Nontarget Screening Exhibits a Seasonal Cycle of PM2.5 Organic Aerosol Composition in Beijing br

The molecular composition of atmospheric particulate matter(PM) in the urban environment is complex, and it remains a challenge toidentify its sources and formation pathways. Here, we report the seasonalvariation of the molecular composition of organic aerosols (OA), based on 172PM2.5filter samples collected in Beijing, China, from February 2018 to March2019. We applied a hierarchical cluster analysis (HCA) on a large nontarget-screening data set and found a strong seasonal difference in the OA chemicalcomposition. Molecularfingerprints of the major compound clusters exhibit aunique molecular pattern in the Van Krevelen-space. We found that summerOA in Beijing features a higher degree of oxidation and a higher proportion oforganosulfates (OSs) in comparison to OA during wintertime, which exhibits ahigh contribution from (nitro-)aromatic compounds. OSs appeared with a highintensity in summer-haze conditions, indicating the importance of anthro-pogenic enhancement of secondary OA in summer Beijing. Furthermore, wequantified the contribution of the four main compound clusters to total OA using surrogate standards. With this approach, we areable to explain a small fraction of the OA (similar to 11-14%) monitored by the Time-of-Flight Aerosol Chemical Speciation Monitor(ToF-ACSM). However, we observe a strong correlation between the sum of the quantified clusters and OA measured by the ToF-ACSM, indicating that the identified clusters represent the major variability of OA seasonal cycles. This study highlights the potentialof using nontarget screening in combination with HCA for gaining a better understanding of the molecular composition and theorigin of OA in the urban environment.