Simultaneous measurements of the concentrations of small, intermediate and large ions and of polar conductivity of both polarities were made at a height of 1 m above ground at Pune (18° 32′N, 73° 51′E, 559 m above msl), India during February 1990 to January 1991. Diurnal and seasonal variations of concentrations of ions of all three categories show their peak values in the morning hours throughout the year. Concentrations of all categories of ions are higher during the nighttime as compared to that during the daytime and are higher in winter than in other seasons. Although small‐ion concentrations show only a small change, intermediate‐ and large‐ion concentrations undergo a change of up to 1 to 2 orders of magnitude over a period of a year. Most of the time, our observations do not exhibit any inverse relationship between the small‐ and large‐ion concentrations. The results have been explained in terms of the stability of the lower atmosphere and accumulation of radioactive gases, aerosols, etc., below nocturnal inversions. The diurnal and seasonal variations of the percentage contribution of three different categories of ions to the polar conductivity show that although contribution of small ions is dominant for most of the day, contributions of intermediate and large ions become very large in the morning hours, especially in winter. Diurnal variations of the mean values of polar conductivity calculated from the ion concentrations are compared with those of the measured values of polar conductivity. The two values show good agreement during the daytime when the polar conductivity is small. However, the measured values of polar conductivity are always higher than its calculated values during nighttime or in the morning hours when polar conductivity is large.

Simultaneous measurements of sma-almost entirely due to small ions [Israel, 1971]. 11-, intermediate-, and large-ion concentrations In the aerosol-free atmosphere, ion-ion recombialong with those of electrical conductivity, made nation causes a decrease in small-ion concentraions usually lies between 1 and 2 x 10 -4 m 2 [1964] observed that the relationship between (V s) -1, whereas that of the intermediate ions total large-ion concentration and total electriand large ions is approximately 1/10 to 1/10,000 cal conductivity differs between daytime and of this value (in terms of the orders of magni-nighttime. tude), the conductivity of atmospheric air is In this study, we have made simultaneous measurements of ion concentration of all the three categories, i.e., small, intermediate, and large,o z o z o E z -15 xIo 50-25 • = 0'921 CASE Tr i 4 8 12 16 ( N s+ N z / I00 )/m -15 5O 25 •= 0.691 -( ß ß ß ß ß ß ß ß ß

Ionization rates for different times on two fair-weather days are computed using Hoppel' s'i 1985) theoretically deduced aerosol-size-dependent attachment coefficients and the aerosol-size spectra derived from our measurements of mobility spectra of atmospheric ions made at a height of I m above grot/nd at Pune (18032 ' N, 73051 ' 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 p.m. 18,526 DHANORKAR AND KAMRA: DIURNAL VARIATION OF IONIZATION RATE

Measurements of mobility spectra of atmospheric ions with mobility between 3.37 × 10−4 and 6.91 × 10−8 m2/(V s), have been made at a height of 1 m above ground at different times of day at Pune (18° 32′ N, 73° 51′ E, 559 m above msl), India. Measurements made for a period of two days only are presented here. Observations demonstrate the presence of all three groups of ions: small, intermediate and large, each having a distinct peak in mobility at all times of day. Diurnal variations of the mobility spectrum show that ion concentrations in all the mobility ranges increase during nighttime and attain their maximum values in the early morning hours. Mobility spectra at these early morning hours suggest the possibility of the presence of ions of mobility even larger than that of 3.37 × 10−4 m2/(V s). Size distribution of atmospheric aerosol particles in the radius range of 0.0023 > r > 0.03 μm, as computed from the mobility spectra, are bimodal in shape throughout the day on either of the two days. One peak that occurs in the nuclei mode, is always observed at 0.003μm, while the position of the other peak which occurs in accumulation mode, changes between 0.01 and 0.03 μm, at different times of the day. The peak in accumulation mode occurs at larger radius when the peak in nuclei mode is higher. Diurnal variation of the concentration of particles ( 0.03 > r > 0.0063 μm) in accumulation mode exhibits single periodicity with a peak in the early morning and that of particles (0.002 < r ≤ 0.0063 μm) in the nuclei mode exhibits double periodicity with the first peak in the afternoon and the second in the early morning. The observations suggest that high concentrations of particles in the nuclei mode at Pune may be due to photochemical activity in the afternoon and to the presence of decay products of radon and the aerosol particles formed by the radiolytic process in the early morning.

It is shown that for a given mobili-acteristic curve gives the measure of this number ty distribution in an air sample a single univer-concentration [Israel, 1931].