Impact of COVID-19 on air quality in the Yangtze River Delta, China.

To investigate the effect of nationwide restrictions due to COVID-19 on air quality in the Yangtze River Delta (YRD), China, we defined four periods named period I (January 1 to 23, 2020), period II (January 24 to February 23), period III (February 24 to April 7), and period IV (April 8 to May 31), which indicated normal period, lockdown period, regional work resumption period, and nationwide work resumption period, respectively. Hourly PM, PM, NO, SO, CO, and O in 41 cities in the YRD region were analyzed. Compared to period I, NO decreased by 58% during period II and increased in periods III and IV. SO remained constant during the four periods (7-8 μg/m). Higher PM concentration was monitored during period II (41 μg/m) when compared to period III (35 μg/m), which was resulted from the enhanced secondary formation. Spatial distribution analysis further indicated that PM in the northern YRD during period II was higher than that during period III, whereas PM in the southern YRD in the period II was similar to that in period III. The results demonstrated that PM shows a nonlinear response to the reduction of its precursors, and this phenomenon varies in different areas. Compared to periods I (36 μg/m) and III (64 μg/m), relatively higher O during period II (64 μg/m) was probably resulted from less NO emission and hence weakened NO titration effect. The study suggested that coordinated and balanced measures are needed to improve air quality.