Fig. 2. Changes in ozone as a function of the HCHO-to-NO₂ ratio.

(A) Changes in ozone by region as a function of the HCHO-to-NO₂ ratio. Circles, squares, and triangles represent European regions, Chinese provinces, and U.S. states, respectively, where NO₂ decreases are found to be statistically significant. (B) Cumulative share of the regions in the “extended transition regime” (1 < HCHO/NO₂ < 4) with a decrease in ozone as a function of the HCHO-to-NO₂ ratio based on pre-lockdown conditions. We find a gradual transition regime between a HCHO-to-NO₂ ratio of 1 and 4. The blue line can be interpreted as the probability that a region at a given HCHO-to-NO₂ ratio will experience a decrease in ozone given a decrease in NO₂. Not all regions with reduced NO₂ due to the lockdown have reduced ozone, as HCHO levels also vary (figs. S5 and S6).