Annual variations of the altitude distribution of aerosols and effect of long-range transport over the southwest Indian Peninsula

Annual variations of the altitude distribution of aerosols and the effect of long-range transport in modulating the aerosol loading over Thiruvananthapuram (8.5°N, 77°E), a relatively clean tropical station located in the southwest coast of Peninsular India, are investigated using dual polarization Micro Pulse Lidar observations carried out during March 2008–May 2011. Combined analysis of these lidar observations with the spatial distribution of aerosols derived from satellite data shows the occurrence of elevated layers of highly non-spherical aerosols in the 1.5–4 km altitude region, which are associated with the wide-spread aerosol plumes over the Arabian Sea during the pre-monsoon and summer-monsoon seasons. In contrast, ∼90% of the column integrated aerosol backscatter coefficient (βa) (below 5 km altitude) occurs below ∼1.5 km during winter. Seasonal variation of mean βa below ∼1 km altitude is <20%. Altitude profiles of βa above ∼1 km during January – characterised by the smallest values of βa, absence of elevated aerosol layers, and weak atmospheric winds – may be considered as the upper limit of the contribution by locally produced aerosols for quantifying the effect of long-range transport during the other months. Compared to January, a 3–10 fold increase in βa occurs in the 2–4 km altitude region during April–May and July–August. The elevated layers contribute ∼20–30% of the total aerosol loading during the above months.