Evolution of ozone formation regimes during different periods in representative regions of China

Ozone (O₃) is produced by photochemical reactions of NO_X and VOCs in the troposphere under sunlight. The column densities of formaldehyde (HCHO) and nitrogen dioxide (NO₂), derived from satellite data, serve as indicators of VOCs and NO_X emissions in the troposphere. Through analyzing the unique characteristics of the threshold range for the HCHO/NO₂ ratio (FNR), the mechanisms of O₃ formation across different regions over a prolonged period can be identified. In this study, we utilized the Empirical Orthogonal Function (EOF) technique to characterize O₃ patterns during the warm season (April to October) spanning 2013–2019. This period is divided into three stages: 2013–2014, 2015–2016, and 2017–2019. Using the third-order fitting model, we assessed the FNR values across different regions in China: BTH (Beijing-Tianjin-Hebei), YRD (Yangtze River Delta), GD (Guangdong), and CY (Chuan-Yu). The FNR value ranges for these regions are as follows: ([1.2,2.0], [1.3,2.1], [2.4,3.2], [1.4,2.2]) during 2013–2014, ([1.1,1.9], [1.2,2.0], [2.0,2.8], [1.2,2.0]) during 2015–2016, and ([1.0,1.8], [1.0,1.8], [1.7,2.5], [1.1,1.9]) during 2017–2019, respectively. Ultimately, our research indicates a shift in certain regions from a VOC-limited regime towards a transitional regime. This shift correlates with a significant decline in anthropogenic NO_X emissions, attributed to the stringent emission control strategies extensively implemented between 2013 and 2019. The spatial expansion of the transitional regime aligns with increasing O₃ concentrations, simultaneously offering guidance for the development of effective emission reduction strategies.