Research On The Temporal And Spatial Distribution Characteristics Of Atmospheric Pollutants And Typical Heavy Pollution Processes In The Sichuan Basin

During the process of economic and social development of Sichuan Basin,its air pollution problem has been attracting much attention due to the special terrain conditions.Along with the execution of preventive and control measures in recent years,the concentration of particulate matter has declined in the basin,whereas its annual average concentration was still higher than the national standard.Besides,rising trends maintained in the annual mean concentration of O₃ and NO₂.It indicated that the prevention and control works in individual cities or some local areas has limited impact on a further reduction of air pollution.Thus,it is necessary to carry out the joint preventive and control works of regional air pollution from the basin scale.In order to maximize the economic benefits and long-term effectiveness of work in air pollution control,varied regions should be focused in different periods.Therefore,we abstracted the monitoring data of atmospheric pollutants in Sichuan Basin from 2015 to 2018 in this study.Firstly,both the temporal and spatial distribution of pollutants in different time scales?year,season,month,and day?were analyzed?chapter 3?.Secondly,for an objective explanation of the actual pollution,according to the Sichuan Heavy Pollution Emergency Plan?revised in 2018?,the regional air pollution processes in the study period in Sichuan Basin was counted in Chapter 4.Thirdly,in Chapter 5,a regional air pollution process was selected,which with the longest period and the highest concentration,and its development and evolution,as well as configuration with the meteorological field were analyzed.At last,in Chapter 6,the WRF-Chem model was used to simulate the air pollution process numerically,and the simulation results were verified.The main conclusions are listed as follows:?1?During the study period,the Sichuan Basin was mainly polluted by particulate matter and O₃,and the NO₂ pollution was relatively light.Particularly,fine particulate matters led to serious pollution generally,and Zigong City was the most polluted city.In the past four years,the annual average of PM₁₀,PM_2.5,CO and SO₂ pollutants dropped significantly,but particulate matters were still higher than national standards.In contrast,the annual mean concentrations of NO₂ and O₃increased.Within a year,the pollution in winter?especially in January?was the most serious.In addition,the high value centers of daily pollutant concentration were mainly located in Chengdu,Zigong,Dazhou and Chongqing.?2?There were 21 regional air pollution processes in the Sichuan Basin from2015 to 2018,within which 19 times occurred in winter and were caused by fine particulate matters,the rest 2 processes occurred with complex pollution characteristics.The spatial distribution of AQI grades of these pollution processes mainly included three types:the Western Type,the Comprehensive Type and the U Type.Besides,Chengdu,Zigong and Dazhou were the heavy polluted centers.?3?The air pollution process from December 19,2017 to January 3,2018 was analyzed in detail.It was revealed that during December 19 to 28,2017,the Sichuan Basin was affected by the weak westerly at 500hPa,as well as a heavily-polluted weather type at 700hPa.At the same time,the temperature and relative humidity of the lower layer?850hPa?rose steadily.Under the combined effect of local pollution source and meteorological conditions,the concentrations of PM₁₀,PM_2.5.5 and CO increased steadily,while the proportion of coarse particles remained a low value.During this period,the air pollution had significant static-stable characteristics with slow change.On December 29,2017,affected by both the cold air activity and northern dust transport,concentration of PM₁₀0 increased much,whereas PM_2.5.5 and CO decreased significantly.Which corresponded to the rapid-changing characteristics of dust pollutions.The dust weathers took place at Guangyuan,Mianyang and Deyang.Moreover,results of particle mass ion mass spectrometry analysis in Chengdu also showed obvious characteristics of dust weather.Due to impact of sand and dust transportation,as well as meteorological field,the concentration of particulate matters varied obviously in different areas of the basin.?4?By evaluating the simulation results of WRF-Chem model,the simulated mean temperature at 2m and wind at 10m were larger than the measured values.And the simulated mean particle concentration were lower than the measured ones.High correlation?82.62%?of temperature was revealed,while lower correlation of wind speed was achieved.In addition,the correlation between PM₁₀0 and PM_2.5.5 before the dust event was about 42%.During the entire period,dust transport changed the source of coarse particulate matter,led to a significant decrease of the correlation of PM₁₀?to 7.65%?,whereas the removal of fine particles by the cold air increased the correlation of PM_2.5.5 to 55.59%.Before the dust event,the correlations in the northwestern part of the basin were the lowest,including Chengdu,Deyang,Mianyang and Ziyang,etc.and The simulation of particulate matter in Ya'an City was the best,whose correlation of PM₁₀0 and PM_2.5.5 reached 79.68%and 77.62%,respectively.In conclusion,the above results revealed the temporal and spatial distribution of air pollution in the Sichuan Basin,as well as summarized the heavy polluted areas and periods in the study region.Not only the scientific basis for the joint preventive and control work of regional air pollution in the Sichuan Basin was provided,but also some valuable scientific information was achieved for further research.