Aerosol characteristics in the marine atmospheric boundary layer over the Bay of Bengal and Arabian Sea during ICARB: spatial distribution and latitudinal and longitudinal gradients

Abstract

Characteristics of aerosols in the marine atmospheric boundary layer (MABL) over the Bay of Bengal (BoB), northern Indian Ocean (NIO), and Arabian Sea (AS) have been investigated using extensive measurements of the mass concentrations and mass size distributions of composite aerosols and mass concentrations of black carbon (BC) aerosols on board an oceanographic research vessel during its 2-month-long cruise of March–May 2006 as a part of Integrated Campaign for Aerosols, Gases, and Radiation Budget (ICARB). The data have been examined for the spatial distribution of aerosols over the oceans around India, for the north-south (latitudinal) and east-west (longitudinal) gradients, and to delineate the distinctiveness of these two oceanic regions with regard to aerosol properties. The results show that in general, aerosol concentrations (total and BC) are significantly higher over the BoB, compared to the AS. Over the BoB, there exists a strong latitudinal gradient with the aerosol concentrations (of total and BC) decreasing rapidly from north to south. On the other hand, over the AS the north-south gradient was very weak and showed an opposite trend; with the concentrations, particularly of BC, increasing from north to south. In the northern BoB, due north of 13°N, BC showed strong longitudinal gradient, with the concentrations increasing from west to east; whereas due south, the longitudinal gradients were weak or insignificant. Over the southern AS, BC tended to decrease westward from the western coast of India, reached a minimum at 60°E, and increased thereafter. No longitudinal gradients were noticed in northern AS. Despite these, the BC mass fraction (F_BC) was nearly constant spatially over the entire BoB, implying an almost uniformly mixed nature of BC with the composite aerosols over the BoB. In contrast, over the AS, F_BC showed an increase southward along the latitude and decrease westward along the longitude. In general, the BC mass fraction was nearly 3 times higher over the BoB compared to the AS implying thereby that the aerosols over the BoB are significantly more absorbing in nature than those over the AS.