Impact of a simulated severe local storm on the redistribution of sulfur dioxide

Abstract

A series of numerical experiments in a two‐dimensional framework have been carried out in order to investigate the impact of a severe summertime, midlatitude storm on the SO2‐S(IV)‐S(VI) transport and oxidation processes by comparing results of numerical simulations with and without consideration of aqueous phase chemistry. The cloud‐transport‐only runs showed considerable dilution of boundary layer air with free tropospheric air in the cloud, such that in its upper reaches and anvil parts only 10% or less originated from the boundary layer. A substantial fraction of the SO2 transported upward into the cloud area is dissolved in the liquid phase particles. As a consequence, about 50% of the SO2 that has gone up in the cloud is removed by precipitation processes under the chosen conditions. Below‐cloud scavenging was likewise an efficient process. Most importantly, the uptake of SO2 in falling cloud drops strongly hinders the transfer of SO2 to the upper reaches of the cloud. Transport of SO2 from the boundary layer to the upper troposphere was therefore a rather inefficient process. Another result of our simulation is that only a small amount of dissolved SO2 is converted into S(VI) during the storm simulation period (1 hour). This is explained by the fact that we assumed H2O2 to be present only in the boundary layer at concentrations about 10 times lower than those of SO2, together with the higher solubility of H2O2. However, because of the large entrainment of free tropospheric air into the cloud, the H2O2 in the free troposphere would be much more efficient in oxidizing S(IV). A number of sensitivity calculations which we conducted, assuming different chemical makeups of the boundary layer did not change the main conclusions of our study.