L]~TT~E AL NUOVO CI~ENTO VOL. 37, N. 15 13 Ag0sto 1983

Atmospheric-Pollution Monitoring through Conductivity Measurements.

M. CA~L~VARO

~stituto di Elettrotecnica, _l~acolt~ di Ingegneria dell'Universit~ - I?Aqui la

]). PAOLETTI

Ist i tuto dl Hisica Tecnica, _Facoltd di Ingegneria del$' Universit~ - I / A q u i l a

(ricevuto il 25 Marzo 1983)

PACS. 92.60. - Meteorology.

Abstract. - The poss ibi l i ty to correlate the densi ty of gaseous pol lutants in the atmosphere (SOs, NO~) and the s ta te of air ionization, defined by the volumetric densi ty of ions of different sizes and mobil i t ies has been exper imental ly investigated. A descrip- t ion is given of a possible method of measurement of atmospheric pollution, based on such a correlation. Pre l iminary experiments are reported.

Introduction. - Several techniques for detection ad measurement of very small den- sities (units or fractions of p.p.m.) of gaseous pol lu tants in the atmosphere have been developed. These are based t rad i t iona l ly on chemical and photochemical methods, which require the use of expensive, and sometimes complex, instruments.

Almost all methods perform a delayed-t ime analysis of samples, taken from the en- vironment , by means of pneumat ic devices and filters. A high-sensit ivity, and possibly low-cost, method for the real- t ime analysis of pol lutants , allowing a continuous air pol- lution monitoring, appears to be highly desirable. The basic objective of the research repor ted in this paper was to develop a method of measurement , based on a possible correlation between small concentrations of gaseous pol lutants and atmospheric electrical conductivity. The conclusions, at present, are qual i ta t ive only, and must be confirmed by systematic experiments.

Principle o/ the method. - Our knowledge on atmospheric ionization has been es- sential ly modified in the last years. Atmospheric ionizat ion is the result of conflicting processes (1), namely pr imary- ions generation and recombinat ion of posit ive and nega-

(1) J. •. CHAL~IERS: Atmospheric Electricity, 2nd Edition (London, 1967).

539

540 ~. CARLEVARO and D. FAOLETTI

t i re ions, c o a g u l a t i o n of small (fast) ions with neutral gaseous molecules, to form large (slow) ions, diffusion, and capture of ions by solid and liquid particles. The average drift velocity impressed to an ion nmving in a gas, when submitted to an electric field, is proportional to the intensity of the field itself, provided the gradient is below the breakdown level. Therefore, it is possible to define, for each type of ion, in given con- ditions of chemical purity, pressure and temperature of the gas, a mobility, essentially depending on the charge-to-mass ratio of the considered ion. Air conductivity, in the presence of a multiplicity of ions of different types, characterized by mobilities q~j, and volumetric densities N j, results from the sum of the individual contributions to the charge transport :

(1) " = ]q~l 5: N~uj ~ - l m -1 , 1

where q is the elementary electric charge. In the atmosphere, polluted by foreign gases, the remaining parameters being unchanged, the ionization cannot be identical to that of pure air. Not all the laws that govern this process are already known ; however, there is evidence that the expected variation depends as much on the density of pollutants as on their nature. When pollution is present the coagulation process of fast ions in slow ions is accelerated. Therefore, a decrease in air conductivity has to be expected. A recurrent topic in scientific reports on the use of atmospheric conductivity measure- ment for air pollution monitoring is the statement that it is a very sensitive tool for meas- uring aerosol content, but it is a very poor one, where exclusively gaseous pollutants are to be monitored. Although the behaviour of some electrical parameters has been investigated previously in open atmosphere, to our knowledge, no study has been ef- fected in ~ controlled environment (2).

The aim of this study is to find a proof of a systematic correlation between the presence of some gaseous pollutants (S02) and air conductivity variation, or even better, ions volumetric densities and mobilities. In the experimental lay-out design, specific care was taken in order to get a pure gaseous mixing of the pollutants, and to minimize the presence of aerosols, which may affect the results.

M e t h o d o/ measuremet~t . - The well-known cylindrical capacitor (Gerdien) has been frequently employed, under different test conditions, in order to measure air conduc- t ivity, or ion density, or ion mobility. A common feature of the above measurements is that a steady state must be obtained before reading the true values of the variables. Continuous recording of one of the variables may be performed, provided its fluctuation is very slow.

The avMlability of powerful and low-cost electronic-data processors has suggested to investigate on the possible use under dynamic t.est condition of the cylindrical ca- pacitor, associtated with real-time processing of output data (3). The objective was the extraction of a greater amount of information frmn the output of the sensor, and eventually the combination in a single instrument of the capabilities of the conductivity, ion density and ion mobility meters.

The axial flow rate of environment air, and contained ions, through the armatures of the capacitor is l imited; therefore, the air behaves as a nonlinear dielectric, whose conductivity decreases for increasing potential gradient of the radial electric field.

(~) M, CARLEVARO and G. CICCtXrtELL~t: Determinazione delIo stato di ionizzazione dell'aria e rilievo emtomatico dello spettro di mobilltdt degli toni, I n t e r n a l R e p o r t No. I (L 'Aqui la , 1980), I s t i t u t o di E le t t ro tecn ica . (3) D. LAURENT and It . PEYROUS: The Science o] the Total Environment , Vol. 13 (1979), p. 55-70.

ATMOSPHERIC-POLLUTION MONITORING THROUGH CONDUCTIVITY MEASUREMENTS 5 4 1

The total radial-current results from the sum of the contributions of each kind of ions as shown by (1); the total contribution of one type of ions cannot exceed the total charge carried by the ions of this type, contained in the air volume let into the capacitor in one second. The conductivity decreases by steps, as the potential gradient increases, and the radial t ransi t t ime of ions decreases. The voltage across the capacitor, at which a conductivity step occurs, is correlated to the mobility of that kind of ions whose radial and axial t ransi t t imes become equal at that voltage. The amplitude of the conduc- t iv i ty step is correlated to the sum of the products of the densities by the mobilities of all kinds of ions whose radial t ransi t t ime is shorter than the axial one.

If a capacitor C is connected in series with a resistor R and a tr iangular wave form voltage generator, the current approaches a square wave form, with exponential fronts, whose rise t ime may be controlled by an appropriate choice of the RC t ime constant. If the capacitor is lossless, the top of the square wave isf la t ; if the capacitor dielectric has a constant conductance G, a ramp is superposed to the top of the square wave. The slope S of the ramp is related to the conductance of the dielectric approximately by the same ratio of peak ampli tude AI of the square wave to the capacitance. Since the geometric parameters of the cylindrical capacitor affect in the same way conductance and capacity, it follows tha t :

8 G (2) AI C eo

thus giving a measurement of the ratio of conductivity a of the air flowing in the ca- pacitor to the permett ivi ty of vacuum %. If the conductivity of the dielectric is non- linear, the top of the square wave becomes a sequence of l inear segments with decreasing slopes. A simple (2) processing of sampled experimental curve of the current through the capacitor allows us to compute the slopes of the segments and the voltages at which changes in the slope occur. These data are sufficient to get the spectrum of ion density or ion mobility. The microprocessor software may perform, if required, digital filtering of output samples, to improve the computation of slopes in the presence of random noise.

E x p e r i m e n t a l a p p a r a t u s . - The laboratory prototype consists of a probe, in which the armatures of the cylindrical aspirant capacitor are constituted by thin steel tubes of the following sizes:

internal diameter of outer armature ~ = 75 mm,

external diameter of inner armature ~ = 65 mm,

length of the armature L = 400 mm.

The inner armature supports are in teflon, shaped to minimize the dispersion conduc- tance between the armatures. The capacitor is fed by a t r iangular wave form voltage generator, incorporated in an electronic electrometer. The voltage drop caused by the capacitor current across a precision resistor is recorded. The velocity of aspirated air is 3.5 m/s. A 6 m 3 mixing chamber was built , in wooden fabric, in order to minimize ion absorption. This volume is sufficient to obtain a quasi-homogeneous air pollutant mixture with the possibility of introducing water vapour also. The chamber is con- nected through a pipe to the central room of a dry box, utilized in open circuit. The probe and the apparatus for temperature and relative humidi ty recording, during the experiment, are placed in this box. An aspirator ensures air circulation and its outlet.

~ 2 M. CARL:EVAI~O and D. r X O n ~ T T ~

The transit time, through the mixing chamber, of the environment air and polluting gas, suitably dosed, is about 10 s. This is sufficient for the expected modifications of ionic composition of the air, due to the presence of a pollutant, to be completed. The measuring device is being modified by the addition of a sampling uni t and an analog/digital converter of the output signal. The development of the processing soft- ware, to be implemented into a dedicated in line microprocessor device, is being carried out. This will allow the direct reading and the printing of processed values of air con- ducitivity, volumetric density and of the mobili ty of every type of ions.

E x p e r i m e ~ t a l resul ts a,~d d i scuss io~ . - Analog recordings on relatively long periods of t ime were carried out in order to define the voltage wave form parameters, slope and duration, most convenient for the purpose of the measurement, and to verify, the ca- pabili ty of the method in different meteorological situations.

Typical examples of these test records are given in fig. 1.

l i l l l i l l l I i l l l i I ) l l r r l l l r l r l I I r

Fig. 1. - Current response to a ramp voltage.

This recording had to be limited to typical winter climatic situation ; the results cannot, therefore, be considered conclusive. I t must be remarked that it is important to observe simultaneously metereological parameters in order to interpret properly any striking variation of air conductivity. The apparatus for quanti tat ive pollutants dosage is not yet developed, but qualitative proofs, carried out by introducing very small quantities of SOs for about ls into the mixing chamber, have shown noticeable deviation in the recorded patterns, when compared to the average behaviour (see fig. 2).

I I I I I I I I I I I I I I l I I I I I I I f I I I I I I I I I I I I

Fig. 2. - Response to the injection of small quantities of SOs.

ATMOSPIf]~RIC-POLLUTION MONITORING Ttt'ROUGH CONDUCTIVITT M~ASUP~M~NTS 5 4 3

These qualitative results appear to indicate tha t i t may be possible to develop a de- tection ins t rument having a fast response to rapid changes of the density of gaseous pollutants, high sensitivity and unlimited life of the sensor.

* * *

This research was supported by C.E.E. We thank R. VIS~NTIN for useful discus- sion an G. NUCCIAR~LLI for the valid help in the measurements.