| Ozone（O3）pollution has shown an obvious increase over the past few years in China,especially in the river delta regions（such as the Pearl River Delta region）with rapid industrialization and urbanization,which has become the new environmental air quality challenge after haze pollution in China.In this study,we analyzed the characteristics and formation mechanisms of O3 in typical urban atmospheres in river delta regions（Dongying in the Yellow River Delta region and Guangzhou in the Pearl River Delta region）,and compared the similarities and differences of O3 pollution in these two river delta regions,and evaluated the effectiveness of "Autumn O3 Peak Reduction Campaigns" in Guangzhou.The long-term trend and pollution characteristics of O3 in typical urban atmospheres in the Yellow River Delta region and Pearl River Delta region were analyzed.The concentration of O3 in typical urban atmospheres in both the Yellow River Delta region and Pearl River Delta region increased gradually from 2015 to 2019（2.5 μg/m3/year and 8.0 μg/m3/year in Dongying and Guangzhou,respectively）,and decreased significantly from 2019 to 2021（-15.5μg/m3/year and-8.5 μg/m3/year,respectively）.Dongying showed the seasonal characteristics in order of summer>spring>autumn>winter,with the severest O3 pollution occurring in June.Guangzhou showed the seasonal characteristics of higher concentrations in summer and autumn and lower concentrations in winter and spring,with the severest O3 pollution occurring in September.The concentration of O3 precursors during the O3 episode days(（maximum daily 8h average ozone concentration（MDA8-O3≥160 μg/m3））in both Dongying and Guangzhou increased significantly（The concentrations of volatile organic compounds（VOCs）and nitrogen oxides（NOx）in Dongying increased by 10.4%and 7.6%,respectively;The concentrations of nitrogen dioxide（NO2）,nitrogen monoxide（NO）,and VOCs in Guangzhou increased by 51.7%,12.8%,and 79.7%,respectively.）.The higher concentrations of O3 precursors and more favorable meteorological conditions（high temperature and low humidity）during the O3 episode days resulted in higher photochemical formation of O3,the in-situ net O3 production rate increased significantly.The O3 formation mechanisms in typical urban atmospheres in the Yellow River Delta region and Pearl River Delta region were analyzed.Based on the model result,O3 formation in Dongying was controlled by both VOCs and NOx,while in Guangzhou it was under the VOCslimited regime.Collaborative emission reductions of VOCs and NOx are an effective way to control O3 pollution in both Dongying and Guangzhou.The emission reduction ratio of VOCs/NOx should be greater than or equal to 1:1 in Dongying,and greater than 1.26 in Guangzhou.Both Dongying and Guangzhou need to increase their efforts in reducing NOx emissions for long-term achievement of O3,and the policy should reflect a transition from the current VOC-limited regime of O3 formation to a NOx-limited regime.The effectiveness of "Autumn O3 Peak Reduction Campaigns" in Guangzhou was evaluated.The concentrations of NOx and VOCs in Guangzhou significantly decreased in autumn 2019（The concentrations of NOx and VOCs decreased by 3.0%and 27.0%,respectively）,however,the concentrations of O3 increased（The concentration of hourly O3 increased by 61.0%）.Significant increase of the supra-regional background O3 concentrations and relative increase of the local production O3 concentrations resulted in the deterioration of O3 pollution in Guangzhou in autumn 2019.The locally produced O3 concentrations slightly increased under the significant emission reductions of VOCs and NOx,with the in-situ net O3 production rate increasing by 2.1%.Emission reductions of anthropogenic VOCs（AVOCs）significantly reduced the O3 formation,while emission reduction of NOx increased O3 formation,and the increase of biogenic VOCs（BVOCs）concentration also increased O3 formation.Overall,the reduction of NOx and the increase of BVOCs concentration negated the benefits of the significant decrease in the emissions of AVOCs on O3 formation control,resulting in the slight increase in O3 formation.This study comprehensively discussed the characteristics and formation mechanism of O3 pollution in typical cities in river delta regions,revealed the similarities and differences of O3 pollution in the different river delta regions,and emphasized the importance of scientific and reasonable precursor emission reduction strategies for O3 pollution control.These results could provide the experience of enlightenment and scientific support for O3 pollution controls in the river delta regions as well as other urban areas facing severe O3 pollution in China. |
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