Optical and physical characteristics of Bay of Bengal aerosols during W-ICARB: spatial and vertical heterogeneities in the marine atmospheric boundary layer and in the vertical column

Abstract

Analysis of the continuous and collocated measurements of columnar spectral aerosol optical depths (AODs) and mass size distributions in the marine atmospheric boundary layer (MABL) over the Bay of Bengal (BoB), carried out from 27 December 2008 to 29 January 2009 during the Winter Integrated Campaign for Aerosols, Gases and Radiation Budget (W-ICARB), revealed distinct regional features in the spatial variations of the aerosol properties in the MABL and column. In general, AODs were high over the northern and northwestern parts of the BoB, with pockets of very high values, within which the AODs were as high as ∼0.8 while the smallest values (∼0.1) were observed over the northeastern BoB, off the Myanmar and Bangladesh coasts. Interestingly, though, this region had the highest Angstrom wavelength exponent α (∼1.5), notwithstanding the generally high values that prevailed over the eastern as well as northern coastal regions of India. Back trajectory analyses revealed the significant role of the advected aerosols in the observed spatial pattern. Within the MABL, high accumulation mode mass concentrations (M_A) prevailed over the entire BoB with the accumulation fraction ranging from 0.6 to 0.95, whereas very high fine-mode (r < 0.1 μm) aerosol mass fractions (∼0.8) were observed over the northeastern and western coastal BoB adjoining the Indian mainland (where α was high to very high). The vertical distributions, inferred from the columnar and MABL properties as well as from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations data, revealed better homogeneity in the northeastern and eastern BoB, whereas significant heterogeneity was seen over other regions.