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Abstract

Black-carbon (BC) aerosol can strongly influence planetary boundary layer (PBL) development and thus severe haze formation, but its distinct role compared with scattering aerosols is not yet fully understood. Here, combining numerical simulation and field observation, we found a “tipping point,” where the daily maximum PBL height decreases abruptly when exceeding a critical threshold of aerosol optical depth (AOD), due to a BC-induced decoupling of mixing zones. Because the threshold AOD decreases with increasing BC mass fraction, our results suggest that the abrupt transition of PBL development to adverse conditions can be avoided by reducing the AOD below the threshold but can be avoided more efficiently by reducing the BC mass fraction to increase the threshold (e.g., up to four to six times more effective in extreme haze events in Beijing). To achieve co-benefits for air quality and climate change, our findings clearly demonstrate that high priority should be given to controlling BC emissions.