Chemical characteristics of airborne particles in Xi'an, inland China during dust storm episodes: Implications for heterogeneous formation of ammonium nitrate and enhancement of N-deposition

Abstract

To identify the sources and heterogeneous reactions of sulfate and nitrate with dust in the atmosphere, airborne particles in Xi'an, inland China during the spring of 2017 were collected and measured for chemical compositions, along with a laboratory simulation of the heterogeneous formation of ammonium nitrate on the dust surface. Our results showed that concentrations of Ca2+, Na+ and Cl− in the TSP samples were enhanced in the dust events, with the values of 41.8, 5.4 and 4.0 μg m−3, respectively, while NO3− (7.1 μg m−3) and NH4+ (2.4 μg m−3) remarkably decreased, compared to those in the non-dust periods. During the dust events, NH4+ correlated only with NO3− (R2 = 0.52) and abundantly occurred in the coarse mode (>2.1 μm), in contrast to that in the non-dust periods, which well correlated with sulfate and nitrate and enriched in the fine mode (<2.1 μm). SO42− in Xi'an during the dust events existed mostly as gypsum (CaSO4·2H2O) and mirabilite (Na2SO4·10H2O) and dominated in the coarse mode, suggesting that they were directly transported from the upwind Gobi Desert region. Our laboratory simulation results showed that during the long-range transport hygroscopic salts in the Gobi dust such as mirabilite can absorb water vapor and form a liquid phase on the particle surface, then gaseous NH3 and HNO3 partition into the aqueous phase and form NH4NO3, resulting in the strong correlation of NH4+ with NO3− and their accumulation on dust particles. The dry deposition flux of total inorganic nitrogen (NH4+ + NO3−) in Xi'an during the dust events was 0.97 mg-N m−2 d−1 and 37% higher than that in the non-dust periods. Such a significant enhanced N-deposition is ascribed to the heterogeneous formation of NH4NO3 on the dust particle surface, which has been ignored and should be included in future model simulations.