Regional PM<sub>2.5</sub> pollution confined by atmospheric internal boundaries in the North China Plain: 2. boundary layer structures and numerical simulation

Abstract. This study reveals and summarizes mesoscale planetary boundary layer (PBL) structures for various pollution patterns in the North China Plain. Three pollution categories have been classified, in terms of the influence of the atmospheric internal boundary (AIB) that significantly determines the distribution and concentration of PM2.5. The Weather Research and Forecast model is used to simulate the PBL structure in this region, and its performance is firstly evaluated using surface observations and intensive soundings data. Observed AIBs and PBL evolution are reasonably reproduced. Simulation results for three pollution categories illustrate respective PBL structures, as well the relationship with the mesoscale AIBs. The first category corresponds to the severest pollution and occurs most frequently (~41 %). The PBL structure is laterally confined by a warm front as a sharp AIB and vertically suppressed by a dome-like elevated temperature inversion, which constitutes a stable and enclosed circumstance, most favorable to pollution formation. The second category is characterized by wind shear line/zone as AIB, with dynamic convergence in the PBL as the dominant cause for PM2.5 accumulation. Three shear modes consist of this category, two of which are related to pressure troughs with the convergence layer of the order of the PBL depth. Another shear mode presents a much thicker convergence layer with a depth of about 3000 m, under the saddle-shaped pressure field. This category corresponds to lighter air pollution, with a frequency of 29 %. The PBL of the third category is laterally delineated by a cold-air damming AIB at the foot of the mountains on the windward side. It manifests as a low-temperature and weak-wind air mass accompanied by an elevated inversion and a convergent flow with a thickness as high as mountains. This PBL structure maintains through day and night within the AIB confined zone, while the ordinary diurnal variation of the PBL occurs outside this zone. 14 % of pollution episodes belong to this category. There remain about 16 % pollution episodes undefined by the AIB influence. They may need to be analyzed separately in the future.