An evaluation of magnitudes of reaction probability of sulfate and nitrate precursors onto East Asian dust particles

We investigate the chemical evolution of dust particles and examine the magnitudes of reaction probability (/gamma) of sulfate and nitrate precursors (such as H2SO4, SO2, N2O5, and HNO3) onto dust particles in East Asia. For this investigation, three data sets from ACE-ASIA C130 Flights #6, #7, and #8 and three data sets from measurements in Seoul are analyzed. During the selected dust storm periods, large amounts of CO32- still remained in fine-mode dust particles (Dp < ~1.3 /mum). The average fractions of CO32- (based on [Ca2+]+[Mg2+] equivalences) for the three C130 flights were 0.87, 0.68, and 0.39, respectively, and the average CO32- fractions for three data sets in Seoul ranged from 0.43 to 0.86. These fractions of carbonate that remained in mineral dust are significantly larger than those reported in dust chemistry-transport modeling studies. Our analysis suggests that this should be due to the uses of overly high /gamma values in the model simulations. This fact is also confirmed in this study with Lagrangian photochemical model simulations conducted with both the constraints observed by ACE-Asia C130 Flights and the initial concentrations obtained from US-EPA Models-3/CMAQ (Community Multi-scale Air Quality) modeling over East Asia. It is inferred from this study that the limited gas-dust interaction is somehow related to a high degree of external mixing of mineral dust particles with urban-derived pollution particles during the East Asian dust events. Closely related with this issue, a possible dependence of the /gamma values on RH is also investigated. Based on the analysis, we suggested that a particular attention should be paid on the issue of the RH- dependent /gamma in the future laboratory works as well as in the future dust chemistry-transport modeling studies. Keywords: Reaction probability; dust particles; East Asia; chemical evolution; carbonate replacement; aerosol mixing state