Sulfate, nitrate and ammonium are the most abundant secondary inorganic aerosols (SIA) in atmospheric fine particle matter (PM_2.5). Meteorological conditions, gas-particle transportation process, and aerosol acidity (pH) can influence SIA formation. In this study, we conducted semi-continuous measurements of water-soluble inorganic ions during a winter extreme pollution event (from January 9th to January 17th, 2015, average PM_2.5 concentrations of ~250 μg m⁻³ and peak concentrations of ~700 μg m⁻³) in Xi'an to elucidate on the SIA formation mechanism. The hourly mean level of the total water-soluble ion was 137.4 μg m⁻³, accounting for 55.3% of PM_2.5 on average. The dominant ions concentrations of SO₄²⁻, NO₃⁻ and NH₄⁺ accounted for 15.8%, 19.0% and 13.2% of PM_2.5 mass, respectively. PM_2.5 was moderately acidic, with an average pH of 4.8 ± 0.4. Compared to the clean periods, sulfate content decreased by 6.9% during the polluted periods, while those of nitrate and ammonium increased by 2.2% and 5.0%, respectively. The increase in nitrogen oxidation ratios (NOR) and ammonia conversion ratio (NHR) from normal days to haze days were greater when comparison to sulfur oxidation ratios (SOR). In the polluted periods, sulfate and nitrate formations were facilitated by water content increase. Strong linear correlation coefficients between SOR (R² = 0.81) and NOR (R² = 0.55) with aerosol water content indicated that the gas-liquid reaction of SO₂ and NO₂ is the major pathway of sulfate and nitrate formation during severe haze episodes. In contrast, the NHR and aerosol water content exhibited a logarithmic relationship, which reveals that when water content was greater than 100 μg m⁻³, the gas-particle partitioning ratio of ammonium was basically unchanged following an increase in water content.

硫酸盐，硝酸盐和铵盐是大气细颗粒物（PM _2.5）中含量最高的次生无机气溶胶（SIA ）。气象条件，气体颗粒的运输过程和气溶胶酸度（pH）会影响SIA的形成。在这项研究中，我们对冬季极端污染事件（2015年1月9日至1月17日，平均PM _2.5浓度为〜250μgm ^-3，峰值浓度为〜700μg）进行了水溶性无机离子的半连续测量。m ^-3），以阐明SIA的形成机理。总水溶性离子的小时平均水平为137.4μgm ^-3，占PM _2.5的55.3％一般。SO的主导离子浓度₄ ^2-，NO ₃ ^-和NH ₄ ⁺的占15.8％，19.0％和PM的13.2％_2.5分别质量。下午_2.5呈中等酸性，平均pH值为4.8±0.4。与清洁期相比，在污染期硫酸盐含量下降了6.9％，而硝酸盐和铵盐含量分别上升了2.2％和5.0％。与硫氧化比（SOR）相比，从正常天到阴霾天的氮氧化比（NOR）和氨转化率（NHR）的增加更大。在污染时期，水含量的增加促进了硫酸盐和硝酸盐的形成。 含气溶胶水的SOR（R ²  = 0.81）和NOR（R ² = 0.55）之间的强线性相关系数表明SO ₂和NO _2的气液反应是严重雾霾发作期间硫酸盐和硝酸盐形成的主要途径。相反，NHR和气溶胶水含量显示出对数关系，这表明当水含量大于100μgm ^-3时，氨的颗粒分布随水含量的增加基本不变。