基于无人机观测的太原盆地PM2.5污染特征

The feature of PM2.5 pollution in Taiyuan Basin based on unmanned aerial vehicle observation

关键词:  太原盆地  局地气象特征  大气边界层  无人机  PM_2.5

Key words:  Taiyuan Basin  local meteorological features  atmospheric boundary layer  UAV  PM_2.5

摘要:

近地面局地环流可以决定污染物的空间分布和浓度峰值，而垂直方向上的大气边界层结构则影响污染物的积累和扩散过程。利用无人机在太原盆地不同方位城市进行5 d污染物垂直观测，综合分析大气边界层结构、局地环流和区域输送对PM_2.5污染的影响。结果表明：观测期间，太原大气逆温层的长时间维持以及较好的垂直水汽条件导致稳定的大气层结；高空下沉气流引起边界层高度下降，大气环境容量降低；谷风输送使PM_2.5持续向盆地北部聚集，三者共同作用导致太原出现快速积累型重污染天气。文水近地面PM_2.5高浓度层低于200 m，表现出本地源的显著影响，垂直扩散条件午后才转好是导致白天浓度持续偏高的主要原因。汾阳和平遥出现系统性偏西风，区域污染物清除明显，但系统风下风向的平遥上午仍有污染物积累，近地面PM_2.5高浓度层超过300 m。

Abstract:

Background, aim, and scope The Taiyuan Basin is situated in North China and is one of the areas with severe air pollution in China. Due to the coal dominated industrial structure and complex terrain conditions, the Taiyuan Basin experiences frequent air pollution events in autumn and winter. Meteorological conditions are also one of the main reasons for the frequent occurrence of pollution events. Influenced by thermal and dynamic conditions, the Taiyuan Basin has obvious valley wind circulation. However, few studies explored the migration and accumulation rules of pollutants in the Taiyuan Basin with mountain valley wind. In addition, the air pollution occurs in the boundary layer, and the dynamic and thermal structure of the boundary layer play an important role in the accumulation and dissipation of pollutants. Therefore, based on vertical observations of the boundary layer, by comprehensively analyzing the effects of local circulation and boundary layer processes, the understanding of the impact of atmospheric boundary layer structure on pollution will be improved. Materials and methods In this study, unmanned aerial vehicle (UAV) was used to carry out vertical observation four times a day for five days in Taiyuan in the north of the basin, Wenshui in the west, Fenyang in the southwest and Pingyao in the east. At the same time, the wind speed and direction data of the ground meteorological stations in the observation area and the meteorological reanalysis data provided by the NCEP were collected. The impacts of the vertical structure of the atmospheric boundary layer, local circulation and pollutant transport on PM_2.5 pollution were analyzed. Results The persistent deep inversion layer, the decreasing height of the boundary layer due to the sinking airflow at high altitudes, and the valley wind transport in the northern part of the basin all contributed to the rapid accumulation of highly PM_2.5 polluted weather in Taiyuan. The near-surface high concentration layer of PM_2.5 in Wenshui was below 200 m during the observation period, indicating the significant inf luence of local sources. The vertical diffusion conditions did not improve until the afternoon, which was the main reason for the sustained high concentration during the daytime. The regional pollutants were cleared obviously under the influence of westerly winds, but there was still pollutant accumulation and the high concentration of PM_2.5 near the surface exceeded 300 m in the morning in Pingyao, downwind of the systematic winds. Discussion Near-surface local circulation can determine the spatial distribution and concentration peaks of pollutants, while vertically the structure of the atmospheric boundary layer influences the accumulation and dispersion processes of pollutants. When the high-altitude air flow is flat westerly or southwesterly, the near-surface circulation is valley wind circulation, and pollutants tend to accumulate in the northern part of the basin because the valley wind is more obvious than the mountain wind; when the high-altitude air flow is northwesterly, the basin is controlled by systematic wind, and atmospheric dispersion conditions are good. Vertical observation by UAV helps to finely depict the vertical distribution characteristics of meteorological elements and atmospheric pollutants, and then accurately obtain the atmospheric boundary layer structure. The impact of the decrease in boundary layer height on heavily polluted weather cannot be obtained through ground-based monitoring. The research results show that the decrease in atmospheric capacity caused by the decrease in boundary layer height may also be one of the important reasons for the rapid accumulation of heavy pollution in Taiyuan in recent years, while the transport of valley winds in the basin further exacerbates the pollution process. In addition to the effects of local accumulation, regional transport and secondary transformation, the decrease of the boundary layer height under special topography may have a more direct impact on air pollution, which also poses a more serious challenge for regional air pollution management-how to maintain low PM_2.5 growth in spite of the rapidly decreasing capacity of the atmospheric environment: firstly, source management of pollutant emissions should be continued; secondly, vertical observation should be strengthened to understand boundary layer evolution and aerosol interactions, and attempts to predict the boundary layer structure; and thirdly, emission reduction should be carried out in advance based on the prediction of air quality. Conclusions Atmospheric temperature stratification, boundary layer height changes and regional transportation have significant effects on accumulation and dispersion of pollutants. The valley wind circulation may aggravate the air pollution in the northern part of the basin. Recommendations and perspectives There are significant differences in the accumulation and dissipation characteristics of pollutants in different parts of the basin, therefore, more targeted treatment measures are needed for the overall improvement of regional pollution.