The effects of cloud processes on the tropospheric photochemistry: An improvement of the eurad model with a coupled gaseous and aqueous chemical mechanism

Abstract

Within the frame of the European Acid Deposition Model (EURAD), the aqueous-phase chemical reactions in the cloud module of the EURAD Chemistry-Transport Model (CTM) are replaced by a comprehensive aqueous-phase chemical reaction mechanism, which is coupled with the RADM II gas-phase chemical mechanism. Results from the CTM column model with our more explicit aqueousphase chemistry are given to show the impacts of cloud processes on tropospheric photochemistry for single cloud events and consecutive cloud events. For single cloud events, the cloud transport generally results in an increase of species concentrations at cloud levels, either through the efficient updraft of near-ground air with much higher species concentrations (e.g. NO x) or the downward transport of air from the upper troposphere (e.g. 0 3). Cloud chemistry for daytime and nighttime clouds can have different impacts on H 2O 2 concentration levels. In the case of consecutive cloud events, cloud chemistry and radiative cloud effects become more important compared to single cloud events, resulting in the reduction of 03 concentrations. Wet deposition can be of importance for 0 3 concentrations in the later period of cloud events. Our results show that the species concentrations at cloud levels may deviate considerably between simulations including and those neglecting clouds even several days after the end of single cloud events.