Enhancing effect of NO₂ on the formation of light-absorbing secondary organic aerosols from toluene photooxidation

Aromatic hydrocarbons are one of the major precursors of atmospheric brown carbon (BrC) and both abundantly co-exist with NO_x in the urban atmosphere especially in winter haze period. However, the impact of NOx on the formation of BrC derived from aromatic hydrocarbons is still not fully understood. In this study, the yield and light absorption of secondary organic aerosols (SOA) from toluene photooxidation under various nitrogen oxides (NO₂) levels were investigated by using a 5 m³ photooxidation smog chamber. A trend of increase at first and then decrease in the SOA yield with an increasing NO₂ concentration was observed. The acid-catalyzed heterogeneous reactions lead to the increase of SOA yield in the low-NO₂ regime. The formation of low-volatility species might be suppressed at high-NO₂ conditions is responsible for the decreased SOA yield. In contrast, light absorption and mass absorption coefficient (MAC) of the toluene-derived SOA continuously increased with the increasing NO₂ concentrations. HR-ToF-AMS results showed that nitrogen-containing organic compounds (NOCs) are the main species that lead to the increase of the SOA light absorption. The ratio of CHN family to the total NOCs, which are derived from the nitro compounds, also increased dominantly with the increasing NO₂ levels and accounted for more than half of the total NOCs when the NO₂ concentration increased to 495 ppbv, indicating that nitro compounds rather than organic nitrates are the major light-absorbing species and preferably formed in the toluene oxidation process.