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

A comprehensive field campaign (STORM) was conducted from October to November 2018 at Peking University Shenzhen Graduate School (DXC, 22.60 °N, 113.97 °E) in Shenzhen, Guangdong Province, to investigate the influence of oxidation on secondary pollution formation under different environmental conditions. Precursor concentrations introduced typical urban characteristics on the modeled OH reactivity (kOH) 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. Abundant photolysis sources (e.g., HONO, HCHO, and O3) intensified photochemistry in the polluted period, and the OH radical concentration peaked 6.0 × 106 cm−3 at noon (11:00–13:00). A low kOH of 12.0 s−1 in the clean atmosphere prolonged the OH lifetime by decreasing the termination efficiency while maintaining 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 immense 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 %. Stable wind direction and velocity reduced the physical dilution loss, which supported the fast Ox rise 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.