Water-soluble ion components of PM 10 during the winter-spring season in a typical polluted city in Northeast China

Abstract

From January 1 to April 22, 2014, an online analyzer for monitoring aerosols and gases (MARGA) was used to measure and analyze water-soluble ions in inhalable particulate matter with a diameter less than 10 μm (PM₁₀) during winter-spring in Shenyang city, China. The results yielded three main findings. (1) During the entire observation period and in seven pollution episodes, SO₄²⁻, NO₃⁻, and NH₄⁺ (SNA) accounted for 84.4–93.1% of the total water-soluble ions (TWSIs). TWSIs accounted for 32% of PM₁₀ mass during the entire observation period, and the contribution of TWSIs in PM₁₀ ranged from 33.4–43.1% in the seven pollution episodes. The contribution of TWSIs components increased during the pollution episodes, but certain differences were observed in different pollution episodes. In terms of ionic equilibrium, the total concentration of negative ions was slightly greater than that of positive ions and the difference was 3.1% of the total ion load on average, indicating that local aerosols are mainly neutral. The water-soluble ions show clear diurnal variation with the high concentration around 09:00 for SO₄²⁻, NH₄⁺, and Cl⁻ which is consistent with the high heating grade index. (2) Pollution episodes often occur in Northeast China, especially during the winter period. Due to the low temperature in the winter, the local coal burning for heating is one of the main sources of pollution besides vehicle exhaust and industrial pollution, which is supported by the higher NO₃⁻/SO₄²⁻ ratio in April than that in January to March. Sometimes, under the prevailing wind directions of W and SSW, the long-distance transport of pollutants from the Beijing-Tianjin-Hebei region and Shandong province superimposed on local pollution leads to the most severe pollution, such as Ep3 and Ep5. (3) SO₄²⁻ concentration is closely related to ambient water vapor pressure (e*), with increase as e* increased depending on the temperature. NO₃⁻ concentration showed a linear relationship of excess NH₄⁺, which suggests homogeneous gas-phase reaction of ammonia and nitric acid is possibly an important pathways of nitrate formation in the haze pollution process in Shenyang City. In addition, our results also suggest the nighttime liquid-phase reaction may cause large increases of nitrate in the haze pollution process.

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