1 M ethodology to D etermine the Global Warming Potential V alues (GWP) of 3M
Chemicals

E12

- 0510

Brian T. Mader 3M Environmental Laboratory August 2, 20 12

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 tha t are not listed in the IPCC assessment reports. The methodology is

de scribed in this repor t. It includes the us e 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 th ese 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 Potential s (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 th ese 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 address es procedures that have been used to provid e inputs for these calculations along with t echniques 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 pote ntial of a compound is calculated using Equation 1

.

Equation 1



## ∫ ∫

∫ ∫

= = TH

CO CO TH

i i

TH

CO

TH

i

i dt t C a dt t C a

dt t RF dt t RF

GWP

0 2 2 0

0 2

0

) ( ) (

) ( ) (

Where: TH is the time horizon (usually 100 years), RF i the radiative forcing of the compound, a i the radiative forcing per unit mass increase of atmospheric concentration of the compound (a

. k

. a the radiative efficiency) and C i ( t )dt the time dependant concentration of the compound. RF CO2 the radiative forcing of the CO 2 , a CO2 the radiative forcing of CO 2 per unit mass increase of in the atmospheric concentration of CO 2 and C CO2 ( t )dt the time dependant concentration of CO 2

.

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

interest ) to the AGWP of CO 2

.

Equation 2 

## ∫ ∫

= = TH

i i TH

i i dt t C a dt t RF AGWP

0 0 ) ( ) (

Equation 3 

## ∫ ∫

= = TH

CO CO TH

CO CO dt t C a dt t RF AGWP

0 2 2

0 2 2 ) ( ) (3 The atmospheric lifetime of the compound of interest is modeled as first order C i (t) dt as

follows:

Equation 4

kT

i 

### e dt t C

− 

### = ) (

Where k i is the first order rate constant of the given compound and t is the time horizon. Equation 2 becomes :

Equation 5 

## ∫ ∫

− = = TH

T k

i TH

i i dt e a dt t RF AGWP i

0 0 ) (

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

Equation 6

i TH

i i MW e a AGWP τ

τ / 1

−

− × × =

Where τ is the atmospheric lifetime

, which is def ined 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

MW i is the molecular weight of the compound. Value s of a i and τ are provided by the IPCC in each assessment report

. Therefore the AGWP of a given compound can be cal c u la ted using Equation 6 should the values of a i 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 the se values

. A description of these methods is provided in the appendix of this report.

The AGWP for CO 2 is more complicated due the numerous processes that control the atmospheric lifetime of CO 2 ( 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 CO 2 emissions). The value of a CO2 is determined using several formula s 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 CO 2 concentration to yield a CO2 . The value of a CO2 reported in the

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

CO 2 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 fi rst order type reaction as was the case for the compound of interests.

Equation 7
dt t C CO ) ( 24 In this case a model is used to determine the change in concentration of CO 2 over the given time

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

CO 2 over the time horizon is calculated from Equation 8 and th e f itting p ara m e t er s provided by

the model

Equation 8

τ t

i i e a a 

## ∑

= + 3

1 0

Where a 0 = 0.217, a 1 = 0.259, a 2 = 0.338, a 3 = 0.186, τ 1 = 172.9, τ 2 = 18.51, τ 3 =1.186

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

Equation 9

44 1 186

. 1 186

. 0 1 51

. 18 338

. 0 1 9

. 172 259

. 0 217

. 0 1

186

. 1 51

. 18 9

. 172 2 / 2

               

− × +        

− × +        

− × + × − × × = =

− − − −

TH TH TH

CO i TH

i CO

i
i

e e e TH a MW e a AGWP

AGWP GWP

τ

τ

Where a i 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), MW i is the molecular weight of the compound in grams per mole, a CO2 is the radiative forcing of CO 2 and is equal to 0.0000141 W m - 2 ppbV - 1
and 44 the molecular weight of CO 2

.

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 short

lived perfluorinated alkenes using the chemical transport model versus the well mixed assumption of Pinnock et al. (1995)

.6 

## Appendix: Methods to Measure or E st imate the Radiative Forcing and Atmospheric Lifetime of C hemicals

A. Introduction

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

values

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

Excel

- based model. T his was originally developed for the Chemical Manufacturers Association
in 1992 by Atmospheric and Environmental Research, Inc (AER)

. 3M has subsequently modified portions of this m odel so that it now also calculates global warming potentials according to IPCC

2007 and WMO 1998 criteria. The WMO 1998 criteria are essentially eq uivalent to Climate Change 2001 criteria. Inputs needed for this model are:

1. Atmospheric lifetime in years

2. Molec ular weight 3. Radiative forcing factor in watts pre square meter per part per billion by volume (W/m 2 /ppbv).



### B. Atmospheric Lifetime

The atmo spheric 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:



## ∑ =

= = n i

i i atm 1 1 1

τ τ Equation 1

Where τ atm is the overall atmospheric lifetime

, and τ i is the i th 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 atmo spheric 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 a nd if the compound has a structure amenable to reaction with hydroxyl radi cal, 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 ch loromethane (CH 3 Cl ) 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. D uring t he initial step in the experimen t, the quartz

cell window i s covered to block any visible and UV light and t he reagent gas concentrations a re monitored for nominally 30 minutes to evaluate wall losses. S uch 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 pre sence of UV light

radiation was monitored until the concentration of ozone become s 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

.

I ntermediates can be formed from the degradation of the compound and t he FTIR spectra of these product(s) can be similar to the FTIR spectrum of the original compound. Therefore

GC/MS i s used to ensure that only the compound of interest i s being measured and to screen for possible d egradation products. The rate constant are reported based on the FTIR data using spectral regions that contain none or minimal portion s of any intermediate chemical s

.

The rate constant for the compound i s calculated using the ratio of the slopes of the c hallenge 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 compound s to value s in the published literature. In a v alidation study utilizing

HFE

- 7100 as the challenge compound and chloromethane (CH 3 Cl ) 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. 199 7.

Table 1. Data Summary

3M Environmental Lab Study E10

- 0322

k HFE7100 / k CH3Cl 0.35

k CH3Cl 3.60E

- 14 cm 3 molecule - 1 s - 1

k HFE7100 1.24E

- 14 cm 3 molecule - 1 s - 1

Wallington et al. data

k HFE7100 / k CH3Cl 0.34

k CH3Cl 3.60E

- 14 cm 3 molecule - 1 s - 1

k HFE7100 1.21E

- 14 cm 3 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 consta nts 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 str ucture. 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 equatio ns 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 reg ression equations a re given in an Excel

worksheet . F our 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 Chem

calculated 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 CF 3 C F 2 CF 2 CF 2 - O

- CH 3 . 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 radica l 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 det ermined by Prinn et

al. in 1995.

y

x k CH k

x

OH OH

OH 6

. 9 ) ( ) ( ) ( 4 × ≅ τ Equation 29 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 mea sured by determining the first order reaction rate of the “challenge” compound relative to a “reference” compound such as ac etaldehyde at 22

- 28 ° C during irradiation with light having a wavelength greater than 280 o r 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. a cetaldehyde ). 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 concentration s 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

min utes to evaluate wall losses. Typically such losses are minor i.e. less than 1% lost in 3 0 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 presen t to provide UV

l ight greater than 280 o r 290 nm ( typical wavelength cutoff for t he troposphere ). The change in the gas concentrations in p resence of UV light radiation i s monitored until the concentration of

the ch allenge and reference compounds become t oo low to accurately quantify using the FTIR.

In a few cases. i ntermediates 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 s are reported based on the FTIR data using spectral regions that contain none or minimal portions of any intermediate chemica ls.

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 acetaldehyd e as the reference compound , t he measur ed rate constant corresponded to an atmospheric lifetime of Novec 1230 of 10.5 ± 0.4 that is within the 7

to 13 d ay 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

k Novec 1230 / k Acetaldehyde 0.48 n/a Atmospheric Lifetime of Acetaldehyde 1 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 st ructure 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 compo und

, the atmospheric lifetime of the reference compound may be used for the challenge compound, or an a v erage of several reference compounds having a similar structure to the challenge compound. For example in some cases adding a CF 2 group to a perflrou oroketone molecule will not greatly influence the atmospheric

lifetime. In add ition 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 a n 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

se veral 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 W ater Droplets

Carbonyl fluorides

, such as COF 2 and CF 3 C(O)F

, are not predicted to react with hydroxyl

radicals

, nor through direct photolysis at signi ficant rates in the troposphere

, and are removed

essential ly 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 st ructure 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 compo und to transfer from the gas phase into the cloud. To esti mate the atmospheric lifetime for a chemical by rainout or transformation in cloud droplets for which no11 lab or literature data is available the structure of the compound is compared to the struct ure 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)

COF 2 5

- 10 0.3

– 2.0

CF 3 C(O)F 5

- 15 0.3

- 3.0

HC(O)F 150

- 1500 80

CF 3 C(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/m 2 /ppbv to calculate the IPCC 1995, 2001 , 2007 reported global warming potentials.

1. P innock Method for Estimating Forcing

If an unsaturated infrared spectrum exists for the compound, 3M calculates the adjusted cloudy

sky radiative forcing in W/m 2 /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 fac tor. 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/m 2 /ppbv. As shown here:

i i

i i

ave F Forcing Rad Inst

σ σ 

## ∑ =

= = 250

1 10

. . Equation 3

where the weighting factor i 

### F

σ (W m - 2 (cm - 1 ) - 1 (cm 2 molecule - 1 ) - 1 ) is the radiative forcing per

unit cross section per inverse ce ntimeter in band i, and i

ave σ (cm 2 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



### Nl T /

) ln(

− = σ Equation 4

(See, for example, page 54 of Finlayson

- Pitts, 200 0 ) In this equation, N is the concentration in

molecules/cm 3

, l is the pathlength in cm, and 

### T is the spe ctral 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 2500Radiative Forcing[10-3W m-2(cm-1)-1(10-18cm2molecule-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 10 13 molecules cm - 3 and 1 m = 100 cm. Thus:

T P

m cm

cm ppmv

molecules X m ppmv Cb

cm molecules Nl 298
* * 100 * 10 46

. 2 * ) (
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 :

A 303

. 2 = σ / 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 a bsorbance, 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 absorban ce 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 band s from 0 to 2500 cm - 1

. Availability of averaged absorbance values simplify Pinnock calculation s. The analyst then pastes the processed tabulated spectrum, or processed and averaged spect ral 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

combinatio n of equations 9 and 10 to convert log 10 absorbance (A) to molecular cross section. This parameter, referred to as sigma ( σ ), has units of cm 2 /molecule.

The Excel file can also be used to determine the average molecular cross sectio n (

ave σ ) for each

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

Th is is accomplished by averaging all of the molecular cross section measurements from the tabulated spectra in each 10

- waven umber 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

ave σ values from a row

into a column of the spreadsheet so that the

ave σ 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 spreadsh eet multiplies each

ave σ 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 the14 "instantaneous forcing". This is the forcing that would oc cur 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 tropop her e that would occur once the stratospheric temperature profile has adjuste d 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), p age 23,233 ) c ompounds with strong absorbance in this wavenumber range would absorb earth

- radiated IR in the troposphere

that would n ormally be absorbed by ozone in the stratosphere. Thus, they would reduce stratospheric heating caused by the O 3 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 cloudy

instantaneous to cloudy

- adjusted radiative forcing, HFC

- 41 needs a

– 6.8% adjustment. T he IR spectrum of HFC

- 41 published in F igure 1 of Pinnock et al. (see F igure 2 below) enabled a rough estimate of its average absorp tion cross section.

Figure 2. IR absorption Spec t r u m 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 would15 likely make the absorption cross secti on 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 1/4 th of this roughly calculated average cross s ection of HFC

- 41 in this band. This 1/4 th 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

s ection 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 cm 2 molecule - 1 ], ( < 1⁄4 tha t of HFC

- 41). The instantaneous radiative forcing is used in GWP calculations for compounds with average cross sections > 0.148 [10 - 18 cm 2 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 approx imate 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 t he 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 s preadsheets 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 wo uld 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 c m 1 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 that16 correlates the number of C

- F bonds in a class of fluorochemicals, i.e. HFCs

, PFCs et c. to the published radiative forcing values in units of W/m 2 /ppbv.

Thus, to e stimate of the radiative forcing factor in W/m 2 / ppb V

, is determine d using the number

of C

- F bonds in the given class of compound , i.e. HFC

, PFC

. T he appropriate regression equation is then used to calculate the radiative forcing factor. 3M currently ha s five sets of regression equations for different classes of fluorochemicals that we developed usi ng 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 estimat ing the radiative forcing of the test compound based on the radiative forcing of the most structurally similar chemical compound. For example, i n the application of this approach to sulfur containing compounds, forcing was assumed to be entirely due to the I R 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 C

F or S

- F bonds relative to the reference compound.

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

When a radia tive forcing value exists for a class of fluorochemicals, i.e. HFC

, PFC, but no

radiat ive 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 HFC s and HFEs (or PFCs and PFEs) a re 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 identical ly. T he details and logic of the s e 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 cou l d 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 the se calculations it w as 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: HFC17 radiativ e 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 th at 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 gr oup 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 "sim ilar" 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 no t s upport 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 ass ume 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. Th erefore , 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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