Composition and Reactivity of Volatile Organic Compounds and the Implications for Ozone Formation in the North China Plain

Enhanced ozone (O-3) pollution has emerged as a pressing environmental concern in China, particularly for densely populated megacities and major city clusters. However, volatile organic compounds (VOCs), the key precursors to O-3 formation, have not been routinely measured. In this study, we characterize the spatial and temporal patterns of VOCs and examine the role of VOCs in O-3 production in five cities (Dongying (DY), Rizhao (RZ), Yantai (YT), Weihai (WH), and Jinan (JN)) in the North China Plain (NCP) for two sampling periods (June and December) in 2021 through continuous field observations. Among various VOC categories, alkanes accounted for the largest proportion of VOCs in the cities. For VOCs, chemical reactivities, aromatic hydrocarbons, and alkenes were dominant contributors to O-3 formation potential (OFP). Unlike inland regions, the contribution to OFP from OVOCs increased greatly at high O-3 concentrations in coastal regions (especially YT). Model simulations during the O-3 episode show that the net O-3 production rates were 27.87, 10.24, and 10.37 ppbv/h in DY, RZ, and JN. The pathway of HO2 + NO contributed the most to O-3 production in JN and RZ, while RO2 + NO was the largest contributor to O-3 production in DY. The relative incremental reactivity (RIR) revealed that O-3 formation in DY was the transitional regime, while it was markedly the VOC-limited regime in JN and RZ. The O-3 production response is influenced by NOx concentration and has a clear daily variation pattern (the sensitivity is greater from 15:00 to 17:00). The most efficiencies in O-3 reduction could be achieved by reducing NOx when the NOx concentration is low (less than 20 ppbv in this study). This study reveals the importance of ambient VOCs in O-3 production over the NCP and demonstrates that a better grasp of VOC sources and profiles is critical for in-depth O-3 regulation in the NCP.