Temporal And Spatial Characteristics Of Aerosol Optical Depth And The Concentration Of PM_2.5 In Guangzhou And Its Influencing Factors

An aerosol is a colloid of fine solid particles or liquid droplets,in air or another gas.The liquid or solid particles have diameter mostly smaller than 102 ?m and larger than 10-3 ?m.Fine particles with a diameter of 2.5 ?m or less are called PM_2.5.Despite the fact that the content of aerosols in the atmosphere is not much,it plays an important role in the balance of atmospheric radiation,global climate change,water cycle,crop growth and the process of ecosystem.Particulates are the deadliest form of air pollution due to their ability to penetrate deep into the lungs and blood streams unfiltered.On the other hand,there are only limited sites on the ground to monitor PM_2.5 concentration,which can not provide the information of PM_2.5 on a region in real time.Therefore,the use of remote sensing technology can help us obtain a more accurate distribution of aerosol optical depth and a wide-ranged,real-time access to PM_2.5 concentration,which is of great applicational significance.In this study,Guangzhou is taken as the research object,and MODIS V5.2 algorithm over land is used to retrieve the aerosol optical depth in the past 15 years.Based on the fact that the retrieval result is quite close to the true value,the temporal and spatial variation of Guangzhou's AOD in recent 15 years is analyzed.And the influencing factors of AOD distribution are analyzed with the methods of unary and multiple regression.Finally,the annual remote sensing estimation model of PM_2.5 concentration in Guangzhou is constructed,and the back trajectory of aerosol is acquired during typical haze days.The main conclusions are as follows:?1?In the past 15 years,the spatial distribution of AOD in Guangzhou is characterized as that the value of AOD is higher in southwest Guangzhou but lower in northeast.?2?The annual average of AOD in Guangzhou is fluctuating from 2001 to 2015,but the overall trend is increasing.There are small peaks in 2003,2007 and 2012,corresponding to AOD values of 0.4032,0.5186 and 0.6238 respectively.After 2013,the air quality of Guangzhou is gradually improved with AOD decreased year by year.The seasonal order of AOD in Guangzhou is: spring> summer> autumn> winter.?3?The trend of precipitation is opposite to that of AOD,that is,when the precipitation gets higher,the AOD value is lower.AOD has a significant negative correlation with altitude,slope and EVI.While the nighttime light intensity shows a significant positive correlation with AOD.The AOD values of the land use types in Guangzhou are in the order of: cultivated area> bare land> farmland> shrub> water> forest land.?4?The spatial distribution of PM_2.5 concentration is that decreases from west to east,but increases from north to south.This is also consistent with the spatial distribution of AOD in Guangzhou.The PM_2.5 concentration of each site in 2016 is selected as the dependent variable,and the AOD of the corresponding sites as independent variable.Then 5 kinds of estimation models are constructed.The fitting degree of the five models from high to low are ordered as univariate quadratic model> linear regression model> exponential function model> logarithmic model> power Function model.By the means of vertical correction and humidity correction,the R square value of the univariate quadratic function model is the highest and the P value is less than 0.001 at the same time.Therefore,the estimation model of PM_2.5 concentration in Guangzhou is: PM_2.5= 0.24AOD²-0.15AOD+ 0.08.And the PM_2.5 concentration in Guangzhou is predicted with RMSE less than 0.1,which proves its high stability.?5?By the analysis of the backward trajectory of the air in a severe haze in Guangzhou,it is found that the sinking of the airflow leads to the accumulation of pollutants and the rise of the airflow leads to the diffusion of pollutants.The warm and humid air is easy to cause the secondary aerosols.And it's easy to spread the pollutants with wind velocity increasing.This study verifies the feasibility of the MODIS terrestrial aerosol algorithm for the inversion of aerosol optical depth in Guangzhou.And to a certain extent,it provides a reference for the variation of atmospheric pollution in metropolitan areas under the background of urbanization in China.It can also provide some advices for relevant departments to improve air quality.In the future,we can explore the mechanism of aerosols' formation and dig out the source and transformation of fine particles,so as to further understand the source,dynamic and dissipation process of haze formation.