Background, aim, and scope: Photochemical smog, characterized by high concentrations of O3 and fine particles, is of great concern in the urban areas, in particular megacities and city clusters like the Pearl River Delta. Materials, methods, and results: Ambient ozone (O3) 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 O3 episode days were identified at WQS and two at TC during the sampling campaign, indicating a more serious O3 pollution in Guangzhou than in Hong Kong. An observation-based model was employed to determine the ozone-precursor relationship. At both sites, O3 production was found to be volatile organic compound (VOC)-limited, which is consistent with previous observations. Anthropogenic hydrocarbons played a key role in O3 production, while reducing nitric oxide emissions aided the buildup of O3 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 O3 formation can be mainly attributed to a small number of VOC species. Discussion and conclusions: A large increment in both simulated HO2 and O3 concentrations was achieved with additional input of hourly carbonyl data. This suggested that apart from hydrocarbons, carbonyls might significantly contribute to the O3 production in the Pearl River Delta. ; Department of Civil and Environmental Engineering