Ionization rates for different times on two fair-weather days are computed using Hoppel's (1985) theoretically deduced aersol-size-dependent attachment coefficients and the aerosol-size spectra derived from our measurements of mobility spectra of atmospheric ions made at a height of 1 m above ground at Pune (18¿32' N, 73¿51' E, 559 m above mean sea level), India. The ionization rates are minimum in the afternoon and increase to their maximum values of 116.52 and 103.93 ion pairs cm-3 s-1 in the early morning. Increase in ionization rate is found to accompany with the increase in the total aerosol concentration. Our results suggest that this increase in ionization rate may be due to addition of some radioactive aerosol particles, primarily in the size range of 0.002 to 0.004 μm. ¿ American Geophysical Union 1994