Effects of relative humidity on heterogeneous reaction of SO2 with CaCO3 particles and formation of CaSO4·2H2O crystal as secondary aerosol

Haze related air pollution has become one of major environmental concerns in some Asian countries. Sulfate is a main component ambient particulate matter (PM) the urban environment during haze episodes. Among pollutants, sulfur dioxide (SO2) an important precursor sulfate and new atmospheric particle formation. However, under different conditions its underlying formation mechanisms are not clearly elucidated. In current work, we collected PM two cities: Zurich (Switzerland) Beijing (China). The morphology composition were investigated by scanning electron microscopy, energy dispersive X-Ray spectroscopy Raman micro-spectroscopy. addition, series lab controlled experiments also performed to study SO2 reaction with CaCO3 aerosol. We found types CaSO4·2H2O crystals Chinese samples, compositions than those from Zurich. experimental data showed that relative humidity played significant role on crystal including size, composition. Relative (RH) above 80% can significantly promote oxidation particles form crystals. contrast, at below 40%, only few be converted CaSO4 particles. results this facilitate understanding secondary inorganic aerosol RHs city environments, provide useful information for control.