Simultaneous measurements of small-, intermediate-, and large-ion concentrations along with those of electrical conductivity, made at Pune (18¿ 32'N, 73¿ 51'E, 559 m above mean sea level), India, show that their diurnal variations have maxima in the morning. However, the difference in times of occurrence of the maxima of the polar conductivity and various ion concentrations indicates that all ions of different categories may contribute to polar conductivity in varying degrees. To assess the degree of contribution of individual ion category to the polar conductivity, the data are divided into four different cases. A good linear correlation is observed between electrical conductivity and small-ion concentration at nighttime in case I when the intermediate- and large-ion concentrations are not very large. However, in other cases when intermediate- and/or large-ion concentrations are more than 2 and 4 orders of magnitude larger than small ion concentration, respectively, the correlation between electrical conductivity and small-ion concentration decreases.

A marked improvement in this correlation is observed when the number of small ions, equivalent to the intermediate-ion and/or large-ion concentration which gives the same value of electrical conductivity, is added to the small-ion concentration. The number of small ions equivalent to intermediate-ion (large-ion) concentration in different cases is calculated by dividing the intermediate-ion (large-ion) concentration by the ratio of the small- to intermediate-ion (large-ion mobilities which is approximately equal to 102 (104). Observations show that at certain times of the day intermediate and/or large ions can contribute significantly to electrical conductivity of the air. Therefore at such times, the small-ion concentration may not be a good indicator of conductivity. The results imply that the contributions of intermediate and large ions can be significant and must be taken into account to get the true conductivity of the air in polluted regions, if one is using ion concentration measurements to estimate the conductivity. ¿ American Geophysical Union 1992