1

Methodology to Determine the Global Warming Potential Values (GWP) of 3M 

Chemicals 

E12-0510

Brian T. Mader

3M Environmental Laboratory

August 2, 2012

Executive Summary

3M has completed a corporate greenhouse gas inventory since 2002. As a part of the effort, and 

for over 20 years, 3M has developed methods for determining the Global Warming Potentials 

(GWP) of compounds that are not listed in the IPCC assessment reports. The methodology is

described in this report. It includes the use of Excel-based programs to conduct calculations that 

are based on IPCC methodology. The inputs into these equations include atmospheric lifetime 

and radiative forcing. Included in the report are descriptions of the methods that are used to 

determine these values. These include analytical measurements and testing and, in the absence 

of this data, methods that can be used to estimate these values. 
2

Introduction

3M has completed a corporate GHG inventory since 2002. As a part of this effort methods have 

been developed to determine the Global Warming Potentials (GWP) of fluorinated greenhouse 

gases (f-GHG) that are discharged at some of its manufacturing operations. This report 

summarizes the methods that have been used to make these determinations. Included is a 

description of the basic methods that are used to determine the GWP’s and the validation of

calculation methods. The appendix addresses procedures that have been used to provide inputs 

for these calculations along with techniques that are used to estimate these values in the absence 

of analytical data.

Determination of Global Warming Potential

This section discusses the methods that 3M uses to determine the GWP for fluorinated-GHG’s. 

They are consistent with published IPCC protocols. Individual calculations are performed for 

each chemical using Excel spreadsheets. These calculations are complicated and our 

methodology was validated by comparing calculated GWP’s to the published IPCC values. 

The global warming potential of a compound is calculated using Equation 1.

Equation 1

∫

∫

∫

∫ = = TH

CO CO

TH

i i

TH

CO

TH

i

i

a C t dt

a C t dt

RF t dt

RF t dt

GWP

0

2 2

0

0

2

0

( )

( )

( )

( )

Where: TH is the time horizon (usually 100 years), RFi the radiative forcing of the compound, ai

the radiative forcing per unit mass increase of atmospheric concentration of the compound (a.k.a 

the radiative efficiency) and Ci(t)dt the time dependant concentration of the compound. RFCO2

the radiative forcing of the CO2, aCO2 the radiative forcing of CO2 per unit mass increase of in the 

atmospheric concentration of CO2 and CCO2(t)dt the time dependant concentration of CO2. 

The GWP is the ratio of the absolute global warming potential (AGWP) of the compound of 

interest) to the AGWP of CO2.

Equation 2

∫ ∫ = =

TH

i i

TH

AGWPi RFi t dt a C t dt

0 0

( ) ( )

Equation 3

∫ ∫ = =

TH

CO CO

TH

AGWPCO RFCO t dt a C t dt

0

2 2

0

2 2 ( ) ( )
3

The atmospheric lifetime of the compound of interest is modeled as first order Ci(t) dt as 

follows:

Equation 4

kT

i C t dt e − ( ) =

Where ki is the first order rate constant of the given compound and t is the time horizon. 

Equation 2 becomes:

Equation 5

∫ ∫ − = =

TH

k T

i

TH

AGWPi RFi t dt a e dt i

0 0

( )

Solving the integral in Equation 5 the AGWP for the given compound is

Equation 6

i

TH

i i MW

e AGWP a

τ

τ

/ 1 − − = × ×

Where τ is the atmospheric lifetime, which is defined as the amount of time it takes for the 

concentration of a compound to decrease to 1/e = 1/2.72 = 37% of its original concentration and 

MWi is the molecular weight of the compound. Values of ai and τ are provided by the IPCC in 

each assessment report. Therefore the AGWP of a given compound can be calculated using 

Equation 6 should the values of ai and τ change to due better measurements or calculations. 

If the radiative forcing and/or atmospheric lifetime are unavailable for a given compound, 

laboratory measurements, quantitative structure activity relationships, or other accepted scientific 

methods are used to determine to these values. A description of these methods is provided in the 

appendix of this report.

The AGWP for CO2 is more complicated due the numerous processes that control the 

atmospheric lifetime of CO2 (i.e. respiration, land use changes, air/ocean exchange, ocean 

chemistry, marine productivity, the rate of mixing of shallow and deep ocean waters, and 

anthropogenic CO2 emissions). The value of aCO2 is determined using several formulas and 

models to calculate the radiative forcing in units of W m-2 and the fraction of this forcing is then 

related to the global average CO2 concentration to yield aCO2. The value of aCO2 reported in the 

2007 AR4 is 0.014134 Wm-2 ppm-1 and 8.7% lower than the 0.01548 Wm-2 ppm-1value listed for 

CO2 in the 2001 TAR. The most complicated value in the AGWP for CO2 is the term given in 

Equation 7. which is non linear and cannot be simplified by first order type reaction as was the 

case for the compound of interests. 

Equation 7

C t dt CO ( ) 2
4

In this case a model is used to determine the change in concentration of CO2 over the given time 

horizon. The model used is the Bern Carbon cycle model. In this case the decay in a pulse of 

CO2 over the time horizon is calculated from Equation 8 and the fitting parameters provided by 

the model

 Equation 8

τ

t

i i a ∑ a e = +

3

1 0

Where a0 = 0.217, a1 = 0.259, a2 = 0.338, a3 = 0.186, τ1 = 172.9, τ2 = 18.51, τ3=1.186

The GWP of a compound can therefore be calculated using Equation 9.

Equation 9

44

0.217 0.259 172.9 1 0.338 18.51 1 0.186 1.186 1

1

172.9 18.51 1.186

2

/

2

































+ × − 















+ × − 















× + × −

− × ×

= =

− − −

−

TH TH TH

CO

i

TH

i

CO

i

i

a TH e e e

MW

e

a

AGWP

AGWP GWP

τ

τ

Where ai the radiative forcing of a given compound per unit mass increase of atmospheric 

concentration in units of W m-2 ppbV-1, τ is the atmospheric lifetime of the given compound in 

years, TH is the time horizon (usually 100 years), MWi is the molecular weight of the compound 

in grams per mole, aCO2 is the radiative forcing of CO2 and is equal to 0.0000141 W m-2 ppbV-1

and 44 the molecular weight of CO2.

Equation 9 was used to calculate the GWPs in this report. To validate Equation 9 the GWPs of 

several PFCs were calculated and compared to the values listed in the IPCC 2007 AR4. The 

calculated values are provided in Table 1 and are essentially the same as those listed in the IPCC 

document. 

For short-lived GHGs, for example having lifetimes less than one week, the methods listed in the 

IPCC will overestimate the GWP as these compounds are not well mixed in the atmosphere. In 

this case transport of the fluorochemicals is determined using a suitable chemical transport 

model to provide a more accurate GWP value. This method is described in the reference by 

Acerboni et al. (2001) who observed that the radiative forcing was 8 to 23 times lower for shortlived perfluorinated alkenes using the chemical transport model versus the well mixed 

assumption of Pinnock et al. (1995).

6

Appendix: Methods to Measure or Estimate the Radiative Forcing 

and Atmospheric Lifetime of Chemicals

A. Introduction

3M estimates GWP’s global warming potentials for compounds which have no IPCC assigned 

values, or for which no global warming potential have been found in the literature by using an 

Excel-based model. This was originally developed for the Chemical Manufacturers Association 

in 1992 by Atmospheric and Environmental Research, Inc (AER). 3M has subsequently modified 

portions of this model so that it now also calculates global warming potentials according to IPCC 

2007 and WMO 1998 criteria. The WMO 1998 criteria are essentially equivalent to Climate 

Change 2001 criteria. Inputs needed for this model are: 

1. Atmospheric lifetime in years

2. Molecular weight

3. Radiative forcing factor in watts pre square meter per part per billion by volume 

(W/m2/ppbv). 

B. Atmospheric Lifetime

The atmospheric lifetime of a chemical may be controlled by several factors such as, reaction of 

the compound with hydroxyl radical, direct photolysis, rainout, or nighttime reaction with nitrate

radicals. In this case, the atmospheric lifetime is calculated as follows:

∑

=

=

=

i n

atm i 1 i

1 1

τ τ

Equation 1

Where τatm is the overall atmospheric lifetime, and τi is the ith factor that is affecting lifetime. In 

many cases one reaction will dominate the overall atmospheric lifetime and this rate is used for 

further GWP calculations. Methodologies for evaluating the atmospheric lifetime of a chemical 

for different mechanism are provided in the following sections.

1. Measurement of the Atmospheric Lifetime for Reaction of the Challenge 

Compound with Hydroxyl Radical

Should a sufficiently pure standard of the compound exist and if the compound has a structure 

amenable to reaction with hydroxyl radical, the atmospheric lifetime can be measured by 

determining the pseudo-first order reaction rate of the “challenge” compound relative to a 

“reference” compound such as methane or chloromethane (CH3Cl) in the presence of the OH 

radical at 22-28°C and a pressure of approximately one atmosphere. The concentrations are 

monitored using continuous in-situ Fourier Transform Infrared (FTIR) spectroscopy as well as 

Gas Chromatography Mass Spectrometry (GC/MS).
7

The compound, the reference compound, humidified nitrogen, dry nitrogen and ozone are mixed 

using mass flow controllers and sent into the FTIR cell. When the concentration of the gases 

reached the target value, the cell is sealed. During the initial step in the experiment, the quartz 

cell window is covered to block any visible and UV light and the reagent gas concentrations are 

monitored for nominally 30 minutes to evaluate wall losses. Such losses are typically minor (i.e. 

less than 1% lost in 30 minutes). The UV light is then switched on and the gases inside the cell 

are exposed to the UV radiation and hydroxyl radicals formed from the photolysis of ozone in 

the presence of gas phase water. The decay of the gas concentrations in presence of UV light 

radiation was monitored until the concentration of ozone becomes low enough such that it is no 

longer in great enough excess for the system to be considered pseudo-first order with respect to 

hydroxyl radical.

Intermediates can be formed from the degradation of the compound and the FTIR spectra of 

these product(s) can be similar to the FTIR spectrum of the original compound. Therefore 

GC/MS is used to ensure that only the compound of interest is being measured and to screen for 

possible degradation products. The rate constant are reported based on the FTIR data using

spectral regions that contain none or minimal portions of any intermediate chemicals. 

The rate constant for the compound is calculated using the ratio of the slopes of the challenge 

compound and reference compound, i.e. the relative rates approach (Pitts, 2000). Method 

validation studies have been done at 3M comparing the rate constants and atmospheric lifetime 

of a challenge compounds to values in the published literature. In a validation study utilizing

HFE-7100 as the challenge compound and chloromethane (CH3Cl) as the reference compound, 

the measured rate constant and atmospheric lifetime of HFE-7100 from the 3M laboratory 

studies are within 3% of the value calculated by Wallington et al. 1997.

Table 1. Data Summary

3M Environmental Lab Study E10-0322

kHFE7100 / kCH3Cl 0.35

kCH3Cl 3.60E-14 cm3 molecule-1 s

-1

kHFE7100 1.24E-14 cm3 molecule-1 s

-1

Wallington et al. data

kHFE7100 / kCH3Cl 0.34

kCH3Cl 3.60E-14 cm3 molecule-1s

-1

kHFE7100 1.21E-14 cm3 molecule-1 s

-1

Percent Difference 3%
8

2. Estimating Atmospheric Lifetime for Reaction of the Challenge Compound 

with Hydroxyl Radical

In some cases, 3M estimates the atmospheric lifetimes using the quantitative structure activity 

relationship (QSAR) based method of Cooper et al. This QSAR method uses regression 

equations correlating ionization potentials and hydroxyl radical (OH•) reaction rate constants of 

hydrogen containing fluorochemicals. The 3M modification uses regression equations correlating 

the ionization potential and the OH• rate constant divided by the number of hydrogens in the 

molecule. The ionization potentials used in these regression equations were estimated using the 

CAChe Chem implementation of MOPAC PM3. CAChe Chem is a QSAR program that can 

calculate many molecular properties of a compound from its molecular structure. The hydroxyl 

radical (OH•) reaction rates used to develop these regression equations were obtained from the 

literature or derived by 3M. Ionization potentials for a few compounds present in 3M emissions 

that were not used in the regression equations were calculated more recently using the same 

MOPAC PM3 model, in a later version of the CAChe software called MOPAC2002 Version 

1.33CAChe. Determination of a compound's ionization potential with this later version is noted 

in its GWP worksheet. It is expected that the ionization potential (also called the highest 

occupied molecular orbital (HOMO) energy) calculations by the newer and older versions would 

be identical. This was confirmed by a recalculation of the HOMO for 1-hydroperfluorobutane by 

3M. The result, IP = -13.512 eV, was precisely the same as that in the spreadsheet from the 

earlier version of the CAChe software. The data used in the preparation of these QSARs, the 

regression equations themselves, and graphs of these regression equations are given in an Excel 

worksheet. Four regression equations have been prepared for four different classes of 

fluorochemicals.

To determine the ionization potential of a new compound, the exact molecular structure is used

in estimating ionization potentials using the CAChe Chem program. The CAChe Chemcalculated ionization potential, referred to as the "HOMO," and the appropriate regression 

equation to estimate the molecule's OH reaction rate constant. For example HFE-n-401 has the 

following structure CF3CF2CF2CF2-O-CH3. Since this compound is an HFE, a regression 

equation for the HFEs was used to produce an estimated rate constant for its reaction with 

hydroxyl radical. For most hydrogen containing halocarbons compounds, the reaction with 

hydroxyl radical in the troposphere will be the most important factor determining its atmospheric 

lifetime. One estimates the atmospheric lifetime (τ) of a compound (x) due to reaction with OH• 

by multiplying the ratio of the OH• reaction rate constants for the compound and methane by 9.6 

years. IPCC (2001) (Page 248) indicates that methane would have an atmospheric lifetime of 9.6 

years if its loss were entirely due to reaction with tropospheric OH•. [Note: earlier 3M estimates 

of (x) OH τ were bases on an atmospheric lifetime for methane of 8.9 years determined by Prinn et 

al. in 1995.

y k x

k CH

x

OH

OH

OH 9.6

( )

( ) ( ) 4 τ ≅ × Equation 2
9

3. Measuring the Atmospheric Lifetime for Direct Photolysis of the Challenge 

Compound

Should sufficiently pure standard of the compound exists and the compound has an absorbance 

in the UV spectrum amenable to direct photolysis, the atmospheric lifetime is measured by 

determining the first order reaction rate of the “challenge” compound relative to a “reference” 

compound such as acetaldehyde at 22-28°C during irradiation with light having a wavelength 

greater than 280 or 290 nm. 

A pure gas standard of challenge compound at a known initial concentration is prepared as well 

as a standard of the reference gas (i.e. acetaldehyde). The gas standards of the challenge and 

reference compounds are then delivered along with nitrogen to the reaction chamber using a 

manifold of mass flow controllers. The reaction chamber is a FTIR cell that is sealed and leak 

checked when concentrations reached the desired IR absorbance level. The initial step in the 

experiment, the quartz cell window was covered with to block any visible and UV light from 

entering the reaction chamber. The reagent gas concentrations are monitored for nominally 30 

minutes to evaluate wall losses. Typically such losses are minor i.e. less than 1% lost in 30 

minutes. The UV light is activated and the gases inside the chamber are exposed to the UV 

radiation from an Oriel model 6251 UV Lamp operated with a UV filter present to provide UV 

light greater than 280 or 290 nm (typical wavelength cutoff for the troposphere). The change in 

the gas concentrations in presence of UV light radiation is monitored until the concentration of 

the challenge and reference compounds become too low to accurately quantify using the FTIR. 

In a few cases. intermediates can be formed from the degradation of the compound and the FTIR 

spectra of these product(s) can be similar to the FTIR spectrum of the original compound. 

Therefore GC/MS is used to ensure that only the compound of interest is being measured and to 

screen for possible degradation products. The rate constants are reported based on the FTIR data 

using spectral regions that contain none or minimal portions of any intermediate chemicals. 

The rate constant for the compound is calculated using the ratio of the slopes of the concentration 

versus time profile of the challenge compound and reference compound, i.e. the relative rates 

approach (Pitts, 2000). Method validation studies have been done at 3M comparing the rate 

constants and atmospheric lifetime of a challenge compounds to values in the published 

literature. In a validation study utilizing perfluoro-2-methly-3-pentanone (3M Novec 1230) as 

the challenge compound and acetaldehyde as the reference compound, the measured rate 

constant corresponded to an atmospheric lifetime of Novec 1230 of 10.5 ± 0.4 that is within the 7 

to 13 day lifetime measured in published studies by Taniguchi et al (2003) and D’Anna et al 

(2005).
10

Table 2. Summary of the atmospheric lifetime for Novec 1230.

Study E12-0337

Parameter value units

kNovec 1230 / kAcetaldehyde 0.48 n/a

Atmospheric Lifetime of 

Acetaldehyde1 5 days

Atmospheric Lifetime of Novec 

1230 10.5 days

1

Seinfeld and Pandis, Atmospheric Chemistry and Physics: from air pollution to climate 

Change, 1998, p. 288,

4. Estimating the Atmospheric Lifetime for Direct Photolysis of the Challenge 

Compound

In some cases 3M may estimate the atmospheric lifetimes for direct photolysis using a structure 

activity relationship. The structure of the challenge compound is compared to one or a series of 

reference compounds for which direct photolysis is known to occur. If a similar chromophore is 

present in both the challenge and reference compound, the atmospheric lifetime of the reference 

compound may be used for the challenge compound, or an average of several reference 

compounds having a similar structure to the challenge compound. For example in some cases 

adding a CF2 group to a perflrouoroketone molecule will not greatly influence the atmospheric 

lifetime. In addition the UV spectrum of the challenge compound can be measured in the 

laboratory and used to assess the atmospheric lifetime. If the UV spectrum of the challenge 

compound has an absorbance feature at wavelengths observed in the troposphere, and the 

absorbance features of the challenge and reference compound are similar, the atmospheric 

lifetime of the reference compound may be used for the challenge compound, or an average of 

several reference compounds having a similar structure to the challenge compound. 

5. Estimating Atmospheric Lifetime for Removal of the Challenge Compound 

by Rainout and/or Transformation in Cloud Water Droplets 

Carbonyl fluorides, such as COF2 and CF3C(O)F, are not predicted to react with hydroxyl 

radicals, nor through direct photolysis at significant rates in the troposphere, and are removed 

essentially entirely into water droplets (Nolle et al., (1992), DeBruyn et al., (1992), Wallington et 

al., (1994). In these cases 3M estimates the atmospheric lifetimes for rainout or transformation 

in cloud water droplets using a structure activity relationship from data in the literature. The 

structure of the challenge compound is compared to one, or a series of reference compounds, for 

which rainout or cloud transformations are known to occur. Data from Wallington et al. (1994) 

is provided in Table 3. Note that in Table 3, the lower limit of five days represents the rate 

limitation for the compound to transfer from the gas phase into the cloud. To estimate the 

atmospheric lifetime for a chemical by rainout or transformation in cloud droplets for which no 
11

lab or literature data is available the structure of the compound is compared to the structure of 

the compounds in Table 3 and an assumption is made regarding its atmospheric lifetime. 

Table 3. Lifetime of Selected Carbonyl Fluorides (Wallington et al., 1994)

Lifetime in Troposphere

Compound Clouds (days) Ocean (years)

COF2 5-10 0.3 – 2.0

CF3C(O)F 5-15 0.3-3.0

HC(O)F 150-1500 80

CF3C(O)Cl 5-30 1-10

C(O)ClF 5-20 0.5 -5.0

C. Estimating Radiative Forcing

In using the 3M model in Excel one needs to express the radiative forcing factor in terms of 

W/m2

/ppbv to calculate the IPCC 1995, 2001, 2007 reported global warming potentials.

1. Pinnock Method for Estimating Forcing

If an unsaturated infrared spectrum exists for the compound, 3M calculates the adjusted cloudysky radiative forcing in W/m2

/ppbv from the infrared spectrum of the compound using the 

method described in 1995 by Pinnock et al. This method divides the infrared spectrum from 0 

cm-1 to 2500 cm-1 into 250, 10-cm-1 bands. For each band, Pinnock et al. have determined a 

weighting factor. This factor is multiplied by the average absorption cross section of the 

compound in the band, which is multiplied by 10 (the band width in cm-1

). The product for each 

band is then summed over the 250 bands. The sum gives the instantaneous cloudy-sky radiative 

forcing of the compound in W/m2/ppbv. As shown here:

i i

i

i Inst Rad Forcing ∑ σ aveFσ

=

= = 2501 . . 10 Equation 3

where the weighting factor i Fσ (W m-2 (cm-1

)

-1 (cm2 molecule-1

)

-1

) is the radiative forcing per 

unit cross section per inverse centimeter in band i, and i σ ave (cm2 molecule-1

) is the cross section 

averaged over each 10 cm-1 band. Pinnock et al. developed these weighting factors by running 

an imaginary compound in their model with the same weak cross section at all wavelengths. 

They obtained the following spectrum:
12

The absorption cross section, σ , is defined as 

σ = −ln(T)/ Nl Equation 4

(See, for example, page 54 of Finlayson-Pitts, 2000) In this equation, N is the concentration in 

molecules/cm3, l is the pathlength in cm, and T is the spectral transmittance as defined by the 

following equation. 

0 I

I T = Equation 5

In this equation, I is the observed (i.e., the reduced) intensity of the radiation after passing 

through the sample and 0 I is the incident intensity upon the sample. 

3M follows the Addendum to EPA Method 320 for FTIR measurement of gaseous emissions. 

This method reports the observed FTIR readings as absorbance ( A ) where

I

I

I

I A 0

10

0

10 = −log = log Equation 6

0

1

2

3

4

0 500 1000 1500 2000 2500

Radiative Forcing

[10-3 W m-2 (cm-1)-1 (10-18 cm2 molecule-1)-1]

Wavenumbers (cm-1)

Figure 1. Instantaneous, cloudy-sky, radiative forcing per unit 

cross section as a function of wavenumber

(Pinnock, et. al., Figure 3)
13

3M reports the concentration of the absorbing gas times the pathlength (Cb ) in units of parts per 

million by volume times meters (ppmV-m). The conversion from A and ppmV-m to σ is done 

as follows:

At one atmosphere and at 25°C, 1 ppmV = 2.46 X 1013 molecules cm-3 and 1 m = 100 cm. Thus: 

T

P

m

cm

ppmv cm

molecules Cb ppmv m X

cm

molecules Nl 298 ( ) * 2.46 10 *100 * *

3

13

2 − = −

Equation 7

Where P is the pressure (atm) in the spectophotometric chamber and T is the temperature (°K). 

The absorption cross section is then calculated as follows:

σ = 2.303A / Nl Equation 8

Where 2.303 is the constant used to convert from a base 10 to a base e logarithm, A is the 

measured log 10 absorbance, and Nl is concentration times pathlength in units of molecules cm-2. 

Thus, to determine the radiative forcing, one obtains a non-saturated IR spectrum with good 

resolution, e.g., 1 cm-1. The spectroscopist then processes the original absorbance spectrum by 

subtracting water features, setting all regions void of absorbance of the molecule of interest to 

zero, and correcting the baseline certain compounds. 3M also has a program to calculate the 

average absorbance values in 10 cm-1 wide bands from 0 to 2500 cm-1

. Availability of 

averaged absorbance values simplify Pinnock calculations. The analyst then pastes the 

processed tabulated spectrum, or processed and averaged spectral bands into two columns of the 

Excel spreadsheet. The left column contains the wavenumber (cm-1); the right column contains 

the base 10 log absorbance (A) at that wavenumber. If the spectrum is in descending 

wavenumber order, it is sorted to put it into ascending order. This spreadsheet uses a 

combination of equations 9 and 10 to convert log10 absorbance (A) to molecular cross section. 

This parameter, referred to as sigma (σ ), has units of cm2

/molecule. 

The Excel file can also be used to determine the average molecular cross section (σ ave ) for each 

10 cm-1 band, starting with the 0 to 10 cm1 band and ending with the 2490 to 2500 cm-1 band. 

This is accomplished by averaging all of the molecular cross section measurements from the 

tabulated spectra in each 10-wavenumber wide band. The spreadsheet then labels these bands 5, 

15, 25, ...2495. The user then uses "paste-special-value" followed by "paste-special-transpose" 

commands to convert the averaging functions to values and to place these σ ave values from a row 

into a column of the spreadsheet so that the σ ave for each wavenumber band is in the same row 

with the weighting factors for that band. These weighting factors come from Table 8 of the 1995 

Pinnock et al. paper.

The spreadsheet multiplies each σ ave by the Pinnock et al. weighting factor for that band, and by 

10 (the width of each band in cm-1

). The spreadsheet then sums the 250 products to give the 
14

"instantaneous forcing". This is the forcing that would occur instantaneously if the compound 

were dispersed in the atmosphere. 

When calculating GWPs, 3M attempts to approximate the adjusted cloudy-sky radiative forcing. 

For many compounds this can be estimated by increasing the instantaneous forcing by 7% as 

described on page 23,237 of Pinnock et al., 1995. The adjusted forcing is the forcing adopted by 

IPCC to calculate GWPs. This forcing is the net change in irradiance at the tropophere that 

would occur once the stratospheric temperature profile has adjusted to the perturbation caused by 

the introduction of the compound, but where the troposphere and surface has not come to 

equilibrium. (See page 353 Climate Change 2001. This simple adjustment should not be made 

for compounds like HFC-41 which have absorbance that strongly overlaps with that of ozone 

(between 1000 to 1100 cm-1). According to Pinnock et al.(1995), page 23,233) compounds with 

strong absorbance in this wavenumber range would absorb earth-radiated IR in the troposphere 

that would normally be absorbed by ozone in the stratosphere. Thus, they would reduce 

stratospheric heating caused by the O3 absorption of this IR. This cooling decreases the adjusted 

radiative forcing. 

Table 4 of Pinnock et al. shows that, instead of needing a +7% correction to adjust from cloudyinstantaneous to cloudy-adjusted radiative forcing, HFC-41 needs a –6.8% adjustment. The IR 

spectrum of HFC-41 published in Figure 1 of Pinnock et al. (see Figure 2 below) enabled a rough 

estimate of its average absorption cross section. 

Figure 2. IR absorption Spectrum of HFC-41 (Figure 1 of Pinnock et al.)

The procedure to make this estimate involved making 25 evenly spaced measurements of the 

cross section [10-18 cm

2 molecule-1

] between 1000 and 1100 cm-1 and averaging them. The 

resulting average cross section in this band was 0.59 [10-18 cm

2 molecule-1

]. The blacked peaks 

in the spectrum of HFC-41 suggest that its absorbance has a lot of fine structure. This would 
15

likely make the absorption cross section of HFC-41 calculated as described above somewhat 

larger than its actual average cross section in the 1000 to 1100 cm-1 band. 

The cut-off that 3M uses for strong absorption between 1000 and 1100 cm-1 is 1/4th of this 

roughly calculated average cross section of HFC-41 in this band. This 1/4th value was chosen 

because, if one assumes that this average cross section in this band decreases adjusted forcing in 

proportion to its magnitude, then the 7% correction applied to a compound with an average cross 

section in this band greater than 1⁄4 that of HFC-41 would give a "corrected" adjusted value that 

would be a less accurate approximation of the actual adjusted radiative forcing than the 

instantaneous radiative forcing calculated by the Pinnock method. Thus, the adjusted radiative 

forcing, determined by multiplying the instantaneous radiative forcing by 1.07, is used in GWP 

calculations for compounds with average cross sections in this 1000 to 1100 cm-1 band that are < 

0.148 [10-18 cm2 molecule-1], ( < 1⁄4 that of HFC-41). The instantaneous radiative forcing is used 

in GWP calculations for compounds with average cross sections > 0.148 [10-18 cm2 molecule-1] 

in this band. 

Therefore, if the average cross section in the 1,000 to 1,100 cm-1 band is < 0.148 [10-18 cm

2

molecule-1] then

Adj. Rad. Forcing = 1.07 * Inst. Rad. Forcing Equation 9

Otherwise we use the instantaneous cloudy-sky radiative forcing in GWP calculations.

The Excel spreadsheet also multiplies the instantaneous forcing by 1.07 to approximate the 

"adjusted radiative forcing" that would be determined by more sophisticated modeling. As 

described above the user of the spreadsheet uses this estimated "adjusted radiative forcing" in the 

subsequent global warming potential calculations unless the compound (like ozone) absorbs 

strongly between 1000 and 1100 cm-1

. In the spreadsheet the average cross section for the test 

compound between 1000 and 1100 cm-1 is calculated and the spreadsheets prepared with the 

current template make a recommendation to the user as to whether to use the adjusted or 

instantaneous forcing based on the criteria described above. 

The user should note the wavenumber range of the spectrum used in the calculations. If the 

spectrum is missing measurements in the 400 to 600 cm-1 ranges, where IR absorption could 

produce significant forcing, the calculated global warming potentials could be low. Some FTIR 

equipment does acquire data between 0 and 640 cm-1 in these cases, should a chemical have an 

absorption in this range it would result in a low GWP estimate. This can normally be determined 

based on compound structure. If this would occur, then an assessment can be made regarding the 

IR absorption in th Spectra used by the Environmental Laboratory typically omit the 0 to 640 cm1 range and thus could give low GWP estimates. 

2. 3M QSAR Method for Estimating Radiative Forcing 

In some cases, estimates of the radiative forcing of a compound are conducted using quantitative 

structure activity relationships (QSAR). The QSAR used here is a linear regression equation that 
16

correlates the number of C-F bonds in a class of fluorochemicals, i.e. HFCs, PFCs etc. to the 

published radiative forcing values in units of W/m2/ppbv. 

Thus, to estimate of the radiative forcing factor in W/m2/ppbV, is determined using the number 

of C-F bonds in the given class of compound, i.e. HFC, PFC. The appropriate regression 

equation is then used to calculate the radiative forcing factor. 3M currently has five sets of 

regression equations for different classes of fluorochemicals that we developed using data from 

the IPCC assessment reports or literature.

3. 3M Method for Estimating Radiative Forcing by Comparison to Analogs

A method somewhat analogous to the QSAR method estimating the radiative forcing of the test 

compound based on the radiative forcing of the most structurally similar chemical compound. 

For example, in the application of this approach to sulfur containing compounds, forcing was 

assumed to be entirely due to the IR absorbance caused C-F or S-F bonds. Thus, the forcing of 

the test compound was normalized to that of the reference compound based on the number of CF or S-F bonds relative to the reference compound. 

4. Estimating Radiative Forcing of HFEs and PFEs from "Corresponding" 

Alkanes

When a radiative forcing value exists for a class of fluorochemicals, i.e. HFC, PFC, but no 

radiative forcing value exists for another class (i.e. HFE, or perfluoroethers (PFE)), 3M may 

estimate the radiative forcing of the HFE or PFE by multiplying the radiative forcing of the 

"corresponding" HFC or PFC by 1.7. As defined in this report, "corresponding" pairs of HFCs 

and HFEs (or PFCs and PFEs) are compounds with identical fluorine and hydrogen substitution. 

Each carbon in these "corresponding" molecules, i.e., the first, second, and third carbons, etc. in 

these pairs of molecules had identical hydrogen and fluorine substitution. Their structure differed 

only in that an oxygen molecule was inserted somewhere in the carbon chain of the HFE or PFE. 

When there is no data available for the "corresponding" HFC, the radiative forcing of the HFE is 

estimated by multiplying the mean radiative forcing of all of the available "similar" HFCs by 1.7. 

As defined in this report, "similar" HFEs and HFCs had the same number of Cs, Hs & Fs. That 

is, the HFCs had the same empirical formula, as the HFE, except for oxygen, meaning they had 

the same number of carbon, hydrogen, and fluorine atoms, although every carbon was not 

substituted identically. The details and logic of these estimation procedure are described below. 

A comparison of HFC and HFE radiative forcing and atmospheric lifetime data in Tables 6.7 and 

6.8 of Climate Change 2001 was made to determine if the forcing or lifetime of HFEs could be 

predicted from data on "corresponding" or "similar" HFCs. Data on four corresponding pairs of 

HFCs and HFEs are found in these tables. Data from these tables were also compared for seven 

sets of "similar" HFEs and HFCs. 

In these calculations it was observed that the radiative forcing values of the HFEs were greater 

than the radiative forcing of the "corresponding" and "similar" HFCs. For both the 

"corresponding" pairs and "similar" groups, the geometric means of the ratio of the HFE: HFC 
17

radiative forcing was 1.7. The range in these HFE/HFC radiative forcing ratios for the 

"corresponding" pairs was 1.3 to 2.5. The ratio of the mean radiative forcing between the 7 

groups of "similar" HFEs and HFCs ranged from 1.2 to 2.7. It therefore appears that radiative 

forcing of a HFE from a "corresponding" HFC can be reasonably predicted by multiplying the 

radiative forcing of the HFC by 1.7. We can also conclude that the radiative forcing of an HFE 

can be estimated from the mean radiative forcing of a group of "similar" HFCs by multiplying 

the mean radiative forcing of the "similar" HFCs by 1.7. This approach was used when no IR 

spectral data was available for an HFE.

We would note that the HFC and HFE comparison also found that there was no consistent trend 

in the relationship between the atmospheric lifetimes of "similar" and "corresponding" HFEs and 

HFCs. The ratio of lifetimes for "corresponding" pairs of HFE to HFC ranged from 0.3 to 5. 

This lifetime ratio ranged from 0.01 to 5 times for the "similar" groups of HFEs to HFCs. HFEs 

that had only hydrogen substitution on one side of the ether linkage had much shorter 

atmospheric lifetimes than HFEs with fluorine substitution on both sides of the ether linkage. 

The atmospheric lifetime data do not support predicting the HFE lifetimes from the lifetimes of 

corresponding or similar HFCs. 

No data are available on the radiative forcing of perfluoroethers (PFEs) in Tables 6.7 and 6.8 of 

Climate Change 2001. 3M believes that it would be logical to assume that PFEs would have 

higher radiative forcings than PFCs and that the radiative forcing ratio would be similar to that 

between "corresponding" HFEs and HFCs. Therefore, in absence of other data, 3M predicts that 

the radiative forcing of a PFE would be 1.7 times the radiative forcing of the "corresponding" 

PFC and this approach has been used when no IR spectral data was available for the PFE.

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