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The effect of the Harmattan dust on atmospheric electric parameters
.roumal of Atmospheric andTerrestrial Physics, 1971, Vol.33,pp.295-SOO.Pergamon Press.Printed in Northern Ireland
SHORT PAPER
The effect of the Harmattan dust on atmospheric electric parameters Department
A. I. I. ETTE of Physics, University of Ibadan, Nigeria, W. Africa (Received 11 June 1970)
Abstract-The vertical air-earth current and the atmospheric potential gradient are known to undergo simultaneous reversals at the beginning and end of the ‘sustausch’ regime during the Harm&tan season. These reversals are attributed to the production of large negative charges through dust dispersal in the dry Harmattan atmosphere by convection and wind, both of which attain maximum strength around noon. 1. INTRODUCTION BETWEEN the months of November and March most of West Africa is under the influence of the dry E.N.E. Harmattan winds which blow southwards from the Sahara desert-filling the air with large concentrations of fine dust particles which considerably reduce visibility. Over the coastal areas the prevailing winds are the moist S.W. monsoon which are actually a continuation of the S.E. trade winds of the South Atlantic. These winds meet the E.N.E. winds along the inter-tropical discontinuity (I.T.D.) which marks the southern boundary of the Harmattan climatic zone characterized by clear skies, high temperatures and low relative humidities by day, large diurnal temperature and humidity ranges; and by the appearance of early morning haze. Within this zone simultaneous reversals of the fair-weather values of the vertical air-earth current and atmospheric potential gradient are common around noon-negative values sometimes approaching those in severe storms often being registered farther north as shown in Fig. 1 obtained by HARRIS (1969) at Zaria (lat. ll”7’N). At Ibadan (lat. 7’24’N) where climatic conditions during the Harmattan season are generally milder, the negative excursions are smaller and occupy a shorter time. Invariably onset of reversal is abrupt; a complete reversal from the normal fair-weather value often being accomplished within about 10 min. No completely satisfactory explanation of these reversals have so far been offered. In this communication the problem is re-examined, and a possible mechanism responsible for the sustained reversals proposed.
2. ELECTRIFICATION OF DUST CLOUDS Intense electrification of dust raised in sandstorms and ‘dust devils’ have been reported in the literature (RUDGE, 1913, 1914b; FREIER, 1960; CROZIER, RUDGIE (1913), working in South Africa observed that dust particles of
widely 1964).
siliceous nature in dust storms had the effect of reducing or even reversing the normal fair-weather potential gradient if present in the air in sufficient concentrations.
He in fact (RUDGE, 1914b) registered potential gradients as high as -10,000 295
A. I. I. ETTE
296
Vm-l
during a dust storm. FREIER (1960), from measurements of potential gradients close to a ‘dust devil’ in the Sahara suggested that the dust cloud behaved like an inverted thunder cloud-with negative charges above positive ones. CROZIER (1964) however obtained results in a New Mexico ‘dust devil’ which he explained in terms of a uniform column of negative charge. The laboratory experiments of RUDGE (1914a) and KUNKEL (1950) have served to throw some light on the mechanism of dust electrification, though the The essential conclusions likely to be of problem is still not fully understood. relevance to the problem of electrification of the Harmattan dust, whose major
2.0 -
I-O -
50
N
-\ .,.---
b x Y E
i
u c(
E ? W
-5-IO
hr,LMT 1000
-
2cix
-15 -
3cm
-20
-
4oco
-25
-
5000
Fig. 1. Air-earth current and potential gradient at Zaria during the Harm&tan. Note scale for negative values. constituent is silica (HAMILTON and ARCHIBOLD, 1945), may be summarised as follows : (a) Electrification of dust particles occurs at the instant of dispersal into a cloud; the energy expended in the process in no way affecting the magnitude or distribution of charges produced (RUDGE, 1914a; KUNKEL, 1950). (b) For silica dust clouds, the larger and heavier particles acquire positive charges, and the finer and lighter ones negative charges. LATHAM (1964) has attempted to explain these charges in terms of a net migration of positive ions from the smaller to the larger particles during collision. KUNKEL’S (1950) results, based on careful measurements of charges on individual particles, that charges of both signs are equally likely among groups of a given size, are hardly relevant to the problem of dust electrification in the atmosphere where particle agglomeration and ion capture cannot be avoided. (c) Small concentrations of dust in the air are enough to give a measurable potential, the magnitude of which increases with the dust concentration (RUDGE, 1914a).
Effect of dust on atmospheric electricity
297
(d) Humidity has no si~~cant effect on the electrification process unless it is so high that the dust particles stick and cannot be dispersed (KUNPEL, 1950). In sandstorms and ‘dust devils’ large quantities of loose sand and dust are carried up to great heights in updraughts in much the same way as hydrometeors in storm clouds. Dispersal of the particles thus results in spontaneous electrification; gravity separating the heavier positively charged particles from the lighter negatively charged ones to produce a dust cloud of negative polarity. With subsidence of turbulence the heavier particles quickly settle out of the dust cloud,
-90 -
---Whole month excluding severe hormatton days - - - Harmottan day,
Xl Feb.
Fig. 2. Potential gradient variation St Ibadan for February 1969.
leaving the finer negatively charged ones-with a maximum size of perhaps 0.5 p (COFFMAN,1963)-in suspension to produce a negative potential gradient and vertical conduction current near the earth’s surface. Under the influence of the prevailing E.N.E. winds some of the fine dust particles originating in the Sahara eventually reach the coastal areas. The primary electrification acquired by the particles in the source region is unlikely to persist for very long, on account of the finite conductivity of the atmosphere; and in any case cannot explain the observed pattern of diurnal reversals in the atmospheric electric parameters. These reversals bear a marked similarity to the ‘austausch’ depressions typical of continental stations (see Fig. 2); and it would appear therefore that there is a secondary mode of dust eleotrification geared to convection in the atmosphere. 3. DIURNAL ELECTRIFICATION PROCESS Harmattan dust particles sampled at Ibadan (MCKEOWN,1958) are found to have diameters ranging between O-1 and O-4 p. During the day these particles are kept aloft by convection currents, but at night when the lapse rate becomes stable
A. I. I. ETTE
298
and surface winds die down the heavier particles slowly settle down under gravity; particles remaining in suspension being trapped below the top of the night inversion layer which builds up to a maximum height just before dawn (GEIQER, 1965). Particles within the lowest layers acquire positive charges probably through the night-time electrode effect (CROZIER, 1963); and constitute condensation nuclei responsible for the familiar early morning haze. With the destruction around sun-rise of the night inversion by convection the electrode effect disappears, the surface haze slowly clears; and drier dust particles
c \
“\, \ *\X\
X 01
J 8
I 20
%
1 22
1 24
hr,LMT
IFig.3. Mean diurnalvariationof space-chargedensityat Ibadan for the period 18-21 February, 1970 (after Oluwafemi).
higher levels are dispersed and brought down through ‘austausch’ to produce a cloud of negative space-charge near the ground. It is suggested that the Harmattan reversals in the vertical air-earth current and potential gradient are initiated by the arrival over the measurement antennas of this negative space-charge blanket; the reversals being intensified during the ‘austausch’ regime as eddy diffusion and wind become vigorous enough to disperse dust on the ground. There would be a gradual reversion to undisturbed weather conditions later in the day when, with weakening turbulence, the negative space-charge density decreases below some value probably dependent on the magnitude of the prevailing potential gradient at the top of the dust cloud and general meteorological conditions. Preliminary measurements made by OLUWAFEMI(1970) late in the last Harmattan season of the diurnal variation of space-charge density for a few days at Ibadan are depicted in Fig. 3. at
4. EFFECT OF WINDSPEED While the Harmattan reversals can be observed in the absence of wind (HARRIS, 1969), surface winds generally intensify the effect through augmentation of the
Effect of dust on atmosphel*ic electricity
299
dust, and hence negative space-charge, concentration at a given level in the atmosphere. Since the terminal velocities of Harm&tan dust particles in suspension in the air are quite negligible compared with windspeed (MCKEOWN, 1958), the SUTTON (1947) distribution equation for pollutants may validly be applied to determine quantitatively the effect of windspeed on dust concentration at a given point. Consider a Cartesian coordinate system with Ox along the ground in the direction of the mean surface wind and Oz pointing vertically upwards. SUTTON(1947) has shown that for pollution released continuously from a line source of strength Q along Oy the pollution concentration at a point (z, z) ignoring windspeed variation with height, is given by
x(44 =
Q I 4----n 2
(
exp(-‘g},
(I)
q-n~s(2-n12,
1
where 3 = mean wind velocity, C, = virtual diffusion coefficient in the vertical direction, and it = turbulence parameter. For the case where the pollution is raised continuously from a rectangular area of infinite cross-wind length the steady-state concentration may be obtained by integrating equation (1) between appropriate limits x1 and x2, say. This procedure clearly leads to an equation of the form
Q is now mass of dust raised per second per unit ares,. Using the method of dimensional analysis, Q may be expressed in the form
where
where r = rj = g = k, a = and so the
radius of dust particles assumed uniform, coefficient of viscosity of air, acceleration due to gravity, and constants, concentrat.ion x may be written explicitly in terms of a in the form x =
?PQ1$(zl,
x2,
2).
(4)
Thus if a > 4 we should expect the dust concentration, hence the negative spacecharge density and locd potential gradient, to increase as some power of the windspeed. KAMRA (1969) has reported noon-time reversals of potential gradient in India during periods of high winds; though he gave no indication of the aerosol condition of the atmosphere during his measurements. The net upward air-mass movement prevailing during the ‘austausch’ regime generates a positive (downward) convection current directed in opposition to the negative conduction current due to the negative local potential gradient. Since
300
A. I. I. ETT~~
however the vertical air-earth current in general varies in a parallel manner with the potential gradient during this period it would appear that there are compensating factors which tend to balance out the convection current component.
5. CoWCLUsIoN The ‘austausch’ reversals in the vertical air-earth current and at’mospheric potential gradient provide a sensitive index of dust concentration in the atmosphere and may be expected to prove of value in deciding ‘Harmattan days’ particularly in borderline cases where conventional meteorological criteria prove indecisive. Detailed experimental studies of the correlation between the atmospheric electric parameters, dust concentration and windspeed are however necessary to establish the reversals as a quantitative meteorological index. REFERENCES COFFMAN M. L. CROZIER W. D. CROZIER W. D. FREIER G. D. GEIGER R.
1963 1963 1964 1960 1965
HAMILTON R. A. and ARCHIBOLD J. W. HARRIS D. J.
1945 1969
&MRA A. K. KNEEL W. B. LATHAM J. MCKEOWN H. D. J. OLUWAFEMI C. 0. RVDGE W. A. D. RUDGE W. A. D. RTJDGE W. A. D.
1969 1950 1964 1958 1970 1913 1914a
SUTTON 0. 0.
1914b 1947
J. geophys. Res. 68, 1565. J. geophys, Res. 98, 3451. J. geophys. Res. 69, 6427. J. geophys. Res. 65, 3504. The Climate Near the Gound, p. 93. Harvard University Press, Cambridge. Q. Jl R. met. Sot. 71,231. Planetary Electrodynamics (Edited by S. C. CORONITI and J. HUGHES), Vol. 1, p. 39. Gordon & Bresch, London. J. Atmosph. Terr. Phys. 81,1281. J. appl. Phys. 21, 820. Q. JI R. met. Sot. 90, 91. Q. Jl R. met. Sot. 84, 280. Private communication. Nature, Lond. Proc. R. Sot. Lond. A90, 256. Proc. R. Sot. Lond. A90, 571. Q. Jl R. met. Sot. 73, 257.
SHORT PAPER
The effect of the Harmattan dust on atmospheric electric parameters Department
A. I. I. ETTE of Physics, University of Ibadan, Nigeria, W. Africa (Received 11 June 1970)
Abstract-The vertical air-earth current and the atmospheric potential gradient are known to undergo simultaneous reversals at the beginning and end of the ‘sustausch’ regime during the Harm&tan season. These reversals are attributed to the production of large negative charges through dust dispersal in the dry Harmattan atmosphere by convection and wind, both of which attain maximum strength around noon. 1. INTRODUCTION BETWEEN the months of November and March most of West Africa is under the influence of the dry E.N.E. Harmattan winds which blow southwards from the Sahara desert-filling the air with large concentrations of fine dust particles which considerably reduce visibility. Over the coastal areas the prevailing winds are the moist S.W. monsoon which are actually a continuation of the S.E. trade winds of the South Atlantic. These winds meet the E.N.E. winds along the inter-tropical discontinuity (I.T.D.) which marks the southern boundary of the Harmattan climatic zone characterized by clear skies, high temperatures and low relative humidities by day, large diurnal temperature and humidity ranges; and by the appearance of early morning haze. Within this zone simultaneous reversals of the fair-weather values of the vertical air-earth current and atmospheric potential gradient are common around noon-negative values sometimes approaching those in severe storms often being registered farther north as shown in Fig. 1 obtained by HARRIS (1969) at Zaria (lat. ll”7’N). At Ibadan (lat. 7’24’N) where climatic conditions during the Harmattan season are generally milder, the negative excursions are smaller and occupy a shorter time. Invariably onset of reversal is abrupt; a complete reversal from the normal fair-weather value often being accomplished within about 10 min. No completely satisfactory explanation of these reversals have so far been offered. In this communication the problem is re-examined, and a possible mechanism responsible for the sustained reversals proposed.
2. ELECTRIFICATION OF DUST CLOUDS Intense electrification of dust raised in sandstorms and ‘dust devils’ have been reported in the literature (RUDGE, 1913, 1914b; FREIER, 1960; CROZIER, RUDGIE (1913), working in South Africa observed that dust particles of
widely 1964).
siliceous nature in dust storms had the effect of reducing or even reversing the normal fair-weather potential gradient if present in the air in sufficient concentrations.
He in fact (RUDGE, 1914b) registered potential gradients as high as -10,000 295
A. I. I. ETTE
296
Vm-l
during a dust storm. FREIER (1960), from measurements of potential gradients close to a ‘dust devil’ in the Sahara suggested that the dust cloud behaved like an inverted thunder cloud-with negative charges above positive ones. CROZIER (1964) however obtained results in a New Mexico ‘dust devil’ which he explained in terms of a uniform column of negative charge. The laboratory experiments of RUDGE (1914a) and KUNKEL (1950) have served to throw some light on the mechanism of dust electrification, though the The essential conclusions likely to be of problem is still not fully understood. relevance to the problem of electrification of the Harmattan dust, whose major
2.0 -
I-O -
50
N
-\ .,.---
b x Y E
i
u c(
E ? W
-5-IO
hr,LMT 1000
-
2cix
-15 -
3cm
-20
-
4oco
-25
-
5000
Fig. 1. Air-earth current and potential gradient at Zaria during the Harm&tan. Note scale for negative values. constituent is silica (HAMILTON and ARCHIBOLD, 1945), may be summarised as follows : (a) Electrification of dust particles occurs at the instant of dispersal into a cloud; the energy expended in the process in no way affecting the magnitude or distribution of charges produced (RUDGE, 1914a; KUNKEL, 1950). (b) For silica dust clouds, the larger and heavier particles acquire positive charges, and the finer and lighter ones negative charges. LATHAM (1964) has attempted to explain these charges in terms of a net migration of positive ions from the smaller to the larger particles during collision. KUNKEL’S (1950) results, based on careful measurements of charges on individual particles, that charges of both signs are equally likely among groups of a given size, are hardly relevant to the problem of dust electrification in the atmosphere where particle agglomeration and ion capture cannot be avoided. (c) Small concentrations of dust in the air are enough to give a measurable potential, the magnitude of which increases with the dust concentration (RUDGE, 1914a).
Effect of dust on atmospheric electricity
297
(d) Humidity has no si~~cant effect on the electrification process unless it is so high that the dust particles stick and cannot be dispersed (KUNPEL, 1950). In sandstorms and ‘dust devils’ large quantities of loose sand and dust are carried up to great heights in updraughts in much the same way as hydrometeors in storm clouds. Dispersal of the particles thus results in spontaneous electrification; gravity separating the heavier positively charged particles from the lighter negatively charged ones to produce a dust cloud of negative polarity. With subsidence of turbulence the heavier particles quickly settle out of the dust cloud,
-90 -
---Whole month excluding severe hormatton days - - - Harmottan day,
Xl Feb.
Fig. 2. Potential gradient variation St Ibadan for February 1969.
leaving the finer negatively charged ones-with a maximum size of perhaps 0.5 p (COFFMAN,1963)-in suspension to produce a negative potential gradient and vertical conduction current near the earth’s surface. Under the influence of the prevailing E.N.E. winds some of the fine dust particles originating in the Sahara eventually reach the coastal areas. The primary electrification acquired by the particles in the source region is unlikely to persist for very long, on account of the finite conductivity of the atmosphere; and in any case cannot explain the observed pattern of diurnal reversals in the atmospheric electric parameters. These reversals bear a marked similarity to the ‘austausch’ depressions typical of continental stations (see Fig. 2); and it would appear therefore that there is a secondary mode of dust eleotrification geared to convection in the atmosphere. 3. DIURNAL ELECTRIFICATION PROCESS Harmattan dust particles sampled at Ibadan (MCKEOWN,1958) are found to have diameters ranging between O-1 and O-4 p. During the day these particles are kept aloft by convection currents, but at night when the lapse rate becomes stable
A. I. I. ETTE
298
and surface winds die down the heavier particles slowly settle down under gravity; particles remaining in suspension being trapped below the top of the night inversion layer which builds up to a maximum height just before dawn (GEIQER, 1965). Particles within the lowest layers acquire positive charges probably through the night-time electrode effect (CROZIER, 1963); and constitute condensation nuclei responsible for the familiar early morning haze. With the destruction around sun-rise of the night inversion by convection the electrode effect disappears, the surface haze slowly clears; and drier dust particles
c \
“\, \ *\X\
X 01
J 8
I 20
%
1 22
1 24
hr,LMT
IFig.3. Mean diurnalvariationof space-chargedensityat Ibadan for the period 18-21 February, 1970 (after Oluwafemi).
higher levels are dispersed and brought down through ‘austausch’ to produce a cloud of negative space-charge near the ground. It is suggested that the Harmattan reversals in the vertical air-earth current and potential gradient are initiated by the arrival over the measurement antennas of this negative space-charge blanket; the reversals being intensified during the ‘austausch’ regime as eddy diffusion and wind become vigorous enough to disperse dust on the ground. There would be a gradual reversion to undisturbed weather conditions later in the day when, with weakening turbulence, the negative space-charge density decreases below some value probably dependent on the magnitude of the prevailing potential gradient at the top of the dust cloud and general meteorological conditions. Preliminary measurements made by OLUWAFEMI(1970) late in the last Harmattan season of the diurnal variation of space-charge density for a few days at Ibadan are depicted in Fig. 3. at
4. EFFECT OF WINDSPEED While the Harmattan reversals can be observed in the absence of wind (HARRIS, 1969), surface winds generally intensify the effect through augmentation of the
Effect of dust on atmosphel*ic electricity
299
dust, and hence negative space-charge, concentration at a given level in the atmosphere. Since the terminal velocities of Harm&tan dust particles in suspension in the air are quite negligible compared with windspeed (MCKEOWN, 1958), the SUTTON (1947) distribution equation for pollutants may validly be applied to determine quantitatively the effect of windspeed on dust concentration at a given point. Consider a Cartesian coordinate system with Ox along the ground in the direction of the mean surface wind and Oz pointing vertically upwards. SUTTON(1947) has shown that for pollution released continuously from a line source of strength Q along Oy the pollution concentration at a point (z, z) ignoring windspeed variation with height, is given by
x(44 =
Q I 4----n 2
(
exp(-‘g},
(I)
q-n~s(2-n12,
1
where 3 = mean wind velocity, C, = virtual diffusion coefficient in the vertical direction, and it = turbulence parameter. For the case where the pollution is raised continuously from a rectangular area of infinite cross-wind length the steady-state concentration may be obtained by integrating equation (1) between appropriate limits x1 and x2, say. This procedure clearly leads to an equation of the form
Q is now mass of dust raised per second per unit ares,. Using the method of dimensional analysis, Q may be expressed in the form
where
where r = rj = g = k, a = and so the
radius of dust particles assumed uniform, coefficient of viscosity of air, acceleration due to gravity, and constants, concentrat.ion x may be written explicitly in terms of a in the form x =
?PQ1$(zl,
x2,
2).
(4)
Thus if a > 4 we should expect the dust concentration, hence the negative spacecharge density and locd potential gradient, to increase as some power of the windspeed. KAMRA (1969) has reported noon-time reversals of potential gradient in India during periods of high winds; though he gave no indication of the aerosol condition of the atmosphere during his measurements. The net upward air-mass movement prevailing during the ‘austausch’ regime generates a positive (downward) convection current directed in opposition to the negative conduction current due to the negative local potential gradient. Since
300
A. I. I. ETT~~
however the vertical air-earth current in general varies in a parallel manner with the potential gradient during this period it would appear that there are compensating factors which tend to balance out the convection current component.
5. CoWCLUsIoN The ‘austausch’ reversals in the vertical air-earth current and at’mospheric potential gradient provide a sensitive index of dust concentration in the atmosphere and may be expected to prove of value in deciding ‘Harmattan days’ particularly in borderline cases where conventional meteorological criteria prove indecisive. Detailed experimental studies of the correlation between the atmospheric electric parameters, dust concentration and windspeed are however necessary to establish the reversals as a quantitative meteorological index. REFERENCES COFFMAN M. L. CROZIER W. D. CROZIER W. D. FREIER G. D. GEIGER R.
1963 1963 1964 1960 1965
HAMILTON R. A. and ARCHIBOLD J. W. HARRIS D. J.
1945 1969
&MRA A. K. KNEEL W. B. LATHAM J. MCKEOWN H. D. J. OLUWAFEMI C. 0. RVDGE W. A. D. RUDGE W. A. D. RTJDGE W. A. D.
1969 1950 1964 1958 1970 1913 1914a
SUTTON 0. 0.
1914b 1947
J. geophys. Res. 68, 1565. J. geophys, Res. 98, 3451. J. geophys. Res. 69, 6427. J. geophys. Res. 65, 3504. The Climate Near the Gound, p. 93. Harvard University Press, Cambridge. Q. Jl R. met. Sot. 71,231. Planetary Electrodynamics (Edited by S. C. CORONITI and J. HUGHES), Vol. 1, p. 39. Gordon & Bresch, London. J. Atmosph. Terr. Phys. 81,1281. J. appl. Phys. 21, 820. Q. JI R. met. Sot. 90, 91. Q. Jl R. met. Sot. 84, 280. Private communication. Nature, Lond. Proc. R. Sot. Lond. A90, 256. Proc. R. Sot. Lond. A90, 571. Q. Jl R. met. Sot. 73, 257.