Spectral aerosol optical characteristics during the northeast monsoon over the Arabian Sea and the tropical Indian Ocean: 2. Ångströ m parameters and anthropogenic influence

Abstract

Ångström parameters are estimated from the spectral aerosol optical depths (AODs) measured over the Arabian Sea (AS) and tropical Indian Ocean (TIO) during the northeast winter monsoon seasons of 1996-2000, and their yearly and monthly mean variabilities are investigated. The 5-year mean Ångström wavelength exponent α is found to be 1.5 over coastal India (CI), AS, and TIO. The 5-year mean β, turbidity parameter, is found to be about 0.14 over CI, 0.10 over AS, and about 0.05 over TIO indicating the less polluted nature of TIO. The α and β values display year-to-year and monthly mean variations. The 5-year mean α and β values over two land locations (Trivandrum (TVM), Vishakapatnam (VSK)) in peninsular India and marine locations (Minicoy (MCP), Kaashidhoo (KCO)) are found to be around 1 and 0.2. An examination of the daily mean wind speeds and the 0.5 μm AODs obtained during 1996-2000 yielded an index of wind dependence 0.05 for AS and 0.11 over TIO. The scaling distance, the distance up to which the effects of continental aerosols are noticeable over the oceanic regions, is estimated to be 1500 km over the Indian Ocean region. The 5-year mean anthropogenic contribution to the 0.5 μ m AODs over CI, AS, and TIO estimated by comparing with the wind speed dependent sea salt aerosol is found to be above 90% over CI and AS, while it is 74% over TIO. The 5-year mean anthropogenic contribution over TVM and VSK is in excess of 80%. Over MCY/KCO the anthropogenic contribution is lower at 70%. Copyright 2004 by the American Geophysical Union.