Removing the effects of meteorological factors on changes in nitrogen dioxide and ozone concentrations in China from...

Previous studies on long-term ozone (O₃) variations in China have reported inconsistent conclusions on the role of meteorological factors in controlling said variations. In this study, we used an observation-based decomposition model to conduct an up-to-date investigation of the effects of meteorological factors on the variations in nitrogen dioxide (NO₂) and O₃ concentrations in China in the summer from 2013 to 2020. The variations in NO₂ and O₃ concentrations after removing the major meteorological effects were then analyzed to improve our understanding of O₃ formation regimes. Ground measurements show that both NO₂ and O₃ concentrations decreased in eastern, central, and southeastern China (e.g., NO₂ and O₃ concentrations in Wuhan reduced by 4.3 and 6.2 ppb, respectively), which was not anticipated. Analyses of meteorological effects showed that reduced wind strength, decreased temperature, and increased relative humidity significantly reduced O₃ concentrations in eastern and central China (e.g., by 10.5 ppb in Wuhan). After removing the major meteorological effects, the O₃ trends were reversed in eastern and central China (e.g., increased by 4.9 ppb in Wuhan). The contrasting trends in NO₂ and O₃ concentrations suggest that their O₃ formations were sensitive to volatile organic compounds (VOC-limited regime). In southeastern China, both NO₂ and O₃ concentrations decreased, implying that the O₃ formation regimes changed to mixed sensitive or nitrogen oxide-limited (NO_x-limited) regimes. The meteorological effects varied by region and may play a dominant role in controlling the long-term O₃ variation. Our results indicate that the attribution of O₃ variation to emission control without accounting for meteorological effects can be misleading.