The MODIS aerosol optical depth/thickness (AOT or AOD) and PM2.5 mass were collocated in space and time for inter comparative and quantitative analysis. To match the data sets in time, all the observations within one hour were averaged. Geographical collocation of MODIS pixels with PM2.5stations is performed by calculating the distance between the MODIS pixel andPM2.5 station using latitude and longitude information. All the MODIS pixels within a distance of 0.25 degrees (about 25 km) are averaged overPM2.5 stations.
For more details on collocation methodology refer Gupta and Christopher, 2008 (ACP).
Traditionally, simple linear regression equations were obtained between PM2.5 and satellite derived AOD. These equations are then used to calculate PM2.5 mass concentration values using satellite AOD as input variable. This accuracy of this method varies as function of local meteorological conditions, location, and season. Following figure provides an example of such relations ship in south-east United States.