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 of the following members of 

the AER staff: 

Shepard A. Clough 

Kevin Gurney 

Curtis Heisey 

Nien Oak Sze 

ii 

Gyula Molnar 

Karen Orth 

Mary Slayter 
Executive 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. 

iv 
1. 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 industries 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 primarily by 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 stratosphere (above the troposphere which is the lower 11 kilometers 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 occur in the liquidphase or on solid surfaces through heterogeneous reactions. Removal in the ocean may also be 

important. 

The rate of removal can be expressed conveniently by a quantity called the total atmospheric 

lifetime. The total atmospheric lifetime is a formal measure of how long (seconds, minutes, hours, 

days, years, and decades), on average, a molecule stays in the atmosphere before being either 

physically removed or chemically altered More specifically, it is the exponential decay time for the 

gas to reach 37% (1/e) of its original concentration following introduction into the atmosphere. 

The total atmospheric lifetime may represent the sum effect of a variety of removal processes 

potentially active within the atmosphere, such as reaction with OH or photolysis in the 

stratosphere. Each one of these individual removal processes has, associated with it, a removal 

lifetime. An estimate of the total atmospheric lifetime is achieved by examining and combining all 

these individual removal processes. 

For example, a compound that is removed 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 of a well-mixed compound associated with a 

particular removal process 'ti, can be given by 

'ti = 1/lq (2) 

where k; is the global-average removal rate constant (sec· I) associated with that particular process. 

A number of useful relationships can be established once the total atmospheric lifetime is 

known. For example, given equal, continuous emissions, a long-lived gas will build up over time 

to a greater concentration than a short-lived one. If such emissions continue until steady-state is 

achieved, the concentration at that time is equal to the product of the atmospheric lifetime and the 

emission rate. Furthermore, transport and mixing processes in the atmosphere will ensure that a 

2 
release 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 

3 
atmosphere. The difference is due to absorption by greenhouse gases. With the exception of the 

spectral region between 750 cm·l and 1300 em· I (the atmospheric window), the radiant energy 

emitted by the earth's surface is totally absorbed by the atmosphere. The energy absorbed by the 

trace gases is re-emitted in the atmosphere isotropically, i.e., radiation is emitted equally in all 

directions. Pan of the energy is directed at the earth's smface while the other pan contributes to 

the upwelling radiation actually seen at the top of the atmosphere. The calculated upwelling 

radiation is shown as a function of wavelength and height in Figure 1 b. A gas which strongly 

absorbs radiation in the window region will funher reduce the radiation emitted by the earth to 

space and thus enhance the greenhouse effect. 

The ability of a particular gas to absorb infrared radiation is primarily a function of two 

intrinsic molecular properties: which atoms constitute the absorbing gas molecules and the internal 

arrangement of those atoms. Most human-produced gases are polyatomic molecules consisting of 

more than two atoms. Polyatomic gas molecules generally absorb radiation in the infrared portion 

of the electromagnetic spectrum. The main issue, as far as enhancement of the global greenhouse 

effect is concerned, is whether or not the infrared absorption spectrum of the gas molecule in 

question coincides with the atmospheric window region of the earth's emission spectrum, and if 

so, the magnitude of the absorption. 

Figure 2 shows the infrared absorption spectrum of CFC-11, one of the 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. 

The effectiveness of a compound to act as a greenhouse gas is measured in two ways 

(IPCC, 1990). For a 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. The other is the resulting increase (or decrease) in the 

equilibrium, surface temperature due to a unit increase in the global concentration of the gas. The 

change in forcing (R) is usually specified in units of watts/m2/ppbv, while the change in surface 

temperature (~T) is generally specified in units of 0C/ppbv (ppbv ==parts per billion by volume). 

In either of these cases, the changes calculated are referred to as direct effects in that the 

distributions of all 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 the troposphere changes. 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, 

4 
for 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 t1T is the direct, equilibrium surface temperature response (°C per ppbv) and~ is the 

molecular weight. 

(3) 

It has been argued that it may be more convenient, for the purpose of emissions 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 or photochemical removal process, the decay 

in concentration cannot be described by a simple exponential function. Calculations using ocean 

models indicate that about 10-15% of C~ emitted could remain in the annosphere indefinitely. 

Rather than integrate the radiative forcing to infinity, it has been suggested that the ratio of time 

integrated forcing over a finite time period, referred to as 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 mass basis 

by 

~TII AT X C(t)x dt 

GWP - -..::==--------

- ~ ATco2 C(t)co2 dt 

where C(t) is the concentration in units of parts per billion by volume (ppbv) at timet, for an 

instantaneous, 1 Kg emission at time t=O; t1T is the direct, equilibrium surface temperature 

response in units of 0C/ppbv and ITH is the integration time horizon. The mixing ratio of 

compound X (in ppbv), resulting from a I Kg mass emission, is given as 

(4) 

(5) 

where 'l"x is the atmospheric lifetime of compound X, and J.lx is the gram molecular weight. Using 

the results of a coupled ocean-atmosphere carbon cycle model developed by Siegenthaler (1983), 

the concentration of added 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 of greenhouse gases in order to estimate future climate 

change. As a consequence, a true assessment 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 regulatory environment associated with greenhouse gases. 

7 
2 . Description of Data Used in Analysis 

2.1 Data used to Estimate Annospheric Lifetime 

The six compounds considered in this study, their chemical formula and molecular weight, 

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 

The reaction rate constants with OH and Q(lD) were measured by researchers at the 

Massachusetts Institute of Technology. All reaction rate measurements were carried out at a 

temperature of 298°K relative to a well-characterized compound. For reaction with OH, two 

different reference compounds were used; methane (koH = 7.7 x 1Q-15 cm3/molec/sec) and 

trifluoromethane OcoH = 2.1 x 1 Q-16 cm3Jmolec/se~:-). OH rate constant measurements relative to 

trifluoromethane are reported here due to the greater sensitivity achieved by this reaction. For 

Q(1D), rates were measured relative to reaction of Q(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 

upper limits of the measured rates. 

8 
Table 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 

9 
average cross-sections for each of the 27 wavenumber intervals 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 most complete at 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 at 0 °C than at 25 °C. 

The calculation for perfluorobutane was carried out using the data received from the Denver 

group in July 1991. In the summer of 1992, we were informed that, because of a problem with 

the sample, these cross-sections should be scaled up by 30%. The results for perfluorobutane 

reported in Table 4 and Table 7 were obtained by multiplying the calculated results by a factor of 

1.3. In September of 1992, we received a new set of IR cross-sections for perfluorobutane from 

the Denver group. We estimated that this new set of cross-sections would produce a calculated 

warming of about -10% larger than the result reported in Table 7. 

Infrared absorption spectra (measured at ~~ °C) for perfluorotributylamine and 

perfluoromorpholine were provided to us by researchers at MIT in the form of already averaged 

cross-sections for each of the 27 wavenumber intervals of the AER model. These values are 

summarized in Table 5 together with the corresponding spectra of average cross-sections. 

Calculations using both the University of Denver and MIT data for perfluorotributylamine 

will be performed throughout this study for comparative purposes. Figures 5a and 4e illustrate 

agreement of the spectral distribution of absorption. However, there are some differences in the 

magnitude of the band-by-band absorption cross-sections. The Denver 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. 

10 
Table 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 no data no data no data 

2 75.00 175.00 no data no data no data 

3 175.00 275.00 no data no data no data 

4 275.00 375.00 no data no data no data 

5 375.00 450.00 no data no data no data 

6 450.00 550.00 no data 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 1.17 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 2 
3. 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 'tx 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) indicates 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) 

13 
where <[MCF]> is the global-average concentration of methyl 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 of 3-D model results from Prather and Spivakovsky (1990) showed that equation 

(10), to first order, can be approximated by 

(11) 

where both the reaction rate constants are specified at a temperature of 277°K. All the rate 

constants measured for this study were performed at a temperature of approximately 298°K. 

Equation (11) can be evaluated at 298°K <kMoPH at 298°K is 1.2 x lQ-14 cm3/molecule-sec) with 

the introduction of some additional uncertainty. The upper limit value of the slower OH reaction 

rate from Table 1 was used to calculate the lifetime due to removal by OH. 

The lifetime associated with removal due to reaction 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. 

To estimate the total annospheric lifetime, the lifetimes associated with the two removal 

processes presented here can be combined as shown in equation (1). This can be expressed as 

(13) 

The results are presented in Table 6. 

14 
Table 6 

Estimated Lower limitt lifetimes Due to Reactions 

with OH and 0(1 D) for the Compounds 

Compound 

Perfluorobutane 

Perfluoropentane 

Perfluorohexane 

Lifetime 

>1800 

>2100 

>2600 

Loss Mechanism 

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 is 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 

15 
calculations performed here include only the direct, equilibrium surface temperature response (£\1) 

caused by the introduction of the compounds considered in this study. Indirect effects such as 

alteration of the lifetime of another greenhouse gas are not included in the computations here. 

Moreover, due to the relative non-reactivity of these compounds, effects caused by reaction 

products are not expected. In addition, time dependant response due to phenomena such as the 

thermal lag of the oceans and other feedbacks are not included. The results are summarized in 

Table 7. 

Table 7 

Calculated Direct Equilibrium Temperature Response (AT) in °C/ppbv 

and Relative AT compared to CFC-11 

Relative £\Ton a per Relative 6T on 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-meth z:lmorpholine 0.27 1.80 0.84 

t See discussion on the perfluorobutane results at the end of section 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 per molecule basis. The 

molecular weight of the compounds are significantly greater than that of CFC-11. As a result, the 

direct, equilibrium surface temperature response on a per unit mass basis (4th column, Table 7) is 

smaller. 

The differenc~s in 6T values are fully consistent with the spectral distributions of the band 

strengths. Note that the above-mentioned temperature dependance of absorption cross-sections 

does not seem to affect the ~ T values significantly. In our particular example of the 

16 
perfluorohexane, 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 values 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 IR cross section from UD is based on fine radiation measurements and 

should be more reliable. 

18 
References 

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 of the earth-atmosphere system. (a) Surface radiation 

obstructed (dashed line) and attenuated by the attnosphere (solid line). (b) 

Upwelling longwave radiation at different heights in the atmosphere. 

20 
c 

~ 

. 

() ~ 

() 

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 absorption cross-section of the compound perfluoro-tributylamine. Source: 

Molina & Tang, 1992. 

21 

1250 

300 
PafhaoropeDiaDo IR abeorpcioa ~acetioa I'Ufluoroheluule IRa 

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'OOUtllle lR a 0.10 

4Ull 

., 

'3 

~ G.06 

lit 

~ 0 

-t< ~ 

13 ~ 

e..ao 

a 

(c) 

Figure 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 of Denver, 1992. 

22 
0 

"'3 

~ 

0 

..§ 

... 

~ ~ 

... 

§ 

(a) 

<I) 

"3 

~ 

0 

J§ 

... 

~ 

>< ... 

§ 

(b) 

Figure 5: 

Perfluorotributylamioe IR absorption cross-section 

r0 . 

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 . 

en 

CJI 

-

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) perfluoroN-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; Wang et 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 are integrated to give the average cross-section in each of the 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 7 40.00 

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 1 cm-1 

resolution is adequate. Second, whether the averaging over the band intervals introduce any 

additional uncertainties. 

A-1 
1 . 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-2 
where X (v) is the measured cross section. 

The change in flux at the tropopause due to the presence of CsFt2 in the atmosphere 

calculated using the fine resolution cross section is 20% smaller than that calculated using the 

cross-section averaged over the 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 on Atmospheric 

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 

(