[Spatiotemporal Distributions and Variations in Summertime Ozone Photochemical Production Regimes over Shanxi]

Satellite-based formaldehyde（HCHO）columns and tropospheric nitrogen dioxide columns were observed using the Ozone Monitoring Instrument（OMI），and groundbased observations of ozone（O₃）for May-August from 2013 to 2022 were connected to calculate the threshold values of the HCHO to NO₂ ratio（FNR）in Shanxi Province. Then，the spatiotemporal distributions and variations in summertime ozone photochemical production regimes were analyzed. The results showed that：① The volatile organic compound（VOC） -sensitive regime area（FNR < 2.3）was obviously reduced，while the VOCs-NO transitional regime（FNR between 2.3-4.1）area increased in the early years and then decreased， and NO x -sensitive regime area expanded significantly in summer from 2013 to 2022 over Shanxi Province. ② The increased summertime FNR during 2013 to 2019 was associated with the co-effect of increased HCHO columns and decreased tropospheric NO₂ columns. The Shanxi Province was generally under an NO regime since 2016，which reflected the remarkable effect of NO x emission reductions；however，there was a shift from a VOC-sensitive regime to a VOCs-NO transitional regime，in which O₃ pollution aggravation was widespread under the background of decreased NO emissions. The decrease in O₃ concentration during 2020 to 2022 followed the synergistical declines in HCHO columns and tropospheric NO₂ columns. ③ The O₃ weekend effects were reversed in Linfen and Yuncheng but were persistent in the other nine cities. Satellite-based weekend HCHO and NO₂ levels were higher than those on weekdays in some cities of Shanxi Province，indicating that the O₃ weekend effect was not only dependent on the changes of precursors emissions but was also closely related to O₃ photochemical production sensitivity. The results indicated the necessity of simultaneous controls in NO emissions and VOCs emissions for ozone abatement plans over Shanxi Province. In addition，Taiyuan，Yangquan，Yuncheng，and Jincheng should continue to promote reduction in NO emissions.