Fig. 1: Evolution of air pollution before and during the COVID-19 lockdown in the YRD region.

Panels a and b present the observations and comparison of model simulations and observations, respectively. Observed values (except VOCs) were daily averages of observations from 41 major cities (list in Supplementary Table 2) in the YRD region. The shade of lines represented the range of mean value ±50% standard deviation across the 41 cities (spatial variability). VOCs data were based on the measurements at SAES supersite in Shanghai. The brown dash line presented the linear fitting result of daily O₃ concentrations from December to February in the past 3 years (2017–2019). The data of solar radiation were the average of the daily maximum at five sites in the YRD region from the national meteorological observing stations. The shade areas in panel b indicate rainy time. Here, we defined the period from 1st December to 23rd January as before lockdown (denoted as Pre-COVID or Pre-C, and the period from 24th January to 29th February as during lockdown (denoted as Lockdown or Lock).