Abstract
Abstract. The transport of aerosols and pollutants from continental India to the adjoining oceanic areas is a major topic of concern and several experimental campaigns have been conducted over the region focusing on aerosol characteristics and their climate implications. The present study analyzes the spectral aerosol optical depth (AOD) variations over Bay of Bengal (BoB) during Winter-Integrated Campaign for Aerosols, gases and Radiation Budget (W-ICARB) from 27 December 2008 to 30 January 2009 and investigates the influence of the adjoining landmass to the marine aerosol field. High AOD500 values (>0.7) occurred over northern BoB due to outflow of aerosols and pollutants from the densely populated Indo-Gangetic Plains (IGP); low AOD500 (0.1–0.2) was observed in central and southern BoB, far away from the mainland. The Angstrom exponent "α" was observed to be high (>1.2) near coastal waters, indicating relative abundance of accumulation-mode continental aerosols. On the other hand, over southern BoB its values dropped below ~0.7. National Center for Environmental Prediction (NCEP) reanalysis data on winds at 850 and 700 hPa, along with air-mass trajectories calculated using Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, suggested transport of continental aerosols from central and northern India over the BoB. On the other hand, when the ship was crossing the eastern BoB, the aerosol loading was strongly affected by air-masses originating from Southeast Asia, causing an increase in AOD and α. Biomass-burning episodes over the region played an important role in the observed aerosol properties. Terra/Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) AOD550 and cruise measured AOD550 showed good agreement (R2 = 0.86 and 0.77, respectively) over BoB, exhibiting similar AOD and α spatio-temporal variation.
Highlights
Tropospheric aerosols affect the radiation budget by scattering and absorbing the incoming solar radiation and by modifying the cloud albedo and droplet size distribution, thereby changing the radiative properties and lifetime of clouds and suppressing precipitation (IPCC, 2007)
The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) payload consists of three instruments: the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), an Imaging Infrared Radiometer (IIR), and a moderate spatial resolution Wide Field-of-view Camera (WFC)
Spectral aerosol optical depth (AOD) in all the clear-sky days were carried out using MICROTOPS-II sun photometer at five wavelengths
Summary
Tropospheric aerosols affect the radiation budget by scattering and absorbing the incoming solar radiation (direct effect) and by modifying the cloud albedo and droplet size distribution, thereby changing the radiative properties and lifetime of clouds (indirect effect) and suppressing precipitation (IPCC, 2007). In continuation of the previous campaign, ICARB during winter season (W-ICARB) was conducted from 27 December 2008 to 30 January 2009 over Indian mainland and BoB aiming to explore the BoB in finer detail, including the northern, eastern part and NIO, focusing on the physical and optical properties of atmospheric aerosols, trace gases and aerosol chemistry over the region. The particular configuration of the cruise track (Fig. 1) enabled measurements on the coastal waters adjoining the anthropogenically-dominated mainland and far off oceanic regions in rapid succession (Moorthy et al, 2010) This provided a nearly homogeneous spatially gridded aerosol database within a time span of about a month, during which the aerosol characteristics are considered to be statistically invariant (Moorthy et al, 2009). This was corroborated by the prevailing meteorology, which was devoid of any major synoptic weather systems, such as cyclones, depressions, or extensive cloud cover during the measuring period
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