Formation and Size Distribution of the Secondary Aerosol Inorganic Ions in Different Intensity of Haze in Qingdao, China

Abstract

Size-segregated atmospheric aerosol samples were collected from September 2015 to February 2016 at a coastal site in Qingdao, and the concentrations of major water-soluble inorganic ions were analyzed by ion chromatography. Characteristics and variation of size distribution of secondary inorganic components in aerosol were discussed, as well as the formation process and influencing factors of SNA(sulfate, nitrate and ammonium). The results indicated that the concentrations of NO3-, SO42-, NH4+, NO2-, Cl- in the aerosols were in the range of 10.32-193.46, 4.42-74.05, 2.21-57.75, 0.05-2.22 and 1.35-17.39 μg·m-3 respectively. And the mass concentration of SNA increased with the intensity of haze pollution. The concentrations of NO3- on the slight, mild, moderate and severe haze days were 55%, 77%, 240% and 537% higher than that on non-haze days respectively, while concentrations of SO42- increased by 4.7%, 35%, 77% and 262% respectively, and concentrations of NH4+ increased by 72%, 83%, 201% and 526% respectively. The contribution of these water-soluble ions to PM2.5 showed that the proportion of sum of NO3-, SO42-, NH4+, NO2- and Cl- to PM2.5 was in range of 62.03%-80.93%. The proportion of ion to PM2.5 decreased in the order of NO3- > SO42- > NH4+ > Cl- > NO2-. With the enhancement of haze pollution, the proportion of NO3- in PM2.5 increased from 29.53% to 45.54%. The correlation analysis showed that NO3- and SO42- in the fine particle were significantly correlated with gaseous precursors NO2 and SO2, and also showed good correlations with relative humidity, visibility, wind speed and other weather conditions. These results indicated that the formation of SNA in fine particles was one of the main reasons for visibility decrease and the formation of air pollution in haze days. Meanwhile, high concentration of gaseous precursors, high relative humidity and low wind speed were the important influencing factors of haze formation. Except for slight haze days, SOR and NOR in the haze days were higher than those on the non-haze days, and increased significantly with the intensifying of haze, especially for 0.43-0.65 and 0.65-1.1 μm particle size. Conversion rates of nitrogen and sulfur in severe haze days were 1.5 times that in non-haze days, which showed nitrate and sulfate in these fine mode were mainly from gas-to-particle conversion. NO3-, SO42-, NH4+ and NO2- increased in haze significantly, which mainly existed in the fine particles. The cloud process played a more important role on haze days. While on non-haze day, cloud process and the heterogeneous reaction were both the main factors. And the highest proportion of fine mode concentration to total one was observed for NO3-(79.4%) and SO42-(74.4%) on severe haze days respectively. NO3- showed a bimodal distribution with peaks in the size-bin of 0.43~0.65 μm and 3.3-4.7 μm on non-haze, slight, mild haze days, and the fine peak moved to 0.65-1.1 μm on moderate haze days, however the bimodal distribution changed to unimodal distribution with peak at 0.65-1.1 μm on severe hazy days. SO42- showed a bimodal distribution with peaks at 0.43-0.65 μm and 2.1-3.3 μm in the non-haze weather, while the size distribution changed to unimodal distribution on hazy days. But the peak sizes were different in different intensity of haze, with peak at 0.43-0.65 μm on mild and slight haze and 0.65-1.1 μm in moderate and severe haze days. NH4+ showed a single peak distribution in the fine mode, with the peak in the particle size of 0.43-0.65 μm on slight and non-hazy days, and 0.65-1.1 μm on mild, moderate and severe hazy days. Therefore, haze has a great influence on the size distribution of SNA.