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APPLICATION OF REACTIVITY CRITERIA I N

OXIDANT-RELATED EMISSION CONTROL IN THE USA

B . Dimitriades and S . B . Joshi*

ABSTRAC T

The u6 e oJ the neacti-v.i,ty cA.i teAian in ox,cdant-neta..ted em.i.abion contAot .i.a

aeex,amined in the tight 06 hecenZ n.ebeanch 6.indinga on po.Q.eci tAn,t t4andpotc.t and

on ovWa.wt chem.i,a tic.y. Occunh.ence o 6 po.U.utan t tAa.nopon.t .i,d ~n.te&pne.ted to

actate that ne.a.c,t.i.v.i,t,i.2a os onganic.d be mea.acviced underi .Co ngen .itkad.i,a.ti.on and

gneaten on.gan.i.c-to-n.iticogen-ox.idez A r-ti.o cond-i.aona than pne.v.i.ou.aty thought.

Re.aati,v,i,t.i,ea unden such cond,i.ti.o ►-cb have been me.a.buned by .the Fnv.i)r.onme.n.taZ

Pno.tec.ti.on Agency Son beveAat ongan,i,ca and neau.P.t6 ate - u.ae.d hete to buppotc,t a

new two-ctada ne.ac,ti.v.i,ty c.ea.4ai6ic.a.ti.on ob onganic emi.ba .i.onb . Pn.ob .2em6 a.aaociated az.i.th the smog chumben meaawcement oS such neacti.viti..e.z and w.lth the uae

ob h.inetl.c indices ob neaa.t,i.v .i,ty ane d.i.acuaae.d.

INTRODUCTION

The reactivity concept was born out of early laboratory studies tha t

showed that organic emissions vary .widely with respect to their ability to cause

photochemical oxidant formation in the ambient air . As of today, the concept

has been in use in control practices-in the U . S . in the form of regulations

calling for selective control of organic solvent emissions . The best known of

these regulations is the Rule 66 (1) . It was designed for use in Los Angeles,

but it was adopted -- 'intact or slightly modified on -- by several air pollution

agencies outside California . The Federal Government has issued a drastically

different guideline for use of the reactivity concept (2), but has not objected

to the use of Rule 66 by the various State and local governmental agencies .

Both the Rule 66 and the Federal Government guideline are not entirely consistent with the latest scientific evidence regarding the role of organic

emissions in the ambient oxidant problem (3) .

In their present form, these regulations impose two limits to organic

solvent emissions : a high limit (e .g . 3000 lb/source/day) for the nonreactive

emissions and a low limit (e .g . 15 or 40 lb/source/day) for the reactive ones .

Enforcement of two such limits means that the thrust of the regulatory measures

is not so much toward emission reduction as it is toward substitution of nonreactive emissions for reactive ones . In fact, in theory at least, these regulations could cause an increase in the amounts of emissions discharged into the

a~~here . This possibility is now thought to have some adverse consequences .

B . Dimitriades, U .S . Environmental Protection Agency, Research Triangle

Park, North Carolin a

S . B . Joshi, Northrop Services, Inc ., Research Triangle Park, North Carolina

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p. 2

Because of these problems, the United States Environmental Protectio n

Agency (USEPA) is in the process of reexamining the use of' the reactivity

concept, in the light of the considerable research findings reported in the last

4 to 5 years . The following is a presentation and discussion of some thoughts

that evolved out of such a reexamination .

REASSESSMENT OF THE USE AND UTILITY OF THE REACTIVITY CONCEPT

RECENT RESEARCH FINDINGS AND IMPLICATION S

There are two aspects of the reactivity issue that need to be reexamined,

First, there is the question of whether or not application of the reactivity

concept has thus far been demonstrably beneficial . The second aspect pertains

to the relative roles of the "reactive" and "nonreactive" organics when such

organics are'subjected to pollutant transport conditions . These aspects need to

be reexamined both separately and in conjunction .

Recent air quality data in the U .S . have indicated that in some localities

the air quality improved, in others it deteriorated, and in others it showed no

effect in the years since the initiation of emission control . From detailed

studies of the impact of emission control on oxi dant air quality '(4,5), it can

be deduced that the impact of the reactivity-related solvent control regulations

cannot be directly assessed . This is simply because such impact is masked by

'14 the overwhelming effects of other factors such as a reduction in organic auto

emission (which is indiscriminate rather than reactivity-related), an increase 4

in nitrogen oxides emissions, growth and/or shifting of human activities, etc .

The judgement submitted here is that the impact of any reactivity-related

control measure upon air quality can be assessed only indirectly based on

laboratory evidence ; direct atmospheric observation is inconclusive .

The second aspect of the reactivity issue, the one that deals with the

relative roles of the "reactives" and "unreactives," likewise can be examined

only indirectly based on laboratory evidence . Relative contributions of the

"reactive" and "unreactive" organics to ambient oxidant and, in general, the

relative roles of such pollutants cannot be determined from aerometric data .

In conclusion, laboratory evidence -- that is, smog chamber evidence --

with all its uncertainties arising from its indirect na ture, continues to

provide the only basis for reassessing the use and utility of the reactivity

concept . For such a reassessment, therefore, it is necessary first that the .~

recent laboratory evidence be presented and *critically examined .

Recent evidence relevant to the issue consists mainly of smog chamber data

suggesting that, under the relatively high organic-to-NO ratio and prolonged

irradiation conditions prevailing in transported air masses, many of, the though-'

to-be unreactive organics could, in fact, cause significant oxidant buildup

(6,7) . This is illustrated by the reported data (6,7) and more pointedly by the

data of Table 1, obtained from an on-going EPA study . These interpretations, *

however, of the recent smog chamber data and associated conclusions have been

questioned by some investigators on grounds related to the smog chamber background contamination problem . Specifically, it has been claimed that, i n

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general, norireactive organics exhibit enhanced reactivity in smog chamber

s

because of the presence of a substantial background of hydroxyl ( OH) radicals in

such chambers (8) . Furthermore, such OH concentrations, were estimated (from

the EPA data on methane ( CH4) disappearance) to be 1 .5 x 10 - 1 .7 x 108 molecule/cm3 ; that is, they were comparable to or higher than the concentrations

observed in the real atmosphere (8) .

Such claims, although qualitatively correct, do not invalidate the original .~ interpretation of the recent smog chamber data, namely, that under optimum

~. conditions many of the thought-to-be unreactive oraanics can in fact cause

J, significant ozone ( 03) buildup . To.explain, consider all possible situation s

that cause chamber background OH buildup . First, background OH could form from

photolysis of nitrogen- compounds desorbing off the chamber walls . Such a

source, however, is unlikely to be important relative to the source created by

the NO reactant itself in the chamber . Second, and more likely, background OH

could ~orm from photochemical reactions of wall-desorbed organics ( and inorganics ganics) and the NOx reactant ) . However, considering that such desorbed organics

are at almost immeasurably low concentrations, it may be deduced that this

S source of OH is also unavoidably present ' in the real atmosphere as natura l

o background contamination . Alternatively, the bias caused by the background OH

radical problem can be reduced - - but not eliminated -- by subtracting from

measured reactivity values the background air reactivity . Finally, irrespectiv e

i ns' of the chamber background OH problem, it has been established that conditions of

, prolonged irradiation and an optimum organic-to-NOx ratio do enhance the reactivities of the less reactive, organics .

In conclusion, the laboratory evidence suggests that in consideration of

.. .,the pollutant transport phenomenon, the existing reactivity classification of

organics should be revised so as to reclassify most . of the "unreactive" organics

into the single class of "reactives ." The question of precisely where the

borderline should be separating the two classes is a crucial ' but extremely

complex one, and it cannot be answered definitively based on the evidence presently available unless some simplifying assumptions are made . In answer to this

question, these authors offer one procedure for defining the boundary between

reactives and unreactives . This procedure is based on use of reactivity data

;obtained in an appropriate chamber under optimum irradiation time and reactant

organic-to-NOx ratio conditions . This procedurp is intended to identify as "nonreactive" those organics which under no circumstances will yield oxidant/03

toncentrations exceeding 0 .08 parts per million ( ppm) . This procedure is illustrated in the next section, using the data of Table 1 . Note, however, that th e

' ;

.COnditions under which these data were obtained may not be ttle optimum ones .

~dditionai tests using a variety of organic-to-NO

,

ratio conditions should be

.-4de -- upon the least reactive organics only -- in order to obtain the reata quisite "optimum " values .

ught . REVISED REACTIVITY CLASSIFICATION

t From the data of Table 1, it would appear rational at first glance that the

ndary point separating the reactive organics from the unreactive ones should

n the reactivity value for propane . Such a judgment, however, presupposes tha t

ppm/0•2 ppm mixture of propane/NO would yield the same concentration of 03 t

he real atmosphere as in EPA's smog chamber .

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1

TABLE I . REACTIVITIES OF ORGANICS MEASURED I N TH E EPA SMOG CHAMBERa1 (11 )

Reactant Maximum 03 Peak-03 time, Average rate of

Organicb/ conc., ppmc-/ hrs organic disappearance,

/

% per hrBackground air 0 .03-Y -- --

Background air + 0

.2 ppm NOx

.01 -- 0

Methane 0 31 0.05

Acetonitri'le, 0 28 0.02

io--s-r Benzaldehyde 0 3.1 2.4

7; ~1,1,1-trichloro- 0 4•6 0.1

ethane

Chloroform 0 2.4 0 .8

~-'Benzene 0.02 1.5 3.1

7;=-o 9-2Methylenechloridec9<<:tz 0 .03 1.8 5 .7

/11 - ()Ethane 0.03 3.0 0.5

74-6-6 Propane 0 .08 3.6 2 . 0

7-; . .,.:'.-.--'Acetylene 0.10 5.3 6. 3

=-0Isobutyl acetate 0 .18 4.3 4.3

l r Acetone 0.18 12.1 0 . 9

-_,n-Butane 0.23 11.9 1. 4

, Methanol 0.25 12.3 1.3

Acetic Acid 0.26 13.7 2.

.28 13.8 1 . 0 't-Butanol 0 1

=,Methyl ethyl ketone 0 .30 .8 .3 1. 5

=i-Propanol 0.32 8.6 3.3

Acrylon.itrile . 0.37 5.5 5.3

=1Perch loroethylene 0 .49 1.8 12. 8

a

/

Chamber was a 400-liter Pyrex glass cylinder irradiated externally with

22 B1ac!<light and 7 Sunlamp fluorescent lamps .

b/Initial reactant concentrations were 4 .0 ppm and 0 .2 ppm for the organic

and NO

.,

respectively . Each organic was tested at least in duplicate .

Irradiation was continued until 03 peaked out .

L/Reported data represent observed maximum 03 concentrations less the maxim um 03 concentration observed for background air (0 .03 ppm) . 14

d/Corrected for dilution . Perchloroethylene, methylene chloride, and chloro

form showed considerably higher rates of disappearance after peaking of 03• ~

e/Value represents highest 03 yield observed in 6 replicated tests

randomly dispersed within the reactivity measurement program .

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While the above presupposition may or may not be correct -- it can never b e

checked directly -- the uncertainties involved are not necessarily prohibitive .

Comparison of the smog chamber and natural sunlight radiation data indicates

that the light intensity in the EPA chamber is comparable to that of natural

Sunlight . Note that small differences in the radiation factor between " smog

chamber and real atmosphere are of no serious consequence because, for low

reactivity systems, such differences affect mainly the peak-03 time and very

-l ittle the maximum 03 concentration . The effect on peak-03 time, however,

(assuming l unlimited irradiation time under pollutant transport conditions)

should not cause the reactivity measurement results to be drastically different

than those in Table 1 . Relevant important factors, other than radiation, are

the chamber background contamination , .the chamber wall surface, and dilution .

The contamination problem has already been discussed ; adjusting the reactivity

measurement by subtracting the background air reactivity is simplistic but

probably the most acceptable solution to this problem . Thus, for example, after

such adjustment, the chamber reactivity of methape is zero, essentially in

agreement with the photochemical model predictions (8,9) . Furthe rmore, th e

~. surface of the EPA chamber ( Pyrex. glass) is not expected to alter drastically

the dominant steps of the oxidant formation process . Finally, dilution in the

smog chamber tests may not be as intensive as in ordinary real atmospheres .

However, the difference can be tolerated with the justification that ( a) it is

not believed to be extremely large, and (b) it makes the chamber data represent

the "most unfavorable" situation, a situation often used in control deliberations .

Summarizing the reasoning in the preceding paragraph, the smog chamber

reactivities listed in Table 1 should not be drastically different than those

expected to be manifested in the real atmosphere . This means that the suggestion that the propane reactivity is the boundary point separating the "re-

.actives" from the "unreactives" would not be grossly wrong . Furthermore, whatever overall error exists is probably in the direction that will produce high

reactivity values, that is, a downward misplacement of the boundary point .

Fortunately, however, raising the boundary point above the propane reactivity

value, e .g ., by as much as 100% ( i .e' ., to 0 .1b ppm 03), does not seem to produce

A much different classification because the 0 .08-0 .16 ppm interval is not expected to be densely "populated" by reactivity values of organics . Thus, for

example, all halogenated paraffins are expected to have reacti v,i ti es comparabl e

to those of chloroform, methylene chloride, and l,l,l-trichloroethane ( i .e . ,

below that of propane), whereas all alcohols, ketones, acids and esters are

expected to be comparable to or more reactive than acetone, methanol, and acetic

a cid, (i .e ., much more reactive than propane) . These expectations, of course,

nted to be verified by testing commonly emitted organics in addition to those

listed in Table 1 . This raises the next question, concerned with the reactivity

10jsurement method .

The reactivity measurement method commonly used thus far is the smog

~er method entailing irradiation of the test organic-NOx mixture and mea14refl ent of resultant oxidant yeild . Use of certain' kinetic entities ( such a s

of organic precursor disappearance or rate constant for the OH-organic O;tioi~ as substitutes for or indices of reactivity has also been proposed

general, the chamber data. on oxidant yield are inherently more vali d

.1e quire extensive and tedious - :smog chamber tests covering a wide range of

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reactant organi c-to-N0 ratio condi ti ons . The kinetic indices of reactivity are

valid only to degrees 8epending on their correlation with the oxidant yield

data . In the case of the simple, two-class classification scheme proposed here,

the most important reactivity data needed are tho se for the least reactive

organics, namely, for the organics comparable to or less reactive than propane .

For those organics, the oxidant yield reactivity is extremely difficult to

measure reliably because such a measurement is sensitive to interferences from

background contamination and organic reagent impurities . Measurements of the

rate of organic reactant disappearance and of the rate- constant for the OHorganic reaction apparently can be made more reliably (10), but the results do

not seem to correlate well with the oxidant yield data, at least, judging from

the . data in Table 1 . Thus, of the 18 organics listed in Table 1, benzaldehyde,

benzene, methylene chloride, and acety3ene showed rates of organic reactant

disappearance that were disproportionately high relative to their oxidant

yields (Table 1) . It is not known to what degree this inconsistency reflects

experimental error or peculiarities ., in the chemistry of these compounds . Clearly,

additional research is needed (a) to establish the relative oxidant-yield reactivities of low-reactivity organics more reliably and for a larger variety of

reactant organic-to-NOX ratio conditions, and (b) to further explore the correlations of oxidant yields with rates of organic precursor reactions and wit

h OH-organic reaction rate constants for a variety of organic compounds, especiall y

halocarbons

. Id

CONCLUSIONS A

1 . Laboratory evidence continues to . provide the only basis for assessing the

utility and recommending the use of the reactivity criterion in the oxidantrelated control'of organic emissions :

2 . Occurrence of the pollutant transport phenomenon suggests that the existing

reactivity classification of organics be revised to consist of two classes,

reactives and unreactives, defined as follows :

• The unreactive organics are . those which under optimum irradiation and

organic-to-NO ratio conditions in an appropriate smog chamber yield no

more than 0 .09 ppm of 03-

v Reactive organics are all organics except those found by measurement or

theoretical estimation to be unreactive .

Alternatively, if the recent EPA smog chamber data are to be accepted, unreaa :tive organics are defined as those with reactivities lower than the reactivity

of propane .

3 . Reactivities of unreactive .organics -are extremely difficult to measure

reliably . Uncertainity problems are caused either by experimental error associated with the smog chamber measurement or by the questionable validity of the

kinetic indices of .reactivity .

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REFERENCES

e 1, Los Angeles Air Pollution Control District . Rule 66 .

2, Federal Register, 36, 15502, Aug . 14, 1971, Appendix B .

3 . Proceedings of the Solvent Reactivity Conference . EPA-650/3-75-010, Nov .

1974, U .S . Environmental Protection Agency, Research Triangle Park, N .C .

27711 .

4 . Trijonis, J ., T .K . Peng, G .T . McRae, and L . Lees . Emissions and Air Quality

Trends in the South Coast Air Basin . EQL Memorandum No . 17, Environmental

Quality Laboratory, California Institute of Technology, Pasadena, Calif ,

' Jan. 1976 .

5 . Wayne, L .G ., K .W . Wilson and C .L . Boyd . "Detection and Interpretation of

irly, Trends in Oxidant Air Quality ." Final Report to EPA from Pacific Environmental Services, Inc . Contract No . 68-02-1840, July 1976 .

of

6 . Altshuller, A .P ., S .L . Kopczynski, D . Wilson and W .A . Lonneman . Photochemical Reactivities of n-Butane and other Paraffinic Hydrocarbons, JAPCA, 19,

p . 787, 1969 .

ally ~'~

7 . Heuss, J . "Smog Chamber Simulation of the Los Angeles Atmosphere ." Scientific Seminar on Automotive Pollutants . EPA-600/9-75-003, Feb . 10-12, 1975 .

~ U.S . Environmental Protection Agency, Research Triangle Park, N .C . 27711 .

8 . Weinstock, W ., and T .Y . Chang . " Methane and NonUrban Ozone ." Paper presented at National APCA Mtg in Portland, Oregon, June 29, 1976 .

9 . Dimitriades, B ., M . Dodge, J . Bufalini, K . Demerjian, and A .P . Altshuller, ing Envir . Science and Technology (In press) 1976 .

~. ;

10 . Darnall, K .R ., A .C . Lloyd, A .M . Winer, and J . Pitts, Jr . "Reactivity Scale

for Atmospheric Hydrocarbons Based on Reaction with Hydroxyl Radical . "

no Envir. Sci & Technology, 10, No . 7, pp . 692-696, July 1976 .

:~.

~1 . Reactivities of Organics Under Pollutant Transport Conditions, S .B . Joshi

or and B .,~ . Dimitriades, EPA report in preparation, 1976 .

~

vity .1, . .-

. ~:.

; othe

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