Climatology of columnar aerosol properties at a continental location in the upper Brahmaputra basin of north east India: Diurnal asymmetry and association with meteorology

The spectro-temporal variation of aerosol optical depth (AOD) and associated physico-optical properties are studied for the period October 2001-November 2010 over a continental location, Dibrugarh (27.3°N, 94.6°E, 111 m amsl) located in the upper Brahmaputra basin of north-east India. The emphasis is on the climatological diurnal asymmetry of AOD and its association with meteorological parameters. AOD is found to be higher during forenoon (FN) hours compared to those in the afternoon (AN) hours in almost all seasons. The mean difference between FN and AN AOD averaged for the period 2001-2010 is 0.18. This variability is found primarily to be driven by the prevailing meteorological conditions including columnar water vapour content. It may also be attributed to the change in the ray path in the forenoon through the polluted industrialised areas located in the east and north-east of Dibrugarh to the cleaner mountain region and river Brahmaputra in the afternoon hours. The estimated CSDs are mostly bimodal in the FN hours while in the AN power law and unimodal character prevails. This indicates dominance of coarse mode aerosols in the forenoon as compared to that in the afternoon. The differences in aerosol modes between FN and AN hours result in the diurnal asymmetry of the modified Ångström coefficients. AOD retrieved from MODIS satellites is also higher in the FN by 0.08 as compared to that in the AN The climatological mean difference between MODIS Terra and Aqua AOD is however, less than the mean difference observed between AOD measured from ground.