| Particulate matter（PM）is one of the main atmospheric pollutants in China,which directly affects the air quality and potentially threats national development and resident’s health.Meteorological conditions play an important role in the occurrence of PM pollution.Accurate identification of meteorological factors and weather conditions that are likely to cause excessive PM concentrations can help pollution prevention and treatment.Based on meteorological observation and pollution monitoring data,this article analyzes the status of PM pollution from 2015 to 2020 in the Cheng-Yu Region of China,with the help of tools such as cost733（weather classification）,Traj Stat（backward trajectory）and WRF-Chem（atmospheric chemistry model）.The impact of weather patterns and meteorological factors on regional PM pollution has been discussed,and a simulation study of a regional PM pollution process in January 2018is done.Conclusions are as follows:1)Annual average concentrations show that the overall PM pollution situation in Cheng-Yu Region were relatively severe in the early years,but improved in recent years.In 2020,the annual averages of PM10 in all cities qualify the national secondary standard,but still half of the cities exceed the standard annual value of PM2.5.The monthly average concentration of PM changes with obvious seasonality.The peak is from November to February（late autumn to early spring of the following year）and the trough is from May to September（late spring to early autumn）,which are related to seasonal changes of human activities and meteorological conditions.The monthly averaged PM2.5/PM10 value of each city presents a trend of high in winter and low in summer.This ratio is generally high and gradually rises over time,which is related to the increased human activities in recent years.The distribution ranges and median values of the concentrations of PM2.5 and PM10 in Chengdu are significantly higher than that in Chongqing during wintertime,and are quite similar in summer.Chengdu is more prone to severe PM pollution（especially in winter）.Statistical analysis of pollutants and meteorological elements shows that wind speed may be an important factor in PM pollution difference between Chengdu and Chongqing.2)The“variance method”is used to classify the synoptic weather patterns,and we get five weather types that affect the PM pollution in the Cheng-Yu Region.By defining a standard for regional PM pollution,we calculate the probability of regional PM pollution under each pattern,and subdivide the existing patterns into polluted and unpolluted days.According to statistics,the probability of regional PM pollution under each weather pattern is classified into Pattern 4,Pattern 2,Pattern 3,Pattern 1 and Pattern 5 from high to low.The seasonal distribution of Pattern 1 is concentrated in autumn and winter,while Pattern 2 and Pattern 3 are more evenly distributed.Pattern 4and Pattern 5 occur more in spring and summer and less in autumn.The pollution of the first four types is concentrated in winter.Comparing the meteorological elements of Chengdu and Chongqing under various weather patterns,it is found that wind speed in Cheng-Yu Region has a major impact on PM pollution.Detailed analysis of the weather and meteorological conditions under each pattern reveals that the foreign westerly wind play an important role in the occurrence of regional PM pollution,while the clean airflow from the southwest ocean can prevent the formation of pollution.If the 850h Pa layer（being the“atmospheric pollutant transport channel”）is mainly controlled by the high-pressure system,the lower layers are convergent and statically stable,and pollution is likely to form;if the high-pressure system is not strong enough,the vertical rise becomes active and the pollutants diffuse in the 850h Pa layer.Also the wet deposition effect of precipitation is more significant.3)The PM pollution process in January 12th-16th,2018 is selected for WRF-Chem simulation,and ground observation data is used for comparison and verification with the simulation results.The results reflect that the model overestimates temperature and wind speed,and underestimates the peak value of relative humidity obviously;the simulation of PM2.5 is more accurate when it is unpolluted,but it is easy to underestimate during severe pollution.The simulation of temperature and relative humidity is better.The simulation value of wind speed deviates most and the deviation of PM2.5 is the least.In general,the model can proper fit the real atmosphere.The pollution process is as follows:[Ⅰ]In the initial stage of pollution,the meteorological conditions are temporarily not favorable for pollution,and local pollutants gradually accumulate on the ground;[Ⅱ]The westerly wind in the area is strong,weather conditions are deteriorating,and PM pollution erupts rapidly;[Ⅲ]The low-level prevailing wind direction turns to easterly,and PM was diluted and removed after being vertically mixed,then the pollution ends.It can be seen that in the heavy PM pollution case,whether the flow field is conducive to the exchange with the outside atmosphere is the main factor that affects the pollution.When the vertical upward movement is active and there are strong winds on the lower level,the pollution is hard to occur.The particularity of the Cheng-Yu Region is embodied in the unique mountainous terrain.Due to the high mountains on the west side of the basin,westerly wind is more responsible to pollution than easterly wind.This article has comprehensively reviewed the PM pollution in Cheng-Yu Region in recent years.We introduce weather classification,backward trajectory,atmospheric chemistry model and other tools to deeply analyze the impact of weather conditions and transmission on the occurrence of particulate pollution.This work is of great value to the treatment of atmospheric PM pollution in Cheng-Yu Region,China,and provides a solid basis for the formulation and implementation of relevant policies and regulations. |
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