Evolution of aerosol chemistry in Xi'an, inland China, during the dust storm period of 2013 – Part 1: Sources, chemical forms and formation mechanisms of nitrate and sulfate

Abstract. A total suspended particulate (TSP) sample was collected hourly in Xi'an, an inland megacity of China near the Loess Plateau, during a dust storm event of 2013 (9 March 18:00−12 March 10:00 LT), along with a size-resolved aerosol sampling and an online measurement of PM_2.5. The TSP and size-resolved samples were determined for elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC) and nitrogen (WSON), inorganic ions and elements to investigate chemistry evolution of dust particles. Hourly concentrations of Cl⁻, NO₃⁻, SO₄²⁻, Na⁺ and Ca²⁺ in the TSP samples reached up to 34, 12, 180, 72 and 28 μg m⁻³, respectively, when dust peak arrived over Xi'an. Chemical compositions of the TSP samples showed that during the whole observation period NH₄⁺ and NO₃⁻ were linearly correlated with each other (r²=0.76) with a molar ratio of 1 : 1, while SO₄²⁻ and Cl⁻ were well correlated with Na⁺, Ca²⁺, Mg²⁺ and K⁺ (r² > 0.85). Size distributions of NH₄⁺ and NO₃⁻ presented a same pattern, which dominated in the coarse mode (> 2.1 μm) during the event and predominated in the fine mode (< 2.1 μm) during the non-event. SO₄²⁻ and Cl⁻ also dominated in the coarse mode during the event hours, but both exhibited two equivalent peaks in both the fine and the coarse modes during the non-event, due to the fine-mode accumulations of secondarily produced SO₄²⁻ and biomass-burning-emitted Cl^- and the coarse-mode enrichments of urban soil-derived SO₄²⁻ and Cl⁻. Linear fit regression analysis further indicated that SO₄²⁻ and Cl⁻ in the dust samples possibly exist as Na₂SO₄, CaSO₄ and NaCl, which directly originated from Gobi desert surface soil, while NH₄⁺ and NO₃⁻ in the dust samples exist as NH₄NO₃. We propose a mechanism to explain these observations in which aqueous phase of dust particle surface is formed via uptake of water vapor by hygroscopic salts such as Na₂SO₄ and NaCl, followed by heterogeneous formation of nitrate on the liquid phase and subsequent absorption of ammonia. Our data indicate that 54 ± 20% and 60 ± 23% of NH₄⁺ and NO₃⁻ during the dust period were secondarily produced via this pathway, with the remaining derived from the Gobi desert and Loess Plateau, while SO₄²⁻ in the event almost entirely originated from the desert regions. Such cases are different from those in the East Asian continental outflow region, where during Asia dust storm events SO₄²⁻ is secondarily produced and concentrates in sub-micrometer particles as (NH₄)₂SO₄ and/or NH₄HSO₄. To the best of our knowledge, the current work for the first time revealed an infant state of the East Asian dust ageing process in the regions near the source, which is helpful for researchers to understand the panorama of East Asian dust ageing process from the desert area to the downwind region.