Analysis On The Causes Of Winter Air Pollution In Yantan Park Of Lanzhou High-Tech Zone

Located in Chengguan District of Lanzhou City,Gansu Province,Yantan Park of Lanzhou High-tech Industrial Development Zone is a comprehensive development zone focusing on high-tech industries,commercial trade and living.The total amount of air pollutants discharged in the park is low,but the concentration of six pollutants in micro-stations of environmental quality monitoring is high.In 2021,the average annual concentrations of PM₁₀,PM_2.5and NO₂in Yantan Park all exceeded the second-level national ambient air quality standards.It is of great significance to investigate the causes of regional air pollution by means of monitoring data analysis and numerical simulation research.In this paper,the air quality monitoring data of Yantan Park in 2021 were firstly used to analyze the time distribution characteristics of pollutant concentration,evaluated the atmospheric environment quality status of Yantan Park,and identified the main overstandard factors and time periods of Yantan Park.Secondly,AERMOD model was used to simulate the spatial distribution characteristics of pollutant concentrations under the influence of emission sources in Yantan Park,local emission sources and regional transport of river valley topography,and to quantify the contribution of major pollution sources.Thirdly,HYSPLIT backward trajectory model and WPSCF weighted potential source contribution analysis method were used to evaluate the influence of air flow in different paths on pollutant transport in the study area.Finally,WRF model was used to simulate the regional meteorological conditions,and the causes of pollution meteorological in Yantan Park were analyzed.The main conclusions are as follows:（1）The analysis of the current situation of atmospheric environmental quality in Yantan Park showed that the annual and daily mean values of NO₂,PM_2.5and PM₁₀all exceeded the national ambient air quality standards in 2021,and the exceeding period was winter.Compared with other areas in the main urban area of the valley terrain,the concentration of particulate matter in Yantan Park was higher than the main urban area,the concentration of NO₂was slightly higher than the main urban area,the concentration of SO₂was significantly lower than the main urban area,and the differences in the concentration of CO and O₃were not obvious.The air pollution factors affecting Yantan Park were particulate matter and NO₂.（2）The analysis of the environmental impact of pollutant emission in Yantan Park showed that the contribution of pollutant emission to pollutant concentration in Yantan Park was less than 5%through the investigation of the emission of major air pollutants in Yantan Park and the simulation by AERMOD model.（3）The analysis of the environmental impact of local pollutant discharge showed that:under the influence of the easterly wind direction in Lanzhou City,the scope of pollution source investigation was expanded upstream to 20km,covering the production enterprises that discharge major air pollutants.In addition,considering the influence of winter heating,the scope of pollution source investigation was expanded to the west to the vicinity of the Yellow River Bridge in Chengguan District.The local pollutant emissions were calculated and simulated by AERMOD model.The results showed that the concentration of NO₂of local pollution source emissions had a great influence on Yantan Park,and the contribution of traffic source to NO₂concentration was 70%.The contribution to the concentration of PM_2.5and SO₂was relatively low,less than 30%.（4）The simulation results of the valley terrain pollution showed that the simulated NO₂concentration and SO₂concentration were close to the monitored concentration,but the simulated PM_2.5concentration only accounts for about 50%of the monitored value.The simulated low PM_2.5concentration may be related to the inaccurate emission inventory.In addition,the impact of regional transport was not considered,which was also the main reason for the low PM_2.5simulation results.（5）The analysis of backward trajectory showed that the regional air quality was mainly affected by three parts of air transport.First,the dust transport path from Central Asia and Xinjiang had a great impact on the regional particulate matter concentration,which was mainly characterized by significantly higher PM₁₀concentration and low PM_2.5/PM₁₀ratio.Second,the air flow from Inner Mongolia to Gansu through the anticyclone path of Ningxia also presented the characteristics of dust path.Third,it was transported by southeast air flow from the southern Sichuan Province through Longnan and Tianshui into the Park,which was a typical pollution transport route with a high PM_2.5/PM₁₀ratio（greater than 0.5）.The contribution analysis of weighted potential sources showed that Tianshui in the south,Baiyin in the north and Dingxi in the east had a greater impact on the particulate matter,and the contribution of WPSCF was greater than 0.6.（6）The analysis of pollution meteorology showed that the development of upper trough led the surface cold air southward to affect Lanzhou,and there was obvious cold advection in Yantan area during the cleaning period.The temperature inversion in the boundary layer was weakened,and the atmospheric diffusion capacity was enhanced.During the pollution period,Lanzhou was continuously controlled by the northwest air flow behind the trough and the downdraft behind the trough,which resulted in the stability and little movement of the regional near-surface air mass and inhibited the development of the boundary layer.The radiosonde data showed that the daily maximum height of the boundary layer decreased obviously,and the inversion layer thickened at night.On the other hand,due to the influence of local topography,Yantan flow field showed the convergence of wind field at night,which was easy to cause the cyclic accumulation of pollutants,and was the main meteorological cause of regional air pollution.