ES&T
The
Environmental Impact of R ecognition of the adverse impact of chlorofluorocarbons (CFCs) on stratospheric ozone has prompted an international effort to replace CFCs with environmentally acceptable alternatives [1-4] such as hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). Examples include HFC-134a (CF 3 CFH 2 ), a replacement for CFC-12 (CF2C12) in domestic refrigeration and automobile air c o n d i t i o n i n g u n i t s ; HCFC-22 (CHF2C1), a replacement for CFC-12 in industrial refrigeration units; and HCFC-141b (CFC12CH3), 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 " o z o n e friendly." HCFCs have
TIMOTHY J . WALLINGTON W I L L I A M F. S C H N E I D E R Ford Motor Company Dearborn, MI 48121-2053
DOUGLAS
R. W O R S N O P
Aerodyne Research, Inc. Billerica, MA 01821-3976
OLE J . N I E L S E N JENS SEHESTED Risø National Laboratory DK-4000 Roskilde, Denmark
WARREN J. DEBRUYN J E F F R E Y A. S H O R T E R Boston College Chestnut Hill, MA 02167 320 A
CFC Replacements— HFCs and HCFCs small but nonnegligible ozone dep l e t i o n p o t e n t i a l s . 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 to 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 a t m o s p h e r e (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 HFC/HCFC 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 C2 haloalkane is given in Figure 1. Reaction with OH radicals gives a haloalkyl radical, which reacts with 0 2 to give the corresponding peroxy radical (R0 2 ). Peroxy radicals can react with three trace species in the atmosphere: NO, N0 2 , or H 0 2 radicals. The importance of these reactions is dictated by the relative abundances and reaction rates of NO, N0 2 , and H0 2 radicals with R0 2 radicals. In the troposphere the concentrations of NO, N0 2> and HQ 2 are compara-
0013-936X/94/0927-320A$04.50/0 © 1994 American Chemical Society
ble (2.5-10 χ 10H cm" 3 ) (4). The reactions of peroxy radicals with NO and N 0 2 have been stud ied extensively [6-10). The peroxy radicals derived from the HFCs and HCFCs considered in this article all react rapidly with NO to give N 0 2 and an alkoxy radical RO (9, 10). T h e a t m o s p h e r i c lifetime of R 0 2 r a d i c a l s w i t h r e s p e c t to r e a c t i o n with NO is 3-7 min (9, 10). Peroxy radicajs react rapidly w i t h N 0 2 to give alkyl peroxynitrates ( R 0 2 N 0 2 ) . By analogy to the measured rate of reaction of CF 2 C10 2 a n d C F 3 C H 2 0 2 radicals w i t h N 0 2 (10, 11), the life time of R 0 2 radicals with respect to reaction with N 0 2 will be approxi mately 10 min. Alkyl peroxynitrates are thermally unstable a n d decom pose rapidly to regenerate R 0 2 radi cals and N 0 2 (12, 13). Peroxy r a d i c a l s react w i t h H 0 2 radicals (7, 8) to give hydroperox ides and possibly, in the case of R 0 2 r a d i c a l s c o n t a i n i n g an a-H a t o m (e.g., CF 3 CFH0 2 ), carbonyl products [e.g., CF 3 C(0)F]. The relative impor tance of the hydroperoxide and al d e h y d e forming channels is uncer tain (14). It s e e m s r e a s o n a b l e to s u p p o s e that t h e p e r o x y r a d i c a l s formed from HFCs and HCFCs react with rates similar to those measured for C F 3 C F H 0 2 , C F 2 C 1 C H 2 0 2 , a n d CF 3 CC1 2 0 2 (i.e., in the range 2-7 χ 10~ 12 cm 3 m o l e c u l e - 1 s"1) (15, 16; Maricq, M. M.; Szentze, }. J., per sonal communication, 1993). Using
an H 0 2 concentration of 10" mole cule cm" 1 gives a lifetime of 2— 8 m i n for CX 3 CXY0 2 radicals with respect to reaction w i t h H 0 2 . The h y d r o p e r o x i d e CX 3 CXYOOH is re t u r n e d to t h e C X 3 C X Y O x r a d i c a l pool via reaction w i t h OH or photol ysis (4). T h e fate of the carbonyl p r o d u c t C X 3 C ( 0 ) X is d i s c u s s e d later. T h e a t m o s p h e r i c fate of t h e alkoxy radical, CX 3 CXYO, is either decomposition or reaction with 0 2 (10, 17-27). Decomposition can oc cur either by C—C b o n d fission or CI atom elimination. Reaction with O z is possible only w h e n an a-H atom is available. In the case of the alkoxy radicals derived from HFC-32 (28), HFC-125 (CF 3 CF 2 H) (19, 21, 22), a n d HCFC-22 (29, 30), only one re a c t i o n p a t h w a y is a v a i l a b l e . T h e alkoxy radicals derived from HFC143a, HCFC-123 (CF 3 CC1 2 H), HCFC124, HCFC-141b and HCFC-142b all have two or more possible fates. For HCFCs 123 and 124 the dominant loss process is elimination of a CI atom to give CF 3 C(0)C1 (17, 21, 25) a n d CF 3 C(0)F (19, 21), respectively. For HFC-143a, HCFC-141b, and HCFC-142b, reaction with 0 2 domi nates, giving CF3CHO (10), CFCl 2 CHO (26, 31), a n d CF 2 ClCHO (26, 31), respectively. T h e case of HFC-134a is the most complex. Un der atmospheric conditions, the alkoxy radical derived from HFC134a, CF3CFHO, decomposes and
reacts w i t h 0 2 at comparable rates (18, 20). The usual m o d e s of alkoxy radi cal loss are not possible for CF :f O. The fate of C F , 0 radicals is reaction w i t h NO (32-35; Zellner, R., per sonal c o m m u n i c a t i o n , 1993), hy d r o c a r b o n s (36-43), a n d possibly water vapor.(44). Reaction of C F 3 0 radicals with NO gives C(0)F 2 . Re action of C F 3 0 radicals with hydro carbons and water produces CF 3 OH. CF 3 OH decomposes heterogeneously and possibly homoge n e o u s l y (44-46) to give C ( 0 ) F 2 a n d HF. CF : i OH is also e x p e c t e d to be incorporated into water droplets (47). T h e p r i n c i p l e g a s - p h a s e at m o s p h e r i c degradation p r o d u c t s of a s e r i e s of i m p o r t a n t HFCs a n d HCFCs are given in Table 2. Reactions of carbonyl intermediates The sequence of gas-phase reac tions that follow from the initial at tack of OH radicals on the parent h a l o c a r b o n are sufficiently r a p i d that heterogeneous and aqueous processes play n o role. In contrast, the lifetimes of the carbonyl prod ucts [e.g., H C ( 0 ) F , C(0)F2, CF3C(0)F] are relatively long (weeks). I n c o r p o r a t i o n into w a t e r droplets followed by hydrolysis plays an i m p o r t a n t role in the re m o v a l of h a l o g e n a t e d c a r b o n y l c o m p o u n d s (48). In t h e cases of HC(0)F, C(0)F2, C(0)FCl, and
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321 A
TABLE 1
Atmospheric lifetimes, ozone depletion, and global warming potentials
Compound
Lifetime years
HFC-32 (CH 2 F 2 ) HFC-125(CF3CF2H) HFC-134a(CF3CFH2) HFC-143a(CF3CH3) H C F C - 2 2 (CHF 2 CI) HCFC-123 (CF 3 CCI 2 H) HCFC-124(CF3CFCIH) HCFC-141b(CFCI2CH3) HCFC-142b(CF2CICH3) C F C - 1 1 (CFCI 3 ) CFC-12(CF2CI2)
6.7 26 14 40 14 1.5 6.0 7.1 17.8 60 105
Ozone depletion6 potential
8
0 0 0 0 0.047 0.016 0.018 0.085 0.053 1.0 e 0.95
co 2 a
Halocarbon global warming potential" 0.094 d 0.58 0.27 0.74 0.36 0.019 0.096 0.092 0.36 1.0 e 3.1 0.00076'
Average of values given by Derwent et al. (4), p. 124. ° Average of values given in Table 4 of Fisher et al. (65). "Average of values given in Table 5 of Fisher et al. (77). ''Estimated from lifetime to be midway between HCFCs 124 and 141b. " By definition. 'The global warming potential of CFC-11 is approximately 1300 times greater than that of C 0 2 (80).
FIGURE 1
Generalized scheme for the atmospheric oxidation of a halogenated organic compound, CX3CXYH (X, Y = H, Cl, or F) a
" Transient radical intermediates are enclosed in ellipses, products with less transitory existence are given in the boxes. Order-of-magnitude lifetime estimates are indicated parenthetically.
CF3C(0)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 CX3C(0)H and CF3C(0)C1 are shown in Figure 2. For CX 3 C(0)H species, reaction with OH radicals is important (51). The lifetimes of CF3C(0)H, CF2C1C(Q)H, 322 A
and CFC12C(0)H with respect to OH attack have been estimated to be 24, 19, and 11 days, respectively (51). Photolysis and scavenging by water probably both play a role in the atmospheric fate of these halogenated aldehydes. CF3C(0)C1 does not react with OH, but undergoes photolysis (27, 52), which competes with incorporation of CF3C(0)C1 into water droplets.
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As shown in Figure 2, photolysis of CF3C(0)C1 gives CF3, CO, and CI (27). In addition, trace amounts (5 χ 10~4 mol) have been reported to ad versely impact wheat and tomato seedlings (79). The concentration of CF3C(0)OH in rainwater expected from the atmospheric degradation of HFCs and HCFCs is
