Weakened Haze Mitigation Induced by Enhanced Aging of Black Carbon in China

A great challenge for haze pollution mitigation with the existing emission control measures in China is ozone (O₃) increase. The chemical processes leading to weakened haze mitigation are still poorly understood. Our work identifies the enhanced aging chemistries of black carbon (BC) with increasing O₃ as an essential driver to weaken haze mitigation based on field observations during autumn/winter haze periods in 2014 and 2018 in North China Plain. The enhanced atmospheric oxidation capacity induced by increasing O₃ promotes the initial aging of accumulated fresh BC from continuous emission under haze pollution conditions and consequently improves the hygroscopicity of BC-containing particles to provide more particulate surfaces and volumes for aqueous and heterogeneous chemistries. The enhanced BC aging amplifies PM_2.5 concentrations by ∼20%, which can be broken by concurrent reductions in multipollutant emissions (i.e., BC, nitrogen oxides, and volatile organic compounds), especially from residential and industrial sources. Moreover, enhanced BC aging implies an adverse effect of O₃ increase on climate change. Observationally enhanced BC aging will help to constrain estimations of the interactions among O₃ increase, haze pollution, and climate warming in recent years in China.