Chemical characteristics of fine aerosols and associated speciated organic compounds in summer Nanjing, China

Along with decrease of fine particulate matter (PM_2.5) concentration in recent years in China, secondary species become increasingly important. This work focuses on characterizing secondary components, and a few important groups of organics including organic nitrogen (ON), organonitrates (OrgN), organosulfates (OS) and polycyclic aromatic hydrocarbons (PAHs), via online measurement of submicron aerosols (PM₁) in Nanjing, China, during 2022 summer. The average PM₁ concentration was 15.39 µg/m³, dominated by secondary components (69.1 %), which were even more important at higher PM₁ levels. The primary organic aerosols (POA) were from traffic, industry and cooking; the two secondary OA factors were both closely linked with photochemistry, with one (OOA1) being relatively fresh and important in early afternoon and another (OOA2) being aged and important in late afternoon. Sulfate formation was also governed by photochemistry but resembled that of OOA2 not OOA1; nitrate formation was associated strongly with heterogeneous hydrolysis and thermodynamic equilibrium. Results also reveal a possible photochemical reaction channel from POA to OOA1, then to OOA2. Case studies show that formations of secondary components responded differently to different weather conditions and governed summer PM₁ pollution. The average ON, OrgN, OS and PAHs concentrations were determined to be 122.8, 84.4, 45.6 and 3.3 ng/m³, respectively. ON was dominated by primary sources (53.8 %). OrgN varied similarly to nitrate. OS formation was linked with aqueous-phase reactions, which were insignificant therefore its level was low. PAHs was mainly from traffic, and photochemical oxidation might be its important sink during afternoon.