Efficient Heterogeneous Formation of Ammonium Nitrate on the Saline Mineral Particle Surface in the Atmosphere of East Asia during Dust Storm Periods

To understand the chemical evolution of dust in the current East Asian atmosphere, the chemistry of PM2.5 and size-resolved aerosols in Shanghai, China, during the 2019 dust storm event was investigated. Our results showed that concentrations of SO42- in the city during the event highly correlated with Ca2+ and Na+ due to the direct emissions of CaSO4 and Na2SO4 from the upwind deserts. In contrast, during the event, NO3- linearly correlated with NH4+ at a molar ratio close to 1:1, and both almost entirely stayed in coarse particles, suggesting they accumulated on the dust surface as NH4NO3. Based on the field observations and laboratory smog chamber simulations, we found that NO2 and O3 in Shanghai during the dust period reacted to form N2O5, which subsequently hydrolyzed into HNO3 on the surface of saline mineral dusts (e.g., CaSO4 and Na2SO4) and was further neutralized by NH3 as NH4NO3. The relative abundances of NO3- and NH4+ in Shanghai during the dust event were notably higher than those a decade ago, indicating that this heterogeneous formation of NH4NO3 on dust was enhanced by the abundantly coexisting NOx, O3, and NH3 in the current East Asian atmosphere, which should be considered in future modeling studies.