Characteristics Of Air Pollution In Xi'an And Reasons For Heavy Polluted Weather

In recent years,affected by climate terrain and other factors,the situation of Xi'an air pollution deteriorated.Xi'an is the one of the top ten cities have relatively poorly air quality in national in 2016.In order to explore the spatial and temporal distribution of pollutants in combination with local characteristics,study the relationship between pollutants and different meteorological factors and the formation mechanism of heavy polluted weather,provide effective decision-making basis for the government to formulate environmental control measures and planning.In this paper,selected atmospheric environmental quality monitoring data and meteorological data of Xi'an from 2014 to 2016.Combined with EXCEL SPASS and DPS statistical analysis software.Used the air quality index calculation method and correlation analysis method to analyze the temporal and spatial characteristics of Xi'an historical haze weather,correlation between the main pollutant concentration and the meteorological condition.Pollution sources in Xi'an were analyzed.The climate statistics and meteorological conditions were analyzed in 2016,and a typical polluted weather process in 2016 was carried out.The paper analyzes the influence of geographical location,meteorological conditions,pollution sources and other factors on the air pollution in Xi'an,and finally gives the relevant suggestions.The main conclusions are as follows:?1?Xi'an is haze hail high incidence area,especially the northeast and central.Particle increase trend to the northwest was concave type.The city have the largest number haze of times from 1970 to 2016.In the past 16 years,the haze center is Lantian-Chang'an area.2000 is the annual change in the haze,first the drop after the rise.High temperature,less precipitation and more sunshine in 2016.There were haze 170 days 543 stations,1-3 months,11-12 months more.Less precipitation and small wind speed is the main weather cause of much haze and lower air quality.?2?The air pollution in Xi'an was serious from 2014 to 2016.Mild and above pollution 531days.In addition to O₃,the other five kinds of pollutants were similar to the seasonal variation,the daily variation curve of each season was consistent,the pollutant accumulation period in time from 19 to 07.Five kinds of pollutants and wind speed was negatively correlated.The highest correlation coefficient in winter.The highest correlation coefficient between NO₂ and wind speed.The maximum wind speed in the range of 2⁵ m/s has the greatest effect on AQI.The pollutant is easy to gather when the SSE wind is taken.The opposite of E,WSW,SE,W and SW.When the temperature is higher than 0?,the higher the PM₁₀,PM_2.5,NO2 and CO concentrations.The pollutant delay to the lowest value of two hours at 4:00 pm,then rise.Show a good negative correlation in 20 to 21 o'clock.The concentration of pollutants rose to a certain until noon.The concentration of pollutants is negatively correlated with the precipitation.The precipitation can significantly reduce the concentration of near-low pollutants in Xi'an,which can remove about20%fine particles and 70%coarse particles in near-surface atmosphere.Medium rain can remove the atmosphere about 35%to 50%of the aerosol content.As long as there will be precipitation on the large particles of dust from the role,and small particles need to reach a certain level of precipitation to play a role.The effluent from the total contaminant concentration is only greater than 10 mm of rainfall.About 1 day after the precipitation can be recovered to the average level of pollution.But the impact of precipitation on O₃ is not significant.In the 20³0mm precipitation conditions,CO concentration increased significantly.Several pollutants were negatively correlated with the monthly variation trend of air pressure,and the concentration decreased with the increase of the pressure field,but it was not controlled by the weak gas pressure field.Humidity is not the primary meteorological element that determines pollutants.?3?Xi'an pollution sources will be transported by the surrounding areas.Air pollution in Guanzhong city group showed obvious U-shaped characteristics within one year.In the past few years,there is no AQI to achieve excellent month,Baoji and Tongchuan AQI index is relatively low,the highest AQI index is Xianyang.Winter pollution is serious in 2014 and 2017,the best air quality in 2015.Most of the time are pollution of the weather on December 2013 and December2016.In the more serious pollution of the Baoji,Weinan and Xi'an,the number of days without pollution days only 2-4 days.Ozone in the year showed a significant N-type characteristics,PM_2.5for the U-shaped features.Baoji and Tongchuan PM_2.5 AQI index is relatively low,Xi'an,Xianyang,Weinan PM_2.5 pollution is more serious.Using the air quality forecast system of Xi'an to analyze the source of pollutants and numerical simulation:the general pollutants will arrive in Xi'an within 6 hours.The marshland contaminants come from Xianyang in the spring,the second in Xi'an.Gaoling from Xi'an up to.From Xianyang,Weinan accounted for 19%.9%from the province.Kangling PM_2.5 main source for Weinan,followed by Xi'an.Half of the pollutants from the Xi'an city of Zhouzhi in the winter.81%of the pollutants from the city of Xi'an in Xiaozhai.Evident,the main pollutants in winter are generated locally from the city.?4?The hourly change was inversely proportional of PM_2.5,PM₁₀ and the visibility in a continuous 15 days of moderate pollution above the process of Xi'an on December 12-21 in 2016.When the primary pollutant concentration is high,the visibility of less than 2 km.The circulation situation are difference between heavy pollution and air turn after good.Before the serious pollution,0.5km below the ground inversion accounted for the vast majority,and the inversion intensity is not,during the serious pollution,in addition to 0.5km below the ground inversion,inversion stratification at several levels have emerged.From the ground up to 3.5km height vertical direction"reverse wet"layer distribution characteristics are not obvious.0.5km below and2.0km above the height of the significant wet layer,ground and low altitude average relative humidity daily 08:00 greater than 20:00.The average relative humidity of the ground and the lower level is more than 65%and 40%respectively.The near surface wind field is mainly controlled by the northeast airflow.The contaminants in the adjacent heavy contaminated areas of Shandong and Hebei are continuously transported to the Guanzhong area.The thickness of the mixed layer is small,the thickness of the mixed layer is generally increased at other times,and the minimum is more than 0.2km and the maximum is more than 1.2km.Aerosols are mainly distributed below 0.5km.Mainly wet haze when serious pollution.Xi'an air quality forecasting system is a good simulation of the process of polluting weather from December 7 to 22 in 2016,and the temporal and spatial evolution of PM_2.5 in Xi'an surrounding area.The results of single station prediction are consistent with the overall trend of the actual situation,non-serious pollution period,the forecast is close to the actual concentration,the serious pollution period,the predicted peak value is significantly lower than the actual situation.For winter heavy weather,the XaWRF-CMAQ mode needs to increase the maximum threshold setting for the predicted product.