Interannual evolution of the chemical composition, sources and processes of PM_2.5 in Chengdu, China: Insights from observations in four winters

The air quality in China has improved significantly in the last decade and, correspondingly, the characteristics of PM_2.5 have also changed. We studied the interannual variation of PM_2.5 in Chengdu, one of the most heavily polluted megacities in southwest China, during the most polluted season (winter). Our results show that the mass concentrations of PM_2.5 decreased significantly year-by-year, from 195.8 ± 91.0 µg/m³ in winter 2016 to 96.1 ± 39.3 µg/m³ in winter 2020. The mass concentrations of organic matter (OM), SO₄²⁻, NH₄⁺ and NO₃⁻ decreased by 49.6%, 57.1%, 49.7% and 28.7%, respectively. The differential reduction in the concentrations of chemical components increased the contributions from secondary organic carbon and NO₃⁻ and there was a larger contribution from mobile sources. The contribution of OM and NO₃⁻ not only increased with increasing levels of pollution, but also increased year-by-year at the same level of pollution. Four sources of PM_2.5 were identified: combustion sources, vehicular emissions, dust and secondary aerosols. Secondary aerosols made the highest contribution and increased year-by-year, from 40.6% in winter 2016 to 46.3% in winter 2020. By contrast, the contribution from combustion sources decreased from 14.4% to 8.7%. Our results show the effectiveness of earlier pollution reduction policies and emphasizes that priority should be given to key pollutants (e.g., OM and NO₃⁻) and sources (secondary aerosols and vehicular emissions) in future policies for the reduction of pollution in Chengdu during the winter months.