| During spring dust storm period, TSP filter samples were collected hourly from 9 to 12 in March 2013 and analyzed for inorganic ions, carbonaceous components and hygroscopicity. Online measurements for inorganic ions of PM2.5, NH3 and size–resolved CCN activity were also performed from December 24, 2013 to January 27, 2014. Both observations were conducted at an urban site of Xi’an. The present research aimed to investigate the chemistry evolution of urban atmospheric particles, including the sources and formation mechanisms of sulfate, nitrate, ammonium and sodium salts and their impacts on the hygroscopicity of dust during the spring dust storm and winter haze period.The results showed that during the winter haze period, PM2.5 was the main factor impairing the urban visibility. Fireworks display in the New Year’s eve of 2014 and the unfavorable meteorological condition for pollutant dispersion caused the sharp increases in concentrations of Cl–, K+ and Mg2+. NO3–, SO42– and NH4+ of PM2.5 were mainly formed by secondary formation and showed increasing concentrations during the hazy days. Na+ was mainly produced by the biomass burning and did not increase. Its concentration ranged from 0.64μg·m–3 to 1.63μg·m–3. The concentration of NO2– was extraordinarily low, because HNO2 in gas phase is very unstable and NO2– is an intermediate and hard to accumulate. Concentration of Ca2+ in PM2.5 was also extremely low, because Ca2+ mainly presented in the coarse particles and wind speed was relatively lower during the winter haze period. Concentration of NH3 ranged from 10.7μg·m–3 to 47.8μg·m–3 with a clear diurnal variation, because NH3 in the urban atmosphere is mainly influenced by human activities, which is more significant in daytime than in nighttime. Analysis using the ZSR rule and the simplified model, both are based on chemical composition of PM2.5, shows that the hygroscopic parameter κ of PM2.5 in the heavy haze days was a little larger than the κ value in light haze days. Furthermore, the hygroscopic parameter κ of PM2.5 showed a noticeable trend of diurnal variation, because of the diurnal variation of atmospheric boundary layer and the enhanced photochemical reactions in daytime. The result of combining the SMPS and CCNC showed that the particle diameter is more important than its chemical composition for aerosol being activated as a cloud condensation nuclei and the hygroscopicity of fine particle was stronger than that of coarse particle.During the spring dust storm period there were two dust storms peaks arriving over Xi’an with the hourly concentrations of TSP being 7527μg·m–3 and 3200μg·m–3and SO42– being 180μg·m–3 and 38μg·m–3, respectively. Concentrations of NH4+and NO3– were almost undetectable at the beginning hour, but both continuously increased with a molar ratio of 1:1 and reached up to 34μg m–3 and 8.7μg m–3 within 48 hr after the dust storm leaving Xi’an. During the dust storm period, SO42– and Cl– strongly correlated with Ca2+ and Na+(r2=0.95), while NH4+ correlated well with NO3–(r2=0.78), which suggests that the sources of SO42–, Cl–, Ca2+ and Na+(r2=0.95) are of the same sources, which are most likely the surface soil of the dried paleolakes in Gobi desert regions, because there are many the dried lakes in Taklimakan and Gobi desert areas, of which the surface soil are enriched in water–soluble salts such as halite, mirabilite, gypsum and glauberite. However, NH4+ and NO3– in the samples were mainly derived from heterogeneous reactions on the dust surface and existed as NH4NO3. Due to the dominant contribution of inorganic ions to the water–soluble matter, the water–soluble fraction of the TSP samples showed a similar level of hygroscopic growth factors, ranging from 1. 27 to1.44 during the whole campaign. |
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