The
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ecognition of the aderse impact of chloDfluorocarbons (CFCs) n stratospheric ozone as prompted an inter.&ational effort to replace CFCs with environmentally -acceptable alternatives (1-4) such as hydrofluorocarbons [HFCs) and hydrochlorofluorocarbons (HCFCs). Examples include HFC-134a (CFJFH,), a replacement for CFC-12 (CF,CI,) in domestic refrigeration and automobile air conditioning u n i t s ; HCFC-22 (CHF,Cl), a replacement for CFC-12 in industrial refrigeration units; and HCFC-14lb (CFCI,CH,,), a replacement for CFC-11 in foam-blowing applications. Both HFCs and HCFCs are volatile and insoluble in water. Following release into the environment these compounds reside in the atmosphere where they are oxidized into a variety of degradation products. The atmospheric chemistry of commercially important HFCs and HCFCs is well established. HFCs are "ozone friendly." HCFCs have
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T I M O T H Y J. W A L L I N G T O N W I L L I A M F. SCHNEIDER Ford Motor Company Dearborn, MI 48121-2053
D O U G L A S R. WORSNOP Aerodyne Research, Inc. Billerico, MA 01821.3976
O L E J. N I E L S E N JENS SEHESTED Rise National Laboratory DK-4000 Roskilde, Denmark
W A R R E N J. D E B R U Y N JEFFREY A. SHORTER Boston College Chestnut Hill, MA 02167 320 A
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Impac~of
CFC ReplacementsHFCs and HCFCs small but nonnegligible ozone depletion potentials. The global warming potentials of HFCs and HCFCs are approximately an order of magnitude less than those of the CFCs they replace. At the concentrations expected from the atmospheric degradation of HFCs and HCFCs, none of the oxidation products can be considered ta be noxious or toxic. The choice of HFCs and HCFCs is motivated by a number of factors. In contrast to CFCs, HFCs and HCFCs contain one or more C-H bonds. Hence, HFCs and HCFCs are susceptible to attack by OH radicals in the lower atmosphere (troposphere). HFCs do not contain chlorine and so do not have the ozone depletion potential associated with the well-established chlorine catalytic cycles. Although HCFCs contain chlorine, the delivery of this chlorine to the stratosphere is relatively inefficient because of the scavenging of HCFCs by OH radicals in the troposphere. To define the environmental impact of HFCs and HCFCs, their ability to destroy stratospheric ozone, contribute to potential global warming, and produce noxious degradation products must be assessed. This assessment requires a detailed knowledge of the atmospheric chemistry of the halocarbons. We present here an evaluation of the environmental impact of HFCs
Environ. Sci. Technol., Vol. 28,NO.7, 1994
and HCFCs in terms of their ozone depletion potentials, global warming potentials, and ability to form noxious degradation products. This evaluation is based on an overview of their atmospheric chemistry and the gas- and liquid-phase loss processes of their halocarbonyl decomposition products. Conversion to carbonyl species The gas-phase atmospheric chemistry of HFCs and HCFCs can be divided into two parts: reactions that convert the HFCIHCFC into halogenated carbonyl species, and reactions that remove these carbonyl compounds. Reaction rates with OH radicals determine the atmospheric lifetimes of all HFCs and HCFCs. Lifetimes are 2-40 years (5, 6) and are listed in Table 1 along with those for CFC-11 and CFC-12 for comparison. A generic scheme for the atmospheric oxidation of a C, haloalkane is given in Figure 1. Reaction with OH radicals gives a haloalkyl radical, which reacts with 0, to give the corresponding peroxy radical (RO,). Peroxy radicals can react with three trace species in the atmosphere: NO, NO,, or HO, radicals. The importance of these reactions is dictated by the relative abundances and reaction rates of NO, NO,, and HO, radicals with RO, radicals. In the troposphere the concentrations of NO, NO,, and HO, are compara-
0013~936X19410927-320/\$04.50/0@ 1994 American Chemical Soclety
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ble 1u L1lL The reactions of peroxy radicals with NO and NO, have been studied extensively (6-10).The peroxy radicals derived from the HFCs and HCFCs considered in this article all react rapidly with NO to give NO, and an alkoxy radical RO ( 9 , 10). The atmospheric lifetime of RO, radicals with respect to reaction with NO is 3-7 min (9, 1 0 ) . Peroxy radicals react rapidly with NO, to give alkyl peroxynitrates (RO,NO,). By analogy to the measured rate of reaction of CF,CIO, and CF,CH,O, radicals with NO, (10, 111, the lifetime of RO, radicals with respect to reaction with NO, will be approximately 10 min. Alkyl peroxynitrates are thermally unstable and decompose rapidly to regenerate RO, radicals and NO, (12, 131. Peroxy radicals react with HO, radicals (7, 8) to give hydroperoxides and possibly, in the case of RO, radicals containing an a-H atom (e.& CF,CFHO,), carbonyl products [e.& CF,C(O)Fl. The relative importance of the hydroperoxide and aldehyde forming channels is uncertain ( 1 4 ) . It seems reasonable to suppose that the peroxy radicals formed from HFCs and HCFCs react with rates similar to those measured for CF,CFHO,, CF,ClCH,O,, and CF,CC1,0, (Le., in the range 2-7 x 10-” cm3 molecule-’ s-’) (15, 16; Maricq, M. M.; Szentze, J. J., personal communication, 1993). Using A
cule cm-3 gives a lifetime of 28 min for CX,CXYO, radicals with respect to reaction with HO,. The hydroperoxide CX,CXYOOH is returned to the CX,CXYOx radical pool via reaction with OH or photolysis (4). The fate of the carbonyl product CX,C(O)X is discussed later. The atmospheric fate of t h e alkoxy radical, CX,CXYO, is either decomposition or reaction with 0, (IO,17-27). Decomposition can occur either by C-C bond fission or C1 atom elimination. Reaction with 0, is possible only when an a-H atom is available. In the case of the alkoxy radicals derived from HFC-32 (281, HFC-125 (CFSCFZHI (19, 21, 22), and HCFC-22 (29, 30). only one reaction pathway is available. The alkoxy radicals derived from HFC143a, HCFC-123 (CF,CCl,H), HCFC124, HCFC-141b and HCFC-142b all have two or more possible fates. For HCFCs 1 2 3 and 124 the dominant loss process is elimination of a C1 atom to give CF,ClOlCl (17, 21, 25) and CF,C(OlF (19,21), respectively. For HFC-l43a, HCFC-l4lb, and HCFC-I42b, reaction with 0, dominates, giving CF,CHO (101, CFC1,CHO (26, 311, and CF,ClCHO (26, 311, respectively. The case of HFC-134a is the most complex. Under atmospheric conditions, the alkoxy radical derived from HFC134a. CF,CFHO, decomposes and
reacts witn u2ar comparaoie rates (18, 20).
The usual modes of alkoxy radical loss are not possible for CF,O. The fate of CF,O radicals is reaction with NO (32-35; Zellner, R., personal communication, 19931, hydrocarbons (36-431, and possibly water vapor,(44).Reaction of CF,O radicals with NO gives C(O)F,. Reaction of CF,O radicals with hydrocarbons a n d water produces CF,OH. CF,OH decomposes heterogeneously and possibly homogeneously (44-461 to give C(O)F, and HF. CF,OH is also expected to be incorporated into water droplets (471. The principle gas-phase atmospheric degradation products of a series of important HFCs a n d HCFCs are given in Table 2. Reactions of carbonyl intermediates The sequence of gas-phase reactions that follow from the initial attack of OH radicals on the parent halocarbon are sufficiently rapid that heterogeneous and aqueous processes play no role. In contrast, the lifetimes of the carbonyl products [e.g., HC(O)F, C(O)F,, CF,C(O)Fl are relatively long (weeks). Incorporation into water droplets followed by hydrolysis plays an important role in the removal of halogenated carbonyl compounds (48). In the cases of HC(O)F, C(O]F,, C(OlFC1, a n d
Envimn. Sci. Technol.. Vol. 28, No. 7,1994 321 A
mpund n < C - 3 2 (CH,F,) HFC~l25 (CF,CF,H) HFC-134a (CF,CFH,) HFC-143a (CF,CH,) HCFC-22 (CHF,CI) HCFC-123 (CF,CCi,H) HCFC-124 (CF,CFCIH) HCFC-141b (CFCI,CH,) H C F C 142b ~ (CF,CICH,) CFC-11 (CFCI,) CFC-12 (CF,CI,)
105
co2 dA~erage01 values given by Dewent el al. (41, p. 124. 4Average 01 Y B I U O ~given in Table 4 of Fisher el al. (65). ‘Average of values q w n in Table 5 of Fisher el al. ( 7 7 ) . *Estimated from lifetime 10 be midway between HCFCs 124 and 141b. *By definition. ‘The global warming potential 01 CFC-11 IS approximately 1300 limes (80).
011
‘Trans!ent r a d i i imrmediales are enclosed in ellipses. pmducts wllh less transitory existem are given in the boxes. Order-of-magnitude lifetime estimates are indicated parenthetically.
CF,C(O)F reaction with OH radicals (49) and photolysis (50)are too slow to be of any significance. These compounds are removed essentially entirely into water droplets. The gas-phase oxidation mechanisms for CX,C(O)H and CF,C(O)Cl are shown i n Figure 2. For CX,C(O)H species, reaction with OH radicals is important (51).The lifetimes of CF,C(O)H, CF,CIC[O)H,
and CFCl,C(O)H with respect to OH attack have been estimated to be 24, 19, and 11 days, respectively (52). Photolysis and scavenging by water probably both play a role in the atmospheric fate of these halogenated aldehydes. CF,C(O)Cl does not react with OH, but undergoes photolysis (27,52),which competes with incorporation of CF,C(O)Cl into water droplets.
322 A Environ. Sci. Technol.. VoI. 28. No. 7, 1994
As shown in Figure 2, photolysis of CF,C(O)Cl gives CF,, CO, and C1 (27).In addition, trace amounts ( 4 % yield) of CF,C1 have been reported (27).CF,CI is a long-lived compound that efficiently transports chlorine to the stratosphere. However, the low yield of CF,Cl from CF,C(O)Cl photolysis renders this pathway environmentally insignificant. T h e photolysis of CF,C(O)H in air may produce CF,H (preliminary data suggest a 56% yield) (53). CF,ClC(O)H a n d CFCl,C(O)H may also be photolyzed to give significant yields of CF,ClH and CFCl,H, respectively. Because these species have shorter atmospheric lifetimes than their parent HCFCs (141b and 142b) (41, the potential formation of CF,ClH and CFCI,H is of little environmental consequence. In contrast, CF,H is approximately an order of magnitude more persistent than HFC143a. CF,H is a potent greenhouse gas, and its formation may increase the global warming potential of HFC-143a. Following reaction of the parent a l d e h y d e with OH radicals, CF,C(O), CF,ClC(O). and CFCI,C(O) radicals can either react with 0, or decompose to give CO and a halogenated methyl radical. Recent work has shown that reaction with 0, is the fate of CF,C(O) radicals (54, 55) a n d possibly CF,ClC[O) a n d CFCl,C[O) radicals. The resulting CX,C(O)O, radical can react with NO or NO,, ultimately yielding CX, radicals and CO, (54-56). Removal of oxidized species The final step in the removal of any species from the atmosphere involves heterogeneous deposition to the Earth’s surface. Removal processes include wet deposition via rainout (following uptake into tropospheric clouds) and dry deposition to the Earths surface, principally to the oceans. The rates of these processes are largely determined by the species’ chemistry in aqueous solution. For the viability of CFC replacements, the question is whether the rate of removal of any degradation product is slow compared to the OH reaction limited lifetime of the parent compounds listed in Table 1. Heterogeneous lifetimes of the parent compounds themselves are on the order of hundreds of years because of their low aqueous solubility and reactivity. The degradation products listed in Table 2 have gas-phase removal rates (via reaction or photolysis)
rameter typically measured in gasliquid mass transfer experiments. For the two related chlorinated species, C(O)Cl, (DeBruyn,w. et al, unpublished results, 1994; 63, 64) and CCl,C(O)Cl (58; above-cited unpublished results: 611, a combination of studies has determined khyd= 100150 s-'. The H*kh,, values for C(O)Cl, and CCl,CIO)CI, 0.2 and 9 M atm-' s-' (above-cited unpublished results), respectively, span the range of the fluorine-containing species listed in Table 3. Assuming khyd= 100 SK' is representative of all the halocarbonyl species, one obtains H = 0.1-0.6 M atn-'. With H and k,,, values, tropospheric lifetimes for heterogeneous uptake into clouds and into the ocean can be estimated, as listed in Table 3 (57,62, 63). For tropospheric cloud processing, the lower limit of 5 days represents transport limitations, that is, the time taken for transport of the materials into the clouds. Two conclusions can be drawn from Table 3. First, and most importantly, despite the considerable uncertainty in the laboratory kinetic results, heterogeneous removal of halocarbonyl species is fast enough to have no effect on the overall halogen lifetime compared to the lifetime of the parent halocarbon. Second, tropospheric cloud rainout will predominate over deposition to the ocean. This conclusion, based on the most recent laboratory results, reverses earlier predictions of ocean-dominated deposition that assumed H z 10 M atm-' (57).
4BLE 2
,as-phaseatmospheric degradation products mpaund
Cahon-containing degradation pOduc16'
FC-32 (CH,F , FC-125(CF3 34a (CF,CFH,) 13a (CF,CH,) CFC-22 (CHF,CI) CFC-123 (CF,CCI,H) CFC-124 (CF,CFCIH) CFC-14lb (CFCI,CH,) CFC-142b (CFX1CH-b ~
iydroperoxider :peeled to be ca
C(O)F, C(O)F,. CF,OH HC(O)F. CF30H, ( , CF,C CF,C(O)H, CF,Ot Fz, CC C(O)F* CF,C(O)CI, CF,OH, C ( 0 ) ^(O)F ,CHO, C(O)FCI, CO, CO, i c H n C(O)F,. CO. CO, uxynilri >,NOz) producls will ais0 be formed but are oxy rad' 4)and then- into the products listed above.
that are slow enough (days or longer) that heterogeneous processing might be significant. They are thought to undergo aqueous interactions sufficiently rapidly for efficient wet and dry deposition (57). For example, although the acid halides are relatively insoluble in water, they hydrolyze to produce the acids HX and CX,C(O)OH (see Table 3). Because the acids are very water soluble, hydrolysis removes the halides from the gas phase irreversibly. It is difficult to measure the relevant aqueous kinetics of such species. Atmospheric removal rates depend on both solubility (expressed in terms of the Henry's law constant, H, M atm-') and hydrolysis rate (k,,,, s-'). Because these spe-
cies do not form stable aqueous solutions, neither parameter is simply measurable in bulk solution. As a result, laboratory determinations involve heterogeneous processes that typically measure combinations of H and khyd.Estimation of atmospheric lifetimes requires deconvolution of these parameters. The relevant aqueous-phase kinetics of the halocarbonyls, (C(O)F,, CF,C(O)F, and CF,C(O)Cl, in particular] have been studied by a number of experimental techniques utilizing droplets ( 4 8 , 581, bubbles (DeBruyn,W. et al., unpublished results, 19941, wetted wall reactors, and aerosol chambers (59).The results of these experiments are summarized in Table 3 in terms of the product H*khYd1'*,which is the pa-
Ozone depletion potentials In discussions of the effects of halocarbons on stratospheric ozone the concept of "ozone depletion potential" (ODP) is useful (64).Ozone depletion potential is defined as the ratio of the calculated ozone column change per mass of a given compound released to the column change for the same mass of CFC-11. Halocarbon ODPs have been calculated by a number of atmospheric modeling groups. Selected results from Fisher et al. (65)are given in Table I. HFCs do not contain any chlorine and so have no ozone depletion potential associated with the wellestablished chlorine-based catalytic ozone destruction cycles. Recently, there has been speculation regarding the possibility of an impact of HFCs on stratospheric ozone by virtue of their degradation into CF,O,, FCO,, and FOX radicals that could
Envimn. Sci. Technol., Voi. 28. No. 7. 1994 323 A
us-phase atmospheric degradation products Llietime Clouds
Ocean
[days)
(years)
r
0.3-2.0 0 5-5 0 0 3-3 0
t t I
55-10
I i
1.0-10.0
participate in catalytic ozone destruction cycles (66-68). However, experimental studies have shown that no such cycles are viable (35, 69-75). The ODPs of HFCs are essentially zero (
