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

摘要

pstrongAbstract./strong 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 1800a??12 March 1000 LT), along with a size-resolved aerosol sampling and an online measurement of PMsub2.5/sub. 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 Clsup−/sup, NOsub3/subsup−/sup, SOsub4/subsup2−/sup, Nasup+/sup and Casup2+/sup in the TSP samples reached up to 34, 12, 180, 72 and 28 ??g msupa??3/sup, respectively, when dust peak arrived over Xi'an. Chemical compositions of the TSP samples showed that during the whole observation period NHsub4/subsup+/sup and NOsub3/subsup−/sup were linearly correlated with each other (ir/isup2/sup=0.76) with a molar ratio of 1 1, while SOsub4/subsup2−/sup and Clsup−/sup were well correlated with Nasup+/sup, Casup2+/sup, Mgsup2+/sup and Ksup+/sup (ir/isup2/sup 0.85). Size distributions of NHsub4/subsup+/sup and NOsub3/subsup−/sup presented a same pattern, which dominated in the coarse mode ( 2.1 ??m) during the event and predominated in the fine mode ( 2.1 ??m) during the non-event. SOsub4/subsup2−/sup and Clsup−/sup 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 SOsub4/subsup2−/sup and biomass-burning-emitted Clsup-/sup and the coarse-mode enrichments of urban soil-derived SOsub4/subsup2−/sup and Clsup−/sup. Linear fit regression analysis further indicated that SOsub4/subsup2−/sup and Clsup−/sup in the dust samples possibly exist as Nasub2/subSOsub4/sub, CaSOsub4/sub and NaCl, which directly originated from Gobi desert surface soil, while NHsub4/subsup+/sup and NOsub3/subsup−/sup in the dust samples exist as NHsub4/subNOsub3/sub. 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 Nasub2/subSOsub4/sub 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 NHsub4/subsup+/sup and NOsub3/subsup−/sup during the dust period were secondarily produced via this pathway, with the remaining derived from the Gobi desert and Loess Plateau, while SOsub4/subsup2−/sup 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 SOsub4/subsup2−/sup is