Climatology of columnar aerosol properties and the influence of synoptic conditions: First-time results from the northeastern region of India

Six years of spectral aerosol optical depths (AODs), from the northeastern part of India (Dibrugarh), are used to evolve a climatology for this region. The results indicate that the seasonal mean AODs at 500 nm go as high as 0.45 +/- 0.05 during premonsoon season (March to May), decrease gradually through the monsoon (June to September) to reach the lowest value of 0.19 +/- 0.06 during the retreating-monsoon season (October and November), and increase to 0.31 +/- 0.04 in winter (December to February). The AOD spectra are generally flatter than those seen typically over continental sites of India (and elsewhere in the neighboring regions) with Angstrom exponent alpha remaining below 1.0 during February through August, indicating a relatively low abundance of fine and accumulation mode aerosols. The columnar size distributions (CSD) retrieved from spectral AODs are, in general, bimodal with primary mode at similar to 0.1 mu m and secondary mode at similar to 1.0 mu m. High mass loading (similar to 309.5 +/- 65.9 mg m(-2)) and effective radius (similar to 0.40 +/- 0.09 mu m) occur during premonsoon and are attributed to significant abundance of coarse ( natural) aerosols. Cluster analysis of air mass back trajectories indicate significant transport of mineral dust from the and regions of west Asia and northwest India across the Indo-Gangetic plains and marine aerosols advected from the Bay of Bengal contributing largely to the coarse mode aerosols during this season. On the other hand, the peculiar topography combined with the local conditions and the widespread rainfall lead to a more pristine environment during retreating-monsoon season with quite low AODs and columnar loading.