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: June 2021

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) type 1 – southerly transport, (b) type 2 – easterly convergence, (c) type 3 –
sinking compression, and (d) type 4 – local accumulation. The explained variances in the four pollution weather types are 43.69 % (type 1),
33.68 % (type 2), 16.51 % (type 3), and 3.92 % (type 4). In persistent haze pollution events, type 1 and type 2 surpass 80 % on the first
and second days, while the other types are present alternately in later stages. The atmospheric structures of type 1, type 2, and type 3 have typical
baroclinic characteristics at mid–high latitudes, indicating that the accumulation and transport of pollutants in the boundary layer are affected by
coupled structures in synoptic-scale systems and local circulation. The atmospheric structure of type 4 has typical barotropic characteristics,
indicating that the accumulation and transport of pollutants is primarily affected by local circulation. In type 1, 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 type 2. The atmospheric vertical structure is not
conducive to upward pollutant diffusion. In type 3, 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, type 3 has the highest PM2.5 concentration. In type 4, 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. Different weather types will shape significantly different structures of the pollution boundary layer. 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.