TRIFLUOROACETIC-ACID FROM DEGRADATION OF HCFCS AND HFCS - A 3-DIMENSIONAL MODELING STUDY

Trifluoroacetic acid (TFA; CF3 COOH) is produced by the degradation of the halocarbon replacements HFC-134a, HCFC-124, and HCFC-123. The for mation of TFA occurs by HFC/HCFC reacting with OH to yield CF3COX (X = F or C1), followed by in-cloud hydrolysis of CF3COX to form TFA. The TFA formed in the clouds may be reevaporated but is finally deposited onto the surface by washout or dry deposition. Concern has been expres sed about the possible long-term accumulation of TFA in certain aquati c environments, pointing to the need to obtain information on the conc entrations of TFA in rainwater over scales ranging from local to conti nental. Based on projected concentrations for HFC-134a, HCFC-124, and HCFC-123 of 80, 10, and 1 pptv in the year 2010, mass conservation arg uments imply an annually averaged global concentration of 0.16 mu g/L if washout were the only removal mechanism for TFA. We present 3-D sim ulations of the HFC/HCFC precursors of TFA that include the rates of f ormation and deposition of TFA based on assumed future emissions. An e stablished (GISS/Harvard/UCI) but coarse-resolution (8 degrees latitud e by 10 degrees longitude) chemical transport model was used. The annu ally averaged rainwater concentration of 0.12 mu g/L (global) was calc ulated for the year 2010, when both washout and dry deposition are inc luded as the loss mechanism for TFA from the atmosphere. For some larg e regions in midnorthern latitudes, values are larger, 0.15-0.20 mu g/ L. The highest monthly averaged rainwater concentrations of TFA for no rthern midlatitudes were calculated for the month of July, correspondi ng to 0.3-0.45 mu g/L in parts of North America and Europe. Recent lab oratory experiments have suggested that a substantial amount of vibrat ionally excited CF3CHFO is produced in the degradation of HFC-134a, de creasing the yield of TFA from this compound by 60%. This decrease wou ld reduce the calculated amounts of TFA in rainwater in the year 2010 by 26%, for the same projected concentrations of precursors.