The Synoptic Type And Meteorological Condition During Extreme Air Pollution Events In Lanzhou City

Due to the dual effects of Pollution source emissions and Meteorological conditions for the spread of unfavorable pollutant,Regional atmospheric environmental problems Characterized by PM₁₀,PM_2.5,and O₃ have become increasingly prominent.Air pollution has seriously affected people’s health and has become a major factor constraining social and economic development.to increase the prevention and control of air pollution.The state has formulated a series of laws,regulations and rules and policy measures.Ambient air pollution problem,although a certain degree of containment,but the environmental situation is still not optimistic.Especially in the event of extreme pollution incidents,the ambient air quality has dropped significantly.The environmental quality bulletin for the first half of 2017 shows:Compared with the same period of last year,the proportion of fine days in 338 prefecture-level cities and above decreased by 2.6percentage points,and the proportion of days of heavy pollution and above increased by 0.7percentage points.In order to effectively prevent and treat heavy pollution,the State Council of the People’s Republic of China issued the“Air Pollution Prevention Action Plan”in September2013.It clearly requires provincial capitals at all levels to strengthen cooperation between environmental protection departments and meteorological departments and establish a system for monitoring and warning heavy pollution weather by the end of 2015,analyze the trend of the heavy pollution weather process.Improve the mechanism of consultation and judgment,Improve the accuracy of monitoring and early warning,Timely release of monitoring and warning information.Therefore,governments at all levels have actively formulated the“Heavy Polluted Weather Emergency Plan”to effectively deal with the hazards of heavy pollution.The accurate judgment of the heavily polluted weather process is the basis for the release of heavy pollution weather warning.Therefore,forecasting and assessing the ambient air quality through statistical analysis and model prediction are crucial for effective early warning of heavy pollution weather.In this paper,the spatial and temporal distribution of pollutant concentration in recent years in Lanzhou City was analyzed using the pollutant concentration data of Lanzhou City in2011-2016,and the effects of temperature,wind speed,relative humidity and precipitation on the distribution of pollutant concentration were discussed;The heavy pollution case with an AQI index of>200 was selected.Taking the statically stable heavy pollution process as an example,it was divided into a sudden increase type heavy pollution process and a cumulative increase type heavy pollution process;Using Lamb-Jenkinson method to Analyze the Synoptic Patterns of Heavy-polluting Processes in Two Types of Silent Steady-state;Finally,500hPa geopotential height map,700hPa geopotential height map,Sea-level atmospheric pressure map,T_lnP chart and meteorological elements of two statically stable heavy pollution processes were analyzed to study the changes in weather patterns and meteorological factors at various stages of the heavy pollution process.In order to further understand the causes of the pollution and the law of atmospheric pollutant diffusion in the region,provide theoretical references for heavy pollution warning and forecasting,provide scientific basis for air pollution control in the region and prevention and control of regional air pollution.The main conclusions of the paper are as follows:1、From the seasonal distribution of pollutant concentration,Except for O₃,the concentration of other pollutants showed a trend of“low summer and high winter”.The increase in the total discharge of pollutants and the unfavorable diffusion conditions during calm weather are the main reasons for the high concentrations of pollutants in winter.O₃ concentration is the highest in summer,which is related to the accelerated photolysis reaction rate between the precursors of O₃due to the increase of solar radiation in the summer,which is conducive to the generation of O₃.2、From the perspective of the daily variation of pollutant concentration Except O₃,other pollutants demonstrate is consistent with seasonal and diurnal variation of boundary layer parameters indicating the pollutant diffusion capacity.The peak hourly concentration of pollutants in winter lags behind other seasons by 1-2h,mainly because the winter inversion time lags behind other seasons.3、The results of pollution meteorological analysis show that temperature and wind speed are important factors affecting the concentration distribution of pollutants;Except O₃,the other pollutants were negatively correlated to air temperature and wind speed.When the relative humidity<40%,the pollutant concentration positively correlated to the relative humidity,while the pollutant mass concentration shows negatively correlated to the relative humidity when the relative humidity>80%,Rainfall shows different removal depends on the type of pollution.4、Using the Lamb-Jenkinson method for weather patterns,the results show,In 2009-2016,the frequency of occurrence in the Lanzhou region was more than 1%.There were 22 weather patterns,25 in the summer and 21 in the winter.There are 12 kinds of circulation types with>3%probability of occurrence of sudden heavy pollution,7 species in summer and 8 species in winter and half a year.The first three circulation types that have the highest frequency in the entire statistical period are E+C+C,E+UD+A,and C+UD+S;the summer and half months are E+A+W,UD+S+W,and S+E+W,C+UD+S,E+C+C,E+UD+A in the winter half year.There are 36 kinds of circulation types with a probability of>3%in the stage of accumulative heavy pollution,7species in summer and 36 species in winter.The first three circulation types with the highest frequency in the entire statistical period are UD+N+E,S+UD+S,C+W+S,and N+UD+C,A+UD+E,and W+W+W in the summer and half year respectively,UD+N+E,N+UD+N,S+UD+S in the winter half year.5、The characteristics of the evolution of high altitude circulation in the process of sudden increase and accumulative heavy pollution are consistent,Both appear as:Before heavy pollution occurs,at 500hPa altitude,Lanzhou area is controlled by the northwest or west-west airflow.At700hPa altitude,there is a warm vortex center in the Qinghai-Tibet plateau area.Lanzhou area has west-west airflow or southwest warm and humid airflow.The formation of a stable layer of high-altitude warm-humid low-altitude cold-dry began to occur,the vertical exchange of air flow was suppressed,and the concentration of pollutants began to accumulate;When heavy pollution occurs,There is a warm ridge formed at an altitude of 500hPa,a warm vortex center is enhanced at 700hPa,and the center of warm vortex is located in the Lanzhou area.Lanzhou has southwestern warm and humid air currents,the stability of the gas layer becomes stronger,and the low temperature forms a strong temperature inversion structure,which is not conducive to the diffusion of pollutants and forms a high peak of pollutant concentration;Heavy pollution subsides,the high-altitude warm center retreats to the Qinghai-Tibet Plateau and weakens or even disappears.The Lanzhou region was affected by the northwestern or western-horizontal air currents,which was transformed from the early warm and humid air control to the effect of cold and dry air.The cold air sank and the low-level pollutants were“squeezed”out.The concentration of pollutants gradually decreased.Environmental air quality is improving.6、Slight changes in the characteristics of ground circulation in the process of sudden increase and accumulative heavy pollution.The sudden change of heavy pollution process is related to the development of the cyclone system.before the outbreak of heavy pollution,Most of China is controlled by the cyclone system.Cyclones have an increasing trend;When heavy pollution broke out,the center of the cyclone is located in the Lanzhou region,where the air flow is highly convergent and pollutants gather;After the heavy pollution process broke out The cyclone system transits and the Lanzhou region is in front of Mongolia’s high pressure.The pressure gradient is large and strong winds are conducive to the diffusion of pollutants.Environmental air quality is improving.Accumulated heavy pollution before the outbreak,the high-pressure anti-cyclone system controls the northwestern region for a long period of time and forms a strong and stable weather condition,causing accumulation of pollutants;When the cyclone system appeared in the Lanzhou area,pollutants accumulated,Heavy pollution outbreak;after heavy pollution broke out Lanzhou gradually transformed into a transitional zone between cyclone and anticyclone systems.The concentration of pollutants decreased.7、The characteristics of inversion structure change in sudden increase and accumulative heavy pollution process are basically the same.Low temperature inversion layer began to appear before heavy pollution occurred;when the occurrence of low-temperature inversion intensity reaches its maximum;after the outbreak of heavy pollution,the inversion layer gradually disappeared.The characteristics of changes in surface meteorological elements during the heavy pollution process are that the change characteristics of meteorological elements in the process of sudden heavy pollution are not obvious.During the process of accumulative heavy pollution,air pressure decreases first and then increases,Temperature increases first and then decreases,Relative humidity decreases first and then increases,The characteristics of wind speed change are not obvious.