Spatio-temporal aerosol optical characteristics over the Arabian Sea during the pre monsoon season
1Atmospheric Research Team, Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Lofos Nymphon, P.O. Box 20048, 11810 Athens, Greece
2Physical Meteorology and Aerology Div., Indian Institute of Tropical Meteorology, Pahan, Pune 411008, India
3Department of Geology and Geoenvironment, University of Athens, University campus 15784 Athens, Greece
4Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, India
Abstract. Ship-borne Aerosol Optical Depth (AOD) measurements obtained by a sunphotometer have been used to retrieve the Ångström wavelength exponent (α). These measurements were obtained in the Arabian Sea (AS), where the surrounded arid-region influence is expected, during the pre-monsoon season. Spectral variation of α is typically considered in this study. It is found that the polynomial fit becomes more accurate in the wavelength band 340–1020 nm rather than the 340–870 nm. The coarse-mode (positive curvature in the ln τα vs ln λ) aerosols are mainly depicted in the Northern part of the AS closely associated with the nearby arid areas and fine-mode aerosols are mainly observed over the far and coastal AS regions. In the study period the mean AOD at 500 nm is 0.246±0.114 and the α340−1020 is 0.904±0.186. The α340−870 exhibits similar values (0.924±0.179), while significant differences revealed for the constant terms of the polynomial fit (a1 and (a2) proportionally to the wavelength band used for their determination. Observed day-to-day variability in the aerosol load and optical properties are direct consequence of the local winds and air-mass trajectories along with the position of the ship.