08 Jun 2022
08 Jun 2022
Status: this preprint is currently under review for the journal ACP.

Diagnosing ozone-NOx-VOC sensitivity and revealing causes of ozone increases in China based on 2013–2021 satellite retrievals

Jie Ren and Shaodong Xie Jie Ren and Shaodong Xie
  • State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China

Abstract. Particulate matter (PM2.5) concentrations in China have decreased significantly in recent years, but surface ozone (O3) concentrations showed upward trends at more than 71 % of air quality monitoring stations from 2015 to 2021. To reveal causes of O3 increases, O3 production sensitivity is accurately diagnosed by deriving regional threshold values of satellite tropospheric formaldehyde-to-NO2 ratio (HCHO / NO2), and O3 responses to precursors changes are evaluated by tracking volatile organic compounds (VOCs) and NOx with satellite HCHO and NO2. Results showed that the HCHO / NO2 ranges of transition from VOC-limited to NOx-limited regimes vary apparently among Chinese regions. VOC-limited regimes are widespread found over megacity clusters (North China Plain, Yangtze River Delta, and Pearl River Delta) and concentrated in developed cities (such as Chengdu, Chongqing, Xi’an, and Wuhan). NOx-limited regimes dominate most of the remaining areas. From 2013 to 2021, satellite NO2 and HCHO columns showed an annual decrease of 3.7 % and an increase of 0.1 %, respectively, indicating an effective reduction in NOx emissions but a failure reduction of VOC emissions. This finding further shows that O3 increases in major cities occur because the Clean Air Action Plan only reduces NOx emissions without effective VOC control. Two cases in Beijing and Chengdu also verified that NOx reduction alone or VOC increase leads to O3 increases. Based on the O3–NOx–VOC relationship by satellite NO2 and HCHO in Beijing, Chengdu, and Guangzhou, the ozone concentration can be substantially reduced if the reduction ratio of VOCs / NOx is between 2 : 1 and 4 : 1.

Jie Ren and Shaodong Xie

Status: open (until 24 Jul 2022)

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Jie Ren and Shaodong Xie

Jie Ren and Shaodong Xie


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Short summary
O3-NOx-VOC sensitivity in China is diagnosed by deriving regional satellite HCHO / NO2 thresholds between O3 production regimes. VOC-limited regimes are widespread found over megacity clusters and developed cities. VOCs and NOx are tracked with satellite HCHO and NO2 to evaluate O3 responses to precursors changes. The significant reduction of NOx emissions without effective VOC control since the Clean Air Action Plan in 2013 is responsible for the increase in O3 concentrations in China.