High Time‐Resolution Measurements of Organic Molecular Markers Reveal the Chemical Evolution and Formation Mechanism of Summertime High‐PM Episodes: A Case Study in Wuhan, Central China

Abstract China has experienced frequent fine particulate matter (PM 2.5 ) episodes in recent years; however, molecular‐level chemical evolution and formation mechanisms of these events remain poorly understood. This study presents the first integrated analysis combining high‐time‐resolution measurements of organic molecular markers from thermal desorption aerosol gas chromatography‐mass spectrometry (TAG) and chemical ionization mass spectrometry (CIMS), with bulk aerosol composition measurements, to investigate the dynamic chemical transformation and formation processes of summer PM episodes in Wuhan, Central China. Using the mass increment ratio (MIR), defined as the concentration during episodes divided by that before episodes, we quantified the enhancement of different species. MIR values >1 were observed for most PM 2.5 major components and organic molecular markers. By examining diagnostic ratios of selected molecular tracer pairs, we revealed distinct aging characteristics of different aerosol types during episodes. Source apportionment analysis incorporating the comprehensive high‐time‐resolution organic molecular tracers identified 11 source factors, six of which were characterized by organic tracers: biomass burning, vehicle emission, cooking emission, aliphatic secondary organic aerosol (SOA), aromatic SOA, and biogenic SOA. Biomass burning and secondary sulfate formation process were two major sources across the campaign period, each contributing to 23% of PM 2.5 . During local episodes, the rapid increase in PM 2.5 was primarily driven by biomass burning emissions and secondary nitrate formation process, with fresh biomass burning tracers and nitro‐aromatic compounds showing the highest enhancements. In contrast, during transport episodes, elevated contributions from biogenic SOA, and secondary nitrate formation process, along with greater mass increments of hDCAs and biogenic SOA tracers, suggested regional transport of aged secondary aerosols. These results advance mechanistic understanding of how competing photochemical and aqueous‐phase pathways drive SOA formation and how source‐specific emissions evolve during different episode types. This study demonstrates the efficacy of high‐time‐resolution organic marker measurements in elucidating rapid chemical evolution and formation mechanisms of PM episodes, providing a scientific basis for formulating region‐specific air quality management strategies with implications for other regions experiencing severe air pollution.