Air quality changes during the COVID-19 lockdown over the Yangtze River Delta Region: An insight into the impact of human activity pattern changes on air pollution variation

Abstract

The outbreak of COVID-19 has spreaded rapidly across the world. To control the rapid dispersion of the virus, China has imposed national lockdown policies to practise social distancing. This has led to reduced human activities and hence primary air pollutant emissions, which caused improvement of air quality as a side-product. To investigate the air quality changes during the COVID-19 lockdown over the YRD Region, we apply the WRF-CAMx modelling system together with monitoring data to investigate the impact of human activity pattern changes on air quality. Results show that human activities were lowered significantly during the period: industrial operations, VKT, constructions in operation, etc. were significantly reduced, leading to lowered SO₂, NO_x, PM_2.5 and VOCs emissions by approximately 16-26%, 29-47%, 27-46% and 37-57% during the Level I and Level II response periods respectively. These emission reduction has played a significant role in the improvement of air quality. Concentrations of PM_2.5, NO₂ and SO₂ decreased by 31.8%, 45.1% and 20.4% during the Level I period; and 33.2%, 27.2% and 7.6% during the Level II period compared with 2019. However, ozone did not show any reduction and increased greatly. Our results also show that even during the lockdown, with primary emissions reduction of 15%-61%, the daily average PM_2.5 concentrations range between 15 and 79 μg m^-3, which shows that background and residual pollutions are still high. Source apportionment results indicate that the residual pollution of PM_2.5 comes from industry (32.2-61.1%), mobile (3.9-8.1%), dust (2.6-7.7%), residential sources (2.1-28.5%) in YRD and 14.0-28.6% contribution from long-range transport coming from northern China. This indicates that in spite of the extreme reductions in primary emissions, it cannot fully tackle the current air pollution. Re-organisation of the energy and industrial strategy together with trans-regional joint-control for a full long-term air pollution plan need to be further taken into account.

Keywords: Air quality; COVID-19; Yangtze River Delta.

Graphical abstract

Fig. 1

Modelling domain and locations of the national observational sites (green triangle). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Scatter plots of observed and simulated PM_2.5 concentrations during pre-lockdown, Level I and Level II response periods at 41 monitoring sites over the YRD region (locations are monitoring sites shown in Fig. 2).

Fig. 3

Relative changes of PM_2.5 during Pre-lockdown, Level I and Level II periods in YRD.

Fig. 4

Yearly changes of PM_2.5, PM₁₀, CO, NO₂, SO₂ and O₃-8h in 41 cities in the YRD during 1st January - 31st Mar, 2020.

Fig. 5

PM_2.5 potential source regions:air mass trajectory analysis during different stages in Shanghai.

Fig. 6

Source contribuitons to PM_2.5 at Shanghai, Hefei, Hangzhou and Nanjing during pre-lockdown, Level I and Level II response periods.

Fig. 7

Sectoral contribuitons to PM_2.5 during pre-lockdown, Level I and Level II response periods in YRD.