Although extensive studies were conducted for water soluble inorganic ions (WSIIs) in PM_2.5, most of them concentrated on developed cities while limited studies in mountainous cities. In this study, daily PM_2.5 was sampled for a year (from Sept. 1, 2017 to Aug. 31, 2018) to comprehensively investigate the characteristics of WSIIs in a typical mountainous city, Chongqing. The results showed that air quality in Chongqing was getting better (annual 43.0 μg m⁻³ PM_2.5) but still higher than the NAAQS annual limit. Consistent with PM_2.5 pattern, the concentration of WSIIs in dry months with high relative humidity (Nov. 2017–Feb. 2018) was also higher than that in wet months (rest months). The ratio of WSIIs/PM_2.5 was similar in dry and wet months, but higher proportion (87.7%) of SNA/WSIIs (SNA: SO₄²⁻, NH₄⁺, and NO₃⁻) were found in dry months, suggesting that secondary ions SNA were enriched in dry months. Importantly, the ratio of NO₃⁻/SO₄²⁻ > 1 in dry months, which was not reported in previous studies, implied that NO₃⁻ replaced SO₄²⁻ and became the predominant pollutant in dry months. Both heterogeneous and homogeneous reactions played important roles in the formation of SO₄²⁻ and NO₃⁻. Positive matrix factorization further demonstrated that secondary aerosol from coal combustion and agriculture activities (38.5%), secondary aerosol from vehicle exhaust (19.5%), soil dust (13.7%) and industry (28.2%) contributed to WSIIs in Chongqing in 2017–2018. In order to improve air quality in Chongqing, relevant measures should be taken to control the sources of WSIIs.

尽管对PM _2.5中的水溶性无机离子（WSIIs）进行了广泛的研究，但其中大多数集中在发达城市，而在山区城市却很少进行研究。在这项研究中，从2017年9月1日至2018年8月31日，每天采样PM _2.5，以全面调查典型山区城市重庆的WSII的特征。结果表明，重庆市的空气质量有所改善（每年43.0μgm ^-3 PM _2.5），但仍高于NAAQS的年度限值。与PM _2.5模式一致，相对湿度高的干燥月份（2017年11月至2018年2月）的WSII浓度也高于潮湿月份（休息月份）。WSII / PM的比率_2.5在SNA / WSIIs的干和湿个月，但较高比例（87.7％）相似（SNA：SO ₄ ^2-，NH ₄ ⁺和NO ₃ ^{- ）}的无水个月被发现，表明二次离子SNA富集在干燥的月份。重要的是，NO的比率₃ ^- / SO ₄ ^2- > 1在干个月，这是不在以往的研究报告，暗示NO ₃ ^-取代SO ₄ ^2-，成为干个月的主要污染物。异质和均相反应在SO ₄ ²⁻和NO ₃ ⁻的形成中均起重要作用。正矩阵分解进一步表明，2017–2018年重庆的WSIIs来自燃煤和农业活动的二次气溶胶（38.5％），汽车尾气的二次气溶胶（19.5％），土壤粉尘（13.7％）和工业（28.2％） 。为了改善重庆市的空气质量，应采取相关措施控制WSII的来源。