Latitude gradient in aerosol properties across the Inter Tropical Convergence Zone: results from the joint Indo-US study onboard Sagar Kanya

Abstract

As part of the Indian Ocean Experiment (INDOEX) Intensive Field Phase (IFP), a cruise by ORV Sagar Kanya was conducted in the Arabian Sea and the Indian Ocean from 20 January to 12 March 1999. Measurements on aerosol properties such as optical depth, mass concentration, size distribution, scattering and absorption coefficients were measured using instruments such as sun-photometer, quartz crystal microbalance, nephelometer and particle-soot absorption photometer. One of the important findings is the large north-south asymmetry in the aerosol characteristics. Aerosol optical depth values were very high, exceeding 0.4, close to the west coast of India and the Arabian Sea, which is greater than by a factor of 4 or more, compared to the values south of the ITCZ. The wavelength exponent α, is found to be in the range of 1.3 to 1.7 in the high optical depth region and is in the range of 0.5 to 0.7 over the pristine region. Aerosol mass concentration data show that the nucleation mode aerosols (radius < 0.1 µm) are systematically higher over the Arabian Sea, with values ranging from 20 to 50 µg/m³. Correlating the aerosol mass with the scattering coefficient, we get a scattering to mass concentration ratio of 2.27 m²/g, for the Arabian Sea region, which is in between the values reported by other workers, 3.3 m²/g for the continent and 1.67 m²/g for the marine regions, elsewhere in the world. The single scattering albedo, ω derived from the scattering and absorption data, is around 0.9 for aerosols found over the Arabian Sea, while near the coastal regions the values are as low as 0.8. Low ω and high optical depth found in the coastal region and Arabian Sea indicate large absorption by aerosols. The results undoubtedly show a large spatial difference in aerosol characteristics between north and south of the ITCZ which could lead to a large asymmetry in aerosol radiative forcing between the two regions.