Hcfc123

HCFC-141b

CH 2FCF3 Decomposition and reaction with 02

CHF2CF3 Decomposition by C-C bond scission

CHC12CF3 Cleavage of C-Cl bond, small fraction by C-C bond scission

CH3CC12F Reaction with 02 primarily hc(0)f, cof2, cf.oh. cf,C(0)F

cof2, cf,oh cf3c(0)c1

hc(0)cfc12

cof2, cf3oh, coci2

c1c(0)f

The fate of CF3OH described earlier is believed to be primarily uptake into clouds. The same is true of COCl2, COF2, HC(0)F, and CF3C(0)F. For all of these species, photolysis at the wavelengths found in the troposphere is negligible, as is reaction with OH (Nolle et al., 1992; Rattigan et al., 1993; Wallington et al., 1994a; Zachariah et al., 1995; World Meteorological Organization, 1995). Table 13.8 summarizes estimates of the lifetimes of these halogenated product species with respect to uptake by the oceans, clouds, and rainwater. Uptake into clouds followed by hydrolysis is the major removal mechanism from the atmosphere for these compounds. Hydrolysis forms HC1 and HF in the case of COCl2, COF2, and HC(0)F as well as formic acid in the latter case. CF3C(0)F hydrolyzes to HF and trifluoroacetic acid, CF3C(0)0H.

The effects on human health of trifluoroacetic acid (TFA) from the oxidation of HFC-f34a, halothane (l,l,l-trifluoro-2-bromo-2-chloroethane, used as an anesthetic), and some of the other CFC replacements, such as HCFC-123 and HCFC-124, have been of some

TABLE 13.8 Estimated Lifetimes of Halogenated Carbonyl Compounds in the Aqueous Phase"

Compound

Ocean

Clouds

Rainout

hc(o)f

3 months

4 days

180 years

cof/

4 months

4 days

72 years

c1c(0)f

2 years

6 days

265 years

cf,c(0)ff

2 years

6 days

675 years

cf,c(o)cr'

3 years

6 days

900 years

h Tropospheric lifetime estimated to be ~0.5-3 days by De Bruyn et al. (1992, 1995) based on measurements of uptake by water surfaces; see also George et al. (1994a,b).

' Tropospheric lifetime estimated to be ~0.5-3 days by De Bruyn et al. (1992, 1995) based on measurements of uptake by water surfaces; see also George et al. (1994a,b).

'' Tropospheric lifetime estimated to be ~2-10 days by De Bruyn et al. (1992, 1995) based on measurements of uptake by water surfaces; see also George et al. (1994a,b).

h Tropospheric lifetime estimated to be ~0.5-3 days by De Bruyn et al. (1992, 1995) based on measurements of uptake by water surfaces; see also George et al. (1994a,b).

' Tropospheric lifetime estimated to be ~0.5-3 days by De Bruyn et al. (1992, 1995) based on measurements of uptake by water surfaces; see also George et al. (1994a,b).

'' Tropospheric lifetime estimated to be ~2-10 days by De Bruyn et al. (1992, 1995) based on measurements of uptake by water surfaces; see also George et al. (1994a,b).

concern. The toxicology of this compound is reviewed by Ball and Wallington (1993) and its effects on plants are discussed by Tromp et al. (1995). The reaction of TFA in the gas phase with OH is relatively slow (k ~ 1.7 X MTn cm3 molecule"1 s_1 at 296 K) and is estimated to account for only ~ 10-20% of the loss of TFA, with the remainder being removed by rainout (M0gelberg et al., 1994a). Calculated maximum concentrations of TFA in rainwater in the future have been suggested to be in the range of 1-80 nmol L_1 (Ball and Wallington, 1993; Tromp et al., 1995), but a 3-D modeling study suggests that global annually averaged rainwater concentrations would be ~ 120 ng L~', and in northern midlatitudes, monthly averaged concentrations could be as large as 450 ng L_1 in the summer (Kotamarthi et al., 1998). These higher estimated values are consistent with measurements of 40-f200 ng L 1 of TFA in rainwater in California and Nevada (Wujcik et al., 1997). Frank and co-workers (1996) report that in 1995, concentrations of TFA in Europe in rainwater were 0.26-2.f nmol L_l, with up to 5.5 nmol L~1 in rivers; in the Dead Sea, where the TFA may have been concentrated through evaporation processes, the concentration was 56 nmol L

CF3C(0)C1 from the oxidation of HCFC-123 is rapidly taken up into cloudwater (Table 13.8). However, it also photolyzes (Rattigan et al., 1993; Wallington, 1994a; WMO, 1995), with an estimated tropospheric lifetime of ~ 33 days assuming a quantum yield for dissociation of unity (Rattigan et al., 1993):

The subsequent reactions of CF3 are as discussed earlier. In the troposphere, the most likely fate of CI is reaction with organics (see Chapter 6).

The aldehyde HC(0)CFC12 formed in the oxidation of HCFC-141b is expected to photolyze in the troposphere, forming in part CFHC12 (Wallington et al., 1994a), which is itself oxidized in the troposphere by reaction with OH. By analogy to the photolysis of nonhalogenated aldehydes, formation of HCO + CFC12 is also expected.

Because HC(0)CFC12 has an abstractable hydrogen atom, it reacts with OH in the troposphere:

With a rate constant at 298 K of 1.2 X fO"12 cm3 molecules-1, the lifetime of this aldehyde with respect to OH at 1 X 106 radicals cm"3 is about 10 days (Scollard et al., 1993). Either decomposition of C(0)CFC12 to CO + CFC12 or its reaction with 02 can occur (Tuazon and Atkinson, 1994):

0 0

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