Dependence of Light Scattering Coefficient of Aerosols on Relative Humidity in the Atmosphere of Nagoya

Abstract

Measurements of scattering coefficient of atmospheric aerosols as a function of relative humidity and electron-microscopic examination of individual particles of 0.1 to 0.5μm radius were carried out during the period from November 1983 to August 1984 in Nagoya. When scattering coefficient ratio is defined to be the ratio of scattering coefficient at ambient relative humidity to that at relative humidity less than 30%, its dependence on relative humidity (humidogram) varied greatly, but most of humidograms are classified into two types. Type A humidogram, which shows the rapid increase in scattering coefficient ratio at relative humidity of about 75%, appeared in the situation of strong wind and ambient relative humidity lower than 80%. In this case, sea-salt particles were often dominant among particles of 0.1 to 0.5μm radius. Type B humidogram, which shows the smooth increase in scattering coefficient ratio with relative humidity, was mainly observed in the situation of relatively weak wind and it was present in a wide range of ambient humidity. Sulfate-containing particles were predominant when type B humidogram was observed.
In Nagoya, scattering coefficient of atmospheric aerosols at relative humidity lower than 80% is almost determined by the dry amount of aerosol particles. At relative humidity below 75%, the scattering coefficient ratio accompanied by type B humidogram tends to show higher value than that accompanied by type A. In the situation of relative humidity higher than 80%, only type B humidogram was observed. It can be concluded that aerosols of type B humidogram are important in causing a high scattering coefficient in the humid atmosphere of Nagoya.