Inversion Of Spatial Changes Of PM2.5 Before And After The COVID-19 Pandemic In Southwest China

Air quality is closely related to socio-economic development,ecological environmental protection and human health.PM_2.5 is an important part of air pollution,when it exceeds a certain concentration limit will cause a series of human health problems,and at the same time aggravate air pollution,so the normalization of monitoring and analysis of atmospheric particulate matter is of great significance to the prevention and treatment of air pollution.With the rapid development of southwest China,it has brought social and economic progress,but also brought about the problem of atmospheric environmental pollution.Among them,Chengdu-Chongqing area is the fourth largest gray haze pollution area in China after the Beijing-Tianjin-Hebei urban agglomeration in the north,the Yangtze River Delta urban agglomeration in the east,and the Pearl River Delta urban agglomeration in the south,the Sichuan Basin is one of the areas with the largest density of Chinese mouth,and the Qianzhong urban agglomeration in Guizhou Province and the Yunnan urban agglomeration in Yunnan Province are important urban agglomerations in the upper reaches of the Yangtze River Economic Belt.The terrain of the southwest region is complex,the surface is fragmented,including plateaus,mountains,hills,plains,basins and other natural landscapes,during which the water network is dense and the weather is changeable,and there is significant spatial distribution heterogeneity of air pollution under this special natural condition.The COVID-19 pandemic swept the world in early 2020,and as of February 2023,more than 9 million people in China had been diagnosed with the coronavirus.In the concentrated outbreak area and local outbreak area of China’s epidemic to take strict activity control measures to curb the spread of the virus,during this period,the social economy was shut down,many factories and enterprises stopped production,the traffic flow was greatly reduced,and the emission intensity of atmospheric particulate matter was reduced.In the context of this social public health event,it can be used as a natural experimental area to explore the changes in atmospheric particulate matter.In this study,the research area of southwest China was taken as the research area,and the three years of 2019,2020 and 2021 were divided into the years before the outbreak of the new crown epidemic,the year of the beginning of the new crown epidemic and the year of normalized control after the occurrence,and the PM_2.5inversion model was constructed within each time scale in the three years.It includes weekly,seasonal and annual scales,where the weekly scale starts on January 23 and includes the first 3 weeks and 9 weeks after January 23,and the quarters are divided by spring,summer,autumn and winter.On the basis of this division,based on PM_2.5 ground station monitoring data,aerosol optical depth data（AOD）,meteorological data,normalized vegetation index and topographic data,multiple linear regression（MLR）model,geographic weighted regression（Geographic Weighted Regression,GWR）models and Geographically and Temporally Weighted Regression（GTWR）models construct datasets of PM_2.5 changes at weekly,inter-seasonal,and interannual scales.The main research contents and achievements are as follows:（1）From the perspective of model construction variables,all variables selected in the annual model meet the model construction requirements,and the variables in the seasonal model are slightly different due to the different climate characteristics of each season.From the construction results of multiple linear regression model,AOD,Relative Humidity（RH）and Surface Pressure（SP）and PM_2.5 showed a positive correlation,Boundary layer height（BLH）,temperature（TEM）,Total precipitation（TP）,Wind Speed（WS）,Vegetation Index（NDVI）and Altitude（DEM）were negatively correlated with PM_2.5.The coefficients of AOD and PM_2.5 in the model were about 0.3,indicating that the relationship between AOD and PM_2.5 was relatively stable,and the contribution of AOD to PM_2.5 was greater.（2）According to the " Fighting Covid-19 China in Action",on January 23,2020,Wuhan,China,implemented the strictest urban control measures to control the spread of the epidemic,using this event as a time node and the change of PM_2.5 concentration and the standardized socio-economic activity index（SSEI）based on nitrogen dioxide（NO2）in the first 3 weeks and 9 weeks after the control respectively,and the results showed roughly the same trend.During these 12 weeks,the PM_2.5 concentration changed significantly,and the PM_2.5 concentration was 52.59,52.38,52.18,43.09,36.32,30.99,26.73,26.20,26.99,27.72,29.00 and 30.19μg/m3,respectively,showing a trend of first declining and then steadily rising.Compared to the same periods in 2019 and 2021,the largest changes were made from the first to the fifth phase after the start of the control.From the perspective of the standardized socio-economic activity index,the stages were 0.59,0.57,0.56,0.51,0.38,0.36,0.38,0.46,0.50,0.58,0.62 and 0.57,which were higher before control,and the socio-economic activity began to weaken in the week before the Spring Festival and SSEI began to decline,and after the beginning of control,it decreased significantly,and finally the steady upward trend was stable.In2019,when there was no epidemic,the SSEI in the same period was 0.64,0.60,0.65,0.60,0.51,0.48,0.51,0.55,0.55,0.56,0.58 and 0.55,and February 4 of the same year corresponded to the Lunar New Year,and SSEI decreased significantly at this stage,but the decline was lower than the same period in 2020.（3）The seasonal distribution pattern of PM_2.5 concentration is low in summer and autumn and high in winter and spring.The winter and spring seasons of 2020 were the seasons with the strictest epidemic control,PM_2.5 concentration decreased the most,and the impact of COVID-19 weakened in the same period of 2021.The seasonal PM_2.5concentration in Chongqing City,Sichuan Province and Guizhou Province showed the characteristics of winter> spring> autumn > summer,and due to the spring planting activities in South Asian and Southeast Asian countries,atmospheric particulate matter was transmitted to the southern region of Yunnan Province through wind action,so the exogenous particulate matter pollution in Yunnan Province was more serious in this season,resulting in a significant increase in PM_2.5 concentration,and the spatial characteristics of spring> winter> autumn > summer were different from those of other three provinces and cities.（4）The average PM_2.5 concentrations in the entire southwest region were 32.08,26.52 and 28.60μg/m3 in three years,respectively,indicating that when the epidemic control was most stringent in 2020,the society stopped production and work,and the atmospheric PM_2.5 concentration was reduced.Although there have been sporadic or local epidemics after 2020,the overall epidemic has entered the post-epidemic era of normal control and control,and the degree of social impact has gradually weakened.（5）Spatially,PM_2.5 in southwest China showed obvious regional differences,highvalue areas were mainly located in the more economically developed Sichuan Basin,PM_2.5 concentration changed significantly,low-value areas were mainly located in West Sichuan and Northwest Yunnan with higher altitude and lagging economic development,PM_2.5 concentration changed little,and the degree of impact of COVID-19 was low.