Chemical composition, source and formation mechanism of urban PM_2.5 in Southwest China

Abstract. Despite significant improvements in air quality in recent years, the Sichuan Basin (SCB) is still facing frequent haze pollution in winter. In this study, the chemical components of PM_2.5 in a typical pollution period at the beginning of 2023 in Chengdu, a megacity in the SCB, were characterized by bulk-chemical and single-particle analysis, and the PM_2.5 sources and formation mechanism of pollution were analyzed. The average mass concentration of PM_2.5 during the study period was 95.4±29.7 μg m^-3. Organic matter (OM) was the most abundant component (35.3 %), followed by nitrate (22.0 %), sulfate (9.2 %) and ammonium (7.8 %). The aerosol particles were classified into five categories: mineral, OM, S-rich, soot and fly ash/metal particles, and most of them were in the state of internal mixing. The entire observation period could be divided into two non-pollution periods (NP-1 and NP-2) and two haze periods (Haze-1 and Haze-2). With the evolution of pollution, the bulk-chemical and single-particle analysis exhibited similar characteristics, suggesting that Haze-1 was mainly caused by pollutants related to fossil fuel combustion, especially mobile sources, while Haze-2 was triggered by the rapidly increasing secondary pollutants. The PM_2.5 sources included dust (8.5 %), biomass burning (3.5 %), coal combustion (15.5 %), industrial processes (6.5 %), vehicular emissions (25.6 %) and secondary sources (40.5 %). Analysis of WRF-Chem model results showed that the average contributions of local sources and regional transmission to pollution in Chengdu were the same (50 % vs 50 %). In addition, the source composition and WRF-Chem simulation results in different periods confirmed our analysis of the formation mechanisms of the two haze events.

This study confirms that, despite the significant reduction in pollution experienced by Chengdu in the past decade, further significant reductions in PM_2.5 are still needed, with particular emphasis on vehicular emissions and secondary sources. High intensity local emissions or large amounts of regional transmission may cause serious haze events, and more effective policies for local emissions reduction or joint prevention and control of regional air pollution are necessary in the future.