Abstract
The unprecedented slowdown in China during the COVID-19 period of November 2019 to April 2020 should have reduced pollution in smog-laden cities. However, moderate resolution imaging spectrometer (MODIS) satellite retrievals of aerosol optical depth (AOD) show a marked increase in aerosols over the Beijing–Tianjin–Hebei (BHT) region and most of Northeast and Central China, compared with the previous winter. Fine particulate (PM2.5) data from ground monitoring stations show an increase of 19.5% in Beijing during January and February 2020, and no reduction for Tianjin. In March and April 2020, a different spatial pattern emerges, with very high AOD levels observed over 50% of the Chinese mainland, and including peripheral regions in the northwest and southwest. At the same time, ozone monitoring instrument (OMI) satellite-derived NO2 concentrations fell drastically across China. The increase in PM2.5 while NO2 decreased in BTH and across China is likely due to enhanced production of secondary particulates. These are formed when reductions in NOx result in increased ozone formation, thus increasing the oxidizing capacity of the atmosphere. Support for this explanation is provided by ground level air quality data showing increased volume of fine mode aerosols throughout February and March 2020, and increased levels of PM2.5, relative humidity (RH), and ozone during haze episodes in the COVID-19 lockdown period. Backward trajectories show the origin of air masses affecting industrial centers of North and East China to be local. Other contributors to increased atmospheric particulates may include inflated industrial production in peripheral regions to compensate loss in the main population and industrial centers, and low wind speeds. Satellite monitoring of the extraordinary atmospheric conditions resulting from the COVID-19 shutdown could enhance understanding of smog formation and attempts to control it.
Highlights
The Beijing–Tianjin–Hebei (BTH) region of Northern China is one of the most economically productive regions in China, generating over 10% of GDP
This study investigates regional air quality indicators over the BTH region and other parts of China during the winter of 2019–2020, when the COVID-19 outbreak in China caused greatly reduced transport and economic activity from November 2019 to February 2020
In March, a slight decline in aerosol optical depth (AOD) levels is seen over the BTH region compared to the previous year (Figure 1), but very significant increases in AOD are seen over other very large peripheral regions, including the Urumqi–Chiangji Economic Zone (UCEZ) in Xinjiang Province in China’s northwest, in the Beibu Gulf Economic Zone (BGEZ) in the far south, and some increase is observed in the northeastern province of Heilongjiang (Figure 1 top left sub-figure)
Summary
The Beijing–Tianjin–Hebei (BTH) region of Northern China is one of the most economically productive regions in China, generating over 10% of GDP It comprises the cities of Beijing and Tianjin, and one province, Hebei, and had a total population of 111 M in 2015. Li et al [3] examined air quality relative to subway density in Beijing and found a 7.7% reduction in Beijing’s air quality index (AQI) in areas surrounding new subway lines, due to reduced car commuting, and Zhang et al [4] found mobile sources to be the greatest contributor to regional haze events in Beijing These studies suggest the major role of vehicular traffic in air quality control. While government pollution control policies appear to have improved air quality in Beijing and Tianjin [5,6,7], pervasive pollution is still evident across a much larger surrounding region, as demonstrated by Yuan and Yang’s [8] report of significant worsening in other cities of the BTH region
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