Spatial heterogeneities in aerosol properties over Bay of Bengal inferred from ship-borne and MODIS observations during ICARB-W cruise campaign: implications to radiative forcing

Abstract

Comprehensive investigations during the last decade have clearly established that aerosols have a significant impact on the climate. This paper reports the results of the spatial variations in aerosol optical depth (AOD) and fine mode fraction (FMF) characteristics as a function of latitude and longitude over the Bay of Bengal (BoB) and the Northern Indian Ocean (NIO) during ICARB-W cruise period of 27th December 2008–30th January 2009 from onboard Sunphotometer and MODIS (Terra, Aqua) satellite measurements. Very high AOD₅₀₀ (0.7–0.8) occurred over the north head BoB adjacent to the northeastern Indian coast and the lowest AOD₅₀₀ (0.1–0.2) occurred in central BoB far away from the coasts, and in a small area in the northeastern part close to Myanmar coast as well as over NIO. The highest values (as high as 1.2) of Ångström exponent, α occurring over northeast BoB (regions close to Bangladesh and Myanmar) indicate relative abundance of accumulation mode particles and very low values of α (below 0.7) over central part of BoB as well as southern BoB/NIO suggesting dominance of coarse-mode sea spray aerosols. Terra/Aqua MODIS AOD₅₅₀ and cruise measured AOD₅₀₀ using Sunphotometer showed good agreement (R2 = 0.92) over the BoB. The total mass concentrations over BoB during cruise period were remarkably high, with a mean value of 28.4 ± 5.7 μg m⁻³. Aerosol FMF was higher than 0.7 over the BoB, while FMF over NIO was about 0.5. NCEP reanalysis data on winds at 850 hPa, along with 5-days airmass back trajectories via HYSPLIT model, suggested transport of continental aerosols from the central and northern India over the BoB by the strong westerly/northwesterly winds. Regionally averaged clear sky aerosol (net) forcing over BoB during the winter is −28.9 W m⁻² at the surface and −10.4 W m⁻² at the top of the atmosphere whereas, the ARF values estimated over NIO at TOA, surface and in the atmosphere are −6.4, −18.3 and +11.9 W m⁻², respectively.