Insight into the formation and evolution of secondary organic aerosol in the megacity of Beijing, China

Organic components are the most abundant fraction of atmospheric submicron particles, but the chemical characteristics of organic aerosols (OA) remain not fully understood. Here, the formation and evolution of secondary organic aerosol (SOA) was evaluated using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer at an urban site in Beijing from 1 to 30 September 2015. The submicron aerosol particles were dominated by organics (accounting for an average of 43.2% of the total mass), followed by nitrate (19.8%), sulfate (19.2%), ammonium (10.2%), black carbon (BC) (8.5%) and chloride (1.4%). Positive matrix factorization of high-resolution organic aerosol mass spectra identified two oxygenated organic aerosol (OOA) components: a less oxidized OOA (LO-OOA) and a more oxidized OOA (MO-OOA). The mass concentrations of MO-OOA and LO-OOA correlated well with the odd oxygen (Ox = O₃+nitrogen dioxide (NO₂)), with slopes of 1.15 μg m⁻³ per 10 ppb Ox and 0.48 μg m⁻³ per 10 ppb Ox, respectively, suggesting the important role of photochemical processing in OOA production. In addition, aqueous-phase processing was also observed to contribute substantially to MO-OOA production when RH > 60%, and RH suppressed LO-OOA formation. Further episode analysis showed that the gas-phase reaction was the main driving force for organic oxidation during the initial pollution stage, during which LO-OOA gradually transformed to MO-OOA as indicated by an increase in the most oxidized ion (CO₂⁺) and a decrease in a moderately oxidized ion (C₂H₃O⁺). Compared with the initial pollution stage, gas-phase oxidation was largely enhanced during the daytime in the peak pollution stage which showed a stronger dependence of MO-OOA on Ox concentration. In addition, enhanced aqueous-phase oxidation and the partitioning process both contributed to the incorporation of oxygenated species into the particle phase during the nighttime in the presence of high aerosol water content.