[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<i>_x</i> 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<i>_x</i> 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<i>_x</i> transitional regime，in which O₃ pollution aggravation was widespread under the background of decreased NO<i>_x</i> 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<i>_x</i> 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<i>_x</i> emissions.