Studies On The Influence Of Foehn On Air Pollution Over Urumqi In Winter

With the rapid development of economy in China and the rapid increase of energy consumption,it becomes worse in air pollution,especially over the cities in metropolitan areas,and more serious in development trend.The primary cause of air pollution was ascribed to excess pollutant discharge,but it has attracted much attention that the disadvantageous air diffusion condition was a major cause of pollution concentration changes,or even some certain air pollution cases.Particularly,there were few researches on the influence of special meteorological conditions such as foehn towards air pollution.Thus,using multi-site and multiple air pollution data and meteorological data?operational data,tethered-sonde data,microwave radiometer data and wind profile radar data?,with the methods of mathematical statistics and multi-source data fusion,the relationship between the air pollution and the meteorological conditions such as foehn was analyzed emphatically and the process and mechanism of the influence of special meteorological conditions such as foehn towards air pollution over Urumqi was revealed.It provided scientific basis for the promotion of the forecast warning level of the air pollution,the urban development planning and the comprehensive control of air pollution.It comes to the following major conclusions.?1?The major pollutant mean concentration such as PM_2.5,PM₁₀ and CO showed highest in winter and lowest in summer.The daily change curve of 6 kinds of pollutant concentration presented relatively coincident.The diurnal variation of main pollutant presented bimodal distribution.The peak in winter appeared at 13:00¹5:00,apparently lagging behind the east plain cities.In the spatial distribution of major pollutant,it's low in city center and high all around.In the winter it's pollution level reached 3 or above.But the main pollutant concentration was more in the north of Urumqi than in the south,and it move towards south in the day.It had a close relationship with the source of pollution,the alternation of mountain-valley wind over Junggar Basin and diurnal variation of superficial Southeast-Wind over Urumqi.?2?The frequency of superficial Southeast-Wind in the winter was quite high.The pollution concentration of 6 kinds of pollutant?except O₃?had a serious effect in accordance from high to low was in foehn days,average case and non-foehn days.It presented some features such as the Southeast-Wind with low airflow bottom,high airflow top,and thick airflow,enhancing the stability of boundary layer,restraining pollution diffusion and aggravating the pollution.It was higher in foehn days than in average case and foehn days for the diurnal variation of 6 kinds of hourly main pollutant,spatial feature and the occurrence frequency of 6th pollution level.That meant that the appearance of foehn increased the air pollution over Urumqi and resulted in the obvious increase of pollutant concentration.?3?The mixing depth maximum of atmosphere over Urumqi was lowest?441m?in winter,and the monthly change presented single-cycle,with lowest?394m?in January and highest?2731m?in June.The frequency of the mixing depth maximum lower than 1000 m reached to 93.1%.And it basically was over 2000 m during Apr to Sep.The mixing depth corresponding to foehn days and non-foehn days in winter was both low,but the mean mixing depth maximum of foehn days was 215 m lower than that of non-foehn days.All kinds of pollutant concentration such as PM2.5 had exponent relation to MMD.With the increase of MMD,the concentration would reduce gradually.The inversion layer mean depth and mean differences present single-cycle.During the critical time of building heating?Nov,Jan,Feb?,the inversion layer mean depth was relatively thick.The mean inversion strength was among 1.13¹.31?/100 m in winter,and among 0.86⁰.96?/100 m in summer.The inversion layer depth of foehn was 282 m thicker than that of non-foehn,with the differences 4.3? higher and strength 0.01?/100 m stronger,in winter in Urumqi.All kinds of pollutant concentration had exponent relation to the mean differences.With the increase of the mean differences,the concentration rose obviously.The mean wind speed's maximum over Urumqi was in spring and the minimum was in winter,so the most severe pollutant concentration appeared in winter.It became maximum in Apr and minimum in Nov,Jan,Feb in monthly change.It presented double-peak in daily change,with the first peak in the afternoon,the second peak in the forenoon,the first valley in the evening and the second valley in the morning.The mean wind speed in winter was even small,and the foehn one was 0.1m/s smaller than the non-foehn one.6 kinds of pollutant concentration had exponent relation to the mean wind speed.With the increase of the wind speed,the concentration would reduce gradually.The correlation coefficients for the pollutant concentration separately with MMD,the inversion layer depth,inversion differences,mean wind speed had got through the notability test of ?=0.05,therefore the stepwise regressive equation of meteorological factor and all kinds of pollution was established.By the application test,the forecast accuracy of all levels?except for individual items?were over 60%.Thereinto the accuracy rate of SO₂ reached 68.4% and the related coefficient between the predicted value of 6 kinds of pollution concentration with the actual value had got through the notability test of ?=0.01.Actual forecast accuracy?except for individual items?with the same level above 60%,A level difference above 80%,O₃ is highest,at 100%,It demonstrated that this prediction equation had well prediction ability,and could provide new technical support for predicting air pollution concentration over Urumqi in the winter.?4?It came into being a severe pollution process during the 15 th to 23 rd in Feb in 2013 over Urumqi,under the background of Xinjiang high-pressure ridge.During the strong phrase,the PM_2.5 daily mean pollution concentration level reached 6,while the concentration maximum of PM₁₀ and PM_2.5 respectively reached 785ug/m³ and 543ug/m³.There were four times low-layer Southeast-Wind with high speed wind and a obvious alternation of mountain-valley wind in these processes.At the meantime,the surface temperature apparently increased under the common influence of free atmosphere foehn and low-layer Southeast-Wind.It showed this foehn effect was very strong.The pressure decreasing made it more evident that it's high in the east and low in the west at the pressure level,more conductive to the development of Southeast-Wind.Southeast-Wind thickened the inversion layer depth,strengthened the inversion strength,restrained the development of the mixing layer height,prevented the pollutant diffusion,and made the pollutant gather to the low boundary layer,resulting in the severe pollution.?5?The foehn mainly made the mixing depth lower,the inversion layer depth thicker,the differences increased,the strength enhanced and the wind speed of surface layer decreasing.And then,it led to weakening the air diffusivity of boundary,degrading the spread conditions,persistently accumulating the pollutant concentration,and singularly increasing pollution concentration.So we should give great attention to the exist of foehn when we made the forecasting and warning of air pollution over Urumqi.