Royal Dutch Meteorological Institute; Ministery Of Infrastructure And The Environment

Publications, presentations and other activities
The impacts of the COVID-19 lockdown on air quality in the Guanzhong Basin, China
by K. Zhang (Chang’an University, Xi’an), G. de Leeuw (KNMI), Z. Yang (Chang’an University, Xi’an), X. Chen (AirCAS, Beijing), J. Jiao (Chang’an University, Xi’an)

The Corona Virus Disease 2019 (COVID-19) appeared in Wuhan, China, at the end of 2019,
spreading from there across China and within weeks across the whole world. In order to control
the rapid spread of the virus, the Chinese government implemented a national lockdown policy.
It restricted human mobility and non-essential economic activities, which, as a side e ect, resulted in
the reduction of the emission of pollutants and thus the improvement of the air quality in many
cities in China. In this paper, we report on a study on the changes in air quality in the Guanzhong
Basin during the COVID-19 lockdown period. We compared the concentrations of PM2.5, PM10, SO2,
NO2, CO and O3 obtained from ground-based monitoring stations before and after the COVID-19
outbreak. The analysis confirmed that the air quality in the Guanzhong Basin was significantly
improved after the COVID-19 outbreak. During the emergency response period with the strictest
restrictions (Level-1), the concentrations of PM2.5, PM10, SO2, NO2 and CO were lower by 37%, 30%,
29%, 52% and 33%, respectively, compared with those before the COVID-19 outbreak. In contrast,
O3 concentrations increased substantially. The changes in the pollutant concentrations varied between
cities during the period of the COVID-19 pandemic. The highest O3 concentration changes were
observed in Xi’an, Weinan and Xianyang city; the SO2 concentration decreased substantially in
Tongchuan city; the air quality had improved the most in Baoji City. Next, to complement the sparsely
distributed air quality ground-based monitoring stations, the geographic and temporally weighted
regression (GTWR) model, combined with satellite observations of the aerosol optical depth (AOD)
and meteorological factors was used to estimate the spatial and temporal distributions of PM2.5 and
PM10 concentrations with a resolution of 6 km  6 km before and after the COVID-19 outbreak.
The model was validated by a comparison with ground-based observations from the air quality
monitoring network in five cities in the Guanzhong Basin with excellent statistical metrics. For PM2.5
and PM10 the correlation coecients R2 were 0.86 and 0.80, the root mean squared errors (RMSE)
were 11.03 g/m3 and 14.87 g/m3 and the biases were 0.19 g/m3 and 􀀀0.27 g/m3, which led to
the conclusion that the GTWR model could be used to estimate the PM concentrations in locations
where monitoring data were not available. Overall, the PM concentrations in the Guanzhong Basin
decreased substantially during the lockdown period, with a strong initial decrease and a slower one
thereafter, although the spatial distributions remained similar.

Bibliographic data
Zhang, K., G. de Leeuw, Z. Yang, X. Chen and J. Jiao, The impacts of the COVID-19 lockdown on air quality in the Guanzhong Basin, China
Remote Sensing, 2020, 12, 3042, 1-23, doi:10.3390/rs12183042.
Abstract (html)  Complete text (pdf: 8 MB)