Unsupervised PM anomalies in China induced by the COVID-19 epidemic.

To stop the spread of COVID-19 (2019 novel coronavirus), China placed lockdown on social activities across China since mid-January 2020. The government actions significantly affected emissions of atmospheric pollutants and unintentionally created a nationwide emission reduction scenario. In order to assess the impacts of COVID-19 on fine particular matter (PM) levels, we developed a "conditional variational autoencoder" (CVAE) algorithm based on the deep learning to discern unsupervised PM anomalies in Chines cities during the COVID-19 epidemic. We show that the timeline of changes in number of cities with unsupervised PM anomalies is consistent with the timeline of WHO's response to COVID-19. Using unsupervised PM anomaly as a time node, we examine changes in PM before and after the time node to assess the response of PM to the COVID-19 lockdown. The rate of decrease of PM around the time node in northern China is 3.5 times faster than southern China, and decreasing PM levels in southern China is 3.5 times of that in northern China. Results were also compared with anomalous PM occurring in Chinese's Spring Festival from 2017 to 2019, PM anomalies during around Chinese New Year in 2020 differ significantly from 2017 to 2019. We demonstrate that this method could be used to detect the response of air quality to sudden changes in social activities.