The annual course of TCA formation in the lower troposphere: A modeling study

We present a modeling study investigating the influence of climate conditions and solar radiation intensity on gas-phase trichloroacetic acid (TCA) formation. As part of the ECCA-project (Ecotoxicological Risk in the Caspian Catchment Area), this modeling study uses climate data specific for the two individual climate regimes, namely "Kalmykia" and "Kola Peninsula". A third regime has also been included in this study, namely "Central Europe", which serves as a reference to somehow more moderate climate conditions. The simulations have been performed with a box modeling package (SBOX, photoRACM), which uses Regional Atmospheric Chemistry Mechanism (RACM) as its chemistry scheme. For this model a mechanism supplement has been developed including the reaction pathways of methyl chloroform photooxidation. The investigations are completed by a detailed sensitivity study addressing the impact of temperature and relative humidity. Atmospheric OH and HO₂ concentrations and the NO_x/HO₂ ratio were identified as the governing quantities controlling the TCA formation trough methyl chloroform oxidation in the gas phase. Model calculations show a TCA production rate ranging between almost zero and 6.5×10³ molecules cm^-3 day^-1 depending on location and season. In the Kalmykia regime the model predicts mean TCA production rates of 1.3×10^-4 and 5.4×10^-5 μg m^-3 year^-1 for the urban and rural environment, respectively. From the comparison of model calculations with measured TCA burdens in the soil ranging between 130 μg m^-3 and 1750 μg m^-3 we conclude that TCA formation through methyl chloroform photooxidation in the gas-phase is probably not the principal atmospheric TCA source in this region.