Study On The Spatiotemporal Distribution Pattern And Influencing Factors Of Urban Aerosol Pollution Island In Xi'a

Urban Pollution Island（UPI）refers to the phenomenon that the concentration of atmospheric pollutants over urban areas is higher than that in suburbs and rural areas.The phenomenon of urban pollution aggregation caused by aerosols can be further defined as Urban Aerosol Pollution Island（UAPI）.Due to the complex interactions between UAPI and urban composite pollution,urban spatial morphology,and urban microclimate,there is still a lack of accurate and systematic understanding of UAPI identification,determination of UAPI intensity,spatial distribution patterns,and formation mechanisms.This paper took Xi’an City as the research area,using air quality monitoring station data,remote sensing data sets,and WRF-Chem model to study the change rule of PM_2.5 concentration and the spatiotemporal distribution characteristics of UAPI in this area.In combination with meteorological reanalysis data,population density,land classification,DEM elevation,and pollution gas data,the influencing factors of Xi’an UAPI were analyzed.The main conclusions are as follows:（1）The research based on the data of the air quality monitoring station shows that the PM_2.5concentration and UAPI intensity in Xi’an show the diurnal variation characteristics of higher at night than in the daytime,the seasonal"U"distribution of higher in winter and lower in summer,and the annual trend of decreasing year by year.The EMD decomposition results of UAPI show that IMF 2,IMF 4 and IMF 6 respectively reflect the multi-scale change characteristics of UAPI in weekly,monthly and annual cycles.The research based on site monitoring results and PM_2.5 remote sensing data sets showed that the UAPI intensity can identify the phenomenon of pollution islands in Xi’an,and the fitting degree between the UAPI results obtained from the data set and the UAPI results calculated by the site is 0.75;UAPI is characterized by weakening from the urban area as the center to the surrounding areas.The profile analysis from the northeast to southwest direction indicates that there is a significant difference in PM_2.5concentration between urban and rural areas.（2）Based on the WRF-Chem model,a typical winter pollution event in Xi’an was simulated.The simulation results of air temperature,relative humidity,wind speed,and surface air pressure had a fitting degree of 0.83,0.91,0.86,and 0.92,while the fitting degrees of PM_2.5,PM₁₀,SO₂,NO₂,O₃ and CO were 0.86,0.85,0.73,0.69,0.7 and 0.72,all within the allowable range.The UAPI in Xi’an presents a gradually decreasing spatial distribution from the urban area as the pollution center to the surrounding areas.With the transmission and diffusion of pollutants,the UAPI tends to spread to the surrounding cities,forming a regional pollution trend.In the late stage of pollution,the urban area pollution has decreased,and the UAPI has been alleviated.（3）The influencing factors of UAPI are analyzed from different perspectives.The research shows that there is a negative correlation between UAPI and boundary layer height and temperature,with the correlation being-0.40 and-0.71,respectively.In calm and stable weather,pollutants tend to accumulate and form UAPI.When the wind speed is greater than 3 m/s,it is beneficial for the transmission and diffusion of pollutants.However,when the wind speed is greater than 5 m/s in winter,due to the impact of cross-border transmission,the concentration of pollutants will rise again.Human activities are the most important influencing factor,and UAPI is positively correlated with human activities.The correlation between PM_2.5concentration and regional population density is 0.68.Urban areas with high population density tend to have higher pollution concentrations.Urban and suburban areas have different land use types,and the distribution of PM_2.5 concentration is:urban and construction land>rural and cultivated land>water and unused land>forest and grassland.The correlation between PM_2.5concentration and elevation is-0.52.The elevation in urban areas is relatively low,and the concentration of PM_2.5 is relatively high.The topographic relief hinders the horizontal movement of pollutants,which prevents their diffusion in a short time,further exacerbating UAPI.The correlation between PM_2.5 and PM₁₀,SO₂,NO₂ and CO is 0.867,0.698,0.689,and0.825,with a consistent spatial distribution.The correlation between PM_2.5 and O₃ is-0.486.SO₂,NO₂ and CO accumulated in urban areas are prone to react to generate secondary inorganic aerosols such as sulfate and nitrate.O₃ accelerates the transformation of secondary aerosols,further exacerbating the UAPI effect.