Markedly enhanced levels of peroxyacetyl nitrate (PAN) during COVID-19 in Beijing

ABSTRACT

Abstract High levels of secondary air pollutants during COVID-19 in China have aroused great concern. In Beijing, measured daily mean peroxyacetyl nitrate (PAN) concentrations reached 4 ppb over the lockdown period (01/24?02/15), whose averages were 2?3 times that before lockdown (01/01?01/23). The lockdown PAN levels also reached a high historical record based on our long-term measurements (2016?2019). Unlike ozone and PM2.5, PAN formation depends on less complex photochemistry between NOx and volatile organic compounds (VOCs), providing a novel approach to investigate the wintertime photochemistry during COVID-19. The GEOS-Chem simulations suggest a markedly enhanced photochemistry by a factor of 2 during the lockdown. Change of meteorology featuring with anomalous wind convergence under higher temperatures is the main reason for enhanced photochemical formation of PAN, while chemically nonlinear feedbacks also play a role. Our results suggest implementing targeted VOC emission controls in the context of increasing photochemical pollution over this complex polluted region.