Dependence of Prevailing Visibility on Relative Humidity at Tokyo International Airport

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 alpha , beta and gamma . The parameter alpha is the ratio of extinction coefficient (evaluated from visibility) to aerosol mass concentration in dry conditions. The parameter gamma is an exponent which determines the ratio alpha (1-[f] ) at relative humidity [f] . The parameter beta is the correction value for measured aerosol mass concentration. In this study, the values of the parameters alpha , beta and gamma were calculated to be 3.0 m super(2) g super(-) super(1) , 32 mu gm super(-) super(3) and -0.51, respectively. The value alpha super(-) super(1) of 0.33 gm super(-) super(2) is almost the same as that of 0.32 plus or minus 0.02 gm super(-) super(2) reported by Waggoner and Weiss (1980), even if the cutoff radius of aerosols in this study ( less than or equal to 5 mu m) 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 alpha , beta and gamma 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.