Quantification of Typical Organosulfurs/Nitrogens in Hourly PM _2.5 : Implication for Atmospheric Organic–Inorganic Conversion

Individual molecular characteristics of organosulfur and organonitrogen compounds (OrgSs and OrgNs) in PM_2.5, resolved at a high time resolution, are critical for assessing their roles in climate change and health impacts. In this study, four typical individual OrgSs and OrgNs were quantitatively analyzed, while six biogenic volatile organic compound (VOC)-derived organic sulfates (OSs) and nine nitro-aromatic compounds (NACs) were semiquantified using a combination of UHPLC-Q-Orbitrap MS/MS and UHPLC/ESI-MS. Hourly resolved data for online sulfate, nitrate, and carbonaceous species, as well as the above 15 OrgSs and OrgNs, were obtained from 187 hourly filter samples collected during summer and winter pollution episodes in Beijing in 2019. Both OrgSs and OrgNs exhibited pronounced seasonal and diurnal variations, influenced by anthropogenic and biogenic VOC emissions in the respective seasons. The findings highlight the dynamic conversion pathways between OrgSs and sulfate (heterogeneous processes vs hydrolysis) and between OrgNs and nitrate (heterogeneous processes vs photolysis), particularly under conditions of high humidity, strong acidity, and enhanced atmospheric oxidation capacity. These mechanisms should be adequately considered in PM_2.5 control strategies and in modeling secondary species formation, especially regarding the additional sulfate and nitrate sources derived from organic compound conversions.