(Invited Presentation) Oxidization of SO2 to sulfate by O3, NO2 and nitrate on the surface of mineral dust particles

CaCO3 is one of the most important components of the atmospheric mineral aerosol. CaCO3 particles can undergo heterogeneous reactions with nitrogen-containing gases (e.g., NO2, gaseous HNO3 and N2O5) in the atmosphere during long-range transport, producing Ca(NO3)2. Ca(NO3)2 is much more hygroscopic than CaCO3, and may have a reactive activity different from CaCO3 in the atmosphere.

Heterogeneous reactions between SO2(50PPM)/O3(50PPM) mixing gas and individual CaCO3, Ca(NO3)2 particles (with the geometric diameter of about 6 to 7 micrometers) under various RH conditions were investigated using Micro-Raman Spectrometry and flow reaction system. Chemical composition and microscopic morphology of the individual particles were determined in situ with Micro-Raman spectrometer during the reactions. Individual CaCO3 particles started reaction with SO2/O3 mixing gas at RH values of 55% and 80%, producing CaSO4. The reactive uptake coefficient (¦ÃSO2) was in the order of 10-7 to 10-8 using the geometric surface area of the individual particles. Individual Ca(NO3)2 particles could react completely with SO2/O3 mixing gas at RH values of ≥ 15%. The product was CaSO4. The reactive uptake coefficient (¦ÃSO2) was in the order of 10-6 to 10-7 using the geometric surface area of the individual particles. The reaction rate increased with the RH. At RH=80%, individual Ca(NO3)2 particles with the geometric diameter of 6 micrometers could react completely with SO2/O3 mixing gas in 480 minutes to produce CaSO4.

Under the same RH and concentration of SO2 and O3, the reaction rate of individual Ca(NO3)2 particles with SO2/O3 mixing gas was much higher than that of individual CaCO3 particles with SO2/O3 mixing gas. Thus the formation of Ca(NO3)2 from CaCO3 particles could accelerate the transform of S(IV) to S(VI) in the atmosphere. The possible gaseous product of the reaction between SO2/O3 mixing gas and Ca(NO3)2 particles was HNO3, indicating the heterogeneous reactions between SO2/O3 mixing gas and Ca(NO3)2 particles were sinks of SO2 and O3 and are a potential source of gaseous HNO3, and might have important impacts on the formation of acid rain and atmospheric oxidizability.