Abstract
We report the results of sun-photometric measurements of Aerosol Optical Thickness (AOT) in India’s Exclusive Economic Zone (EEZ) over the Arabian Sea along with synchronous Ocean Color Monitor (OCM-II) derived AOT estimates during December 12, 2009–January 10, 2010. Relatively higher values of Angstrom exponent (α) around 1.2 near coast and 0.2–0.8 in the India’s EEZ, observed during the cruise period, indicate the presence of smaller particles near the coast due to anthropogenic activities; and larger particles in the India’s EEZ due to advection of pollutants from Indian subcontinent via long-range transport. Results related to α and its derivative reveal four different aerosol types (urban-industrial, desert-dust, clean-marine, and mixed-type) with varying fraction during the study period. Surface radiative forcing due to aerosols is found to be 20 W/m2 over India’s EEZ. OCM-derived AOTs showed good corroboration with in situ measurements with a correlation coefficient of about 0.95. A reasonably good correlation was also observed between AOT and wind speed (R = 0.6); AOT and relative humidity (R = 0.58). The concurrent MODIS AOT data also agree well with those observed by the OCEANSAT (OCM-II) satellite during the campaign period.
Highlights
Aerosols are minute particles, their cumulative radiative effect on the atmosphere is tremendous [1]
Continental aerosols are mainly windblown mineral dust and carbonaceous and sulphate particles produced by forest fires, land-use, and industrial activities, while marine aerosols are mainly sea-salt particles are produced by wave-breaking, and sulphate particles produced by the oxidation of Dimethyl Sulphide (DMS), released by the phytoplankton [5]
A total of four instruments were used concurrently in the present OCM-II calibration and validation experiment. They are (i) Microtops sun-photometer that measures Aerosol Optical Thickness (AOT); (ii) Microtops ozonometer that measures precipitable water content (PWC) and total column ozone (TCO); (iii) Thermohygrometer/AWS (Automated Weather Station) which makes measurements of surface-level meteorological parameters; (iv) a short-wave (SW) pyranometer that determines downwelling solar radiation
Summary
Aerosols are minute particles, their cumulative radiative effect (both direct and indirect) on the atmosphere is tremendous [1]. OCEANSAT-II Ocean Color Monitor (OCM-II) is one such satellite providing ocean color measurements around the sea adjoining the Indian subcontinent. During the months of December through February, north-easterly winds over the Indian subcontinent carry aerosols from the land towards the oceanic atmosphere. These months, become the most ideal period to study the effect of continental aerosols and their dispersal in the oceanic atmosphere. The satellitederived AOT values have been compared with the in situ measurements, and long-range transport of air mass from the Indian subcontinent is examined with the HYSPLIT back trajectories during the cruise period
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