Chemical characteristics and formation mechanisms of PM2.5 during wintertime in two cities with different industrial structures in the Sichuan Basin, China

Abstract

PM2.5 remains one of the critical pollutants involving regional and complex air pollution in many big cities of China, and its improvement has become sluggish in recent years. In this study, we collected PM2.5 samples from two cities, Chengdu and Ya'an, which have different industrial structures in the Sichuan basin during wintertime and investigated the reasons behind their severe PM2.5 pollution. Throughout the entire sampling period, the average PM2.5 concentrations in Chengdu (71.3 ± 24.8 μg/m3) and Ya'an (72.6 ± 27.1 μg/m3) were similar. However, the chemical compositions of PM2.5 showed significant differences. Chengdu exhibited higher concentrations of water-soluble inorganic ions, whereas Ya'an had higher levels of carbonaceous compounds. Both cities experienced an ammonium-rich environment, which promoted the homogeneous generation of secondary pollutants. Moreover, as PM2.5 pollution worsened, the influence of heterogeneous reactions involving SO2 and NOx, as well as the heterogeneous hydrolysis of N2O5, gradually became more pronounced in particle formation. Additionally, adverse meteorological conditions facilitated pollutant accumulation in Ya'an. Using the Positive Matrix Factorization model, we identified 5 sources of PM2.5. The primary source of PM2.5 in both cities was secondary formation (35.4% in Chengdu and 32.5% in Ya'an), while their second-largest contributors varied (26.2% from vehicle emission in Chengdu, and 26.6% from combustion source in Ya'an). These discrepancies highlight the necessity for tailored government interventions, particularly during the winter season. By analyzing the light absorption of the carbonaceous at 370 nm, we discovered that brown carbon was the primary absorber of near-ultraviolet light, with vehicle emissions accounting for the largest portion in Chengdu (37.2%) and combustion emissions being the predominant factor in Ya'an (51.0%). These results could potentially help for having a long-term impact on climate change by simultaneously reducing PM2.5 pollution.