Research On The Pollution Island Effect Of Beijing City Based On Multi-source Remote Sensing Data And WRF-Chem Mode

Under the influence of multiple factors such as human activities,emissions and atmospheric circulation,the concentration of air pollutants in Urban areas with high population density is higher than that in the surrounding suburbs,forming an island-like distribution pattern.We call this phenomenon UPI（Urban Pollution Island）.Traditional studies only focus on urban aerosols,but the formation mechanism and main influencing factors of aerosols are rarely explored.In this paper,Beijing was selected as the research area,PM_2.5was used as the main indicator of air pollution status,air quality monitoring station,multi-source remote sensing satellite data,and WRF-Chem model were used to study the spatio-temporal variation characteristics and formation factors of UPI in this area.The vertical and horizontal distribution characteristics of UPI in Beijing were discussed from the perspectives of annual average,season and typical pollution weather process.To some extent,it makes up for the deficiency that remote sensing methods cannot continuously obtain atmospheric information in the study area due to the influence of imaging conditions.Meteorological data and related auxiliary data were introduced to analyze the main reasons for the formation of urban pollution island effect in Beijing,and the influence of human activities,Meteorological elements and urban heat island effect on UPI was discussed.The main conclusions are as follows:（1）The monitoring results of air pollution in Beijing based on multi-source data show that the phenomenon of urban pollution island is widespread in Beijing,and the seasonal differentiation is obvious.In spring and autumn,Beijing is greatly affected by external dust,so the study was conducted only in summer and winter.Winter is the most significant season of pollution island phenomenon in a year,and the pollution island index is at a high level in this season,with a daily peak of 20μg/m³.The pollution was light in summer,and the peak time was only 10μg/m³.In addition to seasonal variability,the diurnal variation was also significant.In summer,the pollution island index gradually increased at 0:00,and reached the peak at 12:00 and 09:00,then gradually decreased,and reached the diurnal trough at 16:00,and then began to rise steadily.In winter,the pollution island index gradually decreased from0:00 to 09:00 and reached a temporary trough value,followed by a small fluctuation at 18:00,gradually increased after 18:00 and reached the one-day peak at 20:00.（2）WRF-Chem model was used to simulate a typical pollution process in winter,and the simulated meteorological elements and PM_2.5concentration were verified.The results were within a reasonable range and the effect was good.UPI was found in different periods of pollution.At the same time,the vertical distribution of pollutants in different periods was influenced by vertical convection and boundary layer.（3）Based on the spatial distribution of UPI,relevant auxiliary data were introduced to explore its formation factors,and it was found that UPI was affected by wind speed,boundary layer height,temperature,underlying surface characteristics and human activities.The boundary layer height and regional mean temperature are negatively correlated with UPI,while human activities and population density are positively correlated.At the same time,terrain also has a certain influence on the formation of UPI.Undulating terrain can block horizontal turbulent movement,and gravity makes it impossible for pollutants to be transmitted to higher altitudes.The correlation analysis of UPI and UHI shows that the positive correlation between heat island and pollution island occurs in the small scale within 2h in summer and winter,and the heat island is ahead of the pollution island.When the time scale is greater than 2h,UPI and UHI show a negative correlation.In summer,heat island lags behind pollution island,while in winter,heat island leads pollution island.