Unexpected changes in occurrence and sources of chromophoric dissolved organic matter in PM2.5 driven by the clean air action over Xi'an, China

Abstract

Chromophoric dissolved organic matter (CDOM) in atmospheric PM significantly impacts climate and health. However, China's air pollution control on CDOM is unclear. To evaluate the effect of the Blue Sky Defense Battle (BSDB) on the pollution characteristics and sources of CDOM, we comprehensively analyzed 1,428 PM_2.5 samples in Xi'an (2016-2022). Compared with the pre-BSDB period, we found a 17% increase in the air quality index and a 42% reduction in CDOM pollution levels. Furthermore, in winter, carbonaceous aerosol concentrations notably declined, with OC decreased by 25% and EC by 74%. Additionally, CDOM in Xi'an was predominantly composed of humic chromophores (80%), and the highly oxygenated humic-like substances (HO-HULIS) declined by 34%, effectively reducing the potential oxidative toxicity risks. Source apportionment results from the excitation-emission matrix fluorescence fingerprinting technique with the non-negative matrix factorization (NMF) model demonstrated that the effective control of combustion sources was the main driver of changes in atmospheric concentrations, chemical composition, and optical properties. Based on the NMF model analysis of carbonaceous aerosol, the source contributions of atmospheric oxidation and photochemical reactions increased after the BSDB indicating the limited control of the secondary sources. Additionally, the ineffective control of dust sources and the increasing contribution of external transport to CDOM after the BSDB highlighted the importance of effective dust management and regional collaborative control. Despite the decline in PM_2.5 levels, the rise in toxic components (HO-HULIS) indicated ongoing challenges for health-focused pollution control.