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A total suspended particulate (TSP) sample wascollected 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), alongwith a size-resolved aerosol sampling and an online measurement ofPM2.5. 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 andelements to investigate chemistry evolution of dust particles. Hourlyconcentrations of Cl−, NO3−, SO42−, Na+ andCa2+ in the TSP samples reached up to 34, 12, 180, 72 and28 μg m−3, respectively, when dust peak arrived over Xi'an. Chemicalcompositions of the TSP samples showed that during the whole observationperiod NH4+ and NO3− were linearly correlated with each other(r2=0.76) with a molar ratio of 1 : 1, while SO42− andCl− were well correlated with Na+, Ca2+, Mg2+ and K+(r2 > 0.85). Size distributions of NH4+ andNO3− presented a same pattern, which dominated in the coarse mode(> 2.1 μm) during the event and predominated in the finemode (< 2.1 μm) during the non-event. SO42− andCl− also dominated in the coarse mode during the event hours, butboth exhibited two equivalent peaks in both the fine and the coarse modes during thenon-event, due to the fine-mode accumulations of secondarilyproduced SO42− and biomass-burning-emitted Cl- and thecoarse-mode enrichments of urban soil-derived SO42− and Cl−.Linear fit regression analysis further indicated that SO42− andCl− in the dust samples possibly exist as Na2SO4, CaSO4and NaCl, which directly originated from Gobi desert surface soil, whileNH4+ and NO3− in the dust samples exist asNH4NO3. We propose a mechanism to explain theseobservations in which aqueous phase of dust particle surface is formed viauptake of water vapor by hygroscopic salts such as Na2SO4 andNaCl, followed by heterogeneous formation of nitrate on the liquid phase andsubsequent absorption of ammonia. Our data indicate that 54 ± 20% and60 ± 23% of NH4+ and NO3− during the dustperiod were secondarily produced via this pathway, with the remaining derivedfrom the Gobi desert and Loess Plateau, while SO42− in the eventalmost entirely originated from the desert regions. Such cases are differentfrom those in the East Asian continental outflow region, where duringAsia dust storm events SO42− is secondarily produced andconcentrates in sub-micrometer particles as (NH4)2SO4 and/orNH4HSO4. To the best of our knowledge, the current work for thefirst time revealed an infant state of the East Asian dust ageing process in theregions near the source, which is helpful for researchers to understand thepanorama of East Asian dust ageing process from the desert area to thedownwind region

atmospheric chemistry and physics

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