Aerosol-boundary layer feedbacks triggered by both greenhouse gas and aerosol emissions

There is a strong interplay between processes within the planetary boundary layer (PBL) and the number of aerosols within it. Stable weather conditions are conducive to less vertical mixing, a shallower PBL and stronger accumulation of pollutants near the surface. In some cases, this can contribute to episodes of severe haze, with serious health impacts. A change in PBL height, however, may also be driven by changes in anthropogenic emissions and their influence on the atmosphere. In this study, we perform idealized simulation with the earth system model CESM2-CAM6, to investigate the effect of various climate drivers (CO₂, black carbon and sulfate) on turbulence, planetary boundary layer height, and ultimately near-surface pollution. We find that while emissions of all three climate drivers influence the number of severe air pollution episodes, only CO₂ and black carbon emissions do so through an impact on turbulence and PBL height. While black carbon aerosols are known to cause atmospheric heating, increased boundary layer stability and reduced turbulence, we find CO₂ to have a similar albeit opposite effect through surface warming. Our results clearly underline the importance of black carbon mitigation for reducing the most severe exposures to air pollution.