The effects of coexisting Na2SO4 on heterogeneous uptake of NO2 on CaCO3 particles at various RHs

Abstract

Atmospheric particles can undergo nucleation, coagulation, chemical-aging, dissolution-precipitation or other atmospheric processes, resulting in complex multicomponent aerosols. The coexisting species have potentially important consequences in the heterogeneous reactions of multicomponent aerosol particles with polluted gases, which are still poorly understood. The effect of coexisting Na2SO4 on heterogeneous uptake of NO2 on CaCO3 particles is investigated in a broad RH range. The combination of DRIFTS, Raman, SEM and IC provides qualitative and quantitative information about the formation of nitrate and other surface species. Ca(NO3)2 and NaNO3 are generated on mixed CaCO3-Na2SO4 particles under dry condition. Both the amount of NO3− formed and the NO3− formation rates for the mixtures can be predicted based on the linear addition of those for pure CaCO3 and Na2SO4 particles under dry condition. The further reaction of Ca(NO3)2 with Na2SO4 could lead to the formation of crystal NaNO3 and CaSO4·0.5H2O at 30% RH. Coagulation between Ca2+ and SO42− in surface adsorbed water is observed after part conversion of CaCO3 to Ca(NO3)2, resulting in the formation of CaSO4·2H2O at 80% RH. The amount of NO3− formed on the mixtures is dramatically enhanced relative to the predictions at 30% and 80% RH. The findings presented here highlight the role of coexisting species in the heterogeneous reactions of trace gases with multicomponent aerosols due to the complexity of atmospheric particles.