Changes of air quality during the pandemic and airborne transmission issues

The COVID-19 pandemic has killed more than 1 000 000 people within nine months in 2020. The world is changed as the cities were locked down, the traffic reduced, and people forced to work from home and keep social distance. These controlling measures also resulted in drastic reduction of the emission of many air pollutants, providing researchers an unprecedented large-scale natural experiment in examining how the air quality would respond to a strong forcing. In this panel discussion held on 22 September 2020, five experts gathered to discuss their observations and analyses, as well as the current understanding and misconception about airborne transmission. This Forum article is dedicated to Prof. Martin Williams of the Imperial College London, who intended to join the panel discussion but passed away one day before it. Guy Brasseur [Image: see text] Professor of Max Planck Institute for Meteorology, Germany Junji Cao [Image: see text] Professor of Institute of Earth Environment, Chinese Academy of Sciences, China Aijun Ding [Image: see text] Dean and Professor of School of Atmospheric Sciences, Nanjing University, China Lidia Morawska [Image: see text] Professor of Queensland University of Technology, Australia Tong Zhu (Chair) [Image: see text] Dean and Professor of College of Environmental Sciences and Engineering, Peking University, China