Secondary Organic Aerosol Formation from Ambient Air in Summer in Urban Beijing: Contribution of S/IVOCs and Impacts of Heat Waves

Semivolatile and intermediate volatility organic compounds (S/IVOCs) are known as crucial precursors of secondary organic aerosols (SOA), yet their specific contributions to SOA in urban areas remain unclear. Here, we investigate the real-time SOA formation from urban ambient air in summer in Beijing utilizing an oxidation flow reactor (OFR), coupled with aerosol and proton-transfer-reaction mass spectrometers. Our results show that the maximum photochemical formation of SOA in the OFR reached 2.9 μg m^–3 at ∼1.5 days of photochemical age. Primary OA and less oxidized oxygenated OA experience mass loss at high photochemical ages (>3 days) in the OFR, whereas more oxidized oxygenated OA continues to show mass enhancement, indicating the role of heterogeneous processes in the formation of highly aged SOA. Closure studies demonstrate that SOA estimated from the known precursors contribute 50.0 ± 17.3% of the measured SOA. The relatively low contribution (10.3 ± 5.2%) of IVOCs emphasizes the importance of unmeasured S/IVOCs in SOA formation. Furthermore, we illustrate the impact of heat waves on ambient SOA formation by enhancing photochemical oxidation and biogenic emissions in summer.