Characteristics of the chemical processes of organic aerosols by time-of-flight aerosol chemical speciation monitor (TOF-ACSM) in winter in a site of Fenhe Valley, northern China

Fine particulate matter (PM2.5) frequently maintained high concentrations and was accompanied by a sharp increase in organic aerosols (OAs) in winter in the Fenhe Valley (FHV), North China. Few studies have reported the sources and formation mechanisms of OA in this region. In this study, real-time OA was measured using the time-of-flight aerosol chemical speciation monitor (TOF-ACSM) in winter (January 2020) in Jiexiu County, a typical industrial city in the FHV. The results showed that, OA was the main component and accounted for 33.6% of non-refractory-PM2.5 (NR-PM2.5). Positive matrix factorization (PMF) analysis resolved four primary OAs (POA) including hydrocarbon-like OA (HOA, 3.9%), cooking OA (COA, 7.2%), coal combustion OA (CCOA, 15.9%), biomass burning OA (BBOA, 17.2%), and two secondary OAs (SOA) with low oxidation oxygenated OA (LO-OOA, 18.7%) and high oxidation oxygenated OA (MO-OOA, 37.1%). Aqueous-phase processing was the main generation mechanism of LO-OOA and MO-OOA, based on correlations with aerosol liquid water content (ALWC) and odd oxygen (Ox). The average SOA concentration was 49.8 μg/m3 and contributed 54.9% to OA during high relative humidity (RH) pollution periods (NR-PM2.5 > 75 μg/m3, RH > 70%), compared with POA (44.4 μg/m3, 64.3%) during low-RH pollution periods (NR-PM2.5 > 75 μg/m3, RH < 70%). Local emissions were the main sources of POA, and SOA was mainly from the conversion of volatile organic compounds (VOCs) mainly from industrial emissions in the northeast of the city.