Numerical Simulation On Two Typical Heavy Air Pollution Events And Pollution Effect Of Boundary Layer Structure In Lanzhou

There have been great strides in the development of social productivity,industrialization and urbanization since the beginning of industrial revolution.Meanwhile,the environmental pollution problems have become more and more serious,especially the issue of air pollution has become one of the most important environmental problems.It is also the most urgent problem to be solved in China.Lanzhou City is one of the first batch of new China's key construction of industrial cities.As a result of the improper industrial layout and energy structure,the problem of air pollutant emission is very severe,which have critically affected the air quality in Lanzhou.In recent years,the government has introduced a series of policies to improve air quality and it has been improved after taking some measures.However,the air pollution in the mountain region is much more serious than that in the flat area because the meteorological variables are strongly altered under the topographic forcing.In this paper,we summarized characteristics of air pollution in Lanzhou in the past 16 years.The WRF model was used to analyze the idea case of spring dust pollution caused by the cold air.Influences of topographic and thermal factors on the wind field and boundary layer structure of Lanzhou valley were discussed through terrain and surface heat flux sensitivity test.The characteristics of winter static air pollution and its impact factors were analyzed by using WRF-Chem model.The main conclusions are as follows:(1)In the past 16 years,Lanzhou City's air quality had been improved.API/AQI interannual changes were significantly decreased in Lanzhou City,the number of heavy pollution days reduced significantly.The concentration of SO2 in Lanzhou City had markedly declined,the annual average concentration of SO2 had reached the national secondary standards in the past 6 years.PM10 annual average concentration showed a downward trend.NO2 annual average concentration showed a clear linear increase trend in the past 4 years.The primary pollutants are PM10 and PM2.5 in Lanzhou City in the past 3 years.The main pollutants in January and December were PM2.5,and that in the other months were PM10.(2)In the cases of sand-dust air pollution caused by cold air,the type of ascending-descending heavy pollution incident was caused by the dust carried by the cold air into Lanzhou.WRF model successfully simulated the change of temperature and wind field during the process of cold air intrusion into Lanzhou valley.Results showed that cold air invaded Lanzhou along the northeast side of the valley below 2.5 km.There was westly gradient wind above 3.5 km and a weak wind layer(wind speed less than 4 m·s-1)was found at the height of 2.5-3.5 km.As the temperature rapidly reduced at the height of 2.5 km,an inversion layer was formed above 2.5 km.Wind speed and potential temperature vertical gradient were very small.A mixed layer with a thickness of more than 700 m was formed at the night of March 19 th.(3)The surface heat flux had little effect on wind speed and potential temperature during cold air transit,and it had a great effect on daytime potential temperature with no noticeable weather process.The increase of local heat forcing induce east wind.The effect of surface heat flux on the height of the boundary layer was mainly reduced during the day,and the PBLH(planetary boundary layer height)decreased with the decreasing of surface heat flux.The most significant period for the response of terrain forcing was the cold air transit phase with a large wind speed.During this period,the wind speed would be increased by reducing the terrain height of both north and south sides of the valley.Whereas,there would be little impact on the height of the weak wind layer.It suggested that the formation of weak wind layer may be related to the large terrain of the Lanzhou Valley,not to the local terrain.In the afternoon,a new inversion layer with a thickness of 200 m formed when the southern side mountain terrain of the valley were reduced,and resulting in the boundary layer height decreased from 1.8 km to 1.0 km.(4)WRF-Chem model has a good skill in simulating the air pollution in Lanzhou area.The YSU scheme on meteorological field and pollutant concentration is slightly better than the ACM2 scheme.There is a big difference of the daily variation peak between the simulated particulate pollutants and the monitoring value,which is mainly caused by the diurnal variation of the pollutant source and the height of the boundary layer.(5)The numerical simulation results show that there was mostly calm or breeze within 500 m AGL in Lanzhou.Due to the special mountainous terrain,there was mainly downdraft and an untouched ground temperature inversion in the day time.Weak updraft and a strong temperature inversion in nighttime.Those conditions were not conducive to the diffusion of air pollutants.During the heaviest period of pollution,there was also a strong touched ground temperature inversion during the daytime,which leads to a high level of pollutant concentration.The boundary layer height can significantly affect the particle pollutants concentration,and they have a good negative correlation.