Black-carbon-induced regime transition of boundary layer development strongly amplifies severe haze

•High concentration of black carbon (BC) can trigger a tipping point PBL development•With aerosol load above the point, maximum height decreases abruptly•The is caused by BC-induced decoupling vertical mixing zones•Reducing BC much more efficient to avoid than reducing other aerosols Air pollution major threat human health. Severe haze often an unexpected extremely shallow planetary boundary layer (PBL), lowest part atmosphere where most pollutants are concentrated. Insufficient understanding formation mechanism leads failure air quality forecast and effective prevention. We found that high atmospheric concentrations black-carbon particles (an aerosol) “tipping point” in height. Above this these suppress layers create stable PBL, trapping near ground greatly deteriorating quality. This scenario be avoided through targeted reductions emissions (rather targeting total particulate matter reductions). Results further show mega-wildfires climate change nuclear disasters result enormous cause extreme stratification persistent layers. Black-carbon strongly influence (PBL) development thus severe formation, but its distinct role compared with scattering not yet fully understood. Here, combining numerical simulation field observation, we point,” daily abruptly when exceeding critical threshold optical depth (AOD), due zones. Because AOD increasing mass fraction, our results suggest abrupt transition adverse conditions below efficiently fraction increase (e.g., up four six times events Beijing). To achieve co-benefits for change, findings clearly demonstrate priority should given controlling emissions. In recent decades, heavy winter have frequently occurred Chinese megacity regions, threatening health millions.1Lelieveld J. Evans J.S. Fnais M. Giannadaki D. Pozzer A. 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