| As the lowest layer of the atmosphere,the physical and chemical processes of the atmospheric boundary layer have a direct impact on human activities.The structure of the wind field in the atmospheric boundary layer greatly affects the transport and diffusion of atmospheric pollutants,thereby changing the local air quality.Characterizing the atmospheric boundary layer is of great scientific significance and application value for understanding the changes of the atmospheric environment.The Sichuan Basin is one of the densely polluted areas in our country,and the western edge of the basin is immediately close to the Qinghai-Tibet Plateau,where the special mountain terrain forms the complex wind field structure in the atmospheric boundary layer with the unique effects on local air quality.This paper,analyzes the meteorological elements of atmospheric boundary layer with the intensive sounding data of atmospheric pollutants in the Mingshan area of Ya'an City in the represented months of four seasons in year 2017,and conducts air quality simulation on the temporal and spatial distribution of PM2.5 affected by the atmospheric boundary layer low-level jets.The structures of atmospheric boundary layer under complex terrain background and their impacts on atmospheric pollutants in the western edge of the Sichuan Basin was revealed.The main research contents and conclusions are as follows:?1?Mountain-Valley Breeze circulation changes and their impacts on O3The occurrence order of the Mountain-Valley Breeze in Mingshan area was summer>winter>autumn>spring.The surface wind speed was 1.6 times strengthened by the valley breeze and reduced by 38%under the impact of mountain breeze.The valley breeze circulation influence height was as high as 1500m while the influence height of mountain breeze was only 500m.The horizontal ranges of valley and mountain breeze circulations were respectively about300km and 200km.The presence of Mountain Valley Breeze was related to the surface solar radiation intensity at daytime and the rate of the radiation cooling at night.Mountain Valley Breeze appeared easily on clear and less cloud days.The Mountain Valley Breeze had a significant transporting effect on O3.The basin area was heavily polluted while the mountain area was clean.During the daytime,the valley breeze circulation transported O3 from the heavily polluted basin to the clean mountains,and under the influence of the strong updraft,O3was accumulated at 100-600m.The plateau clean airflow in the mountain breeze reduced the O3 concentrations at nighttime.?2?Low-level jets in the atmospheric boundary layer and their impacts on PM2.5The low-level jet?LLJ?of atmospheric boundary layer occurred most frequently in summer and least in autumn and winter in 2017.The frequency of occurrence at night was higher than that during daytime,mainly at altitudes of 100-500m.The wind speeds of LLJ were mainly distributed at 5-8m·s-1.The two low-level jets of the atmospheric boundary layer on January5th and 6th in 2017 showed the different effects on the change of atmospheric fine particles PM2.5concentrations.The atmospheric boundary layer low-level jets in the westerly wind showed a clean effect on air pollutants.The clean plateau airflow brought by the westerly wind rapidly reduced the local PM2.5concentrations.The number concentrations of the particles with the diameter above 0.5?m below the jet axis were reduced by more than 50%.The low-level jets in the easterly wind showed a process of air pollutant accumulation,and the easterly wind carried the atmospheric pollutant from the heavily PM2.5 polluted area in the basin to the local area,further aggravating local PM2.5 pollution.Under the condition of the wind speed above 5m·s-1,the number concentrations of fine particles showed a significant change,while the easterly and westerly wind below 5m·s-1 exerted less impact on the number concentration of the fine particles.?3?Numerical simulation of atmospheric boundary layer structures and their impacts on PM2.5 under mountainous terrainThe different performances of PM2.5 simulation with air quality model WRF-Chem were validated in the different surface PM2.5levels under the background of complex mountainous terrain in the western edge of the Sichuan Basin.The adjustment of the atmospheric boundary layer schemes almost had no clear effect on the simulation of atmospheric boundary layer low-level jets in the heavy PM2.5pollution.The simulation of atmospheric boundary layer structures with the MYJ scheme performed better than the simulations with YSU and BL schemes,although the simulated starting time of the atmospheric boundary layer low-level jets was earlier with the underestimated wind speed of the low-level jet in moderate PM2.5 pollution.The YSU scheme could best simulate the atmospheric boundary layer with the period of the low-level jet,the height of the jet axis,and the maximum wind speed compared to the observation in the clean PM2.5 environment,indicating that the weather background of free atmosphere resulting PM2.5 pollution could affect the performance of atmospheric boundary layer simulation during the process of PM2.5 pollution.We selected the MYJ scheme for simulation of moderate PM2.5 pollution and the YSU scheme for the clean PM2.5 environment to explore the mechanism of atmospheric boundary layer low-level jets on PM2.5 in the western edge of Sichuan Basin.The westerly jet removed more than 50%of atmospheric fine particles,and easterly jet could double the local PM2.5pollution levels. |
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