Heterogeneous atmospheric degradation of current-use pesticides by nitrate radicals

In the atmosphere, pesticides are distributed between gaseous and particulate phases according to their physicochemical properties. In these two phases, they can react with atmospheric oxidants such as ozone, hydroxyl radical and nitrate radicals. Heterogeneous kinetics of the degradation by nighttime nitrate radicals are not well described. In this study, the heterogeneous reactivity with nitrate radicals of eight current-use pesticides (i.e., difenoconazole, tetraconazole, cyprodinil, fipronil, oxadiazon, pendimethalin, deltamethrin, and permethrin) adsorbed on silica model particles was investigated using laboratory experiments with in-situ nitrate radicals generation and concentration measurement. Under these experimental conditions, all pesticides were degraded. Atmospheric half-lives calculated with a Langmuir-Rideal model ranged between 8 days and 16 days and between 2 days and 11 days according to a Langmuir-Hinshelwood model for an atmospheric nitrate radicals concentration of 20 ppt. Results obtained can contribute to a better understanding of the atmospheric fate of pesticides in the particulate phase and show the importance of their degradation by nitrate radical compared to their degradation by other oxidants such as ozone and hydroxyl radicals.