Diurnal Variation of Electrophilic Organic Compounds in Urban PM_2.5 through Nontargeted Analyses: A Focus on Atmospheric Transformation

Electrophilic compounds belong to a group of critical components of fine particulate matter (PM_2.5) with significant health risk, but little is known regarding their occurrence and sources in the atmosphere. In this study, we developed a nontargeted approach to analyze particulate electrophilic compounds in urban Beijing and identified 147 ones with significant diurnal variation, among which oxygenated aromatic compounds (OACs) are the predominant species. The discrepancy in hierarchical clustering analysis of atmospheric transformation markers, e.g., 9-nitroanthracene (9NANT) and citraconic anhydride (CA), and PAHs, e.g., fluoranthene (FLT), suggested a potential secondary formation pathway in the urban atmosphere. The isomeric ratio of acephenanthrylene (ACP) to FLT, and the product-to-precursor ratio of 11<i>H</i>-benzo[a]fluorene-11-one (BaFO) to FLT, further demonstrated the prevailing existence of atmospheric oxidation. The relationship between night-to-day variation and molecular characteristics, particularly the significant negative correlation with oxygen-related features such as the carbon oxidation state, confirmed that multigenerational oxidation processes during daytime predominantly drive diurnal/nocturnal differences. This study achieved a systematic identification of electrophilic compounds and their transformation in the atmosphere, revealing the pivotal role of oxidation on the fate of organic chemicals that resulted in their diurnal variation.