The formation mechanisms of particulate pollution in Lanzhou, which used to be one of the most polluted cities across the world, remain unclear even though air pollution in Lanzhou has been improved in recent decades. Multiple online data during 2019–2020 winter was used to analyze the characteristics and reveal the formation mechanisms of particulate pollution in Lanzhou. Organic matter, nitrate, sulfate, and ammonium accounted for 35.8%, 16.6%, 12.7%, and 10.8%, respectively, of PM_2.5 mass. The contribution of secondary inorganic aerosols decreased from low to high particulate levels. Nitrate and ammonium concentrations consistently increased with increasing PM_2.5 while sulfate concentrations firstly increased but remained stagnant under high particulate conditions. The sulfur and nitrogen oxidation ratios exhibited relatively low values of 0.18 ± 0.12 and 0.11 ± 0.05, respectively. Nitrogen oxidation ratio showed little dependence on relative humidity because of limited aqueous phase generation of nitrate. The increasing nitrate to sulfate ratio and decreasing organic carbon to elemental carbon ratio under high particulate concentrations indicate relative decreasing contribution of coal burning and increasing contribution of vehicle exhaust emissions on particulate pollution. The rapid decreasing trend of secondary organic carbon to elemental carbon ratio under high particulate concentrations was attributed to a combination of the relatively decreasing photochemical generations and increasing accumulation of primary emissions by the weakened atmospheric diffusion ability. Our results suggest that controlling primary carbonaceous species and vehicle exhaust emissions is the most effective measure in order to sustainably mitigate particulate pollution in Lanzhou in the future.

中文翻译：

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西北兰州冬季颗粒物污染特征及形成机理

尽管近几十年来兰州的空气污染有所改善，但兰州曾经是世界上污染最严重的城市之一，其颗粒物污染的形成机制仍不清楚。利用2019-2020年冬季的多个在线数据来分析其特征并揭示兰州颗粒物污染的形成机理。有机物，硝酸盐，硫酸盐和铵分别占PM _2.5质量的35.8％，16.6％，12.7％和10.8％。二次无机气溶胶的贡献从低到高的颗粒水平降低。硝酸盐和铵浓度随着PM _2.5的增加而持续增加而硫酸盐浓度首先增加，但在高颗粒条件下仍停滞不前。硫和氮的氧化比分别显示相对较低的值，分别为0.18±0.12和0.11±0.05。氮的氧化比显示出对相对湿度的依赖性很小，因为硝酸盐的水相生成有限。在高颗粒物浓度下，硝酸盐与硫酸盐的比值增加，而有机碳与元素碳的比值降低，表明燃煤的贡献相对减少，而汽车尾气排放对颗粒物污染的贡献则增加。在高颗粒物浓度下，次生有机碳与元素碳之比的快速下降趋势归因于相对较弱的光化学生成和由于大气扩散能力减弱而增加的一次排放积累。我们的研究结果表明，为了持续减轻兰州的颗粒物污染，控制主要碳质物质和汽车尾气排放是最有效的措施。