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_2.5, PM₁₀, NO₂, SO₂, CO, and O₃ in 41 cities in the YRD region were analyzed. Compared to period I, NO₂ decreased by 58% during period II and increased in periods III and IV. SO₂ remained constant during the four periods (7-8 μg/m³). Higher PM_2.5 concentration was monitored during period II (41 μg/m³) when compared to period III (35 μg/m³), which was resulted from the enhanced secondary formation. Spatial distribution analysis further indicated that PM_2.5 in the northern YRD during period II was higher than that during period III, whereas PM_2.5 in the southern YRD in the period II was similar to that in period III. The results demonstrated that PM_2.5 shows a nonlinear response to the reduction of its precursors, and this phenomenon varies in different areas. Compared to periods I (36 μg/m³) and III (64 μg/m³), relatively higher O₃ during period II (64 μg/m³) 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.