Chemical Composition and Size Distribution of Airborne Particulate Water-Soluble Species in Xi'an,Central China during Spring 2009:Implications for Heterogeneous Reactions of Anthropogenic Pollutants with Mineral Dust

【摘要】：PM10 and size-resolved (9-stage) aerosol samples in the urban air of Xi'an, China were collected during the spring of 2009 including a massive dust storm occurring on April 24th and measured for inorganic ions, water-soluble organic (WSOC) and inorganic carbon (WSIC), water-soluble organic nitrogen (WSON), EC and OC. Characteristics of chemical compositions and size-distributions of those measured species in the urban samples were investigated and further compared with those simultaneously observed on the mountaintop of Mt. Hua, which is located about 150 km east to the city, to recognize the impact of dust storm on aerosol chemistry of the downwind region. PM10 in Xi'an during the non-dust storm period ranged from 58-420 g m-3 (257±79 g m-3), and sharply increased to a 3-hr maximum of 890 g m-3 (av. 589±273 g m-3) on the dust storm day (April 24th). Mass-closure study results showed that during the non-dust storm period organic matters accounted for 25% of the PM10 particle mass, followed by SO42- (13%), NO3- (10%), water-soluble carbonate (10%), EC (5%), NH4+ (4%), and other ions (7%). In contrast, water-soluble carbonate became the most abundant among the measured species during the dust storm period, accounting for 12% of the PM10 mass, followed by organic matters (11%), SO42- (3%), Ca2+(2%) and NO3- (1.5%). Such a changes in chemical compositions suggests a significant heterogeneous reaction of acidic gases (e.g., H2SO4, HNO3, SO2 and N2O5) with dust to form water-soluble mineral compounds. A bimodal size distribution of particles was observed during the non-dust time with two equivalent peaks in the fine (2.1 m) and coarse (2.1 m) ranges, in contrast to a unimodal pattern (i.e., coarse mode) observed on the dusty day. All the measured ions except NH4+ significant shifted toward larger particles during the dust storm time. Similar size-distribution patterns of particles were found on the mountaintop of Mt. Hua, which are bimodal during the non-dust period and unimodal during the dust period. However, Cl- and NO3- presented a different size distribution between the urban and the mountaintop atmospheres. Cl- showed a bimodal pattern in the urban area during the whole sampling periods with an increased coarse mode during the dusty day, in contrast to a unimodal coarse mode observed at the mountaintop in either the non-dust or the dust periods, which is most likely caused by heterogeneous reactions of sulfuric and/or nitric acids with chloride-containing compounds (e.g., KCl) during the long-range transport of biomass burning emissions from lowland regions to the mountaintop. NO3-presented a bimodal pattern in the urban air with a dominant peak in the fine mode during the non-dust period and two equivalent peaks in the fine and coarse ranges during the dust storm period. Whereas in the mountain atmosphere NO3- presented two equivalent peaks in the fine and coarse modes on the non-dust storm days and a unimodal coarse mode on the dust storm day. Such a difference can be explained by an evaporation of HNO3 from the fine mode particles and a subsequent re-adsorption onto alkaline coarse particles, which was more significant at the mountaintop during the dust-storm period.