Seasonal Transmission And Influence Characteristics Of Air Pollutants In Xigu Petrochemical Industrial Zone

Xigu Petrochemical Industrial Zone of Lanzhou City is one of the few industrial zones in the main urban area in China.Under the combined influence of the special complex terrain and the stable meteorological conditions in winter,the pollutants exhausted by the zone will have a non-negligible impact on the ambient air quality in the main downtown area.With the increasingly stringent environmental protection requirements at the national level,the zone has adopted a series of environmental rectification measures to improve the environmental air quality in Lanzhou City.However,the Xigu Petrochemical Industrial Zone is still one of the largest pollution discharge units in Xigu.In order to further improve the environmental air quality,reasonably solve the air pollution problem,formulate specific measures for industrial source control,effective air pollution prevention and control programs in Lanzhou City,this paper uses four control points status monitoring data in Lanzhou City to analyze the current status of atmospheric environmental quality,the annual and diurnal variation characteristics of pollutant concentration distribution from year 2015 to 2016.Use source area pollution source survey statistics to estimate source intensity parameters and establish a list of key pollution emission sources in Xigu petrochemical industrial zone.Use CALPUFF model to simulate the pollution concentration distribution of conventional pollution factors(SO₂,NOx,PM_2.5)and characteristic pollution factors（NMHC）in different seasons and analyze its contribution to the environmental impact of Lanzhou City’s downtown area.Combined with the WRF model analysis of regional weather conditions on typical days of each season,analyze the transmission and influence characteristics of atmospheric pollutants in Xigu Petrochemical Industrial Zone in different seasons.These are expected to provide technical support for the control of key air pollution sources,the formulation of heavy pollution weather environmental emergency plan,pollution prevention and control in the main urban area of Lanzhou City.The main conclusions of the paper are as follows:（1）Statistics on the four control points status monitoring data of the atmospheric environmental quality in Lanzhou City during year 2015-2016.The results shows that the main pollution period in the main urban area of Lanzhou is winter（November to March）,followed by spring（April-May）,and the least pollution days are in summer and autumn（June-October）.The most frequent primary pollutant in the main urban area of Lanzhou City is PM₁₀.The seasonal changes of the average concentration of SO₂,O₃ in each season and each control points shows obvious characteristics of“one peak,one grain”diurnal variation;NO₂,CO,PM₁₀,and PM_2.5shows obvious“double peaks,double grains”diurnal variation.（2）Through the statistical analysis of the pollution sources in the Xigu Petrochemical Industrial Zone,the parameters of the source intensity of the pollution are estimated,and a list of key pollution emission sources in the petrochemical industry in the concentrated area is established.It is determined that there are a total of 46 emission sources with annual total smoke emissions of 458.59 tons,SO₂ 1374.62 tons,NOx 3102.88 tons,and VOCs 1205.07 tons.There are 54 unorganized emission sources with annual total VOCs emissions of 3181.027 tons.（3）The CALPUFF model is used to simulate the contribution of pollutants SO₂,NOx,PM_2.5,and NMHC to the environmental impact of Lanzhou City.The simulation results shows that the average daily concentration of SO₂ is:Xigu District（4.20%）>Anning District（3.62%）>Qilihe District（3.45%）>Chengguan District（0.83%）;NOx is:Xigu District（8.62%）>Anning District（8.15%）>Qilihe District（6.80%）>Chengguan District（1.42%）;PM_2.5 is:Xigu District（4.85%）>Qilihe District（4.20%）>Anning District（4.13%）>Chengguan District（1.52%）;the average hourly concentration of NMHC is:Xigu District（111.99%）>Anning District（58.50%）>Qilihe District（26.93%）>Chengguan District（3.03%）.From this we can see that the SO₂,NOx,and PM_2.5 emissions from the Xigu Petrochemical Industrial Zone have the most significant impact on the environmental quality in Xigu District,followed by Anning District and Qilihe District,and have least impact in Chengguan District.The main reason is that due to the influence of horizontal wind field with altitude changes,SO₂,NOx,and PM_2.5 emitted from elevated point sources are affected by the high-level west air flow,and they can spread to the east of Anning District,Qilihe District,and Chengguan District.Affects the concentration of near-surface pollutants.The impact of NMHC on the environmental quality of Xigu District is most obvious,followed by the impact on Anning District,followed by Qilihe District.Compared with the contribution to Xigu District,the impact of pollutants on Chengguan District is minimal.The main reason is that low-lying sources are affected by low-level easterly air currents.Emissions of NMHC mainly transmit to the west,and they have less impact on Anning District,Qilihe District,and Chengyuan District,which are further away.（4）Selecting NOx as the representative pollution factor,using the CALPUFF model to simulate the diurnal variation process of pollutant concentration,combined with comprehensive analysis of the regional meteorological conditions analyzed by the WRF model,it can be known that the height of the mixed layer above the concentration area is low and the regional mountain circulation is prevalent at 02:00-05:00.The impact scope and degree of pollutants are relatively large;at the time of 08:00,due to the influence of solar radiation,the height of the mixed layer above the concentration area gradually increases,the conditions of transmission and diffusion of pollution become better,and the scope and degree of influence begin to change;from 11:00 to17:00,the height of the upper mixed layer is high,and the valley wind circulation is prevalent in the area,which significantly reduces the scope and degree of pollutants.At 20:00,due to the influence of solar radiation,the height of the mixed layer over the concentrated area is reduced,the regional mountain circulation is prevalent,and the pollution is transmitted and diffused.The complex conditions deteriorates again,and the scope and degree of influence increases significantly.