Abstract
Abstract. Ship-borne observations of spectral aerosol optical depth (AOD) have been carried out over the entire Bay of Bengal (BoB) as part of the W-ICARB cruise campaign during the period 27 December 2008–30 January 2009. The results reveal a pronounced temporal and spatial variability in the optical characteristics of aerosols mainly due to anthropogenic emissions and their dispersion controlled by local meteorology. The highest aerosol amount, with mean AOD500>0.4, being even above 1.0 on specific days, is found close to the coastal regions in the western and northern parts of BoB. In these regions the Ångström exponent is also found to be high (~1.2–1.25) indicating transport of strong anthropogenic emissions from continental regions, while very high AOD500 (0.39±0.07) and α380–870 values (1.27±0.09) are found over the eastern BoB. Except from the large α380–870 values, an indication of strong fine-mode dominance is also observed from the AOD curvature, which is negative in the vast majority of the cases, suggesting dominance of an anthropogenic-pollution aerosol type. On the other hand, clean maritime conditions are rather rare over the region, while the aerosol types are further examined through a classification scheme based on the relationship between α and dα. It was found that even for the same α values the fine-mode dominance is larger for higher AODs showing the strong continental influence over the marine environment of BoB. Furthermore, there is also an evidence of aerosol-size growth under more turbid conditions indicative of coagulation and/or humidification over specific BoB regions. The results obtained using OPAC model show significant fraction of soot aerosols (~6 %–8 %) over the eastern and northwestern BoB, while coarse-mode sea salt particles are found to dominate in the southern parts of BoB.
Highlights
In the last decades aerosols have been recognized as a major source in determining global climatic change, since they play an important role in solar and thermal radiative transfer in the atmosphere (e.g. Wild, 2009)
The use of OPAC model for the simulation of the main aerosol properties over entire Bay of Bengal (BoB) is another unique topic of the present work, while our results were found to be in considerable agreement with optical and chemical aerosol characteristics measured on board by various scientific teams
During the W-ICARB campaign plenty of instrumentation was in operation on board oceanic research vessel (ORV) by several scientific teams focusing on investigating the aerosol optical, physical and chemical properties
Summary
In the last decades aerosols have been recognized as a major source in determining global climatic change, since they play an important role in solar and thermal radiative transfer in the atmosphere (e.g. Wild, 2009). Recent cruise campaigns (e.g. Indian Ocean Experiment, INDOEX; Arabian Sea Monsoon Experiment, ARMEX; Integration Campaign for Aerosols, gases and Radiation Budget, ICARB) focused on region-specific characterization of the aerosol properties in oceanic regions surrounding India. Results from these campaigns have shown that large amounts of wind-blown dust particles and other anthropogenically produced aerosols get transported from the Asian landmass over oceanic regions, thousands of kilometers away from their sources (e.g. Ramanathan et al, 2001b; Ganguly et al, 2005; Moorthy et al, 2005, 2008). The use of OPAC model for the simulation of the main aerosol properties over entire BoB is another unique topic of the present work, while our results were found to be in considerable agreement with optical and chemical aerosol characteristics measured on board by various scientific teams
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