Dependence of Prevailing Visibility on Relative Humidity at Tokyo International Airport.

Abstract

The relation between prevailing visibility and relative humidity measured at Tokyo International Airport was studied by taking aerosol mass concentration into consideration. The period used for the analysis is from December 1995 to March 1996, and from December 1996 to March 1997. An empirical equation which shows the relation was proposed and it has three parameters α, β and γ. The parameter α is the ratio of extinction coefficient (evaluated from visibility) to aerosol mass concentration in dry conditions. The parameter γ is an exponent which determines the ratio α (1-ƒ) γ at relative humidity ƒ. The parameter β is the correction value for measured aerosol mass concentration. In this study, the values of the parameters α, β and γ were calculated to be 3.0m2g-1, 32μgm-3 and -0.51, respectively. The value α -1 of 0.33gm-2 is almost the same as that of 0.32±0.02gm-2 reported by Waggoner and Weiss (1980), even if the cutoff radius of aerosols in this study (≤5μm) was larger than that in their measurements. The growth rate of aerosol particles with increasing relative humidity estimated in this study was equal to the value for the “normal” particles described in Kasten (1969).   The calculated visibility in another period of 15-21 February 1994 was obtained by using the parameters α, β and γ evaluated in this study, aerosol mass concentration and relative humidity measured during this period. The comparison showed that the calculated visibility coincided with observed visibility in this case.