Bihourly Monitoring Reveals the Significance of Photochemical Removal of PAHs in the Urban Atmosphere

Atmospheric processing significantly influences the fate and health risk of polycyclic aromatic hydrocarbons (PAHs), yet their real-world transformation remains unclear. Here, we conducted bihourly measurements of 28 PAHs and 19 methylated derivatives (MePAHs) in urban Beijing to investigate their diurnal variation and loss pathways. Both gaseous and particulate PAHs exhibited pronounced diurnal patterns, with lower concentrations during the daytime. Chemical oxidation was identified as the major atmospheric sink. The methylnaphthalene-to-naphthalene (MeNAP/NAP) ratio showed pronounced midday declines, indicating OH radical-driven oxidation as a key removal route for gaseous PAHs. For particulate PAHs, mainly four- to five-ring species, diagnostic isomer ratios exhibited midday minima, aligning with heterogeneous photochemical reactivity. Strong correlations were observed between gaseous PAHs and their OH radical rate coefficients ( r = 0.92, p < 0.001), and between particulate PAHs and total photochemical residence time ( r = −0.67, p < 0.001), demonstrating the critical role of photochemical oxidation in PAH removal. Given the predominance of particle-bound PAHs, heterogeneous reactions represent a significant atmospheric sink. These findings highlight the critical role of photochemical processes in PAH transformation under realistic urban conditions and highlight the need to account for these dynamics in future exposure and health risk assessments.