Study On The Causes And Reduction Mechanism Of Persistent Pollution Process In Autumn And Winter In Chengduchongqing Area

The Chengdu-Chongqing area is affected by the complex terrain of the plateau-basin-hills and the rapid development of urbanization(cluster)and the large impact of large pollution emissions.The light wind,calmness,high humidity,multiple temperature inversions and reduced precipitation have caused frequent and long-term pollution in the autumn and winter areas of the Sichuan basin(November,December,January,February).This paper uses the PM_2.5 pollution concentration in the Sichuan Basin from 2014 to 2018in autumn and winter to propose a continuous regional pollution classification index,analyze the temporal and spatial distribution characteristics of persistent regional pollution and the abnormal weather situation,and maintain and remove the persistent regional pollution,and focus on the study of the high-low altitude atmospheric circulation structure,the characteristics of meteorological elements change,and the low-level atmospheric dynamic-thermal structure in the process of sustained regional pollution maintenance and weaken dissipation.The main research results are as follows:(1)The air pollution in autumn and winter in Chengdu-Chongqing area is mainly PM_2.5 pollution.In the past five years,the persistent regional pollution concentration has fluctuated and declined.Among them,the persistent regional pollution in 2014 was the most serious,followed by 2016,and the air quality in 2018 was relatively good,and the pollution concentration in the western,southern and northeastern urban agglomerations of the Chengdu-Chongqing region.(2)For the average daily PM_2.5 concentration of 17 cities in the study area reaching moderate or higher air pollution(greater than 115?g/m³),the PM_2.5 persistent regional pollution is divided into four levels according to the pollution impact range and duration.Grading indicators are:Grade I persistent regional pollution(at least 50%of cities in the area but at least 3 neighboring cities or 5 non-adjacent cities have pollution at the same time,lasting 3 days or more),Grade II persistent area pollution(50%and above cities in the area,duration 1-2 days),Grade III persistent regional pollution(50%and above cities in the area,duration 3-6 days),Grade IV persistent regional pollution(50%and above cities in the area,with a duration of 7 days and above).The pollution intensity is ranked from high to low in order of IV>III>II>I.The first-level persistent regional pollution mainly occurred in the western and southern urban agglomerations of Chengdu-Chongqing area,the second-level persistent regional pollution concentrated in the southern urban agglomeration of Chengdu-Chongqing,and the third-level and fourth-level persistent regional pollution are distributed in the whole region.(3)The mechanism of the effect of different high and low altitude circulation structures on the causes of graded persistent regional pollution shows that:500h Pa in the Chengdu-Chongqing area is affected by the southwest or westward air flow before the trough,700h Pa is affected by the change of the high ridge in the west,and the circulation structure controlled by the pressure field on the ground is conducive to the formation of Grade I persistent regional pollution;the 500h Pa circulation situation in the second-level persistent regional pollution began to change,the 700h Pa began to be dominated by the pre-slot weather type,and the ground pressure field was mainly the high-low front type;under the third-level and fourth-level persistent regional pollution,the upper altitude of500h Pa in Chengdu-Chongqing area is mostly affected by the low vortex of the Indian Peninsula and the southward trough moving eastward,and 700h Pa is frequently affected by the pre-slot weather,which enhances the transport of warm and humid advection to Chengdu-Chongqing area.(4)The meteorological conditions during the continuous period of regional pollution with different intensities are basically high humidity,small wind speed and temperature inversion,but the specific characteristics of each level are different,such as the inversion increase and increase step by step,etc.Among them,in the process of long-term and wide-ranging continuous pollution of grade III and IV,the temperature decreases,the relative humidity increases,the wind speed gradually decreases,the air pressure increases,the ground inverse temperature increases.There is a humid atmosphere in the upper air,which is close to and below the high-temperature inversion layer.Under the high-temperature inversion layer,the middle and low-level circulations in the troposphere in Chengdu and Chongqing are continuously developed and enhanced,and the scope is expanded.At the same time,winds above 700h Pa are dominated by westerly winds,while winds below 700h Pa are mostly southerly winds with weak wind speeds,which is also conducive to the maintenance of persistent regional pollution.(5)Straight westerly small wave pattern,two troughs one ridge type,north-ridge south-trough type are the main large-scale circulation situation of 500h Pa in the process of regional pollution weaken dissipation.The difference with the pollution type is that the high altitude in the Chengdu-Chongqing area is mostly behind the trough.In the process of slowly weakening pollution,the phenomenon of the Chengdu-Chongqing area still in front of or above the trough still exists,the 500h Pa is mainly dominated by the flat westerly wind and the north ridge south trough,and the 700h Pa pollution weather type still accounts for a large proportion,so the pollution removal speed is slow;during the rapid dissipating process,the 500h Pa is located behind the high-altitude trough in Chengdu-Chongqing area,the 700h Pa is controlled by the clean weather type to speed up the removal of pollutants.(6)During the pollution dissipating process,the 24h regional temperature change at700h Pa was negative,and the 24h regional negative temperature change increases during the rapid dissipating process,the effect of cold air or cold advection is enhanced.During the slow dissipating process,due to the short-term impact of the adverse weather situation at high altitude,the low-level inversion temperature,high-temperature inversion temperature,and southerly wind appear weakly,and maintain a small-scale low-level circulation,and the pollutant removal speed is slow;during the rapid dissipating process,the inversion temperature of the ground quickly disappeared,and the inversion temperature at high altitude remained stable at about 600h Pa(affected by the large terrain of the plateau),the cold air in Chengdu-Chongqing area is active and the atmosphere is humid,which is beneficial to the formation of precipitation.In both dissipating processes,the atmospheric dynamic structure is"lower level upward convergence and higher level outward divergence",and the rapid dissipating process develops stronger convergence movements,accelerating the vertical weakening and dissipation of pollutants.