On the relationship between ozone and its precursors in the Pearl River Delta: application of an observation-based model (OBM)

Photochemical smog, characterized by high concentrations of O-3 and fine particles, is of great concern in the urban areas, in particular megacities and city clusters like the Pearl River Delta. Ambient ozone (O-3) and its precursors were simultaneously measured at two sites in the Pearl River Delta, namely, Wan Qing Sha (WQS) in Guangzhou and Tung Chung (TC) in Hong Kong, from 23 October to 01 December 2007 in order to explore their potential relationship. Eight high O-3 episode days were identified at WQS and two at TC during the sampling campaign, indicating a more serious O-3 pollution in Guangzhou than in Hong Kong. An observation-based model was employed to determine the ozone-precursor relationship. At both sites, O-3 production was found to be volatile organic compound (VOC)-limited, which is consistent with previous observations. Anthropogenic hydrocarbons played a key role in O-3 production, while reducing nitric oxide emissions aided the buildup of O-3 concentrations. Among VOC species, the summed relative incremental reactivity (RIR) of the top 12 compounds accounted for 89% and 85% of the total RIR at WQS and TC, respectively, indicating that local photochemical O-3 formation can be mainly attributed to a small number of VOC species. A large increment in both simulated HO2 and O-3 concentrations was achieved with additional input of hourly carbonyl data. This suggested that apart from hydrocarbons, carbonyls might significantly contribute to the O-3 production in the Pearl River Delta.