The dynamic-thermal structures of the planetary boundary layer dominated by 1 synoptic circulations and the regular effect on air pollution in Beijing 2

100029, China 16 Abstract. Synoptic circulations play important roles in meteorological conditions and air quality within the 17 planetary boundary layer (PBL). Based on Lamb-Jenkinson weather typing and multiple field measurements, this 18 study reveals the mechanism of how the coupling effects of multiscale circulations influence PBL structure and 19 pollution. Due to the topographic blocking in the daytime, pollutants accumulate in the plain areas horizontally. 20 The sinking divergent flows overlying on the rising convergent flows within the PBL inhibit the continuously 21 upward dispersion of aerosols vertically. At night, the horizontal and vertical coupling mechanisms synergistically 22 worsen the pollution. The large-scale environmental winds and regional-scale breezes affect the pollution directly 23 via the horizontal coupling effect, which generates a pollution convergent zone of different directional flows. The 24 relative strength of flows causes the severely polluted area to move around horizontally from 39°N to 41°N. In 25 addition, the multiscale circulations regulate the mixing and diffusion of pollutants indirectly the vertical 26 coupling effect, which changes the PBL dynamic-thermal structure. The warm advection transported the upper 27 environmental winds overlies the cold advection transported by the lower regional generating strong 28 wind direction shear and advective inversion. The capping inversion and the convergent sinking within the 29 PBL suppress massive pollutants below the zero speed zone. The multilayer PBL under cyclonic circulation has no 30 diurnal variation. Weak ambient winds strengthen the mountain breezes observably at the temperature 31 inversion can reach 900 m. The nocturnal shallower PBL, consistent with the zero velocity between ambient 32 and mountain winds, can reach 600 m. By contrast, the PBL under southwesterly circulation is mono-layer with 33 obvious diurnal variation, reaching 2000 m in the daytime. The strong winds restrain the 34 development of regional breezes, the zero speed zone is located at 400 m and the inversion is lower 200 m 35 at night. The PBL under westerly circulation has a hybrid structure with both multiple aerosol and diurnal 36 variation. The inversion is generated by the vertical shear of zonal winds. Clean and strong winds are 37 dominated under anticyclone circulation, the vertical shear and the diurnal variation of field disappear 38 because of strong turbulent mixing, and there is no significant PBL structure. Our results imply the algorithm 39 of atmospheric environmental capacity under synoptic circulations, such as the cyclonic type, a multilayer 40 PBL needs the turbulence decays and a stable boundary 82 layer forms with weak turbulence. A radiation inversion on the ground caps the pollutants and leads to the 83 accumulation near the surface. Hu et al. (2014) found that westerly warm advection above the Loess Plateau was 84 NE; easterly, E; southeasterly, SE; southerly, S; southwesterly, SW; westerly, W; northwesterly, NW; 149 and northerly, N), and sixteen hybrid types (CN, CNE, CE, CSE, CS, CSW, CW, CNW, AN, ANE, AE, ASE, AS, ASW, AW, 150


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According to the pollution intensity, three pollution types (cyclonic C, southwesterly SW and westerly W) and one 172 clean type (anticyclonic A) occurring most frequently in the NCP were selected as the studied circulation patterns.

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It was consistent with the results of Li et al. (2020) on the relationship between pollutant concentration and 174 circulation types in northern China. Weather types with high PM2.5 concentration but occurring no more than 175 ten times, such as type CE and type CW, were not discussed in this article. The average and extreme PM2.5 176 concentrations of type C reached 77 μg/m 3 and 270 μg/m 3, respectively, and were much stronger than the other 177 pollution types. Clearly, the cyclonic circulation pattern was more conducive to severe pollution events. The 178 circulation of type A was the most common type, and the PM2.5 concentration was 28 μg/m 3 , which was the 179 lowest.

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As shown in Fig. 3, the locations of the high and low pressures and the intensity of the wind fields at 925 181 hPa under different circulation patterns were clearly distinct. In type C, Beijing was located in the center of low 182 pressure, and the sea to the east of China was controlled by an anticyclone (Fig. 3a). Southwesterly winds 183 prevailed, flowing northward to Beijing along the periphery of the anticyclone with an average wind speed of 3 184 m/s. In type SW, Beijing lay southeast of the low pressure in Mongolia, and the high pressure over the sea was 185 significantly enhanced compared with type C (Fig. 3b). Therefore, southeasterly winds prevailed to the south of 186 Beijing and shifted southwesterly after flowing by. In type W, westerly winds were dominant and converged with 187 southwesterly flows to the north of Beijing (Fig. 3c). The mean velocity of environmental flows in type SW and 188 type W was observably larger than that in type C. In general, the mainland was mainly controlled by low pressure  The mainland was governed by low pressure under type C synoptic circulations, and the ambient winds 223 were mainly southwesterly (Fig. 3a). On the afternoon of the 22nd, the plain breezes in central Hebei, which were 224 induced by thermal contrast between the mountain and plain, blocked weak environmental winds and the direct 225 transportation of pollutants to Beijing (Fig. 5a). The westerly and the northerly mountain breezes began to prevail 226 at night while the conversion from sea breeze to land breeze was not obvious (Fig. 5b). The onshore winds in the 227 coastal area were notably larger than the northerly mountain breezes in southern Chengde (SCD), which were breezes and the ambient southerly flows (Fig. 5f). The four directional airflows formed a convergent zone that 240 caused pollutants to accumulate dramatically in the plain areas. This convergent region that is generated by the 241 coupling effect of large-scale circulation and regional-scale mountain breezes at night also appeared in other 242 pollution types, as will be discussed later.

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The PBL under type C circulation presented a multilayer structure without diurnal variation (Fig. 6a). The 244 highly stable structure and weak ambient winds resulted in a higher aerosol concentration near the surface than 245 that in the other pollution types (Fig. 4). The pollution decreased from bottom to top within the PBL and was 246 characterized by a gradient distribution. It is consistent with previous research (Jiang et al., 2020) that the top PBL 247 height is equal to the maximum detection range of wind Lidar. In the daytime, environmental southwesterly 248 winds dominated within the PBL. On the night of the 22nd, meridional winds turned to easterly (Fig. 5b, 6b), and 249 the northerly downslope winds were strengthened simultaneously in the lower PBL (Fig. 5c, 6c). Easterly and 250 northerly winds were up to 600 m so that the directional shear of meridional and zonal winds ascended 251 considerably. The shallower nocturnal PBL coincided with the zero speed zone between the upper environmental 252 winds and lower regional-scale breezes with the largest directional shear (Fig. 6b, c). Variations of the vertical 253 dynamic structure in the PBL drove the thermal structure to adjust. Warm air advected by large-scale 254 southwesterly winds overlay on the cold air advected by regional-scale northeasterly breezes. Consequently, a 255 conspicuous advective temperature inversion occurred from 600 to 900 m (Fig. 6d) accompanied by stable 256 stratification (Fig. 6e). However, the relatively stronger northerly breezes compared to the environmental winds 257 made the pollutants recirculate southward horizontally (Fig. 5c, 6c). Furthermore, the wind shear developed so 258 high that the stable stratification was above 300 m and the inversion was above 600 m; the pollutants dispersed 259 vertically to some extent. Compared to the previous night, the ambient winds on the night of the 23rd were 260 stronger; thus, both south and east transport channels were established, along with the pollution convergent 261 zone (Fig. 5f). The weak easterly and northerly winds were lower than 300 m (Fig. 6b, c), resulting in temperature 262 inversion and stable stratification connected to the ground. A high concentration of pollution was accumulated in 263 the convergent zone horizontally and trapped below the lowest PBL vertically. Thus, the PM2.5 concentration on 264 the night of the 23rd was significantly higher than that on the 22nd. and the velocity of environmental winds was appreciably higher than that in type C. (Fig. 3b). On the early 277 https://doi.org/10.5194/acp-2020-1123 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License. morning of the 26th, mountain breezes carrying clean air masses prevailed in Beijing, and the air quality was good 278 (Fig. 7a). The basic southerly winds dominated in the Beijing-Tianjin-Hebei region in the afternoon, transporting 279 pollutants northward and causing airflow to converge in plain areas (Fig. 7b). However, pollutants were ventilated 280 horizontally by strong ambient winds and diffused vertically by the intensified turbulent mixing within the 281 growing ML, so the aerosol concentration grew slowly during the day (Fig. 8a). At night, the mountain breezes 282 were strengthened while the ambient southerly winds were weakened; hence, the pollutants were transported to 283 Beijing via the east pollution channel (Fig. 7c). Multiscale circulations of different directions joined and generated 284 a convergent zone in the plain area. Afterwards, easterly flows were further strengthened and transported 285 pollutants to Beijing continuously, the severely polluted area moved westward (Fig. 7d, 8a). In the daytime of the 286 27th, the ambient winds prevailed again, and strong ambient winds removed pollutants by enhancing the 287 ventilation and turbulent mixing (Fig. 7e, 8a). Therefore, the PM2.5 concentration decreased instantly and the air 288 quality in the Beijing-Tianjin-Hebei region improved markedly (Fig. 7f).

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Unlike type C, the PBL presented a monolayer structure in type SW, and the aerosol within the PBL was 290 uniformly distributed (Fig. 8a). Furthermore, the PBL had an obvious diurnal variation and the maximum 291 detection distance of wind Lidar was only consistent with the top ML in type SW. The nocturnal PBL and the 292 growing or collapsing ML were usually lower than the maximum detection distance, indicating that there were 293 residual aerosols above the PBL. In the daytime of the 26th, southwesterly winds dominated within the PBL, and 294 the temperature lapse rate was greater than 0.5 °C/100 m. Along with radiation reinforcing turbulent kinetic 295 energy, the PBL rose to 1200 m. Pollutants were transported to Beijing but mixed vertically (Fig. 8a), so the PM2.5 296 concentration near the surface grew slowly (Fig. 7b). On the night of the 26th, the regional-scale circulation 297 developed upward, and the vertical wind shears between the lower regional breezes and upper environmental 298 winds were strengthened prominently (Fig. 8b, c). The warm advection overlay on the cold advection resulted in 299 advective inversion, forcing the PBL to adjust to become stable, correspondingly (Fig. 8d, e). Similar to type C, a 300 high concentration of pollutants was trapped below the zero wind speed zone where the nocturnal PBL was 301 located. In the daytime of the 27th, large-scale environmental winds within the PBL were strengthened greatly.

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The PBL height was 800 m higher than that of the previous day; thus, the pollutants were advected horizontally 303 and diffused vertically (Fig. 8a). The basic southerly winds with high speed prevailed in central and southern 304 Beijing on the night of the 27th, preventing the mountain winds from flowing southward (Fig. 7f). As a result, no 305 vertical shear of meridional winds occurred in the dynamic field (Fig. 8c) and no temperature inversion occurred 306 in the thermal field (Fig. 8d). The PM2.5 concentration was further reduced. It can be inferred that the 307 temperature inversion in type SW was generated by the vertical thermal contrast of meridional winds. When the 308 meridional winds were uniformly southerly winds within and above the PBL, the air masses in the upper layer had 309 the same thermal properties as that in the lower layer, which will reduce the vertical wind shear and destroy the   (Fig. 9a). Taking the mountain as the boundary, environmental westerly winds prevailed in northwestern 323 https://doi.org/10.5194/acp-2020-1123 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License. 324 pollutants and formed a convergent belt along the western mountains (Fig. 3c, 9b). This distribution of synoptic 325 circulations in type W was conducive to the occurrence of severe pollution around mountains. Similar to other 326 pollution types, the ambient winds converged with region-scale mountain breezes at night, forming a convergent 327 zone (Fig. 9c). The convergent zone moved southward later because of intensified mountain breezes (Fig. 9d). The 328 large velocity of environmental winds leads to strong ventilation (Fig. 9e). In addition, the increasing PBL made 329 the pollutants diluted vertically, and the air pollution was alleviated temporarily. On night of the 16th (Fig. 9f), the 330 synergistic effects of multiscale circulations led to the convergent zone again, and pollution occurred in the 331 easterly flows with a high PM2.5 concentration.

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The PBL under type W circulation presented a hybrid structure, having similar characteristics of types C and 333 SW simultaneously. Similar to type C, the aerosol concentration was characterized by a gradient distribution 334 within the multilayer PBL (Fig. 10a). However, the PBL had an obvious diurnal variation, and the maximum 335 detection distance of wind Lidar was only consistent with the top ML in the daytime, similar to type SW. Although 336 the PBL height reached 1600 m in the daytime (Fig. 10a), the PM2.5 concentration at the surface did not decrease 337 observably because of the massive pollution accumulated previously and the continuous emissions and 338 transportation of pollutants (Fig. 9b). The mixing layer collapsed along the zero wind speed of meridional winds 339 after sunset, and the breezes within nocturnal PBL shifted northwesterly at night (Fig. 10b, c). In type W, zonal 340 circulation dominated. The vertical shear of zonal winds was intensified significantly at night, while the vertical 341 shear of meridional winds diminished. Therefore, it can be assumed that the temperature inversion in type W was 342 produced by the vertical shear of zonal winds. The thermal contrast between the upper westerly winds and the 343 lower easterly winds produced a deep inversion layer that existed from the surface to 500 m (Fig. 10d), as well as 344 a stable stratification with a depth exceeding 600 m (Fig. 10e). This is consistent with the findings of Hu et al.  (Fig. 11a, b). The lapse rate of temperature was greater than 1 °C/100 m, and Ri was less than 0.25 364 within the PBL (not shown). Although the aerosol concentration of the clean type was far less than that of 365 pollution types, the PBL height was only 500 m at night (Fig. 11a). Sometimes, the PBL in the clean type was even  Fig. 12. In the daytime, the NCP region was controlled by a rising motion below 900 379 hPa with a sinking motion overlaying it (Fig. 12a). Correspondingly, the basic flows below 900 hPa presented a 380 convergence, while that above 900 hPa presented a divergence (Fig. 12b). Airflows inside the PBL converged and 381 rose, while the sinking and divergent flows superposed above the PBL, preventing the pollutants from moving 382 upward continuously and making it difficult for the aerosol particulates to diffuse beyond. As a consequence, the 383 pollutants accumulate gradually in the daytime because of the common influences of horizontal topographic 384 blocking and vertical upward mixing with the ML rise. At night, the winds presented a consistent sinking motion 385 below 500 hPa with the largest sinking velocity occurring near the surface (Fig. 12a). Wu et al. (2017) found that 386 the descending motion of synoptic circulations contributed to a reduction in the PBLH by compressing the air 387 mass. In general, the airflow of pollution types is always convergent inside the PBL with the strongest 388 convergence occurring at 950 hPa, regardless of whether it is daytime or nighttime. The height of the nocturnal 389 PBL reduced observably and simultaneously with the convergence zone; meanwhile, divergent downdrafts above 390 the PBL make it difficult for pollutants to diffuse upward (Fig. 12b). Thus, massive pollutants were capped near 391 the surface and accumulated rapidly.