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 AOD 500 (0.7–0.8) occurred over the north head BoB adjacent to the northeastern Indian coast and the lowest AOD 500 (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 AOD 550 and cruise measured AOD 500 using Sunphotometer showed good agreement ( R 2 = 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. ► The spatial variation of columnar measurements of AOD 500 observed high values over northern/northwestern parts of the BoB in which AODs were as high as ∼0.7; while the low values of AOD 500 (<0.2) were observed in the northeastern regions of BoB (Myanmar/Bangladesh coast) and southern BoB which opens to the vast Indian Ocean. ► α showed highest values of ∼1.5, even though generally high values prevailed over the eastern as well as northern coastal regions of India and β followed the pattern similar to that of AOD with a mean value of 0.17 ± 0.09 over BoB. ► Total mass concentration showed low values over the central BoB and equatorial Indian Ocean with a mean value of 23.6 ± 4.3 μg m −3 , high values over northern BoB (mean 45.4 ± 10.2 μg m −3 ) and moderate mass concentrations with a mean value of 32.3 ± 7.1 μg m −3 , persisted along the east coast of India with higher values very near to the ports. ► FMF is found to be higher than 0.7 over BoB indicates the dominance of fine mode particles which were produced mainly due to manmade sources in the surrounding regions and got transported across the oceanic region. ► The diurnally averaged aerosol forcing over entire BoB at the surface is −28.9 W m −2 and at the TOA is −10.4 W m −2 . The difference between the TOA and the surface forcing is +18.5 W m −2 which is absorbed in the atmosphere. Similarly, 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.