Does lockdown reduce air pollution? Evidence from 44 cities in northern China.

Responding to the ongoing novel coronavirus (agent of COVID-19) outbreak, China implemented "the largest quarantine in human history" in Wuhan on 23 January 2020. Similar quarantine measures were imposed on other Chinese cities within days. Human mobility and relevant production and consumption activities have since decreased significantly. As a likely side effect of this decrease, many regions have recorded significant reductions in air pollution. We employed daily air pollution data and Intracity Migration Index (IMI) data form Baidu between 1 January and 21 March 2020 for 44 cities in northern China to examine whether, how, and to what extent travel restrictions affected air quality. On the basis of this quantitative analysis, we reached the following conclusions: (1) The reduction of air pollution was strongly associated with travel restrictions during this pandemic-on average, the air quality index (AQI) decreased by 7.80%, and five air pollutants (i.e., SO, PM, PM, NO, and CO) decreased by 6.76%, 5.93%, 13.66%, 24.67%, and 4.58%, respectively. (2) Mechanism analysis illustrated that the lockdowns of 44 cities reduced human movements by 69.85%, and a reduction in the AQI, PM, and CO was partially mediated by human mobility, and SO, PM, and NO were completely mediated. (3) Our findings highlight the importance of understanding the role of green production and consumption.