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 NO(x)-limited regime when HO(2)/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 HO(2)/OH ratio higher than 5 and lower than 2 was indicative of NO(x)-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 HO(2)/OH ratio. In comparison with other commonly used photochemical indicators, HO(2)/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.