A. K. Kamra

The atmospheric global electric circuit: An overview

Research work in the area of the Global Electric Circuit (GEC) has rapidly expanded in recent years mainly through observations of lightning from satellites and ground-based networks and observations of optical emissions between cloud and the ionosphere. After reviewing this progress, we critically examine the role of various generators of the currents flowing in the lower and upper atmosphere and supplying currents to the GEC. The role of aerosols and cosmic rays in controlling the GEC and linkage between climate, solar-terrestrial relationships and the GEC has been briefly discussed. Some unsolved problems in this area are reported for future investigations.

Radon and its short-lived progeny: variations near the ground

Abstract In the atmosphere above the surface of the Earth the concentrations of radon and its progeny are measured, along with the meteorological parameters from December 1997 to December 2000 for a continental location, Pune (18°N,74°E), India. The concentrations show maxima in the early morning hours when the turbulence mixing is minimum; whereas in the afternoon the turbulence mixing is maximum and concentrations exhibit minima. The median values of concentration are 9.70 and 2.84 Bq m −3 , respectively, for radon and its progeny, during the observation period. Ionization rates in the atmosphere are derived for the same period. It is found that the ionization rate exhibits a median value of 5.48 ion pairs cm −3 s −1 . The diurnal and seasonal variations in the concentrations of radon and its progeny, and the ionization rate due to radioactivity are found to exhibit correlation with the relative humidity, and anti-correlation with the temperature.

Background aerosol concentration derived from the atmospheric electric conductivity measurements made over the Indian Ocean during INDOEX

Measurements of the atmospheric electric conductivity on board ORV Sagarkanya during her three cruises over the Indian Ocean (17°N to 20°S, 57°E to 79°E) during the periods of December to March 1996–1997, 1998, and 1999 are reported. The results show that the values of atmospheric conductivity over the southern hemisphere are 2 to 3 times ofthat over the northern Indian Ocean and the north-to-south gradients extend up to the Intertropical Convergence Zone (ITCZ) and have large interseasonal and intraseasonal variations. The values of electric conductivity have been used to calculate the aerosol concentrations. The latitudinal variations of the aerosol concentration have been observed to have positive gradients from the Indian coastline to the ITCZ, and the gradients are different during the three cruises. The aerosol concentrations attain their pristine level only at 15°–20°S in this season. Because of the large interseasonal variability of the aerosol concentration observed over the northern Indian Ocean, it is concluded that estimating any secular change in the background aerosol pollution may be a futile exercise in this area.

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.

Collection efficiencies of large water drops collecting aerosol particles of various densities

Abstract A laboratory experiment to study the effect of the density of aerosol material on collection efficiency of water drops, is conducted with MgS0 4 and MnCl 2 particles. When compared with the earlier results for NaCl and other particles, our observations show that the collection efficiency of water drops increases wish increase in impaction parameter, irrespective of whether the change in impaction parameter is caused by the change in the density of particles or the size of particles/water drop. Our results are correlated through the aerodynamical diameter parameter and with earlier experimental results of other investigators for particles of different densities. The agreement between our experimental results and the theory of Slinn (1983, Precipitation Scavenging in Atmospheric Sciences and Power Production , Chap. 11. Division of Biomedical Environmental Research, U.S. Department of Energy) increases with the increase in the density or size of the particles. Moreover, our experimental results confirm the prediction of Slinns theory that inertial impaction mechanism is dominant for smaller particles if their density is higher.

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.

Spontaneous breakup of charged and uncharged water drops freely suspended in a wind tunnel

Time for which charged or uncharged water drops of different sizes can be suspended over a vertical wind tunnel before their spontaneous breakup and the size distribution of droplets resulting from their breakup have been determined. Probability of spontaneous breakup of a drop has been found to increase with the size and charge of the drop. It has been observed that water drops carrying a charge of 5×10−10 C breakup immediately after their suspension if their diameter > 8 mm. Total number of droplets produced on spontaneous breakup of a drop increases with the size of the drop, and if the drop size is > 6.6 mm, the total number of droplets is more when the drop is uncharged than that when it is charged. However, the number of droplets larger than a critical size is more if the parent drop is charged and the number of droplets smaller than that critical size is more if the parent drop is uncharged. It has been attempted to qualitatively explain the experimental results as the result of enhanced surface charge density around the waist of the drop during its oscillation. Charge on the drop has been suggested to cause an increase in width of the base of suspended drop.

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.

Scavenging of aerosol particles by large water drops: 1. Neutral case

A laboratory experiment to measure the collection efficiency of millimeter-sized neutral water drops collecting micron-sized aerosol particles has been conducted. In agreement with the previous studies, the results show that the collection efficiency increases with the decrease in drop size and/or increase in particle size. Further, the collection efficiency is observed to increase with impaction parameter and decrease with the drop Reynolds number. The limited and sparse data obtained in our experiment suggets that the collection efficiency of large drops may attain an upper limit which may be less than unity as the particle diameter approaches 10 μm. Moreover, the maximum collection efficiency attained by such large drops seems to decrease with the increasing drop size. Shape oscillations and the eddy shedding capability of large water drops have been proposed to explain the observed results.