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

Understanding ozone (O) formation regime is a prerequisite in formulating an effective O pollution control strategy. Photochemical indicator is a simple and direct method in identifying O 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 formation. Thus, it may impact accuracy in signaling O formation regimes. In this study, an advanced three-dimensional numerical modeling system was used to investigate the relationship between atmospheric oxidation and O formation regimes during a long-lasting O exceedance event in September 2017 over the Pearl River Delta (PRD) of China. We discovered a clear relationship between atmospheric oxidative capacity and O formation regime. Over eastern PRD, O formation was mainly in a NO-limited regime when HO/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/OH ratio higher than 5 and lower than 2 was indicative of NO-limited and VOC-limited regime, respectively. Physical contribution, including horizontal transport and vertical transport, may pose uncertainties on the indication of O formation regime by HO/OH ratio. In comparison with other commonly used photochemical indicators, HO/OH ratio had the best performance in differentiating O formation regimes. This study highlighted the necessities in using an atmospheric oxidative capacity-based indicator to infer O formation regime, and underscored the importance of characterizing behaviors of radicals to gain insight in atmospheric processes leading to O pollution over a photochemically active region.