Roles of Regional Transport and Vertical Mixing in Aerosol Pollution in Shanghai Over the COVID-19 Lockdown Period Observed Above Urban Canopy

Regional transport and vertical mixing are important for aerosol pollution of megacities, but their roles are often challenging to assess via ground observations. In this study, we measured aerosol chemical composition simultaneously above urban canopy (Shanghai Tower, 609 m), a site representative of aerosols from regional scale, and a nearby ground site and investigated the roles of regional transport and vertical mixing over 2020 COVID-19 lockdown period (1 January to 18 April), when local emissions were first drastically reduced and then recovered. During the lockdown period, regional transport was the major source of most aerosol species (organics, NO3-, SO42-, and NH4+) at both heights according to the high correlation coefficients (R = 0.81-0.87) between both heights. Correlation coefficients and vertical ratios (609 m/ground) of most aerosol components showed similar diurnal variations with the evolution of planetary boundary layer height, indicating the role of vertical mixing in aerosol pollution. Moreover, the concentrations of aerosol components at 609 m generally preceded those at ground level by 1-2 hr, indicating that aerosols were first transported at upper boundary layer, and then were mixed downwards to ground level. At 609 m, highly oxidized oxygenated organic aerosol (OOA; a surrogate of secondary organic aerosol (SOA)) dominated in organic aerosol (=75%). The high correlations (R = 0.96) between OOA and hydrocarbon-like organic aerosol (HOA; a surrogate of primary organic aerosol (POA)) at 609 m indicated that they originated similarly from regional transport. This study highlights the importance of regional joint prevention and control of pollutant emissions and observation above urban canopy.