Identification of close relationship between atmospheric oxidation and ozone formation regimes in a photochemically active region

Understanding ozone (O-3) formation regime is a prerequisite in formulating an effective O-3 pollution control strategy. Photochemical indicator is a simple and direct method in identifying O-3 formation regimes. Most used indicators are derived from observations, whereas the role of atmospheric oxidation is not in consideration, which is the core driver of O-3 formation. Thus, it may impact accuracy in signaling O-3 formation regimes. In this study, an advanced three-dimensional numerical modeling system was used to investigate the relationship between atmospheric oxidation and O-3 formation regimes during a long-lasting O-3 exceedance event in September 2017 over the Pearl River Delta (PRD) of China. We discovered a clear relationship between atmospheric oxidative capacity and O-3 formation regime. Over eastern PRD, O-3 formation was mainly in a NOx-limited regime when HO2/OH ratio was higher than 11, while in a VOC-limited regime when the ratio was lower than 9.5. Over central and western PRD, an HO2/OH ratio higher than 5 and lower than 2 was indicative of NOx-limited and VOC-limited regime, respectively. Physical contribution, including horizontal transport and vertical transport, may pose uncertainties on the indication of O-3 formation regime by HO2/OH ratio. In comparison with other commonly used photochemical indicators, HO2/OH ratio had the best performance in differentiating O-3 formation regimes. This study highlighted the necessities in using an atmospheric oxidative capacity-based indicator to infer O(3 )formation regime, and underscored the importance of characterizing behaviors of radicals to gain insight in atmospheric processes leading to O-3 pollution over a photochemically active region. (C) 2020 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.