Local radical chemistry driven ozone pollution in a megacity: A case study

To investigate the effect of oxidation on the formation of secondary pollution in different environmental conditions, a comprehensive field campaign (STORM) was carried out from October to November 2018 at Peking University Shenzhen Graduate School (DXC, 22.60 °N, 113.97 °E) in Shenzhen, Guangdong Province. Precursor concentrations introduced typical urban characteristics on the modeled OH reactivity (k_OH) levels with a broad range between 15.0 and 30.0 s^-1. The daily maxima of the observed OH and HO₂ radical concentrations were (2.3–12.8) × 10⁶ cm^-3 and (1.3–9.1) × 10⁸ cm^-3, respectively. In the polluted period, abundant photolysis sources (e.g., HONO, HCHO, and O₃) intensified photochemistry. The OH radical concentration peaked 6.0 × 10⁶ cm^-3 at noon (11:00-13:00). The low k_OH (12.0 s^-1) in the clean atmosphere suggested that the reduction in termination efficiency prolonged the OH lifetime, so that the period maintained a sustained OH concentration comparable to the polluted period. In terms of meteorology, the dominating air mass was isolated from the east and northeast directions, which promoted the transition of ozone from mild pollution to severe pollution. The abundant precursor emissions at urban sites first compensated for the negative effect of declined solar radiation, and then they amplified radical propagation. Simultaneously amplified radical propagation promoted oxidation capacity, and increased the chain length (ChL) from 3.6 to 4.1, and P(O₃) has lifted effectively by approximately 13.3 %. The stable wind direction and velocity reduced physical dilution losses and thus led to the rapid rise in O_x around 16:00. Further detailed investigations are required on the environmental causes of ozone pollution to address the influence of the oxidation processes on secondary pollution formation under other environmental conditions.