Reply on RC2

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.