This paper reports the results of a modeling study of the formation and deposition of trifluoroacetic acid (TFA) following the atmospheric oxidation of HFO-1234yf (CF₃CF=CH₂) emitted in India, China, and the Middle East.  The concentrations of trifluoroacetic acid expected in precipitation are shown to be in a similar range to those estimated for emissions of HFO-1234yf in North America, Europe, and China (Henne et al., 2012; Kazil et al., 2014; Luecken et al., 2010; Wang et al., 2018).  Similar to the previous studies modeling emissions in North America and Europe it is concluded that deposition of trifluoroacetic acid following the atmospheric oxidation of HFO-1234yf emitted in India, China, and the Middle East has a negligible impact on human or ecosystem health.  I recommend publication after the authors have considered the following minor comments.

(1) Line 56, the latest GWP estimate for HFC-134a is 1,600 (Hodnebrog et al., 2020).

(2) Line 65, allowance for the decomposition of chemically activated CF₃CFHO radicals needs to be made in assessing the yield of CF₃COF and hence trifluoroacetic acid from the atmospheric oxidation of HFC-134a (Wallington et al., 1996).  Hence, the yield of TFA from atmospheric oxidation of HFC-134a is substantially lower than 30%.

(3) The negligible impact of TFA formation during the atmospheric oxidation of HFCs, HCFCs, and HFOs has been established for some time.  In 2007, the WHO Ozone report concluded ‘‘trifluoroacetic acid from the degradation of HCFCs and HFCs will not result in environmental concentrations capable of significant ecosystem damage’’.  In 2008, Hurley et al. concluded that “the products of the atmospheric oxidation of CF₃CF=CH₂ have negligible environmental impact”.  In 2016, Solomon et al. concluded that “concentrations of TFA and its salts in the environment that result from degradation of HCFCs, HFCs, and HFOs in the atmosphere do not present a risk to humans and environment”.  Although some of these are cited in the latest review by Neale et al. (2021) which is cited, it would be appropriate to explicitly mention these previous reports for context.

References

Henne, S., Shallcross, D. E., Reimann, S., Xiao, P., Brunner, D., O’Doherty, S. and Buchmann, B.: Future emissions and atmospheric fate of HFC-1234yf from mobile air conditioners in Europe, Environ. Sci. Technol., 46, 1650–1658, 2012.

Hodnebrog, Ø., Aamaas, B., Fuglestvedt, J. S., Marston, G., Myhre, G., Nielsen, C. J., Sandstad, M., Shine, K. P., and Wallington, T. J.: Updated global warming potentials and radiative efficiencies of halocarbons and related compounds, Rev. Geophys., 58, e2019RG000691, 2020.

Hurley, M. D., Wallington, T. J., Javadi, M. S., and Nielsen, O. J.: Atmospheric Chemistry of CF₃CF=CH₂: Products and Mechanisms of Cl Atom and OH Radical Initiated Oxidation, Chem. Phys. Lett., 450, 263-267, 2008.

Kazil, J., McKeen, S., Kim, S. W., Ahmadov, R., Grell, G. A., Talukdar, R. K. and Ravishankara, A. R.: Deposition and rainwater concentrations of trifluoroacetic acid in the United States from the use of Hfo-1234yf, J. Geophys. Res., 119, 14,059-14,079, 2014.

Luecken, D. J., Waterland, R. L., Papasavva, S., Taddonio, K. N., Hutzell, W. T., Rugh, J. P. and Andersen, S. O.: Ozone and TFA impacts in North America from degradation of 2,3,3,3- tetrafluoropropene (HFO-1234yf), A potential greenhouse gas replacement, Environ. Sci. Technol., 44, 343–348, 2010.

Solomon, K. R., Velders, G. J. M., Wilson, S. R., Madronich, S., Longstreth, J., Aucamp, P. J., Bornman, J. F.: Sources, fates, toxicity, and risks of trifluoroacetic acid and its salts: relevance to substances regulated under the Montreal and Kyoto Protocols. J. Toxicol. Environ. Health. Part B, Crit. Rev., 19, 289-304, 2016.

Wallington, T. J., Hurley, M. D., Fracheboud, J. M., Orlando, J. J., Tyndall, G. S., Sehested, J., Møgelberg, T. E., and Nielsen, O. J.:  Role of excited CF₃CFHO radicals in the atmospheric chemistry of HFC-134a, J. Phys. Chem., 100, 18116-18122, 1996.

WMO (World Meteorological Organization), 2007. Scientific Assessment of Ozone Depletion: 2006, Global Ozone, Research and Monitoring Project – Report 50, Geneva, Switzerland.

The authors present a modelling study of deposition of trifluoroacetic acid (TFA) resulting from emissions of HFO-1234f in India, China and Middle east. The HFO-1234f emissions are expected to increase in these regions in the future and such study is of significance to the atmospheric community. GEOS-Chem and WRF-Chem models were used. The models were characterized systematically, and I think the overall conclusions are scientifically sound and the paper is publishable in ACP. Below are few questions and comments I have for the authors:

-TFA rainwater concentration is dependent of the frequency of rain. I assume that the rainfall pattern will likely change in the next 20 years. Could the authors comment on how this will impact TFA rainwater concentration over the middle east region? A graphical relationship between amount of rainfall and TFA rainwater concentration for each region would be helpful for the readers

-TFA rainwater concentration in China using WRF-Chem is significantly different (factor of 2) compared with the results from GEOS-Chem and previous study by Wang et al. (shown in Figure 4). However, the percentage of wet deposition seem relatively similar in China for GEOS-Chem and WRF-Chem. Could the authors explain this difference?  

-Recent study by Holland et al. ACS Earth Space Chem. 2021, 5, 849-857 suggest Criegee intermediates in forested region can significantly decrease the lifetime of TFA. The GEOS-Chem simulations in this study show that the overall significance is small except for the forested regions in south east Asia. I wonder if this contribution will be higher in these regions during the dry seasons.

-Is there any reason why the loss from reaction with Criegee intermediates is relatively higher in the India region in Figure 14 b) compared with 14 a)? Is this because of seasonality in rainfall?

Minor comments:

-The dashes in the figures are hard to see. Please put bigger gaps in between the dashes to make them obvious.

-In the kinetic scheme, could the authors label the phases of all the fluorinated species. I also suggest labelling each step and then citing them in the following paragraph.

-dissociation coefficient in line 288 should have unit of mol l^-1

-Change ‘the’ to ‘they’ in the last line of caption for figure 8

-It is difficult to see some of the datapoints in Figure 9 and 10. I suggest using panel with one column and scaling the vertical axis better