Overview on the spatial–temporal characteristics of the ozone formation regime in China

Abstract

Ozone (O₃), a main component in photochemical smog, is a secondary pollutant formed through complex photochemical reactions involving nitrogen oxides (NO_x) and volatile organic compounds (VOCs). In the past few decades, with the rapid economic development, industrialization and urbanization, the mixing ratio of O₃ has increased substantially in China. O₃ non-attainment days have been frequently observed. Despite great efforts made in the past few years, it is still difficult to alleviate O₃ pollution in China, due to its non-linear relationship with the precursors. In view of the severe situation in China, this study presents a comprehensive review on the spatial–temporal variations of the relationship between O₃ and its precursors (i.e. O₃ formation regime), built upon the previous reviews of the spatial–temporal variations of O₃ and its precursor levels. Valuable findings from previous studies are laid out for a better understanding of O₃ pollution, followed by implications for the control of O₃ pollution. This literature review indicates that O₃ formation in most areas of the North China Plain (NCP), Yangtze River Delta (YRD) and Pearl River Delta (PRD) regions is in a VOC-limited regime during the high-O₃ seasons due to dramatic emissions from human activities in cities. Outside these metropolitan areas, a NO_x-limited regime dominates rural/remote areas. From summer to winter, the O₃ formation regime over China shows a tendency to shift to a VOC-limited regime. Furthermore, O₃ formation in China shifted toward increasing sensitivity to VOC emissions before the 12^th Five-Year-Plan. However, after the 12^th Five-Year-Plan, successful reduction of NO_x slowed down this trend. Further effective control of VOCs is expected to achieve sustained O₃ attainment in the future. To timely solve the current O₃ pollution problem, precise control of O₃ precursors is proposed, together with the joint prevention and control of regional air pollution.