Characteristics and Formation Mechanisms of Winter Particulate Pollution in Lanzhou, Northwest China

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.