Secondary Organic Aerosol Formation through Photochemical Oxidation of NO and SO2 in an Indoor Smog Chamber

Abstract

Nitric oxide (NO) and sulfur dioxide (SO2) are atmospheric pollutants that play a crucial role in the formation of secondary organic aerosols (SOAs) with volatile organic compounds. However, there have been few studies on the combined effect of NO and SO2 in the formation of SOA in various environments. This study used the Ajou University Smog Chamber (AUC), where a pillow-shaped fluorinated ethylene propylene Teflon film reactor was fixed, to investigate SOA formation under NO-toluene conditions with light intensity and SO2 concentration as influencing factors. The cumulative concentration of SOA in the NO-toluene reaction was 702–968/cm3, which was 1.96–2.11 times higher than the cumulative concentration in the NOx-toluene reaction (333−493/cm3). The maximum cumulative number concentrations were the highest at 4 mW/cm2 and proportionally increased with light intensity. The particle size distribution showed that SOA was predominantly formed in the 0–50 nm range and it also increased as the light intensity increased. Additionally, experiments were conducted by varying the light intensity and SO2 injection concentration under the condition [toluene]/[NO]=3. When [SO2]/[NO]=1, the concentration of SOA increased with light intensity (30, 49, and 91/cm3 at 0, 2, and 4 mW/cm2, respectively); however, at other ratios, the highest concentration of SOA was observed at a light intensity of 2 mW/cm2. The overall SOA concentration increased with higher SO2 concentration, although the SO2 and SOA concentrations did not positively correlate. This study implies that factors such as SO2 concentration, type of oxidants, and light intensity can influence atmospheric SOA formation.