Ozone Pollution and Its Response to Nitrogen Dioxide Change from a Dense Ground-Based Network in the Yangtze River Delta: Implications for Ozone Abatement in Urban Agglomeration

Understanding the response of tropospheric ozone (O₃) to nitrogen dioxide (NO₂) change is important for local O₃ control. The relationship between O₃ and NO₂ at county scale in China has been extensively studied using models, but there is a lack of results from direct measurements. In this study, we used measurements of O₃, NO₂ and meteorological conditions from a dense network in the Yangtze River Delta (YRD), and satellite observed formaldehyde (HCHO) and NO₂ column densities for the analysis of O₃ variabilities and its relationship to NO₂. As a result, severe O₃ pollution occurred mainly in Shanghai city, southern Jiangsu and northern Zhejiang provinces in YRD during April–September. In addition, meteorological conditions could explain 54% the diurnal O₃ variation over YRD. During April–September 2015–2021, O₃ showed a significant positive relationship (r = 0.61 ± 0.10) with NO₂ after removing the impact from meteorological conditions. However, the relationship could be reversed with NO₂ concentration change. Our result suggested that the controllable O₃ related to NO₂ change is up to 100 μg·m⁻³ in megacities over Shanghai and northern Zhejiang province. The O₃ is much more sensitive to the NO₂ reduction in megacities than surrounding areas. Our results evaluate the different impacts of NO₂ changes on O₃ formation, which provides explanation for the simultaneously alleviated O₃ pollution and reduced NO₂ in 2020 in Shanghai and northern Zhejiang, as well as the increased O₃ in most counties before 2019 with reduced NO₂ during October–March. The driving mechanism as revealed from this study for O₃ and NO₂ will be valuable for the O₃ abatement through NO₂ reduction at sub-county scale over YRD in China.