Weakened Aerosol-PBL Interaction During COVID-19 Lockdown in Northern China

Anthropogenic emissions were greatly constrained during COVID-19 lockdown in China. Nevertheless, observations still showed high loadings of fine particles (PM2.5) over northern China with secondary aerosols increasing by 15 mu g/m(3) yet a similar to 10% drop in light-absorbing black carbon (BC). Such a chemical transition in aerosol composition tended to make the atmosphere more scattering, indicated by satellite-retrieved aerosol absorption optical depth falling by 60%. Comparison between weather forecast and radiosonde observations illustrated that, without upper-level heating induced by BC, the stabilized stratification diminished, which was conducive for planetary boundary layer (PBL) mixing and thus near-surface pollution dispersion. Furthermore, coupled dynamic-chemistry simulations estimated that emission reduction during the lockdown weakened aerosol-PBL interaction and thus a reduction of 25 mu g/m(3) (similar to 50%) in PM2.5 enhancement. Based on the unique natural experiment, this work observationally confirmed and numerically quantified the importance of BC-induced meteorological feedback, further highlighting the priority of BC control in haze mitigation.

Automated summary

During the COVID-19 lockdown in China, anthropogenic emissions were significantly reduced, yet high levels of PM2.5 and secondary aerosols were still observed, along with a decrease in light-absorbing black carbon. Changes in meteorology led to a transition in aerosol composition, resulting in a lower aerosol absorption optical depth. The study suggests that emission reductions weaken aerosol-Planetary Boundary Layer (PBL) interactions, leading to a decrease in PM2.5 enhancement.