Surface particulate concentration has often been estimated from column-integrated aerosol optical depth (AOD). Their relationship is affected by various factors, such as the planetary layer height, meteorology (atmospheric stability, wind, relative humidity, etc.), and topography, which are investigated thoroughly using a combination of ~1500 surface station datasets, two ground-based lidars, and CALIPSO space-based lidar measurements made across China. Improved estimation of PM_2.5 is achieved.