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

Abstract

To understand the chemical evolution of dust in the current East Asian atmosphere, the chemistry of PM_2.5 and size-resolved aerosols in Shanghai, China, during the 2019 dust storm event was investigated. Our results showed that concentrations of SO₄^2- 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 NO_x, O₃, and NH₃ in the current East Asian atmosphere, which should be considered in future modeling studies.