Boundary layer structure characteristics under objective classification of persistent pollution weather types in the Beijing area
Authors: Sun Zhaobin , Zhao Xiujuan , Li Ziming , Tang Guiqian , Miao Shiguang 
Published: October 2020

Abstract. Different types of pollution boundary layer structures form via the coupling of different synoptic systems and local mesoscale circulation in the boundary layer; this coupling contributes toward the formation and continuation of haze pollution. In this study, we objectively classify the 32 heavy haze pollution events using integrated meteorological and environmental data and ERA-Interim analysis data based on the rotated empirical orthogonal function method. The thermodynamic and dynamic structures of the boundary layer for different pollution weather types are synthesized, and the corresponding three-dimensional boundary layer conceptual models for haze pollution are constructed. The results show that four weather types mainly influence haze pollution events in the Beijing area: (a) type1: southerly transport, (b) type2: easterly convergence, (c) type3: sinking compression, and (d) type4: local accumulation. The explained variance in the four pollution weather types are 43.69 % (type1), 33.68 % (type2), 16.51 % (type3), and 3.92 % (type4). In persistent haze pollution events, type1 and type2 surpass 80 % on the first and second days, while the other types are present alternately in later stages. The atmospheric structures of type1, type2, and type3 have typical baroclinic characteristics at mid-high latitudes, indicating that the accumulation and transport of pollutants in the boundary layer is affected by coupled structures in synoptic-scale systems and local circulation. The atmospheric structure of type4 has typical barotropic characteristics, indicating that the accumulation and transport of pollutants is primarily affected by local circulation. In type1, southerly winds with a specific thickness and intensity prevail in the boundary layer, which is favorable for the accumulation of pollutants in plain areas along the Yan and Taihang Mountains, whereas haze pollution levels in other areas are relatively low. Due to the interaction between weak easterly winds and the western mountains, pollutants accumulate mainly in the plain areas along the Taihang Mountains in type2. The atmospheric vertical structure is not conducive to upward pollutant diffusion. In type3, the heights of the inversion and boundary layers are the lowest due to a weak sinking motion while relative humidity is the highest among the four types. The atmosphere has a small capacity for pollutant dispersion and is favorable to particulate matter hygroscopic growth; as a result, the type3 has highest PM2.5 concentration. In type4, the boundary layer is the highest among the four types, the relative humidity is the lowest, and the PM2.5 concentration is relatively lower under the influence of local mountain–plain winds. The findings of this study allow us to understand the inherent difference among heavy pollution boundary layers; in addition, they reveal the formation mechanism of haze pollution from an integrated synoptic scale and boundary layer structure perspective. We also provide scientific support for the scientific reduction of emissions and air quality prediction in the Beijing–Tianjin–Hebei region of China.