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 μg/m3 yet a ∼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 μg/m3 (∼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.
Key Points Heavy haze still engulfed northern China despite great emission reductions during COVID‐19 lockdownEnhanced secondary aerosols and decreased black carbon (BC) led to a more scattering atmosphere and weakened aerosol‐planetary boundary layer interactionObservational evidences and quantitative modeling confirmed the importance of BC during the unique natural experiment