Upper tropospheric SO2 conversion into sulfuric acid aerosols and cloud condensation nuclei

We have investigated the potential of upper tropospheric cloud condensation nuclei (CCN) formation hom sulfur dioxide, This process involves three ste ps including SO2 conversion to gaseous sulfuric acid (GSA), homogeneous bim olecular nucleation of GSA and water vapor leading to new aerosol particles , and new aerosol particle growth via mutual coagulation and GSA-H2O conden sation. Our investigation includes both measurements of SO2 and GSA as well as model calculations of SO2 conversion to new aerosol particles ill the p resence of preexisting aerosols which act as a competing sink for GSA, The model reproduces the measured abundance ratio GSA-SO2 reasonably well. Furt hermore, the model predicts that efficient upper tropospheric CCN formation from SO2 may, in fact, occur under certain plausible conditions. These con ditions are explored utilizing the factor F-CCN: defined as a ratio of the source and sink terms of gaseous sulfuric acid. In particular, a threshold F-CCN value for efficient CCN formation is calculated with the model.