Air Pollution Characteristics Of Persistent Cold Air Pool In Lanzhou And Its Formation And Dissipation Processes

With the accelerating process of industrialization and urbanization,the problem of urban pollution has become increasingly serious,and has gradually attracted extensive attention.The air pollution problems of urban areas in China involve a wide area.It is largely different in specific areas,including complex formation causes.Lanzhou City is located in the basin of the Yellow River valley on the northeast side of the Tibetan Plateau.It is both a typical semi-arid valley city and an important industrial base in Northwestern China.It has a dense population and buildings,being one of the most polluted cities in China or even in the world once.The diffusion and transport processes of atmospheric pollutants are far more complex than ordinary plain cities due to its special valley terrain and the climatic effects that it produces.The mutual feedback between different influence factors has become more prominent in the valley basin,especially.Based on this,the air pollution characteristics and the frequency of occurrence and monthly changes of the persistent cold air pool（PCAP）in Lanzhou are counted,using the sounding data from 2005 to 2016.Then,the characteristics of the changes in synoptic circulation and meteorological factors of a typical PCAP are analyzed.Finally,the WRF-Chem model is used to explore the causes of the formation,maintenance,and dissipation of PCAP.Terrain sensitivity tests are further carried out in consideration of the effects of terrain on the air pollution.The main conclusions are as follows:（1）The air quality status in Lanzhou is dominated by good and lightly polluted,and excellent and heavily polluted are less frequent.The mass concentration of PM₁₀in Lanzhou shows a decreasing trend.The concentration of annual average SO₂ also decreases after 2008.However,the concentration of NO₂ increases year by year after2013.PM_2.5.5 and PM₁₀0 mass concentrations decrease,but the frequency of PM_2.5.5 and PM₁₀0 episode day increase significantly in 2016.The correlation coefficient between the concentration of PM_2.5.5 and PM₁₀0 reaches R²=0.56（α=0.01）.The concentration of PM_2.5.5 accounts for about 28%of PM₁₀0 in Lanzhou（47%in China）,and the proportion of fine particulate matter is very small.There are 192 PCAPs in Lanzhou,most of which occur in fall and winter from 2005 to 2016.The frequency of PCAP is the biggest in December and January.When the PCAP occur in Lanzhou,the frequency of air pollution is 63%.The proportion of all heavy pollution days related to PCAP is26%,and that of medium and light pollution is 49%and 27%,respectively.（2）The circulation fields on 850hPa are dominated by the high pressure regime during the periods of the PCAP,which brings cold air to the Lanzhou Valley Basin.The pressure gradient near the high pressure center is very small,with the weak wind or the calm near the surface.East Asia is controlled by one trough-one ridge at500hPa.And Lanzhou is located in front of the ridge,in response to a significant warming at 700hPa.The vertical configuration of upper warm and lower cold layers is conducive to intensification of the temperature inversion,which in turn facilitates the production and maintenance of the PCAP.The humidity changes little during the whole process,and the pressure gradually decreases during the continuous air pollution process.Until Air Pollution Index becomes lightly polluted,the pressure drops to the minimum and then increases.（3）The upper layers are managed by the west wind at 08 o’clock and 20 o’clock during the PCAP,and the intensity is slightly weakened.The wind speed near the surface is small,and the wind direction is uncertain.Lanzhou is covered by ground-hugging inversion from December 31,2014 to January 1,2015.The intensity of inversion increases significantly from January 2 to January 3,accompanying the increasing thickness,bringing in middle level pollution.The ground-hugging inversion is quite shallow at 20:00 on January 4.There is a strong inversion layer above 1600m and its altitude is beyond the height of the surrounding mountains.The capping effect on the lower atmosphere disappears.At the same time,the wind speed at the middle and high levels increases significantly,which contributes to the diffusion of pollutants.（4）Comparisons of the WRF-Chem simulation with surface observations and upper air soundings indicate that WRF-Chem is capable of simulating the daily variations of wind speed,temperature,and boundary layer.Simultaneously,it can simulate the daily variation of particulate matter concentration,the peak valley value,and the sharp increase and decrease in concentration.The hourly HD（heat deficit）of the valley basin has a 2h results in a later or slower change in the hourly particulate matter concentration.There is a good positive correlation between HD and PM₁₀concentrations.The larger the HD,the greater the concentration of PM₁₀,with the correlation coefficient of R²=0.50（α=0.01）.During the stage of the PCAP buildup,the upper temperature increase is caused by vertical temperature advection and non-adiabatic source.The temperature advection of upper level during the maintenance phase makes the inversion always exist,and its intensity is not weakened.The contributions from the vertical and horizontal temperature advection lead to upper warming during the breakup phase of this PCAP episode.（5）The wind speed,boundary layer height and surface temperature of the terrain sensitivity test during the day is larger than the control experiment’and has no significant change at night after reducing the terrain height.The thickness of temperature inversion decreases and the intensity weakens at 08:00 and 22:00 on January 2.The height of the boundary layer is obviously decreased by 400m than that of the control experiment at 08:00.On January 3,the intensity of inversion weakens,and the temperature near the surface decreases.After the height of the terrain is reduced,the intensity of the PCAP weakens and the duration of the PCAP decreases at the same time.After the reduction of terrain height,the Qinghai landform on the west side of Lanzhou is reduced.The southerly winds and easterly winds diffuse the pollutants that accumulate in the Valley Basin to the west and north of Lanzhou,and the pollutants accumulate in Qinghai province.The concentration of pollutants in the Lanzhou Valley Basin decreases,and the maximum decreases can be up to 36μg/m³.The south winds in the east and south sides of Lanzhou are strengthened.On one hand,it benefits the northward spread of pollutants in Lanzhou.On the other hand,the blocking effect of the mountains weakens.When a relatively strong southerly wind appears,the pollutants in the south of China can be transported more smoothly to the north,which affects the air quality in Lanzhou in turn.