Significance Regional haze adversely affects human health and possibly counteracts global warming by greenhouse gases, but its formation and radiative forcing remain unclear. Here we present field measurements to show reduced frequency but not severity for heavy haze and little-improved moderate haze, when SO ₂ is significantly reduced in China. Our laboratory experiments identify black carbon (BC)-catalyzed sulfate formation involving NO ₂ and NH ₃ . Radiative transfer calculations accounting for this BC chemistry indicate small net climatic cooling/warming but large surface cooling, atmospheric heating, and air stagnation. Our work reveals that this BC catalytic chemistry dominates the formation, trend, and radiative forcing of regional haze, suggesting the necessity of simultaneous reduction in emissions of SO ₂ , NO _x , NH ₃ , and BC for air-quality improvement.