When we need to make a simplified model of the air pollution phenomena for planning use, it is necessary to choose an optimal appropriate form and dimension of the source vector in some defined sense. There are various kinds and sizes of pollution sources in an urban area. It is desirable to divide them into some groups and set them to representative pointwise sources, taking their distribution and emission intensities into consideration. This paper proposes a quantitative measure to divide sources into groups on the basis of the Monte-Carlo variance reduction method. We develop the concept of the representative pointwise source as a potential function of the air pollution which represents the actual types of pollution sources. The results are illustrated by a numerical example and the application to a real air-shed is presented. The availability of the new measure for the practical optimal location plan of monitoring stations referrred.