Savita Dhanorkar

Diurnal and seasonal variations of the small-, intermediate-, and large-ion concentrations and their contributions to polar conductivity

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.

Measurement of mobility spectrum and concentration of all atmospheric ions with a single apparatus

It is shown that for a given mobility distribution in an air sample a single universal characteristic curve can be obtained for all Gerdien condensers. An apparatus has been developed which can measure the mobility spectrum and concentration of small, intermediate, and large atmospheric ions simultaneously in much shorter time. Sources of error and some precautions taken to minimize these errors in this apparatus are described. Some representative results showing the validity and usefulness of the apparatus are described.

Diurnal variations of the mobility spectrum of ions and size distribution of fine aerosols in the atmosphere

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.

Diurnal variation of ionization rate close to ground

Ionization rates for different times on two fair-weather days are computed using Hoppels (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 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.

Calculation of electrical conductivity from ion-aerosol balance equations

The ion-aerosol balance equations are solved to study the effect of aerosol concentration on electrical conductivity for different ionization rates taking bimodal lognormal size distribution for the grand average continental aerosol particles and using size dependent attachment and coagulation coefficients for the aerosol particles. The results show that the inverse relationship between the polar conductivity and aerosol concentration exists only up to a certain critical value of aerosol concentration which depends upon the prevailing ionization rate. Further increase in aerosol concentration causes an increase in polar conductivity due to the contribution made to it by large ions. However, this increase in conductivity is again restricted by recombination of charged aerosols in a highly polluted atmosphere. Further, it is shown that in polluted urban environments where particulate concentration is large and variable the charge equilibrium is attained faster when the effect of recombination of charged aerosols is included.

Observations of some atmospheric electrical parameters in the surface layer

Measurements of atmospheric potential gradient, conductivity of both polarities and space charge are reported for Pune (18°32′N, 73°51′E, 559 m above msl). Observations show negative excursions of potential gradient at night, frequent observations of excess values of one polarity of conductivity and the presence of very high values of space charge of either polarity at night. Results are discussed in terms of a lower thin layer of positive space charge due to the electrode effect and an upper layer of negative space charge due to radioactive elements and their emanations close to ground and mutual mixing of these two layers under different meteorological conditions.

Effect of coagulation on the particle charge distribution and air conductivity

The steady state charge distribution on submicron-size charged aerosols is calculated from the ion-aerosol balance equations including the effects of both the ion-particle charging mechanism as well as the coagulation of charged aerosols. The particle charge distribution is found to depend on the aerosol concentration and the ionization rate. The results show that for a given ionization rate the fraction of singly charged aerosols increases up to a critical value of aerosol concentration and then decreases with further increase in the aerosol concentration. However, the fractions of multiply charged aerosols show a steady increase with the aerosol concentration. The variation of the polar conductivity with aerosol concentration shows that the inverse relationship between them exists only up to a critical value of aerosol concentration which is decided by the prevailing ionization rate. Above this critical value the polar conductivity shows an increase with the aerosol concentration. Further, it is shown that though the polar conductivity is mainly due to small ions in a clean environment, the charged aerosols may dominantly contribute to the polar conductivity in a polluted atmosphere. For aerosols having larger mean radii the contribution of multiply charged aerosols becomes appreciable for higher values of aerosol concentration and is almost comparable to that of the small ions.

Atmospheric electricity measurements at Pune during the solar eclipse of 18 March 1988

Measurements of atmospheric electric potential gradient, conductivity of both polarities and space charge at the ground surface at Pune during the partial solar eclipse of 18 March 1988 have been made. In spite of no appreciable change in atmospheric temperature at the ground surface, all the atmospheric electric parameters showed remarkable changes during the period of eclipse. Results do not support any vertical transport of charge, either by conduction or by convection, near to the ground surface.

Effect of coagulation on the asymmetric charging of aerosols

The asymmetric charge distribution on submicron aerosols has been examined by solving the ion–aerosol balance equations including the effects of both the ion–aerosol attachment mechanism and the coagulation of charged aerosols. Our results for the case of asymmetric charging show a decrease in the charging asymmetry as compared to Hoppel and Fricks [Aerosol Sci. Technol. 5 (1986) 1] results where the effect of coagulation is not included. Further, the results show an excess of negative aerosols when the number of elementary charges on the aerosols p≤3 and an excess of positive aerosols when p≥4. As in the case of symmetric charging, in asymmetric charging, the particle charge distribution is found to depend on the aerosol concentration and the rate of ionization. The contribution of the coagulation process becomes significant when the aerosol concentration exceeds ∼105 particles cm−3. Moreover, the charging asymmetry increases with increasing aerosol concentrations but decreases with the increasing ionization rate.

Asymmetric charging of the aerosols including the coagulation mechanism

The charge distribution on the submicron size charged aerosols is computed from ion-aerosol balance equations including the effect of coagulation of the charged aerosols. The asymmetry in the charge distribution caused by the unequal diffusion of positive and negative ions to the aerosols and by their mutual coagulation is examined for different mean values of the aerosol radii. Our results show a decrease in the charging asymmetry for the aerosols having up to 3 elementary charges when the effect of coagulation is included.