## Estimates of the Global Warming Potential of Proposed Chemical Compounds

Prepared for 3M Industrial Chemical Products Division 3M Center Building 236-1 8-33 St. Paul, Minnesota 55144-1000 

## by Malcolm K.W. Ko David T. Chang
Atmospheric and Environmental Research, Inc. 840 Memorial Drive Cambridge, MA 02139 October 1992

### Table of Contents Executive Summary ......................................... .....................................................................iii 1 . Background.......................................... .... ... ... ...... ... ............................... ............ ......... 1 1.1 Atmospheric Ufetime... ............................................................................ ...... 1 1.2 Greenhouse Effect, Radiative Forcing and Equilibrium Temperature Response .................................................... .................... .. ...... 3 1.3 Global Warming Potential ................................... ..................... ..... ..... .. ........5 2. Description of Data Used in Analysis....... ......... ... ...... .............................. .............. 8 2.1 Data Used to Estimate Atmospheric Lifetime ...........................................8 2.2 Infrared Data Used to Calculate GWP ... ............. .... ................................... 9 3. Results ... .....:.:.............. ........ ............. ........ .... .............. ...... ..... .. ............... .. ..... ..... ....... 1 3 3. 1 Atmospheric Ufetime.... ....... ....... ........ .. ......................... ... ... .... .. ....... ........... 1 3 3.2 Direct, Equilibrium Surface Temperature Response ...... .......... ............ 15 3.3 Halocarbon Global Warming Potential and Global Warming Potential .. ...... ......... ... ............ .... .. ...... ......... .... ... ..... ........ ....... ... .... 17 References ... .......... ...................................................... .... ........ .......................... ................... 19 Appendix: Effect of Spectral Overlap in Greenhouse Forcing Calculations This report was prepared with the assistance ofthe following members of the AERstaff: ShepardA. 

### Clough Kevin Gurney Curtis Heisey Nien Oak Sze

ii 

### Gyula Molnar Karen Orth Mary SlayterExecutive Summary AER was asked by 3M to provide estimates for the radiative forcing and global warming potentials for the 6 chemical compounds listed below. Using infrared transmission data provided by researchers at the University of Denver and Massachusetts Institute of Technology (MIT), AER computed the IR absorption cross - section and calculated the radiative forcing for each compound using the AER 1-D RC model. With UV cross-sections and hydroxyl radical (OH) and excited oxygen atom O(lD) reaction rates provided by researchers at MIT, AER also calculated the lifetimes for each compound due to these removal processes. The radiative forcing and lifetime results were then used to calculate the global warming potentials at different integration time horizons .It should be emphasized that the estimates presented here will be subject to a variety of uncertainties such as the model calculated radiative forcing, the reaction rate measurements and the concentration of atmospheric OH, a naturally occurring species that reacts readily with many

compounds. The uncertainty has not been explicitly calculated, but order-of-magnitude estimates will be noted. The following table summarizes the calculated direct, equilibrium surface temperature response

**(~T).** lifetimes, halocarbon global warming potential, and global warming potential

(GWP) at three different time horizons for the given compounds. Values for CFC-11 are included for comparison. The lifetime values are based on upper limits for reaction rate constants with O(lD) and OH radical. If these are the primary removal mechanisms, these lifetime estimates represent lower limits. Lifetimes would likely be longer if the reaction rates with Q(lD) and OH are slower than estimated. However, lifetimes could also be shorter if more rapid removal processes exist for any of these compounds. iii**~T** 

### Lifetime* Compound (OC/ppbv) 

### (years) HGWP GWP(20) GWP(100) GWP(500) CFC-11 0.15 55** 1.0 4500 

### 3400 1400 Perfluorobutane 0.20 >1800 >25 4100 5500 8400 Perfluoropentane 0.24 >2100 >29 4100 5500 8600 Perfluorohexane 0.26 >2600 >33 3700 5100 8000

**Perlhro-2-00ty~** 

### 0.33 >2300 >30 3800 5200 8200 Perfluorotributylamine (MIT) 

### 0.48 >390 >4.6 3400 4300 4700 Perfluorotributylamine (UD) 

### 0.63 >390 >6.1 4500 5600 6100

**Perfluoro-N-meth~lmorpholine** 

### 0.27 >1600 >24 4500 6100 9200 * Based on O(lD) 

### and OH loss mechanisms only for all compounds except CFC-11. ** Lifetime cited

### in IPCC 1990.

iv1. Background Over the past two decades, two atmospheric pollution problems have elicited international concern. These are global wanning due to an enhanced greenhouse effect and stratospheric ozone depletion. The types of pollutants contributing to these problems range from ubiquitous emissions from fossil fuel combustion to chemicals used in specific i n dustries for particular tasks, such as the

use of chlorofluorocarbons to clean electronic circuit boards . In an effort to lessen these global atmospheric pollution problems, chemical companies are screening the chemicals they plan to produce by estimating their global warming potential and ozone depletion potential . The focus of this study is on the first of these two impacts, global warming . The following sections give a brief description of global warming and the index commonly used to compare the contribution different chemicals may make to this problem.

1.1 Atmospheric Lifetime

In order for a particular compound to warrant concern as a greenhouse gas, its concentration must build up to certain critical levels in the atmosphere. Generally speaking, the higher the concentration, the greater the chance of global warming. The concentration of a gas in the atmosphere at any given time is a function of both the rate at which it is emitted to the atmosphere and the rate at which it is removed. The emission rate of a human-produced gas is not a property of the compound, of course, but is determined by the nature and extent of its use. The rate of removal, on the other hand , is determined primaril y b y the molecular make-up of the compound, how it reacts with the radiation field and the concentration of certain oxidizing agents in the atmosphere, such as the hydroxyl radical (OH), excited oxygen atoms Q(ID), Cl, 0:3, and NO] . Given typical concentrations of these compounds in the troposphere, reaction with OH is likely to be the most important The chemical structure of a molecule can also be altered by photolysis in which radiant energy from the sun breaks the bonds holding a particular molecule

together. This process usually requires relatively energetic radiation. Consequently, photolysis occurs primarily in the stratosphe r e (above the troposphere which is the lower 11 kilo m eters of the atmosphere), a region where radiation of short wavelengths (UV) is found. This process is, therefore, less effective in the troposphere. A molecule, assuming it is soluble, can also be removed physically from the atmosphere by being absorbed onto a water (cloud, fog, etc) droplet and falling as rain. It could also adhere to a particle (dust, soot, etc) without chemically reacting with it, and be removed as dry deposition 1(note that these processes are reversible). In addition, chemical reactions can occurin the liquidphase oron solid surfaces through heterogeneousreactions. Removal in the ocean may also 

### be important. Therate ofremoval can beexpressed conveniently bya quantity called the total atmospheric lifetime. The total atmospheric lifetimeis a formal measure ofhow long (seconds, minutes, hours, days, years, and decades), on average, a molecule stays in the atmosphere before being either physically removed orchemically altered More specifically, it 

### is theexponential decaytime for the gas toreach 37% (1/e) ofitsoriginal concentration following introduction into the atmosphere. The total atmospheric lifetime mayrepresent the sumeffect ofa variety ofremoval processes potentially active within the atmosphere, such asreaction with OH orphotolysisin the stratosphere. Each oneofthese individual removal processes has, associated with it, a removal lifetime. Anestimate ofthe total atmospheric lifetimeisachievedby examining andcombining all these individual removal processes. Forexample, a compound that isremoved in the atmosphere by N mechanisms,each witl:l a globally-averaged atmospheric lifetime '!i, will have a global-average atmospheric lifetime '!atm• given by 

### i=N 1 

# L 1 

### -- - - 'tatm 'ti 

### i=l (1) In general, the global-average atmospheric lifetime ofa well-mixed compound associated with a particularremoval process 

### 'ti, can be given by 'ti 

## = 1/lq (2) where k; is the global-averageremoval rate constant (sec·I) associated with that particularprocess. A number ofuseful relationships can 

### be established oncethe total atmospheric lifetime is known. Forexample, given equal, continuous emissions, a long-lived gas will build up overtime to a greaterconcentration than a short-lived one. Ifsuch emissions continue until steady-state is achieved, the concentration at that time is equal to the product ofthe atmospheric lifetime and the emission rate. Furthermore, transport and mixing processes in the atmosphere will ensure that a 2release of a long-lived gas at a particular location will, in due time, exhibit a relatively uniform concentration over the entire globe. In this way, an initially local or regional phenomenon can eventually grow into a global presence. 1.2 Greenhouse Effect, Radiative Forcing and Equilibrium Temperature Response The temperature of the earth's surface and the atmosphere is maintained by energy balance between absorbed solar radiation and the radiation emitted by the earth. The atmosphere, however, plays a crucial role in regulating the temperature at the surface. Solar radiation entering at the top of the atmosphere is acted upon by a number of processes . Some of this radiation is immediately reflected back to space by clouds, sulfate aerosol and the surface of the earth and some is absorbed by the atmosphere. The majority, however, passes through the atmosphere and is absorbed by the surface of the earth. The temperature of the surface is regulated, in part, by this absorption. The

earth, acting as a blackbody, radiates energy back to space in the infrared portion of the electromagnetic spectnun thus achieving equilibrium with the incoming radiation. However, not all of this outgoing radiation passes directly to space. Some is absorbed by particular trace gases and clouds in the atmosphere and is re-radiated back to the surface. This process, known as the greenhouse effect, keeps the temperature of the earth approximately 32 °C higher than it would be in the absence of an infrared absorbing atmosphere. Gases responsible for this infrared absorption are referred to as greenhouse gases. Water vapor is one of the most effective naturally occurring greenhouse gases in the atmosphere.

In recent years, increasing amounts of human-produced chemical compounds have been emitted into the atmosphere. Some of these are greenhouse gases. As they build up and add to the naturally occurring greenhouse gases, the concern is that they may enhance the greenhouse effect and thereby increase the mean global temperature, a phenomenon generally referred to as global warming. Among the greenhouse gases which are increasing on a global scale are C(h, N20, Cl-4, and CFCs. There is some recent evidence that atmospheric sulfate aerosol is also increasing which may act as an anti-greenhouse agent (Charlson et. al., 1992). For a human-produced gaseous compound to contribute significantly to global warming, it must absorb infrared radiation in the so-called "atmospheric window" region of the earth's emission spectrum. This is illustrated in Figure la which shows (as a dashed line) the Planck function for a blackbody at 288 K, roughly corresponding to the radiance emitted by the earth's surface. However, only part of this energy reaches the top of the atmosphere. Also shown in the figure (as a bold line) is the actual radiance emitted by the surface that reaches the top of the 3atmosphere. The difference isdue toabsorption by greenhouse gases. With the exception ofthe spectral region between 750 

### cm·l and 1300em·I (theatmospheric window), the radiant energy emitted bythe earth's surface is totally absorbed by the atmosphere. The energy absorbed by the trace gases 

### isre-emitted in the atmosphereisotropically,i.e.,radiation isemittedequally in all directions. Panofthe energy 

### is directed at theearth's smface while theotherpan contributes to theupwellingradiation actually seen at the top ofthe atmosphere. Thecalculated upwelling radiation is shown as a function ofwavelength and height in Figure 1 b. A gas which strongly absorbs radiation in the window region will funherreduce the radiation emitted by the earth to space and thus enhance the greenhouse effect. Theability ofa particular gas to absorbinfraredradiation isprimarily a function oftwo intrinsic molecularproperties: which atomsconstitutethe absorbing gas molecules and the internal arrangement ofthose atoms. Most human-produced gases are polyatomic moleculesconsisting of more than two atoms. Polyatomic gas moleculesgenerally abso rb radiation in the infraredportion ofthe electromagnetic spectrum. The main issue, as far as enhancement ofthe global greenhouse effect isconcerned, is whetherornotthe infraredabsorption spectrum ofthe gas molecule in question coincides with the atmospheric windowregion ofthe earth's emission spectrum, and if so, the magnitude ofthe absorption. Figure 2 shows the infrared absorption spectrumofCFC-11, one ofthe human-produced compounds being emitted into the atmosphere. As shown in the figure, this gas strongly absorbs infrared radiation in the earth's atmospheric window. Theeffectiveness ofa compound to act asa greenhouse gas is measured in two ways (IPCC, 1990). Fora gas which is uniformly mixed throughout the atmosphere (a well-mixed gas), one measure is the change in radiative forcing at the tropopause, the boundary between the troposphere and the stratosphere. Theotheris the resulting increase (ordecrease) in the equilibrium, surface temperature due to a unit increase in the global concentration ofthe gas. The change in forcing (R) is usually specified in units ofwatts/m2/ppbv, while the change in surface temperature

**(~T)** is generally specified in units of0 C/ppbv (ppb v ==parts perbillion by volume). Ineitherofthese cases, the changes calculated are referred to as direct effects in that the distributions ofall other trace gases (except H20)remain fixed in the calculations. In the calculation, the H20 concentration in the troposphere is adjusted by fixing the relative humidity so that the concentration changes as the temperature in thetropospherechanges. In addition, the change in temperature corresponds to the changes at steady-state after the system has adjusted to changes in the temperature profile. Previous studies (see, e.g., Fisher et al., 1990) showed that, 4for cases where the bulk of the species resides in the troposphere, the calculated change in forcing is proportional to the calculated change in surface temperature. For cenain greenhouse gases, especially 0:3 and CO:z, the equilibrium surface temperature response will also depend on how stratospheric temperatures readjust as a result of the initial radiative forcing. Since the radiative forcing of a gas depends on the extent to which its absorption spectrally coincides with the absorption of other atmospheric greenhouse gases, it cannot be measured in the laboratory but must be computed from known annospheric properties. 1.3 Global Warming Potential

In an effort to reflect both the relative radiative impact and the lifetimes of different greenhouse gases in the atmosphere, scientists have developed the Global Warming Potential (GWP) as a relative measure of the integrated warming impact. The GWP is an index which defines the time-integrated warming effect due to an instantaneous release of a unit mass of a given greenhouse gas relative to that of a reference gas such as carbon dioxide or CFC-11. This measure takes into consideration both the infrared absorption characteristics of a gas (through the use of computed radiative forcing) and its lifetime and compares these to either CFC-11 (Halocarbon Global Warming Potential or HGWP) or carbon dioxide (Global Warming Potential or GWP). It is a measure used by policy makers to assess the environmental acceptability of human-derived compounds thought to contribute to global warming. Consequently, HGWP and GWP have taken on a great deal of significance to the policy making community. A shortcoming of comparing compounds by their GWP is that it does not consider the relative rate at which compounds used in

competing technologies enter the environment. The fundamental concern, from an environmental perspective, is the actual size of the warming that results from the presence of a specific amount of the greenhouse gas in the atmosphere rather than its GWP. The concept of the direct halocarbon global warming potential (HGWP) was introduced as a measure of the greenhouse impact per unit mass of halocarbon substitutes relative to CFC-11 (Fisher et al, 1990). The HGWP value for a gas is defined as the ratio of the steady - state calculated warming for a fixed continuous release of a gas relative to the calculated warming for the same release (by mass) of CFC-11. One can also define the total radiative forcing of a kilogram of the replacement compound as the time-integrated forcing from the time it is emitted until the time it

is completely purged from the atmosphere. For a compound X with lifetime -rx, the HGWP is given on a mass basis by 5

### HGWP 

## = 6 T x f.LCFC-11 'tx ATcFC- 11

**~x** 'tCFC-11 where 

### t1Tis the 

### direct, equilibrium surface temperatureresponse (°C perppbv)

**and~** is the molecularweight. (3)

It has been argued that 

### itmay be more convenient, for thepurpose ofemissions control policy, to use 

### C02 as the equivalent standard in place of

### CFC-11. Because

**C~** is not removed from the annosphere by a simple first-order chemical 

### orphotochemicalremoval process, the decay inconcentration cannot bedescribed by a simpleexponential function. Calculations using ocean models indicate that about 

### 10-15% of

**C~** emitted couldremain in the annosphere indefinitely. Rather than integrate theradiative forcing toinfinity, it has been suggested that theratio oftime integrated forcing overa finite timeperiod, referred toas the integration time horizon (TI1l), would represent a more appropriate, comparative measure. With 

### C02 as a reference gas, the 

### GWP for compound

### X is given on a mass per massbasis by



## ~TII AT X C(t)x dt 

### GWP - 

### -..::==--------

-



## ~ ATco2 C(t)co2 dt where C(t) is the concentration in units ofpartsperbillion by volume (ppbv) at timet, for an instantaneous, 1 Kg emission at time 

### t=O; 

### t1Tis the direct, equilibrium surface temperature response in units of 0 C/ppbv and ITH is the integration time horizon. Themixing ratio of compoundX (in ppbv), resulting from a I Kg mass emission, is given as (4)

(5) where 'l"xis the atmospheric lifetime ofcompoundX, and J.lx is the gram molecular weight. Using the results ofa coupled ocean-atmospherecarbon cycle model developed by Siegenthaler (1983), the concentration ofadded carbon dioxide resulting from a unit mass emission, can be expressed as 

### C(t)c02 

## = 1.3 

### x 10-10 

# I Ai c<·tl'ti) 

### i=l
6 

## (6)The coefficients, Ai and the lifetimes, 'ti have been estimated by fitting the atmospheric

**C~** decay results of Siegenthaler (1983) and are given as At - A3 = 0.30036,0.34278, 0.35686 and 'tJ - 't3 = 6.993, 71.108, 815.73. GWP values for some important greenhouse gases presently being added to the atmosphere, are given in Table 1 (IPCC. 1992). Table 1 GWP of Important Greenhouse Gases Over Several ITHs* ITH Gas 20 years 100 years 500 years **~** 1 1 1 Cf4 35 11 4 N20 260 270 170 CFC-11 4500 3400 1400 * GWP on a mass per mass basis The GWP index is useful as a comparative scaling factor for different chemical compounds when used in the production of, or within, different produced goods. However, the uncertainties associated with the lifetimes and radiative characteristics of **e** compared gases should be considered when making decisions based on this measure. By combining the most important properties of a greenhouse gas, its direct, equilibrium surface temperature response .1T, and its atmospheric lifetime, the GWP provides a measure to

assess the relative global warming potential of different radiatively active gases. While the GWP has been used extensively in policy discussions and debates, it does not include any measure of the absolute concentration of a compound in the atmosphere. As the IPCC Report (1990) notes, we need to know the future concentration o f greenhouse gases in order to estimate future climate change . As a consequence, a true asses s ment of the climate change due to the release of greenhouse gases from human sources should, at the very least, include projections of the emission rates of these compounds into the atmosphere. This is being attempted by various researchers and is approached by estimating future emissions over time given assumptions about growth , utilization patterns and the regu l atory environment associated with greenhouse gases. 7

### 2 . Description of Data Used in Analysis 2. 1 Data used toEstimate AnnosphericLifetime The six compoundsconsidered in this study, theirchemical formula and molecularweight, are given in Table 2. 

### Table 2 Chemical Compound Description Compound Perfluorobutane Perfluoropentane Perfluorohexane Perfluoro-2-butyloxacyclopentane Perfluorotributylamine Perfluoro-N-methylmorpholine Formula CF3(CF2)2CF3 CF3(CF2bCF3 CF3(CF2)4CF3 CF3(CF2)3CF(CF2hO (CF3(CF2)3)3N CF3N(CF2hO(CF2h Molecular Weight 238 288 338 

### 416 671 299 Thereaction rate constants with OH and Q(lD) weremeasured byresearchers at the MassachusettsInstitute ofTechnology. Allreactionratemeasurements were carriedoutata temperature of298°K relative to a well-characterized compound. Forreaction with OH, two differentreference compounds were used; methane (koH = 7.7 x 1Q-15 cm3/molec/sec) and trifluoromethane 

## OcoH 

## =2.1 x 1 Q-16

**cm3Jmolec/se~:-** ). OHrate constantmeasurements relative to trifluoromethane arereported here due to the greater sensitivity achieved by this reaction. For Q(1D), rates were measuredrelative toreaction ofQ(lD) with CFC-12 

## <ko{Io) 

## = 1.4 x 1o-10 cm3Jmolec/sec). The kinetic data is supplied in Table 3. Note that all rates presented here are upperlimits ofthe measured rates.

8Table 3 OH and 0(10) Reaction Rate Data (Molina & Tang) Compound Perfluorobutane Perfluoropentane Perfluorohexane koH (cm3/molec/sec) <2.1 X 10-11 <1.3 X tQ-17 <1.1 X tQ-17 ko<1o) ( cm3frnolec/sec) <9.5 X tQ-12 <9.8 X tQ-12

<J . 6 X tQ-12 Perfluoro-2 - butyloxacyclopentane <1.7 x 10-11 <6.7 x 10 - 12 Perfluorotributylamine* <1.7 x tQ-16 <1.7 x tQ-11 Perfluoro-N-methylmorpholine <9.9 x tQ-18 < 1.5 x 10-11 * The loss of this compound during the rate constant measurements may have been entirely due to photolysis (Molina & Tang, 1992) . Photolysis of fully-fluorinated compounds is considered unlikely. Two analogous compounds to those considered in this study, carbon tetrafluoride (CF4) and perlluoroethane (C2F6), have been measured in the atmosphere and have estimated lifetimes in excess of 10,000

years (Cicerone, 1979) . There is preliminary evidence that the loss measured for perfluorotributylamine may have been due to photolysis rather than reaction with OH and O(lD) (Molina and Tang, 1992). The UV cross - section between 190 and 300 nm for perfluorotributylamine is given in Figure 3 showing

UV absorption in the 190 to 210 nm wavelength range. 2.2 Infrared Data used to Calculate Global Wanning Potentials The complete, high-resolution infrared absorption cross-sections for perfluoropentane, perfluorohexane, perfluoro- 2-butyloxacyclopentane, perfluorotributy lamine, and perfluorobutane were provided to us by researchers at the University of Denver. The absorption spectra were measured at up to four temperatures between -20 °C and +25 °C. The 25 °C spectra are given in

Figures 4a through 4e. As part of their work, the University of Denver also measured theIR cross sections for

CFC-11. The radiative forcing calculated using the University of Denver data is in good agreement with that calculated using the cross sections from NCAR. Table 4 summarizes the corresponding 9average cross-sections for each ofthe 27 wavenumberintervals used in the AER 1-D RC (radiative-convective) model. Note that we selected to use the 25

# oc cross-sections because the data were mostcompleteat this temperature. In addition, the band strengths showed a rather weak temperature dependance. For instance, for perfluorohexane, the integrated band strength was approximately 9% greater at0 °C than at 25 °C. The calculation forperfluorobutane was carried outusing the datareceived from theDenver group in July 1991. In the summerof 1992, we were informed that, because ofa problem with the sample, these cross-sections should be scaled up by 30%. The results for perfluorobutane reported in Table 4 and Table7 were obtained by multiplying thecalculatedresults by a factor of 1.3. In September of 1992, we received a new set ofIR cross-sections for perfluorobutane from the Denver group. Weestimated that this new setofcross-sections would produce a calculated warming ofabout 

### -10% larger than the resultreported in Table 7. Infrared absorption spectra (measured at

**~~** °C) forperfluorotributylamine and perfluoromorpholine were provided to us byresearchers at MITin the form ofalready averaged cross-sections for each ofthe 27 wavenumberintervals ofthe AER model. These values are summarized in Table 5 together with the corresponding spectra ofaverage cross-sections. Calculations using both the University ofDenverand MITdata forperfluorotributylamine will be performed throughout this study for comparative purposes. Figures 5a and 4e illustrate agreement ofthe spectral distribution ofabsorption. However, there are somedifferences in the magnitude ofthe band-by-band absorption cross-sections. TheDenver group attempted to generate high-precision, high-resolution spectra. The Denver spectra were scanned hundreds of times. The MIT spectra are simple IR scans made with less precise instrumentation and with less painstaking effort.

10Table 4 Cross-Sections (cm-1 cm-atmosphere-1) for Model Spectral Intervals Based on University of Denver Spectra at 25°C AER Perfluoro-2- model Umiting Perfluoro- Perfluoro- butyloxacyclo- Perfluoroband# Wavenumbers **~ntane** hexane **~ntane** butane 1 0.00 75.00 0.00 0.00 0.00 0.00 2 75.00 175.00 0.00 0.00 0.00 0.00 3 175.00 275.00 0.00 0.00 00 0.00 4 275.00 375.00 0.00 0.00 0.00 0.00 5 375.00 450.00 0.00 0.00 0.00 0.00 6 450.00 550.00 0.00 0.00 0.00 0.00 7 550.00 640.00 0.38 0.21 0.18 0.70 8 640.00 700.00 0.35 1.50 1.72 0.00 9 700.00 740.00 10.00 11.15 7.17 6.33 10 740.00 800.00 1.50 2.43 2.71 0.50 11 800.00 850.00 3.81 3.11 3.82 0.00 12 850.00 900 . 00 4.88 2.35 0.93 2.69 13 900.00 945.00 0.70 0.15 1.38 3.50 14 945.00 1001.00 1.39 2.02 2.01 3.47 15 1001.00 1019.00 1.49 0.44 4.96 0.73 16 1019.00 1063.00 1.11 0.23 5.70 0.25 17 1063.00 1081.00 0.15 2.09 8.85 4.36 18 1081.00 1161.00 7.76 10.56 20.38 6.51 19 1161.00 1225.00 21.85 23.97 29.22 8.06 20 1225.00 1325.00 50.00 53.87 53.21 53.42 21 1325.00 1425.00 4.34 4.69 4.03 1.29 22 1425.00 1525.00 0.00 0.00 0.00 0.00 23 1525.00 1625.00 0.00 0.00 0.00 0.00 **.~** 24 1625.00 1725.00 0.00 0.00 .'. 0.00 0.00 25 1725.00 1825.00 0.00 0.00 0.00 0.00 26 1825.00 2075.00 0.00 0.00 0.00 0.00 27 2075.00 2325.00 0.00 0.00 0.00 0.00 Integrated Band Strength 8620 9384 10704 7556 (em-atmosphere )-1 11

### Table 5 Cross-Sections (cm·1 cm-atmosphere·1) for Model Spectral Intervals Based on MIT and University of Denver Spectra at 25°C AER limiting Perfluoro-N- Perfluorotri Perfluorotrimodel Wavenumbers morpho line butylamine butylamine band# (Ml1) (M11) (Denver) 

### 1 0.00 75.00 nodata no data nodata 

### 2 75.00 175.00 nodata nodata nodata 

### 3 175.00 275.00 nodata nodata nodata 

### 4 275.00 375.00 nodata nodata nodata 

### 5 375.00 450.00 nodata no data nodata 

### 6 450.00 550.00 nodata 

### 2.17 0.00 7 550.00 640.00 0.98 2.55 3.39 8 640.00 700.00 1.38 2.04 2.27

... 

### 9 700.00 740.00 1.55 26.44 34.4 10 740.00 800.00 0.63 9 .89 11.9 11 800.00 850.00 0.00 6.08 6.5 12 850.00 900.00 2.78 1.81 0.99 13 900.00 945.00 8.28 2.23 1.35 14 945.00 1001.00 1.1 7 7.26 10.23 15 1001.00 1019.00 0.24 1.63 0.574 16 1019.00 1063.00 0.25 2.69 3.12 17 1063.00 1081.00 1.14 2.80 4.58 18 1081.00 1161.00 12.33 15.10 26.53 19 1161.00 1225.00 40.56 36.79 54.76 20 1225.00 1325.00 25.84 92.32 109.91 21 1325.00 1425.00 14.05 10.52 11.6 22 1425.00 1525.00 0.74 0.72 0.00 23 1525.00 1625.00 0.27 0.38 0.00 24 1625.00 1725.00 0.10 0.18 0.00 25 1725.00 1825.00 0.04 0.12 0.00 26 1825.00 2075.00 0.03 0.29 0.00 27 2075.00 2325.00 0.05 0.40 0.00 Integrated Band Strength 

### 8589 17478 21540 (em-atmosphere)- I
1 23. Results 3.1 Atmospheric Lifetime Given the reaction rate for reaction with the hydroxyl radical (OH), the global average lifetime for a compound X against this mechanism is generally given by <[X]> 1 'tx = <kxOH(T} [OH] [X]> - <kxOH(T) [OH]> (7) where [X] is the local concentration of the species (molecules/cm3), [OH] is the local OH concentration (molecules/cm3), kxOH (cm3/molecule-sec), is the reaction rate constant which is a function of the local temperature T eK), and < > denotes global averaging . Evaluation of 't x requires knowledge of the global distribution of the hydroxyl radical. Direct measurements of OH in the troposphere do not presently exist. Furthennore, even if the technology for such measurements were available, the large variability expected for OH as a function of location would make it difficult to derive global average values. Two approaches can then be followed to estimate <OH>: a) results of a photochemical model can be used to derive global average OH; b) observations of a trace compound which is primarily removed by OH can be used to estimate the global average concentration. Because of its observational basis, approach b) is preferred for evaluation of present-day removal. Modeling, however, would play an important role in assessing possible changes in tropospheric OH in response to global change. The global average OH concentration can be estimated from measurements of a given trace

compound if it satisfies the following: 1) its emission into the atmosphere is well known; 2) it is

primarily removed by tropospheric OH; 3) long-term measurements exist at different locations.

Methyl chloroform (MCF; CH3CCl3) satisfies these criteria, and has been used as a standard to determine the tropospheric OH concentration. Analysis of measurements of methyl chloroform by the ALE/GAGE network (Prinn et al., 1987; Prather et. al., 19?2; I7ather and Spivakovsky, 1990) indicat es that the tropospheric lifetime of MCF due to reaction with OH is about 7 years. Thus, 'tMCF = <kMCFOH(T) [OH] [MCF]> <[MCF]> <kMCFOH(T) [OH)> = 7 years 1 (8) 13where <[MCF]> is the global-averageconcentration ofmethyl chloroform (molecules/cm3). Equations (7) and (8) imply that

_ <kMcf>H(T) [OH] [MCF]> <[X]> 'tx - 7 years <[MCF]> <kxOH(T) [OH] [X]> <kMcf>H(T) [OH]>

- 7 years <kxOH(T) [OH]> 

### (9) (10) The analysis of3-D model results from Prather and Spivakovsk y (1990) showed that equation (10), to first order, can be approximated by (11) where boththereactionrateconstants are specified ata temperature of277°K. All the rate constantsmeasured for this study were performed at a temperature ofapproximately 298°K. Equation (11) can beevaluated at 298°K 

### <kMoPH at 298°K is 1.2 x 

### lQ-14 cm3/molecule-sec) with the introduction ofsomeadditional uncertainty. The upperlimitvalue ofthe slowerOHrea ction rate from Table 1was usedtocalculate the lifetime due toremoval by OH. The lifetime associated with removal due toreaction with O(lD) can be approximated by

""xO(lD) 

## = 1 X 

# w -

11

.. 3000 years 

### x kO(lD) (12) X which represents a simple fit to general results calculated in the 

### AER 2-D model. Toestimate the total annospheric lifetime, the lifetimes associated with the tworemoval processes presented herecan be combined as shown in equation (1). This can beexpressed as (13) The results are presented in Table 6.
14Table 6 Estimated Lower limitt lif e time s Due to Reactions with OH and 0(1 D) for the Compounds Compound Perfluorobutane Perfluoropentane Perfluorohexane Lifetime >1800 >2100 >2600
Loss Mechani sm O(lD), OH reaction Q(lD), OH reaction O(lD), OH reaction Perfluoro-2-butyloxacyclopentane >2300 O(lD), OH reaction Perfluorotributylamine* >390 OH, O(lD) Perfluoro-N-methylmorpholine > 1600 O(lD), OH reaction * The lower limit lifetimes due to the stated loss mechanisms could be greater than reported here because of possible photolysis effects in the rate determining experiment (Molina & Tang, 1992). t Lifetimes could be shorter than the lower limits shown in this table if other loss mechanisms exist for any of these compounds. For perfluorotributylamine, equation (13) can be modified to include removal due to photolysis. Using the absorption cross-section shown in Figure 3, and assuming that the quantum yield is 1 and that the cross-section is zero for wavelengths greater than 230 nm, the 1-D model calculated lifetime due to photolysis is 130 years. Combining this with the lifetime due to reaction with O(lD) and OH results in an atmospheric lifetime of97 years. Thus, the lifetime is determined primarily by photolysis. However, the calculated lifetime is very sensitive to the cut-off wavelength. If the cross section i s non-vanishing for wavelengths larger than 300 nm, the lifetime would be much shorter. The UV absorption of the remaining compounds is relatively weak compared to perfluorotributylamine. Nevertheless, because the lifetimes of the remaining compounds due to reaction with O(lD) and OHare long, even weak UV absorption can represent the most significant loss mechanism. For this reason, further work on determining the UV cross-sections and quantum yield of all the compounds may be necessary. 3.2 Direct Equilibrium Surface Temperature Response We used the AER 1-D RC model to calculate the equilibrium warming due to the introduction of 1 ppbv of the gas into the present day atmosphere. The calculations assumed that the gas has a relatively long lifetime such that it is well-mixed throughout the atmosphere. The 15calculations performed here include only the direct, equilibrium surface temperature response (£\1) caused by the introduction ofthe compounds considered in this study. Indirect effects such as alteration ofthe lifetime ofanother greenhouse gas are not included in the computations here. Moreover, due to therelative non-reactivity ofthese compounds, effects caused by reaction products are not expected. In addition, time dependant response due to phenomena such as the thermal lagofthe oceans and other feedbacks are not included. Theresults are summarized in Table7 . 

### Table 7 Cal culated Direct Equilibrium Temperature Response (AT) in °C/ppbv and Relative AT compared to CFC-11 Relative £\Ton a per Relative 6Ton a per 6T molecule basis unit mass basis Compound

**C/~~bv)** compared to CFC-11 compared to CFC-11 CFC-11 0.15 1.00 1.00 tperfluorobutane 0.20 1.32 0.76 Perfluoropentane 0.24 1.60 0.76 Perfluorohexane 0.26 1.70 0.69 Perfluoro-2-butyloxacyclopentane 0.33 2.20 0.72 Perfluorotributylamine (MIT) 0.48 3.20 0.66 Perfluorotributylamine (UD) 0.63 4 .20 0.85 Perfluoro-N-methz:lmorpholine 0.27 1.80 0.84 t See discussion on the perfluorobutane results at the end ofsection 2.2 The third and fourth columns in the table show the direct, equilibrium surface temperature response relativ- e to CFC-11 on a per molecule and mass basis, respectively. The 6T values for the compounds shown are up to four times larger than CFC-11 on a permolecule basis. The molecular weight ofthe compounds are significantly greaterthan that ofCFC-11. As a result, the direct, equilibrium surface temperature response on a perunit mass basis (4th column, Table 7) is smaller. The

**differenc~s** in 6T values are fully consistent with the spectral distributions ofthe band strengths. Note that the above-mentioned temperature dependance ofabsorption cross-sections does not seem to affect the

**~**T values significantly. In ourparticular example ofthe 16perfluorohexane, the .1T increased about 9% when the 25 °C cross-section values were replaced by the 0 °C values. 3.3 Halocarbon Global Wanning Potentials and Global Wanning Potentials The HGWP values for three different integration time horizons for the compounds are given in Table 8. The HGWPs with an infmite time horizon correspond to the definition given in equation (3) (see Fisher et al., 1990). They are calculated using the lifetimes given in Table 6. As a result, they are to be interpreted as lower limits. While not explicitly stated in Table 8, including removal by photolysis reduces the HGWP values ofperfluorotributylamine by factors of 4, 1 . 1, 1.4, and 2.9 for rrn of oo, 20, 100 and 500 years respectively. Note that while HGWP (oo) is proportional to the calculated lifetime of the species, values for HGWP (20) and HGWP (100) are not sensitive to uncenainties in lifetimes for species with lifetimes of 400 years and longer . Table 8 Calculated Halocarbon Global Warming Potential Compound HGWP (oo) HGWP (20) HGWP (100) HGWP(500) CFC - 11 1.00 1.00 1.00 1.00 Perfluorobutane 25 0.90 1.60 6.00 Perfluoropentane 29 0.90 1.60 6.20 Perfluorohexane 33 0.82 1.50 5 . 70 Perfluoro- 2-butyloxacyclopentane 30 0.85 1.50 5.90 Perfluorotributylamine (MIT) t 4.6 0.76 1.30 3.40 Perfluorotributylamine (UD)t 5.1 0.99 1.60 4.40 Perfluoro-N - methylmorpholine 24 0.99 1.80 6.60 t The IR cross section from UD is based on fine radiation measurements and should be more reliable. The GWP values relative to CO:l for the same three integration time horizons are given in Table 9 . Again, these values should be interpreted as lower limits. As in Table 8, including removal by photolysis reduces the GWP v a lues of perfluorotributylamine by factors of 1.1, 1.4, and 2.9 for ITH of 20, 100 and 500 years respectively. 17

### Table 9 

### Calculated GWP with Different Integration Time 

### Horizons Compound 20:tears 100 :tears 500 :tears CFC-11 4500 3400 1400 Perfluorobutane 4100 5500 8400 Perfluoropentane 4100 5500 8600 Perfluorohexane 3700 5100 8000 Perfluoro-2-butyloxacyclopentane 3800 5200 8200 Perfluoronibutylamine (Ml1) t 

### 3400 4300 4700 Perfluoronibutylamine (UD)t 

### 4500 5600 6100 Perfluoro-N-meth:tlmorpholine 4500 6100 9200 

### t The IRcross section from UD is based on fine radiation measurements and should 

### be more reliable.

18References Charlson R. J . , S. E. Schwartz, J. M. Hales, R. D. Cess, J. A. Coakley, Jr., J. E. Hansen, and D. J. Hoffmann (1992) Climate Forcing by Anthropogenic Aerosols, Science, 255, 423- 430. Cicerone, R. J. (1979) Annospheric Carbon Tetrafluoride: A Nearly Inert Gas, Science, 206, 59-

61. Fisher, D.A., C.H., Hales, W. - C. Wang, M . K.W. Ko, and N.D. Sze (1990) Model calculations of the relative effects of CFCs and their replacements on global warming. Nature, 344, 513-516. IPCC (1990). Scientific Assessment of Climate Change. Intergovernmental Panel on Climate Change. IPCC (1992) . The Supplementary Repon to the IPCC Scientific Assessment. Intergovernmental Panel on Climate Change. Ko, M.K.W . , N.D. Sze, and K. Orth (1991) On the use of global warming potential values in energy efficiency studies. Final Report, Contract CTR91-21/P91-055, AFEAS, Washington, D.C. Molina and Tang (1992) Private communication. Prather, M., and C.M. Spivakovsky (1990) Tropospheric OH and the li . fetimes of hydrochlorofluorocarbons. J. Geophys. Res., 95 (Dll), 18,723-18,629. Prather, M., A . M. A. Ibrahim, T. Sasaki, F. Stordal and G. Visconti (1991) Calculations of Future CVBr and Ozone Depletion. In: Scientific Assessment of Ozone Depletion: 1991. In preparation. Prinn, R., D. Cunnold, R. Rasmussen, P. Simmonds, F. Alyea, A. Crawford, P. Fraser, and R. Rosen (1987) Atmospheric trends in methyl chloroform and the global average for the hydroxyl radical. Science, 238, 945-950. Siegenthaler, U., (1983) Uptake of Excess CO:z by an Outcrop - Diffusion Model of the Ocean. J. Geophys. Res., 88 (C6), 3599 - 3608. Varanasi, P., and S. Chudamani (1988) Infrared intensities of some chlorofluorocarbons capable of perturbing the global climate. J. Geophys. Res., 93 (D2), 1666 - 1668. 19(a) Figure 1: 8 

### ;

**~**

**~** 

## • 8 **--Tapd~** -too• ··-·-200· 

# ---400-


# ---- --100·

- s.f ...

1000 1500 2000 Wavenumber[an"1 ] (b) Radiative characteristics ofthe earth-atmosphere system. (a) Surfaceradiation obstructed (dashed line) and attenuated by the attnosphere (solid line). (b) Upwelling longwave radiation at different heights in the atmosphere.

20c **~** ()
**~**. () en• ·c;: E 2N oe () c .2<0 ....... a. • ... ..... **o~** • Sl < Figure 2: Figure 3:

0.05 0.04 0 . 03 0.02 0.01 0

CFC-11 Absorption Cross-Section 750 850 950 1050 1150 Wavenumber (cm-1) CFC-11 Absorption cross-section. Source: National Center for Atmospheric Research . . 4.0 ·-' ,_. ,_. 3.5 3.0 -. 2.5 :.:. 2.0 1.5 1.0 0.5 ..

0 . 0.5 _1 I I I I I I I 190 200 210 220 230 240 250 260 270 280 290 **Wavelen~h** (nm) UV abs orption cross-section of the compound perfluoro-tributylamine. Source: Molina & Tan g, 1992. 21

1250 300PafhaoropeDiaDoIRabeorpcioa

**~acetioa** I'UfluoroheluuleIRa 0.10 4UO 4lOI 41.01 

### ., 

# .. 'il '3

**~** t!

**~** 406 0 ll

**~** 

# ..

**~ ~**  t< CUM

**~** 

# .. '"e 

### fi <> G.Gl e..ao OM toO I«» 

### a 1400

**Wave~~umber** (llcm) (a) (b) Pafluoro-2-blllyiOABcyCiopt:lllaDC .IR CI'OS5-3CCIIOD PafliiOr'OOUtlllelRa 0.10 4Ull 

### ., '3

**~** G.06



### ~ lit 0 

# -

**~** t< 

### 13

**~** e..ao 

### a (c)



### Figur e 4: 0.1 o..o& 

## -a

**~** 15

**~ ~**  

# .. "Q >< 004 

# .. ij

10 1400 (d)

(e) 

### Infrared absorption cross-sections for (a) perfluoropentane, (b) perfluorohexane, (c) perfluoro-2-butyloxacyclopentane, (d) perlluorobutane, and (e) perfluoronibutylamine. Source: University ofDenver, 1992. 220 "'3 **~** 0 ..§ ... **~ ~** ... § (a) <I) "3 **~** 0 J§ ... **~** ><... § (b) Figure 5:

Perfluorotributylamioe IR absorption cross-sectionr0 . co q IB 0 .- 0 .a N 0 .- 0 .

0 0 . Wavenumber N CD .,., Perfluoro-N-methylmorpboline IR absorption cross-section 0 en CD .#' 0 .- CJI en 0 ., <D N 0 .co <D 0 ,.._ c;n 0 0 - 0 0 0 0 Wavenumber Infrared absorption cross-sections for (a) perfluorotributylamine and (b) perfluoro N-methylmorpholine. Source: Molina and Tang, 1992. 23

### Appendix : Effect of Spectral Overlap In Greenhouse Forcing Calculations The radiative forcing in this 

### report is 

### calculated using AER one-dimensional radiativeconvective (1-D R-C) model (Wang and Molnar, 1985; Wanget al., 1990). The model uses the mean absorption coefficient approximation in simulating the absorption in the annosphere. In 

### calculating the greenhouse forcing for the species, the IR absorption cross-section with resolution of 1 cm-1 areintegrated to give the average cross-section ineach ofthe wavelength band intervals as listed in table l. 

### Table 1. Band intervals used in the AER R-C model. 

### Interval 1 2 

### 3 4 

### 5 6 7 

### 8 9 10 11 12 13 14 15 16 17 18 19 20 21 

### 22 23 24 25 26 27 

### Limiting Wavenumbers 0.00 75.00 75.00 175.00 175.00 275.00 275.00 375.00 375.00 450.00 450.00 550.00 550.00 640.00 640.00 700.00 700.00 740.00 740.00 800.00 800.00 850.00 850.00 900.00 900.00 945.00 945.00 1001.00 1001.00 1019.00 1019.00 1063.00 1063.00 1081.00 1081.00 1161.00 1161.00 1225.00 1225.00 1325.00 1325.00 1425.00 1425.00 1525.00 1525.00 1625.00 1625.00 1725.00 1725.00 1825.00 1825.00 2075.00 2075.00 2325.00 This appendix addresses two questions. First, whether the cross-section reported at 1cm-1 resolution is adequate. Second, whether the averaging overthe band intervals introduce any additional uncertainties.
A-11 . Effects of fine structure in the target molecule The effect of the fine line structure has to be taken into account if both the target molecule and the background atmosphere exhibit structures with resolution smaller than 1 cm·l. The spectra for molecules that are heavy and have complex molecular structures do not exhibit high spectral content. This is because of the fact that the spectral spacing between the vibrational-rotational transitions depends on the inverse of the molecular moments of inertia and is small compared with the voigt width of the spectral transitions themselves. In other words, the lines are sufficiently overlapped in the atmospheric regime of interest as to provide a spectrum which may be considered smooth with respect to the spectral absorption of the atmosphere. Tilis approach is particularly valid for absorption in the 1 Omm spectral window in the tropospheric regime of the atmosphere.

For pressures from 1000mb to 100mb, appropriate to the troposphere, the width of a typical collisionally broadened line ranges from 0.08 cm-1 at the surface to 0.008 cm-1 at the tropopause. Although the latter is small, the spectral spacing between lines is even smaller for these heavy molecules so as to make this an acceptable approach. For the troposphere the sharpest spectral features for these types of molecules are typically of the order of one cm-1 . 2. Averaging over band The effect of averaging on atmospheric radiative transfer has been assessed using a

vectorized version of the FASCODE line-by-line model (Clough et al., 1986). In addition to having the capability of computing atmospheric fluxes and cooling rates, the model is able to provide changes in those quantities to extremely high precision, of the order of one part in 107 for the fluxes. Tilis capability enables the assessment of changes in atmospheric radiative transfer due

to small perturbations in the atmospheric absorption resulting from the introduction of trace species into the annosphere. Two calculations were performed. In the first calculation, the CsF12 cross-section at 1 cm- 1 resolution was used. In the second calculation, the cross-section is averaged across the band interval given in Table 1. The value of the constant value for the cross section extinction over a given spectral interval was obtained by requiring that the integrated absorption over the given spectral interval be the same as that for the measured cross sections. Specifically, the value, Xb, for the spectral interval ( v max - v min )b was obtained from the relation Xb = 1 J::max;b X(v)dv (Vmax-Vmin)b Vmin;b A-2where X (v) is the measured cross section. Thechange 

### in flux atthe tropopause due tothepresence ofCsFt2 

### in the atmosphere calculated using the fineresolution cross section is 20% smallerthan that calculated using the cross-section averaged overthe spectral bands. We feel that this is a realistic estimate for the uncertainties given by the averaging. 

### References Clough, S.A., F.X. Kneizys, E.P. Shettle, and G.P. Anderson (1986) Annospheric radiance and transmittance: FASCOD2. In: 

### Proceedings of the Sixth Conference onAtmospheric Radiation, Williamsburg, Virginia, pp. 141-144. Wang, W.-C., and G. Molnar (1985) A model study ofthe greenhouse effects due to increasing atmospheric CH4, N20, CF2Cl2 and CFCI3. 

### J. 

### Geophys. Res., 90, 12971 -12980. Wang, W.-C., G. Molnar, M.K.W. Ko, S . Goldenberg, and N.D. Sze (1990) Annospheric trace gases and global climate: a seasonal model study. 

### Te/lus, 428, 149-161.

A-3 (