total volume) over the whole 

soil surface by means of an appropriate syringe (e.g. Hamilton micro liter syringe or by a Hamilton 

dispenser unit). In case an organic solvent has to be used, it should not be a photosensitizer and should be 

allowed to evaporate from the soil thin-layers prior to the start of the irradiation process. As light 

penetration in soils is generally lower than 200 μm (28) it is important to apply the test item in a very low 

volume of solvent to minimize its penetration into deeper soil layers. 

35.76. The amount of test item applied to the soil thin-layers should correspond, for general chemicals, 

to the estimated amount normally reaching the soil, or for crop protection chemicals, to the maximum 

recommended single use rate, and should be related to the surface area of the soil thin-layers used. An 

example for calculating the application rate is shown in the following for a single maximum field use rate 

of 288 g/ha calculated for the top 1-cm soil layer based on a bulk density of 1.5 g/cm3

. Then it corresponds 

to a dose of approx. 5.8 μg if 3 g soil is used for each test system. 

288 g test item ha 1 x 106 μg test item 1 cm3 1.92 μg test item

ha x 1 x 108 cm2 x g test item x cm x 1.5 g soil = g soil 

1.92 μg test item 3.0 g soil 5.8 μg test item

g soil x test system = test system 

In case the dose rate together with the available analytical method is borderline, it might be reasonable to 

reduce the depth of soil layer to <1 cm (finally to 0.1 cm.) for calculation or to exclude the terms on soil 

depth and bulk density (cursive typed above), i.e. just calculating a dose rate per surface area, then ignoring 

the depth of soil. 

Test conditions

Test temperature 

36.77. During the test, the treated soil thin-layers are kept in the incubation system described in 

paragraphs 21-24 at a constant temperature of 20 ± 2°C. The temperature is monitored during the test by a 

thermocouple which is inserted in one soil thin-layer. Adequate temperature control is necessary to help 

maintain also soil moisture within the light-exposed and dark controls. 

Moisture content 

37.78. Soil moisture should be measured and recorded. The soil thin-layers (see paragraphs 21, 

24 and 32) are adjusted to moisture of a pF of 2.0-2.5 15, corresponding to a water holding capacity of 40 

to-60 %. For sure it is inevitable that the very top soil surface is getting dried during each irradiation 

interval. However, soil moisture is re-adjusted in the remaining samples each time when samples are 

 

15 For determination of water holding capacity (WHC) or field capacity (pF) see OECD TG 307. 
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removed for analysis or at least every 48 hours by adding lost portion of water to the soil thin-layers when 

necessary. This procedure (thus including to some extent drying and re-wetting) is regarded as best 

simulation of outdoor irradiation conditions of contaminated top soil. 

Dark controls 

38.79. Additional soil thin-layers are kept under identical conditions in the dark to distinguish between 

photochemical and other reactions (5). Experience shows that it might happen that the dark controls show a 

faster transformation of test item than the irradiated samples. On the one hand this finding clearly indicates 

that light has no greater degradation potential for the test item. On the other hand it shows that a kind of 

sterilization occurs under the influence of intense light then lowering the degree of biodegradation. 

Test duration 

39.80. For rate and pathway studies duration of exposure to sunlight (natural or artificial) should 

normally not exceed 30 environmental solar days. The soil layers should be incubated in appropriate static 

or flow-through systems for time periods corresponding to the above limits. For example, a period of 240 

hours (= 10 days) corresponds to approx. 30 days of natural midsummer sunlight (calculated by 

information given in paragraphs 17-20 or using intensity values of Annex 3). 

Performance of the test

40.81. After treatment with the test item, the soil thin-layers are placed in an incubation system such as 

the ones illustrated in Annex 4. The system is then sealed and placed beneath the xenon light source or 

exposed outside to natural sunlight. The soil thin-layers are then irradiated for appropriate time intervals 

under the test conditions described above (see paragraphs 36-39). Irradiation in the laboratory may be 

carried out continuously or discontinuously simulating day and night cycles (e.g. 12 hours each). 

41.82. At least, duplicate soil thin-layers are removed at appropriate time intervals and the soil samples 

are extracted with appropriate solvents and analyzed for test item and/or phototransformation products. If 

applicable, absorption solutions are removed at the same time intervals and analyzed for volatile products. 

Besides a soil sample taken directly after application (0-hour sample) at least 6 sampling points should be 

included. Time intervals should be chosen in such a way that pattern of decline of the test item and patterns 

of formation and decline of phototransformation products can be established (e.g. 0, 17, 24, 41, 65, 137 

and 240 hours). 

42.83. When using 14C-labeled test item, non-extractable residues will be quantified by combustion and 

a mass balance will be calculated for each sampling interval. 

DATA AND REPORTING

Treatment of results

43.84. The amounts of test item, phototransformation products, volatile products and non-extractable 

should be given as % of applied initial dose for each sampling interval. Also a mass balance should be 

given in percentage of the applied initial dose for each sampling interval. A graphical presentation of the 

test item percentages against time will allow an estimation of its phototransformation half-life or DT50 
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under the experimental test conditions. Major phototransformation products should be identified and their 

percentages should also be plotted against time to show their rates of formation and decline. 

44.85. More accurate determinations of half-lives or DT50 values and, if appropriate, DT90 values should 

be obtained by applying appropriate kinetic model calculations and their conversion to environmental 

sunlight conditions (see paragraphs. 17-20). The half-life and DT50 values should be reported together with 

the description of the model used, the order of kinetics and the correlation coefficient (r). If appropriate, the 

calculations should also be applied to the major phototransformation products. Examples of appropriate 

models are described in references (29) to (34). 

Test report

45.86. The test report must include: 

Test item: 

 - Common name, chemical name, CAS number, chemical structure (indicating position of label 

when radiolabeled material is used) and relevant physical-chemical properties (see para. 7) 

 - Purity (impurities) 

 - Radiochemical purity of labeled chemical and specific activity (where appropriate). 

 Test soil: 

 - Details of collection site 

 - Date and procedure of soil sampling 

 - Properties of soil, such as pH, organic carbon content, texture and water retention 

characteristics 

- Length of soil storage and storage conditions. 

 Xenon light source: 

- Manufacturer and model 

- Filters and their purpose 

 Spectrum of the light source between 290 and 800 nm given by the manufacturer or measured 

at the beginning and the end of the test 

- Comparison of the spectrum of the light with that of natural sunlight 

- Light intensity measured at sample level (Watt • m-2) and area irradiated. 

 Test conditions: 

 - Dates of the performance of the studies 

 - Amount of test item applied 

 - Solvents used and method of application for the test item 

- Area of soil thin-layers treated 

- Description of the incubation system used 

- Air flow rates (if applicable) 

- Temperature of experimental set-up 

- Soil moisture content during incubation 

- Method(s) of extraction 

- Methods for quantification and identification of the test item and major phototransformation 

products in soil and absorption solutions or traps including LOD and LOQ 

- Number of replicates. 

 Test results: 

- Tables of results expressed as % of applied dose for the soil thin-layers 
ENV/JM/TG/RD(2006)1/REV 

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- Mass balance during and at the end of the studies 

- Quantification of non-extractable (bound) radioactivity or residues in soil 

- Quantification of volatile products 

- Plots of percentages in soil versus time for the test item and, where appropriate, for the major 

phototransformation products 

- Assessment of phototransformation kinetics and half-life or DT50 for the test item and, where 

appropriate, for major phototransformation products including confidence limits 

- Proposed pathway of phototransformation, where appropriate 

- Discussion and interpretation of results, i.e. significance of results to environmental behavior 

of test item. 

LITERATURE

(1) US-Environmental Protection Agency (1982). Pesticide Assessment Guidelines, Subdivision N. 

Chemistry: Environmental Fate. 

(2) Agriculture Canada (1987). Environmental Chemistry and Fate. Guidelines for registration of 

pesticides in Canada. 

(3) European Union (EU) (1995). Commission Directive 95/36/EC amending Council Directive 

91/414/EEC concerning the placing of plant protection products on the market. Annex I: Fate and 

Behaviour in the Environment. 

(4) Dutch Commission for Registration of Pesticides (1995). Application for registration of a 

pesticide. Section G: Behaviour of the product and its metabolites in soil, water and air. 

(5) SETAC (1995). Procedures for Assessing the Environmental Fate and Ecotoxicity of 

 Pesticides. Mark R. Lynch, Ed. 

(6)(7) OECD (1993). Guidelines for the Testing of Chemicals. Paris. OECD (1994 – 2000): 

Addenda 

 6-11 to Guidelines for the Testing of Chemicals. 

(7) OECD (2000). Draft Guideline “Phototransformation of Chemicals in Water” – Direct and 

Indirect Photolysis. 

(8) ISO 10977 (1993). Photography – Processed photographic colour films and paper prints – 

Methods for measuring image stability. 

(9) Zepp, R.G. and Cline, D.M. (1977). Rates of direct photolysis in aquatic environment. Environm. 

Science and Technology, 11, 359-366. 

(10) OECD (1997). Guidance Document on Direct Phototransformation of Chemicals in Water 

Series on Testing and Assessment No. 7. 

(11) Miller, G.C. and Zepp, R.G. (1983). Extrapolating photolysis rates from the laboratory to the 

environment. Residue Reviews, 85, 89-110. 
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(12) Mill,T., Mabey, W.R., Bomberger, D.C., Chou, T.-W., Hendry, D.G. and Smith, J.H. 

(1982).Laboratory Protocols for Evaluation of the Fate of Organic Chemicals in Air and Water. 

U.S. Environmental Protection Agency; EPA 600/3-82-002. 

(13) Parker, S. and Leahey, J.P. (1988): Development of a method to investigate the photodegradation 

of pesticides. Brighton Crop Protection Conference – Pests and Diseases – 1988, 663 – 668. 

(14) Wypych, G. (19..?). Handbook of Material Weathering. Second Edition, Chapter 4, 61-94. 

(15) Burkhard, N. and Guth, J.A. (1979). Photolysis of organophosphorus insecticides on soil 

surfaces. Pestic. Sci. 10, 313 – 319. 

(16) Guth, J.A. (1980). The study of transformations. In Interactions between Herbicides and the Soil. 

R.J. Hance, Ed., Academic Press, 123 – 157. 

(17) Guth, J.A. (1981). Experimental approaches to studying the fate of pesticides in soil. Progress in 

Pesticide Biochemistry. D.H. Hutson, T.R. Roberts, Eds. J. Wiley & Sons. Vol. 1, 85 – 114. 

(18) Soil Texture Classification (US and FAO systems). Weed Science, 33, Suppl. I (1985) and Soil 

Sci. Soc. Amer. Proc. 26, 305 (1962). 

(19) Methods of Soil Analysis (1986). Part I, Physical and Mineralogical Methods (A. Klute, Ed.). 

Agronomy Series No. 9, 2nd Edition. 

(20) Methods of Soil Analysis (1982). Part 2, Chemical and Microbiological Properties (A.L. Page, 

R.H. Miller and D.R. Kelney, Eds.). Agronomy Series No. 9, 2nd Edition. 

(21) ISO Standard Compendium Environment (1994). Soil Quality – General aspects; chemical and 

physical methods of analysis; biological methods of analysis. First Edition. 

(22) Mückenhausen, E. (1975). Die Bodenkunde und ihre geologischen, geomorphologischen, 

mineralogischen und petrologischen Grundlagen. DLG-Verlag, Frankfurt/Main. 

(23) Scheffer, F. and Schachtschabel, P. (1998). Lehrbuch der Bodenkunde. F. Enke Verlag, Stuttgart. 

(24) Schlichting, E., Blume, H.-P. and Stahr, K. (1995). Bodenkundliches Praktikum: eine 

Einführung in pedologisches Arbeiten für Oekologen. 2nd Edition. Blackwell, Berlin-Oxford. 

(25) OECD (1995). Final Report of the OECD Workshop on Selection of Soils/Sediments. Belgirate, 

Italy, 18-20 January 1995. 

(26) ISO 10381-6 (1993). Soil Quality – Sampling – Part 6: Guidance on the collection, handling and 

storage of soil for the assessment of aerobic microbial processes in the laboratory. 

(27) Anderson, J.P.E. (1987). Handling and storage of soils for pesticide experiments. In Pesticide 

Effects on Soil Microflora. L. Somerville, M.P. Greaves, Eds. Taylor & Francis, 45-60. 

(28) Ciani, A. (2003). Photolysis processes in soils and other porous media. Diss. ETH No. 15306. 
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(29) Hamaker, J.W. (1976), The application of mathematical modelling to the soil persistence and 

accumulation of pesticides. Proc. BCPC Symposium: Persistence of Insecticides and Herbicides, 

181 – 199. 

(30) Goring, C.A.I., Laskowski, D.A., Hamaker, J.W. and Meikle, R.W. (1975). Principles of 

pesticide degradation in soil. In Environmental Dynamics of Pesticides. R. Haque and V.H. 

Freed, Eds., 135 – 172. 

(31) Timme, G. and Frehse, M. (1980). Statistical interpretation and graphic representation of the 

degradational behaviour of pesticide residues. I. Pflanzenschutz – Nachrichten Bayer 33, 47 – 60. 

(32) Timme, G., Frehse, H. and Laska, V. (1986). Statistical interpretation and graphic representation 

of the degradational behaviour of pesticide residues. II. Pflanzenschutz – Nachrichten Bayer 39, 

188 – 204. 

(33)(35) Hill, B.D. and Schaalje, G.B. (1985). A two compartment model for the dissipation of 

deltamethrin on soil. J. Agri. Food Chem., 33,1001-1006. 

(34)(36) Gustafson, D.I. and Holden, R.L. (1990). Nonlinear pesticide dissipation in soil: A new model 

based on spatial variability. Environm. Science and Technology, 24, 1032-1038. 
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ANNEX 1: DEFINITIONS AND UNITS

Test item: any substance, whether the parent compound or relevant transformation products. 

Transformation products: all substances resulting from biotic or abiotic transformation reactions of the 

test item including CO2 and products that are in bound residues. 

Bound residues: “Bound residues” represent compounds in soil, plant or animal, which persist in the 

matrix in the form of the parent substance or its metabolite(s)/transformation product(s) after extraction. 

The extraction method must not substantially change the compounds themselves or the structure of the 

matrix. The nature of the bond can be clarified in part by matrix-altering extraction methods and 

sophisticated analytical techniques. To date, for example, covalent ionic and sorptive bonds, as well as 

entrapments, have been identified in this way. In general, the formation of bound residues reduces the 

bioaccessibility and the bioavailability significantly (1) [modified from IUPAC 1984 (2)]. 

Phototransformation: reactions occurring on soil thin-layers under the influence of light (> 290 nm). 

Soil is a mixture of mineral and organic chemical constituents, the latter containing compounds of high 

carbon and nitrogen content and of high molecular weights, animated by small (mostly micro-) organisms. 

Soil may be handled in two states: 

(a)(c) undisturbed, as it has developed with time, in characteristic layers of a variety of soil types; 

(b)(d) disturbed, as it is usually found in arable fields or as occurs when samples are taken by 

digging and used in this guideline (3). 

Xenon lamp: An intensive source of ultraviolet, visible and near-infrared light produced by electrical 

discharge in Xenon under high pressure. 

Intensity: Traditional term for photon flux, irradiance or radiant power (radiant flux). In terms of an 

object exposed to radiation. The term should now be used only for qualitative descriptions. 

Irradiance (E) [W • m-2]: The radiant flux or radiant power P of a all wavelengths incident on an 

infinitesimal element of surface containing the point under consideration divided by the area of the element 

(dP/dS; simplified expression: E = P/S when the radiant power is constant over the surface area 

considered). 

Radiant Power (P), [J • s-1 or W]: Power emitted, transferred or received as radiation. 

Half-Life, t0.5 is the time taken for 50 % transformation of a test item when the transformation can be 

described by first-order kinetics; it is independent of the concentration. 

DT50 (Disappearance Time 50) is the time within which the concentration of the test item is reduced by 

50 %; it is different from the half-life t0.5 when transformation does not follow first order kinetics. 

DT90 (Disappearance Time 90) is the time within which the concentration of the test item is reduced by 

90 %. 

The limit of detection (LOD) is the concentration of a substance below which the identity of the substance 

cannot be distinguished from analytical artifacts. 

The limit of quantification (LOQ) is the concentration of a substance below which the concentration cannot 

be determined with an acceptable accuracy. 
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LITERATURE REFFERRED IN THE ANNEX 1

(1)(4) DFG: Pesticide Bound Residues in Soil. Wiley – VCH (1998). 

(2)(5) T.R. Roberts: Non-extractable pesticide residues in soils and plants. Pure Appl. Chem. 56, 945 – 

956 (IUPAC 1984). 

(3)(6) OECD Test Guideline 304 A: Inherent Biodegradability in Soil (adopted 12 May 1981). 
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ANNEX 2: SPECIMEN LIGHT SOURCE AND APPARATUS (if artificial light is used)

A.C. Suntest® CPS+ Irradiation Unit

1: Refrigerated circulating chiller 

connected to cooling platform, etc. 

2. SUNTEST unit with irradiation 

chamber inside (not visible here), 

usually equipped with xenon lamp 

NXE 1500 B 

3. Cooling platform. 

4. Data logger connected to a 

thermocouple 
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ANNEX 2 (continued)

B.D.Spectrum Of Unfiltered Xenon Arc Compared To Sunlight

C.E.GLOBAL RADIATION

Global radiation (data from CIE Publication No. 20, 1972) 

Wavelength Irradiance Percentage of total radiation in each range

(nm) (W*m-2) (%)

280-400 68 6.1

400-800 580 51.8

800-1400 329 29.4

1400-3000 143 12.7

Total 1120 100

Sum (280-800): 57.9

Compiled data taken from J. Boxhammer (Original Hanau Heraeus GmbH, Hanau) : XenonBestrahlungseinheiten und Ultratest Bestrahlungseinheiten – Basiselemente zur Projektierung von 

Expositions- und Prüfanlagen. 
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ANNEX 3 

LIGHT INTENSITIES OF A XENON LAMP AND OF NATURAL SUMMER SUNLIGHT

A. XENON LAMP1

Position 

No. 

Irradiance E (300 – 400 nm) 

[W/m2

] 

r* 

 Start End Mean 

1 + 69.2 69.7 69.4 1.036 

2 78.0 78.9 78.4 1.171 

3 79.6 80.7 80.1 1.196 

4 ++ 79.2 80.8 80.0 1.194 

5 78.5 79.9 79.2 1.182 

6 75.3 76.4 75.9 1.132 

7 + 63.3 65.0 64.1 0.957 

Mean 74.7 ± 6.2 75.9 ±6.2 75.3 1.124 ± 0.09 

 75.3 ± 6.0 

(1) U. Morgenroth, personal communication. 

+ edge positions 

++ central position 

* r = ratio between artificial and natural light 

B. NATURAL SUMMER SUNLIGHT

[Values taken from references (12) and (13)] 

Region latitude Irradiance E (300 – 400 nm) 

[W/m2

] 

Florida, USA 30°N 66.98 

Greensboro, NC, USA 40°N 68.98 

Basel, Switzerland 50°N 65.32 

Mean 30 – 50°N 67 ±3 % 
ENV/JM/TG/RD(2006)1/REV 

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ANNEX 4A

EXAMPLE OF A TEST SYSTEM FOR PHOTOTRANSFORMATION EXPERIMENTS ON SOIL 

SURFACE (15)
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ANNEX 4B

EXAMPLE OF A TEST SYSTEM FOR PHOTOTRANSFORMATION EXPERIMENTS ON SOIL 

SURFACE (13)

A. INCUBATION AND COOLING TANK 

B. SOIL PHOTOLYSIS PLATE 
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ANNEX 4C 

EXAMPLE OF A TEST SYSTEM FOR PHOTOTRANSFORMATION EXPERIMENTS ON SOIL 

SURFACE (Static system)

A. Test Vessel for Soil Photolysis Studies 

trap for CO2 and volatile organic compounds

Quartz wool

Soda lime (for adsorption of atmospheric CO2 ): 0.5g

Quartz wool

Soda lime (for adsorption of CO2 emanating from soil): 2g

Quartz wool

Polyurethane plug for adsorption of volatile organic compounds

quartz glass cover

 soil layer

E.g. the test systems consist of glass vessels 

(36-mm inner diameter, 35-mm height, base 

area 10.2 cm2

) each containing 3 g soil (dry 

weight), which provide an about 3-mm soil 

depth. A glass neck with ground joint (NS 10) 

is attached to the side of the wall. There, the 

flask is closed with a solid trap attachment, a 

small glass tube of 90 mm length and 12 mm 

inner diameter, in which volatile compounds 

are bound to soda lime and polyurethane foam. 

The trap is packed in the sequence: PU-foam 

plug → quartz wool → 2 g soda lime → quartz 

wool → 0.5 g soda lime → quartz wool (see 

Figure). The quartz glass cover (thickness 3-

mm) is sealed with metallic clips and the 

ground glass joint was covered with a rubber 

septum. 

B. Test Vessels Position in the Suntest® CPS+ Irradiation Unit 

Light source

Measuring cell Irradiation vessel

a b c d

Sample platform

Distance of light source ..from the cooling 

..from the measuring 

..from the soil 

..from the quartz lid (top) of the 

a: approx. 230 

b: approx. 230 

c: approx. 226 

d: approx. 195 

Measuring cell: e.g. XenoCal WB 300-800 nm (Atlas MTT GmbH, Germany)