Day-by-Day Ambient Determination of Nighttime Heterogeneous Oxidation of Particle-Phase Benzo[a]pyrene in an Urban Environment

Benzo[a]pyrene (BaP) is widely used as a model compound for studying the heterogeneous oxidation of particle-borne polycyclic aromatic hydrocarbons (PAHs), yet its decay kinetics under real atmospheric environments remain poorly constrained. Here, we present the first day-by-day determination of nighttime decay rates of particulate BaP using bihourly measurements in urban Shanghai. Two decay patterns were observed: incomplete decay with substantial remaining BaP, and complete decay with little residual BaP. Across 88 nights, decay rates averaged 0.21 ± 0.15 h⁻¹, with 0-87% of BaP remaining unreacted by the end of night. Decay rates increased linearly with gas-phase ozone and showed a temperature-dependent relationship with initial BaP concentration across three temperature regimes (TR₁: 5-16 °C; TR₂: 16-25 °C; TR₃: 25-30 °C). Effective activation energies differed markedly between those of TR₁₋₂ and TR₃. Residue BaP was negatively correlated with relative humidity across all regimes, whereas its positive correlation with organic carbon mass was confined to TR₁ and absent in TR₃, suggesting a transition in the aerosol phase state. Nighttime atmospheric lifetimes of BaP were 5.2, 9.3, and 13.1 h under TR₁, TR₂, and TR₃, respectively. These ambient constraints provide needed kinetic parameters for modeling the fate, transport, and health impacts of atmospheric PAHs.