Formation and Evolution Mechanisms for Two Extreme Haze Episodes in the Yangtze River Delta Region of China During Winter 2016

Abstract Severe haze pollution frequently occurred in China during winter. Mechanisms for the formation and evolution of high PM 2.5 (particulate matter with aerodynamic diameter of 2.5 μm or less) episodes, however, remain poorly understood. We characterize two extreme haze episodes in the Yangtze River Delta region of China from 1 to 9 December (Episode I) and 19 to 24 December (Episode II) in 2016 using comprehensive measurements and model analyses. The aqueous sulfur dioxide (SO 2 ) oxidation catalyzed by mineral ions and the heterogeneous uptakes of SO 2 , sulfuric acid (H 2 SO 4 ), nitrogen dioxide (NO 2 ), nitrogen trioxide (NO 3 ), nitrogen pentoxide (N 2 O 5 ), and nitric acid (HNO 3 ) on mineral aerosols are included in the model to better represent the formation of sulfate‐nitrate‐ammonium. The optimized mechanisms substantially improve the simulations of PM 2.5 composition, particularly for sulfate and nitrate. The two episodes show different synoptic conditions and evolution stages, with gradual PM 2.5 increase under stagnant weather conditions in Episode I ( Stage I: Slow Increase Stage , Stage II: Rapid Formation Stage , and Stage III: Dissipation Stage ) and with explosive PM 2.5 increase mostly associated with cross‐border transport from North China in Episode II ( Stage I′: Clean Stage , Stage II′: Fast Transport Stage , and Stage III′: Clear Stage ). The concentrations of sulfate‐nitrate‐ammonium increased evidently and became the key components of PM 2.5 during haze episodes. The heterogeneous conversion from SO 2 to sulfate on mineral aerosols is the main reason for sulfate increase, accounting for more than 50% of sulfate production. This study provides a better understanding of the causes for winter haze in China.