Urban Activity Shifts Reshape S/IVOC Composition and SOA Formation: Insights from Wintertime Observations in Beijing

Semivolatile and intermediate-volatility organic compounds (S/IVOCs) are key but under-constrained precursors of urban secondary organic aerosol (SOA). We leveraged the COVID-19 lockdown-to-reopening transition in Beijing as a natural experiment to track molecularly resolved gaseous organics using comprehensive two-dimensional gas chromatography-mass spectrometry (GC × GC-MS). Across 305 quantified species, IVOCs accounted for ∼37% of the measured gas-phase loading yet dominated diagnosed incremental SOA formation (∼89%) in a simplified observation-constrained 0D diagnostic implementation based on two-dimensional volatility basis set (2D-VBS) concepts. Partial least squares discriminant analysis (PLS-DA) revealed compositional shifts, with n-C₈, 2-butyl-1-octanol, and hexamethylcyclotrisiloxane (D3) enriched after reopening, indicating rebounds in traffic and volatile chemical product-related emissions; within the VMS mixture, D3 became dominant while lockdown-phase D3 remained near the low-end quantification regime. Meteorological normalization indicated that the decline in total gas-phase organics could not be explained by local meteorology alone and was consistent with a weakened regional background. Despite stronger anthropogenic tracers after reopening, diagnosed daily SOA yields decreased. This efficiency drop appears more strongly associated with thermodynamic constraints than with a broad reduction in atmospheric oxidation capacity. These results indicate that IVOC-enriched noncombustion and evaporative emissions may materially influence diagnosed urban SOA formation within the measured gas-phase precursor pool.