On the aerosol number concentration-wind speed relationship during a severe cyclonic storm over south Indian Ocean

Abstract

Measurements of the number concentration and size distribution of aerosols in the range of 0.5-20 μm 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^-1, 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^-1 and then maintains an almost constant value up to the maximum wind speed of 33.1 m s^-1. The total aerosol number concentration varies a little with wind speed in the range of 11-16 m s^-1. While increase in the total aerosol number concentration corresponding to the increase in wind speed from 4 to 11 m s^-1 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^-1 is mainly due to decrease in the concentration of particles of <1 μm. 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.