The impact of the control measures during the COVID-19 outbreak on air pollution in China

The outbreak of the COVID-19 virus in Wuhan, China, in January 2020 just before the
Spring Festival and subsequent country-wide measures to contain the virus, eectively resulted in the
lock-down of the country. Most industries and businesses were closed, trac was largely reduced,
and people were restrained to their homes. This resulted in the reduction of emissions of trace gases
and aerosols, the concentrations of which were strongly reduced in many cities around the country.
Satellite imagery from the TROPOspheric Monitoring Instrument (TROPOMI) showed an enormous
reduction of tropospheric NO2 concentrations, but aerosol optical depth (AOD), as a measure of
the amount of aerosols, was less aected, likely due to the dierent formation mechanisms and the
influence of meteorological factors. In this study, satellite data and ground-based observations were
used together to estimate the separate eects of the Spring Festival and the COVID-19 containment
measures on atmospheric composition in the winter of 2020. To achieve this, data were analyzed
for a period from 30 days before to 60 days after the Spring Festivals in 2017–2020. This extended
period of time, including similar periods in previous years, were selected to account for both the
decreasing concentrations in response to air pollution control measures, and meteorological eects
on concentrations of trace gases and aerosols. Satellite data from TROPOMI provided the spatial
distributions over mainland China of the tropospheric vertical column density (VCD) of NO2,
and VCD of SO2 and CO. The MODerate resolution Imaging Spectroradiometer (MODIS) provided
the aerosol optical depth (AOD). The comparison of the satellite data for dierent periods showed
a large reduction of, e.g., NO2 tropospheric VCDs due to the Spring Festival of up to 80% in some
regions, and an additional reduction due to the COVID-19 containment measures of up to 70% in
highly populated areas with intensive anthropogenic activities. In other areas, both eects are very
small. Ground-based in situ observations from 26 provincial capitals provided concentrations of
NO2, SO2, CO, O3, PM2.5, and PM10. The analysis of these data was focused on the situation in
Wuhan, based on daily averaged concentrations. The NO2 concentrations started to decrease a few
days before the Spring Festival and increased after about two weeks, except in 2020 when they
continued to be low. SO2 concentrations behaved in a similar way, whereas CO, PM2.5, and PM10 also decreased during the Spring Festival but did not trace NO2 concentrations as SO2 did. As could be
expected from atmospheric chemistry considerations, O3 concentrations increased. The analysis of
the eects of the Spring Festival and the COVID-19 containment measures was complicated due to
meteorological influences. Uncertainties contributing to the estimates of the dierent eects on the
trace gas concentrations are discussed. The situation inWuhan is compared with that in 26 provincial
capitals based on 30-day averages for four years, showing dierent eects across China.