Formation and characteristics of secondary aerosols in an industrialized environment during cold seasons

Abstract. Secondary aerosols including inorganic and organic components often dominate the fine aerosol mass, it is thus important to elucidate the formation and characteristics of these species. In this work, we measured the submicron aerosols (PM₁) by using an Aerodyne high resolution soot-particle aerosol mass spectrometer in suburban Nanjing, China. The site was surrounded by industry plants, and the measurement was conducted during cold seasons (February–March 2015). We found that under such environment, the PM₁ was predominantly comprised of secondary species (on average 63.2 % from ammonium sulfate and nitrate). Results show that moisture plays a key role to enhance both nitrate and sulfate formations. The moisture promotes the gas-particle partitioning and nocturnal heterogeneous production of nitrate, while transformation of SO₂ into sulfate directly in aqueous phase is more significant. The organic aerosol (OA) occupied ~1/4 of total PM₁ mass, and the primary OA (POA) and secondary OA (SOA) contributions were almost equal. A specific industry-related OA was separated and a modified graphical method was introduced to describe the evolution of OA. Results further show that the most abundant OA factor, which is the one with highest oxidation degree, is also mainly driven by aqueous-phase processing, while the other two less oxygenated SOA factors are mainly governed by photochemical processing. Peak sizes of sulfate, nitrate and OA all shifted towards larger sizes with the increases of relative humidity, reflecting the effects of aqueousphase processing too. Aqueous-phase driven secondary aerosols were found to be very important in enhancing the PM1 pollution, while photochemical processed SOA was important to OA pollution, leading to a fresher OA at higher OA concentrations. We further demonstrated influences of the aqueous-phase processing and photochemical processing on formation of secondary aerosols by using two typical cases, respectively. This paper highlights the importance of aqueous-phase chemistry on sulfate and nitrate formations, and that different portions of SOA can be dominated by different mechanisms in an industrialized environment.