Socioeconomic Data and Applications Center Environmental Effects of Ozone Depletion 1998 Assessment

Atmospheric Production and Fate of Trifluoroacetic Acid

    Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are used as CFC substitutes in a variety of applications. A substantial body of experimental and theoretical work has been undertaken to determine the atmospheric chemistry and environmental impact of these compounds. While some minor uncertainties exist, our current understanding of the atmospheric chemistry of the commercially important HFCs and HCFCs is now well established (Wallington et al., 1994; 1995). A generalized scheme for the atmospheric oxidation of a haloalkane that could degrade to give trifluoroacetic acid is given in Figure 6.4.
 
 

Fig. 6.4. Generalized scheme for the atmospheric oxidation of a halogenated organic compound, CF3CXYH (X = Cl or F, and Y = Cl, H, Br, or CF3). Radical intermediates are enclosed in ellipses. Typical lifetime estimates are given in parenthesis.

Oxidation is initiated by reaction with OH radicals giving a halogenated alkyl radical which adds O₂ to give the corresponding peroxy radical (RO₂). Peroxy radicals react with three important trace species in the atmosphere: NO, NO₂, and HO₂ radicals. Reactions with HO₂ and NO₂ delay, but do not prevent, the conversion of peroxy (CF₃CXYO₂) into alkoxy (CF₃CXYO) radicals. Reactions of haloperoxy radicals with NO are rapid and give the alkoxy radical with essentially 100% yield. The atmospheric fate of the alkoxy radical, CF₃CXYO, is either decomposition or reaction with O₂. Decomposition occurs by C-C bond fission, or by the elimination of a Br, Cl, or CF₃ group. The atmospheric fate of CF₃C(O)X (X=F or Cl) is dominated by incorporation into rain-cloud-sea water followed by rapid hydrolysis to trifluoroacetic acid. Photolysis is a competing loss mechanism for CF₃C(O)Cl and limits its conversion into CF₃C(O)OH to 60% (Cox et al., 1995). There are no competing loss processes for CF₃C(O)F; it is converted entirely into CF₃C(O)OH. Although CF₃C(O)OH is produced in aqueous phase chemistry, is highly soluble and partitions into the water phase (Bowden et al., 1996), the evaporation of cloud droplets can transfer CF₃C(O)OH to the gas phase where it can react with OH radicals. However, this reaction is slow (Carr et al., 1994; Møgelberg et al., 1994) and is only a minor (<5%, Kanakidou et al., 1995) loss of CF₃C(O)OH. The main atmospheric fate of CF₃C(O)OH is rain-out to the surface.

    To assess the potential for a halocarbon to produce trifluoroacetic acid, it is necessary to quantify the yield of trifluoroacetyl halide in its gas phase oxidation mechanism. This has been established for all the commercially significant halocarbons. Six compounds, listed in Table 6.2, have been identified that degrade to give trifluoroacetyl halide. With the exception of HFC-134a, the atmospheric oxidation mechanisms of the compounds given in Table 6.2 are relatively simple. Atmospheric oxidation of halothane (Bilde et al., 1998), isoflurane, and HCFC-123 (Edney et al., 1991; Tuazon and Atkinson, 1993a; Hayman et al., 1994) gives CF₃C(O)Cl of which approximately 60% undergoes hydrolysis to give CF₃C(O)OH. Oxidation of HCFC-124 (Tuazon and Atkinson, 1993a; Edney and Driscoll, 1992) and HFC-227ea (Zellner et al., 1994; Møgelberg et al., 1996) gives CF₃C(O)F which is then converted into CF₃C(O)OH. Current and projected use of HFC-227ea is minor. The oxidation mechanism of HFC-134a is complicated by two factors. First, vibrationally excited CF₃CFHO* radicals are formed in the CF₃CFHO₂ + NO reaction and two thirds of these vibrationally excited radicals undergo decomposition via C-C bond scission on a time scale short compared to that needed for chemical reactions (Wallington et al., 1996). Second, reaction with O₂ competes with decomposition via C-C bond scission for the available thermalized CF₃CFHO radicals (Wallington et al., 1992; Tuazon and Atkinson, 1993b; Rattigan et al., 1994). The net effect is that the molar yield of CF₃C(O)F, and hence CF₃C(O)OH, is approximately 0.13 (Wallington et al., 1996).

    An estimate for the concentration of CF₃C(O)OH in rain water can be obtained as follows. Halothane and isoflurane have been used as anesthetics for many years. It is estimated that the current emissions of halothane and isoflurane are 1500 and 750 tonnes/yr, respectively (Boutonnet et al., 1998). The atmospheric lifetimes of halothane and isoflurane are short (see Table 6.2) compared to the time scale over which they have been emitted into the atmosphere. Assuming that these compounds are in steady state then their degradation gives a combined yield of 1540 tonnes/year of CF₃C(O)Cl. Accounting for photolysis, this results in a global deposition rate of 800 tonnes/year of CF₃C(O)OH. Current atmospheric concentrations of HCFC-123 and HCFC-124 are at, or below the detection limit of 0.1 ppt (Boutonnet et al., 1998). Thus, upper limits for atmospheric burdens of these gases are 2500 tonnes for HCFC-123 and 2300 tonnes for HCFC-124. Combining these burdens with the lifetime values given in Table 6.2 and accounting for photolysis of CF₃C(O)Cl it can be concluded that upper limits for the flux of CF₃C(O)OH from HCFC-123 and HCFC-124 are 760 and 320 tonnes/year, respectively. HFC-134a is present at a concentration of 2.5 pptv in the Northern Hemisphere and 1.2 pptv in the Southern Hemisphere (Montzka et al., 1996; Oram et al., 1996) from which it can be estimated that the present atmospheric burden of HFC-134a is 31,000 tonnes. Combining this burden with the lifetime and yield values in Table 6.2 gives a CF₃C(O)OH flux of 300 tonnes/year from the atmospheric oxidation of HFC-134a. Combining the contributions from these known sources gives an upper limit for the estimated contemporary TFA formation rate of 800 + 760 + 320 + 300 = 2180 tonnes/year. The known TFA precursors are relatively long lived and therefore distributed on a global scale. The annual global rainfall is 4.9 x 10¹⁷ liters (Erchel, 1975) and the global average TFA concentration in rainwater is expected to be less than 5 ng/l. Detailed computer modeling studies using appropriate global OH fields, halocarbon emissions, and rainfall patterns have confirmed that the simple approach adopted above provides a reasonable estimate of expected TFA concentrations in rainwater (Rodriguez et al., 1993; Kanakidou et al., 1995; Wild et al., 1996).

    To assess the future contribution of HFCs and HCFCs to levels of TFA in rainwater we need to consider future emission scenarios for these compounds which are reviewed in detail elsewhere (Boutonnet et al., 1998). Use of halothane and isoflurane anesthetics is anticipated to remain constant, or perhaps decline. Production of HCFC-123 and HCFC-124 is regulated under the Montreal Protocol and will be phased out. Production of HFC-134a is not regulated and anticipated to increase significantly. Using an emission scenario from the US Environmental Protection Agency (EPA) combined with a three-dimensional atmospheric model, Rodriguez et al. (1994) calculated that in the year 2020 the atmospheric decomposition fluxes of HFC-134a, HCFC-123, and HCFC-124 will be 115, 20, and 20 kTonnes/yr. Multiplying by the appropriate CF₃C(O)OH molar yields in Table 6.2 and making the necessary adjustments for the molecular weights involved, a flux of 16.7 + 8.9 + 16.7 = 42.3 kTonnes/yr is estimated and translates to a global average concentration in rainwater of 86 ng/l. Kotamarti et al. (1998) estimated the global average TFA concentration in rain water for 2010 to be in the range 100-160 ng/l.

Table 6.2: Compounds known to produce TFA (CF₃C(O)OH) in the atmosphere.

Compound	Molecular weight	Common name	Molar CF3C(O)OH yield	Atmospheric  lifetime
CF3CHClBr	197.5	Halothane	0.6	1.2 years [1]
CF3CHClOCHF2	184.5	Isoflurane	0.6	5 years [2]
CF3CHCl2	153	HCFC-123	0.6	1.5 years [3]
CF3CHFCl	136.5	HCFC-124	1.0	6.0 years [3]
CF3CH2F	102	HFC-134a	0.13	14.6 years [4]
CF3CHFCF3	170	HFC-227ea	1.0	36.5 years [4]

[1] Orkin and Khamagonov (1993); [2] Brown et al. (1989); [3] WMO (1989); [4] IPCC (1996).

    Water bodies characterized by little or no outflow and high evaporation rates may have the potential to accumulate TFA. Tromp et al. (1995) developed a concentration-dependent model and estimated that TFA concentrations of 100 mg/l could be achieved in this type of water body in as few as 30 years, with rainfall concentration only 1 mg/l. In contrast, a recent analysis by Boutonnet et al. (1998) questioned the validity of the assumptions inherent in the Tromp et al. study, concluding that accumulation of TFA in seasonal wetlands "appears to be highly improbable". The potential for accumulation of TFA remains unclear.

    High concentrations of TFA have been observed in contemporary water and air samples, suggesting the existence of one or more large unknown sources. Samples of rain and surface waters (oceans, rivers, lakes, and springs ) have been obtained from many geographical areas (USA, Canada, Australia, South Africa, Germany, Israel, Ireland, France, Switzerland, Finland) and show that TFA is a ubiquitous contaminant of the hydrosphere (Frank et al., 1996; Zehavi and Seiber, 1996; Grimvall et al., 1997; Wujcik et al., 1998), with values up to 40900 ng/l (Zehavi and Seiber, 1996). The average TFA concentration in rain water observed in Bayreuth during 1995 was 100 ng/l (Frank et al., 1996). The source of the currently observed levels is unknown and puzzling. The observed TFA concentrations are orders of magnitude larger than those predicted to result from the atmospheric degradation of the replacement HCFCs and HFCs.

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