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Observations of some atmospheric electrical parameters in the surface layer
OBSERVATIONS OF SOME ATMOSPHERIC ELECTRICAL PARAMETERS IN THE SURFACE LAYER S. S. DHANORKAR,C. G. DESHPANDEand A. K. KAMRA Indian Institute of Tropical Met~rolo~,
Pune-411005, India
~~~c~-M~surements of atmospheric potential gradient, conductivity of both polarities and space charge are reported for Pnne (18”32’N,73”51’E,559 m above msl). Observations show negative excursions of potential gradient at night, frequent abservations 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. Key word index: Atmospheric electrical measurements, atmospheric electricity in surface layer, diurnal variations of electrical parameters, electric dimatoIogy.
Measurements of atmospheric eiectric potential gradient at ground level and conductivity of both polarities and space charge at 12 cm above the ground have been made irregularly on 2-3 fairweather days every month during 1985-1987 except for monsoon periods (June-September). Behaviour of diurnal variations of these parameters in different seasons has been studied. The objective of these measurements is to have a preliminary unde~tanding of electric climatology of the region where these and other observations are planned to be made reguhuly in the future.
APPARATUS
Atmospheric electric fields have been measured w[th a d.c. electric field-mill kept flush with the ground. For measurement of electrical conductivity of bath polarities, a Gerdien’s apparatus with two identical tubes and a common fan to suck the air through them is used. For space charge measurements a direct filtratian technique using an absolute filter with 99.97% ef%iency for 0.3 ,um particles, is used. Both apparatus for measurements of conductivity and space charge are kept with their intakes at 12 cm above ground surface Signals from all sensors are recorded, after amplification, on Esterline-Angus strip chart recorders. Air-earth conduction current is computed from observations of atmospheric electric field and conductivity. OBSERVATIONS
Figure 1 shows a record of parameters on 27 February 1986.Electrical conductivity is Iower during up mb4l
839
the day than during the night. fn the morning after sunrise conductivities of both polarities suddenly decrease and attain small values and remain low throughout the day. Potential gradient, however, increases in the morning and shows a maximum at sunrise. Potential gradient shows another maximum at about 1900-2100 LMT. However, unlike morning maximum in potential gradient, there is no decrease in conductivity associated with this maximum in potential gradient. Air-earth current, a product of conductivity and potential gradient, remains almost constant with somewhat higher values at night than during the day. Positive and negative conducti~t~es are approximately always equal and their diurnal variations are generally mirror-images of each other. Sometimes, however, conductivity of one polarity becomes much larger than the other. For example on 4 March 1986 in Fig. 2 negative conductivity at night-time is much higher than the positive one. It gives rise to negative space charge at the ground. As a consequence, the fairweather positive potential gradient is either sup pressed or sometimes it even becomes negative. Large negative excursjo~ of potential gradient have been observed during the night. This is in conformity with earlier observations (Kamra, 1982) which were made about IO km away from the present locarion. Atmospheric space charge is small, positive and fluctuating during daytime and comparatively large and negative at night. With the decrease in conductivity and increase in potential gradient at sunrise, space charge becames positive and continues to be so during the whole day. On some days, however, space charge attains large and positive values at night. An example of this is shown in Fig. 3. On such days high values of positive space charge reduce to normal small daytime values at sunrise. It is interesting to note,
840
t 4
r
-40
27
FEBRUARY
1986
400-
LOCAL
TIME
(Hrr.1
Fig. I. Diurnal variations of atmospheric space charge, p, potential gradient, F. and atmospheric conductivity of both polarities, II, at ground level on 27 February 1986. Dotted curve represents computed air-earth current, i.
4 MARCH i
E L: IA
LOCAL
TIME
1986 2
_
*
‘“E a
-2
,P
(Hrr.)
Fig. 2. Diurnal variations of atmospheric space charge, p, potential gradient, f’. and atmospheric conductivity of both polarities, L, at ground level on 4 March 1986. Dotted curve represents computed air-earth current, i.
however, that irrespective of the polarity of the space charge, fairweather potential gradient at night continues to be suppressed or even of negative polarity. It has been observed that days with large positive space charge at night are more frequent in winter when the atmosphere is more stable. Also, whenever positive space charge appears at night, its values are generally very high, often an order of magnitude higher than the daytime values.
DISCUSSION
Convection currents in the morning, carry aerosol particles up from the ground surface which, by combination, change the high mobility small ions to low
mobility large ions. Thus conductivity decreases and potential gradient increases after sunrise. (Chalmers, 1967). In the evening, however, the top of the mixed layer moves downwards and consequently the depth of this layer reduces. This will cause some decrease in the columnar resistance of the atmosphere. Consequently, air-earth current will increase if the value of potential of the equalizing layer is assumed to be constant during this period. Computed values of airearth conduction currents from our data do show an increase in magnitude of the airearth current during this period, as shown in Fig. I. Two experiments (Dhanorkar et al., personal communication, 1987; Sathe et al., personal communication, 1987) carried out earlier at the same location, to examine the effect of electric field and of the
841
Atmospheric electrical parameters in surface layer
a P
w
1 0000
I 0600 LOCAL
I
1200 TIME
8
I800
I 2400
(Hrr.)
Fig. 3. Diurnal variations of atmospheric space charge, p. potential gradient, F, and atmospheric conductivity of both polarities, 1, at ground level on 11 January 1987. Dotted curve represents computed air-earth current, i.
development of the mixed layer on the space charge concentration near to the ground surface, indicate that space charge in the lower atmosphere may be stratified in two layers (Hoppel, 1967). A lower thin layer of positive space charge due to the electrode effect (Crozier, 1963) may be topped by an upper layer of negative space charge due to radioactive substances and their emanations in the ground and lower atmosphere. Potential gradient at the ground will, therefore, be the net sum of that due to an equipotential layer and that generated by each of these space charge layers. Polarity of space charge will be determined by the layer in which the measurements are made and therefore the height above the ground at which the measurements are made. The two space charge layers will intermix with each other to the extent determined by the atmospheric stability and thus by the prevailing meteorological conditions in the lower atmosphere. Thus at night atmospheric space charge will have positive or negative values depending upon whether the apparatus, which is placed 12 cm above the
ground surface, sucks air from the lower positive space charge layer or upper negative space charge layer. With the increase in turbulence in the morning this stratification in the lower atmosphere is broken and normal small positive values of atmospheric space charge are measured. Observations indicate that a lower layer of positive space charge exists only when the atmosphere is very stable and turbulence is very small.
REFERENCES
Chalmers J. A. (1967) Atmospheric Electricity. Pergamon Press, London. Crozier W. D. (1963) Electrode effect during night-time low wind conditions. J. geophys. Res. 68, 3451-3458. Hoppel W. A. (1967) Theory of electrode effect. J. atmos. &err. Phys. 29, 709-721. Kamra A. K. (1982) Fairweather space charge distribution in the lowest 2 meters of the atmosphere. .r. geophys. Res. 87, 4257-4263.
Pune-411005, India
~~~c~-M~surements of atmospheric potential gradient, conductivity of both polarities and space charge are reported for Pnne (18”32’N,73”51’E,559 m above msl). Observations show negative excursions of potential gradient at night, frequent abservations 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. Key word index: Atmospheric electrical measurements, atmospheric electricity in surface layer, diurnal variations of electrical parameters, electric dimatoIogy.
Measurements of atmospheric eiectric potential gradient at ground level and conductivity of both polarities and space charge at 12 cm above the ground have been made irregularly on 2-3 fairweather days every month during 1985-1987 except for monsoon periods (June-September). Behaviour of diurnal variations of these parameters in different seasons has been studied. The objective of these measurements is to have a preliminary unde~tanding of electric climatology of the region where these and other observations are planned to be made reguhuly in the future.
APPARATUS
Atmospheric electric fields have been measured w[th a d.c. electric field-mill kept flush with the ground. For measurement of electrical conductivity of bath polarities, a Gerdien’s apparatus with two identical tubes and a common fan to suck the air through them is used. For space charge measurements a direct filtratian technique using an absolute filter with 99.97% ef%iency for 0.3 ,um particles, is used. Both apparatus for measurements of conductivity and space charge are kept with their intakes at 12 cm above ground surface Signals from all sensors are recorded, after amplification, on Esterline-Angus strip chart recorders. Air-earth conduction current is computed from observations of atmospheric electric field and conductivity. OBSERVATIONS
Figure 1 shows a record of parameters on 27 February 1986.Electrical conductivity is Iower during up mb4l
839
the day than during the night. fn the morning after sunrise conductivities of both polarities suddenly decrease and attain small values and remain low throughout the day. Potential gradient, however, increases in the morning and shows a maximum at sunrise. Potential gradient shows another maximum at about 1900-2100 LMT. However, unlike morning maximum in potential gradient, there is no decrease in conductivity associated with this maximum in potential gradient. Air-earth current, a product of conductivity and potential gradient, remains almost constant with somewhat higher values at night than during the day. Positive and negative conducti~t~es are approximately always equal and their diurnal variations are generally mirror-images of each other. Sometimes, however, conductivity of one polarity becomes much larger than the other. For example on 4 March 1986 in Fig. 2 negative conductivity at night-time is much higher than the positive one. It gives rise to negative space charge at the ground. As a consequence, the fairweather positive potential gradient is either sup pressed or sometimes it even becomes negative. Large negative excursjo~ of potential gradient have been observed during the night. This is in conformity with earlier observations (Kamra, 1982) which were made about IO km away from the present locarion. Atmospheric space charge is small, positive and fluctuating during daytime and comparatively large and negative at night. With the decrease in conductivity and increase in potential gradient at sunrise, space charge becames positive and continues to be so during the whole day. On some days, however, space charge attains large and positive values at night. An example of this is shown in Fig. 3. On such days high values of positive space charge reduce to normal small daytime values at sunrise. It is interesting to note,
840
t 4
r
-40
27
FEBRUARY
1986
400-
LOCAL
TIME
(Hrr.1
Fig. I. Diurnal variations of atmospheric space charge, p, potential gradient, F. and atmospheric conductivity of both polarities, II, at ground level on 27 February 1986. Dotted curve represents computed air-earth current, i.
4 MARCH i
E L: IA
LOCAL
TIME
1986 2
_
*
‘“E a
-2
,P
(Hrr.)
Fig. 2. Diurnal variations of atmospheric space charge, p, potential gradient, f’. and atmospheric conductivity of both polarities, L, at ground level on 4 March 1986. Dotted curve represents computed air-earth current, i.
however, that irrespective of the polarity of the space charge, fairweather potential gradient at night continues to be suppressed or even of negative polarity. It has been observed that days with large positive space charge at night are more frequent in winter when the atmosphere is more stable. Also, whenever positive space charge appears at night, its values are generally very high, often an order of magnitude higher than the daytime values.
DISCUSSION
Convection currents in the morning, carry aerosol particles up from the ground surface which, by combination, change the high mobility small ions to low
mobility large ions. Thus conductivity decreases and potential gradient increases after sunrise. (Chalmers, 1967). In the evening, however, the top of the mixed layer moves downwards and consequently the depth of this layer reduces. This will cause some decrease in the columnar resistance of the atmosphere. Consequently, air-earth current will increase if the value of potential of the equalizing layer is assumed to be constant during this period. Computed values of airearth conduction currents from our data do show an increase in magnitude of the airearth current during this period, as shown in Fig. I. Two experiments (Dhanorkar et al., personal communication, 1987; Sathe et al., personal communication, 1987) carried out earlier at the same location, to examine the effect of electric field and of the
841
Atmospheric electrical parameters in surface layer
a P
w
1 0000
I 0600 LOCAL
I
1200 TIME
8
I800
I 2400
(Hrr.)
Fig. 3. Diurnal variations of atmospheric space charge, p. potential gradient, F, and atmospheric conductivity of both polarities, 1, at ground level on 11 January 1987. Dotted curve represents computed air-earth current, i.
development of the mixed layer on the space charge concentration near to the ground surface, indicate that space charge in the lower atmosphere may be stratified in two layers (Hoppel, 1967). A lower thin layer of positive space charge due to the electrode effect (Crozier, 1963) may be topped by an upper layer of negative space charge due to radioactive substances and their emanations in the ground and lower atmosphere. Potential gradient at the ground will, therefore, be the net sum of that due to an equipotential layer and that generated by each of these space charge layers. Polarity of space charge will be determined by the layer in which the measurements are made and therefore the height above the ground at which the measurements are made. The two space charge layers will intermix with each other to the extent determined by the atmospheric stability and thus by the prevailing meteorological conditions in the lower atmosphere. Thus at night atmospheric space charge will have positive or negative values depending upon whether the apparatus, which is placed 12 cm above the
ground surface, sucks air from the lower positive space charge layer or upper negative space charge layer. With the increase in turbulence in the morning this stratification in the lower atmosphere is broken and normal small positive values of atmospheric space charge are measured. Observations indicate that a lower layer of positive space charge exists only when the atmosphere is very stable and turbulence is very small.
REFERENCES
Chalmers J. A. (1967) Atmospheric Electricity. Pergamon Press, London. Crozier W. D. (1963) Electrode effect during night-time low wind conditions. J. geophys. Res. 68, 3451-3458. Hoppel W. A. (1967) Theory of electrode effect. J. atmos. &err. Phys. 29, 709-721. Kamra A. K. (1982) Fairweather space charge distribution in the lowest 2 meters of the atmosphere. .r. geophys. Res. 87, 4257-4263.