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1] Measurements of the number concentration and size distribution of aerosols in the range of 0.5-20 mm diameter were made during a severe cyclonic storm in the Southern Hemisphere at 53.3°S, 52.5°E on 19 February 2004. Data were analyzed to study the aerosol number concentration-wind speed relationship. It was found that in conformity with past observations, total aerosol number concentration increases with increase in wind speed from 4 to 11 m s À1 , measured at 10 m above sea level. However, contrary to most of the earlier studies, total aerosol number concentration decreases with increase in the wind speed from 16 to 22 m s À1 and then maintains an almost constant value up to the maximum wind speed of 33.1 m s À1 . The total aerosol number concentration varies a little with wind speed in the range of 11-16 m s À1 . While increase in the total aerosol number concentration corresponding to the increase in wind speed from 4 to 11 m s À1 is spread over the whole range of particle sizes, decrease in the aerosol number concentration associated with increase in wind speed from 16 to 22 m s À1 is mainly due to decrease in the concentration of particles of <1 mm. The increase in aerosol number concentration is well recognized as being due to the enhanced bubble-breaking activity at the sea surface and increased entrainment of sea-salt particles, once produced. We propose that decrease in the aerosol number concentration may possibly occur because of the scavenging of aerosols by larger seawater drops injected into the atmosphere at high wind speeds. This scavenging process may act as a built-in sink, which becomes operative at high wind speeds and restricts the enhancement in concentration of marine aerosols.Citation: Pant, V., C. G. Deshpande, and A. K. Kamra (2008), On the aerosol number concentration -wind speed relationship during a severe cyclonic storm over south Indian Ocean,

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Measurements of the atmospheric electric conductivity made in the equatorial Indian Ocean and Arabian Sea in August and September 1991 show that the value of conductivity decreases from 2.3 x mho m-l in the equatorial Indian Ocean, where relative humidity of the surface air is 7&80%, to 1.1 x mho m-l in the region of the Somali current, where relative humidity of the surface air increases to 8&90%. The inverse relationship observed between conductivity and relative humidity, in spite of showing a large scatter, is stronger in the case of negative than positive conductivity. The sharp increase in the sizes of ions and marine aerosol particles when the relative humidity exceeds 7 5 4 0 % is proposed as the cause of the observed decrease in conductivity in the region of the Somali current.

An aerosol charge analyzer has been constructed to measure the charge distribution of NaCl particles generated in the laboratory. A radioactive electrostatic charge neutralizer utilizing Po‐210 was used to neutralize the electrostatic charge of the particles. The atomization technique was used to generate NaCl particles with diameters of 0.2 to 0.8 μm, while the evaporation and condensation method was adopted to generate particles of 0.01 to 0.2 μm in diameter. The experimental data demonstrates that the absolute average particle charge depends on the particle diameter, and is higher than that calculated by the Boltzmann charge equilibrium for particles within the range of 0.2 to 0.8 μm. The charge increases with decreasing NaCl concentration. When these particles are neutralized using the Po‐210 neutralizer, it is found that the electrostatic charge reaches the Boltzmann charge equilibrium. For 0.01 to 0.2 μm NaCl particles generated using the evaporation and condensation method, test results show that the absolute average particle charge is higher than that calculated by the Boltzmann charge equilibrium for particles larger than 0.03 to 0.05 μm in diameter, while it is lower than that predicted by the Fuchs theory [1], for particles smaller than 0.03 to 0.05 μm. However, after charge neutralization, particles with diameter above 0.05 μm reach the Boltzmann charge equilibrium condition, and the charges for particles with diameters of 0.010 to 0.05 μm, agree well with Fuchs' theory.