Patent Description:
Solid and/or liquid particles suspended in a gas are referred to as aerosols, which are present naturally in the ambient atmosphere or as a result of man-made activities. Method and apparatus for measuring particles suspended in a gas are important for atmospheric aerosol research and in other scientific and technical disciplines where small particles suspended in a gas play a significant role.

An important method to measure the concentration and size distribution of aerosol particles is differential mobility spectrometry. For such a measurement the aerosol particles, i.e. particles suspended in a gas, must be conditioned in order to create a specific charge distribution on the particles. Particles not properly charge conditioned will give rise to erroneous results. The Wide-Range Particle Spectrometer (WPS™) manufactured by MSP Corporation (<NPL>)) is one such instrument capable of measuring aerosol size distributions from <NUM> to <NUM> in diameter. In this instrument, a sub-range of aerosol particle size from <NUM> to <NUM> is measured by differential mobility or scanning mobility spectrometry. For the purpose of this disclosure we will refer to both measurement approaches as differential mobility spectrometry, or DMS, since both are based on the differential mobility measuring principle. Instruments based on differential mobility spectrometry are available from several manufacturers. The electrical ionizer described in this disclosure can in principle be used with any one of these aerosol measuring instruments.

Traditionally, aerosol charge conditioning for DMS is accomplished by means of a radioactive ionizer. Two commonly used radioactive ionizers are Krypton <NUM> and Polonium <NUM> (<NPL>) and <NPL>)). These radioactive ionizers make use of high-energy subatomic particles produced by radioactive decay to ionize the gas to form positive and negative ions needed for charge conditioning. Krypton <NUM> is a beta emitter, producing high-energy beta particles, i.e. electrons, by radioactive decay. Polonium <NUM> is an alpha emitter producing energetic subatomic alpha particles, which are the nuclei of helium atoms. These energetic sub-atomic particles then collide with gas molecules to form positive and negative ions for charge conditioning. These sub-atomic particles are much smaller than the size of a single atom, which is approximately <NUM>Ǻ in the case of hydrogen. Alpha and beta particles are considerably smaller than <NUM>Ǻ in size.

In comparison, aerosol particles are considerably larger. An aerosol particle with a diameter of <NUM>, which is <NUM>Ǻ, is considered very small in aerosol studies and is near the lower size limit of particle measurement by DMS. Aerosol of such a small size is therefore much larger than particles of nuclear physics. Particles of nuclear physics are very different from the particles of interest in aerosol studies. These two types of particles are not the same and should be clearly distinguished. For the purpose of this disclosure, unless otherwise noted, the particles of interest are aerosol particles rather than sub-atomic particles of nuclear physics.

When gas containing suspended aerosol particles is exposed to energetic sub-atomic particles produced by radioactive decay, the gas becomes ionized to form positive and negative ions. The gaseous ions then collide with the suspended aerosol particles to produce a characteristic charge distribution referred to as a Boltzmann distribution (<NPL>)), <MAT> where e is the elementary unit of charge, d is the particle diameter, k is Boltzmann's constant, T is the absolute temperature, n is the number of elementary units of charge on the particles and fn is the fraction of particles in the aerosol carrying n elementary units of charge. Table <NUM> shows the particle charge distribution according to the Boltzmann's law.

An aerosol in Boltzmann charge equilibrium will develop a charge distribution with substantially equal concentration of positively and negatively charged particles. The total charge on the particles, i.e. the sum of all positive and negative charges carried by the particles, is equal to zero. As a result, an aerosol in Boltzmann charge equilibrium has no overall net charge. Overall the aerosol is electrically neutral while the individual particles in the aerosol may carry a charge, although not all particles are charged. The conditions needed to produce Boltzmann charge distribution are discussed in <NPL>) and <NPL>).

When an aerosol carrying suspended particles are charge-conditioned by flowing the aerosol through a radioactive ionizer under suitable operating conditions, it will emerge from the ionizer carrying the charge distribution shown in Table <NUM>. This specific charge distribution is then used for size distribution analysis by DMS.

An electrical ionizer for aerosol charge conditioning and measurement by DMS, therefore, must generate a charge distribution similar to the Boltzmann charge distribution generated by a radioactive ionizer in order to achieve accurate measurement results. One difference between radioactive ionizer and electric ionizer is that ionization by sub-atomic particles produced by radioactive decay occurs in the absence of an external electric field, whereas charge conditioning by ions generated by corona discharge frequently occurs when there is a significant electric field present. Not all electrical ionizers are thus capable of charge conditioning an aerosol to a sufficient degree to produce a Boltzmann distribution. As a result, an electrical ionizer capable of generating a charge distribution similar to the Boltzmann distribution is needed for high accuracy aerosol measurement by DMS. Such an ionizer is now needed because of the increased regulation on the use of radioactive material, which makes the use of radioactive ionizers less attractive or convenient for scientific research and technical applications.

Other developments in electrical ionizers include those described in <NPL>) and in <CIT>. Both use a DC corona discharge to generate separate streams of positive and negative ions in clean air, which are then mixed with an aerosol to provide positive and negative ions for charge conditioning. The aerosol is thus diluted, which is a disadvantage in some applications. Both devices have failed to achieve wide spread acceptance, perhaps as a result of complexity, reliability, and/or cost. Another approach to aerosol charge neutralization is by means of an AC corona discharge as described by<CIT>. <CIT> discloses a device for charging or adjusting the charge of gas-borne particles into a defined charge distribution under utilization of corona discharge in the aerosol space.

<CIT> discloses a method and apparatus for generating charged particles for size measurement by electrical mobility.

This disclosure includes an apparatus for exposing particles in a gas to ions in order to cause a charge on said particles to change, the apparatus comprising a chamber with an inlet for the gas to enter and an outlet for said gas to exit. The chamber is surrounded by an enclosure with a conductive wall, the wall being held at a ground potential, and the enclosure includes a first wall surface, a second wall surface opposite to and facing the first wall surface, and chamber walls that space the first wall surface from the second wall surface. An electrode with an exposed tip is in contact with the gas in the chamber and located adjacent to the first wall surface, the electrode being held at a different potential from the ground potential of the chamber wall. The electrode is connected to a source of voltage sufficient to cause a corona discharge to occur, forming an electric field and ions in the chamber. The inlet and outlet define a nominal gas flow path along a line connecting the centre of said inlet with the centre of said outlet, which passes between the first wall surface and the second wall surface and is closer to said second wall surface than said first wall surface and passes through said second region between said exposed tip and said second wall surface such that the gas flow path passes mainly through a region of space with a low electric field intensity since the electric field has a potential gradient having an electric field intensity in a second region of said chamber adjacent the second wall surface which is lower than in a first region of said chamber adjacent said exposed tip and said first wall surface.

A method is also disclosed for conditioning a charge on particles in a gas to produce substantially equal concentrations of positively and negatively charged particles and a substantially zero total particle charge in said gas for aerosol measurements by differential mobility spectrometry, said method comprising causing said gas to flow through a chamber, the chamber having an inlet for gas to enter and an outlet for gas to exit, said chamber being surrounded by an enclosure with a conductive wall, said conductive wall being held at a ground potential, wherein the enclosure includes a first wall surface, a second wall surface opposite to and facing the first wall surface, and chamber walls that space the first wall surface from the second wall surface and causing the gas to flow through the chamber along a nominal gas flow path along a line connecting the centre of said inlet with the centre of said outlet and passing between the first wall surface and the second wall surface, and creating an AC corona discharge within the chamber having a relatively higher electric field intensity region adjacent the first wall surface and a relatively lower electric field intensity region adjacent the second wall surface, wherein the nominal gas flow path is closer to said second wall surface than said first wall surface the gas flow from the inlet to the outlet thereby occurring mainly in the relatively lower intensity region thereby creating substantially equal concentrations of positively and negatively charged particles, and a substantially zero total particle charge in the gas.

The chamber may comprise a volume in the approximate range between <NUM> and <NUM> cc, and may include flowing said gas with suspended particles through said chamber at a rate of gas flow to achieve a gas residence time in the chamber in the approximate range between <NUM> and <NUM> seconds. An AC voltage may be used to create the AC corona discharge, said AC voltage having a root-mean-square value in the approximate range between <NUM> to <NUM> volts, sufficient to create an AC current with a root-mean-square current in the approximate range between <NUM> and <NUM> mA.

<FIG> is a schematic diagram of the system for measuring the concentration and size distribution of aerosol particles by DMS using the electrical ionizer of the present disclosure as an aerosol charge conditioner. The electrical ionizer is shown generally at <NUM> in its preferred embodiment and is comprised of a housing, <NUM>, which is electrically conductive and grounded, to provide an enclosure around the ionization chamber <NUM> whose walls are thus also conductive and grounded. Ionization chamber <NUM> has an inlet, <NUM>, for the aerosol to enter and an outlet, <NUM>, for the aerosol to exit. A high voltage electrode, <NUM>, is held by insulator <NUM> and placed in the ionization chamber with an exposed electrode tip, <NUM>, near one wall, <NUM>, of the enclosure. The tip is exposed to the aerosol flowing through the ionization chamber carrying suspended particles for charge conditioning. The electrode is placed in a position close to the surface of wall <NUM> of the ionization chamber but separated from it by a sufficient distance so that a suitably high voltage applied to the electrode will cause a stable corona discharge to develop without arcing. A DC voltage of an appropriate polarity can be used to generate a DC corona of either a positive or a negative polarity for aerosol charging. An AC voltage can be used to create an AC corona discharge thus generating both positive and negatively charged ions in the ionization chamber for charge conditioning for DMS.

Aerosol for charge conditioning and measurement by DMS comes from source <NUM>, which can be an aerosol in the ambient atmosphere for measurement by DMS for concentration and size distribution analysis. It can be an aerosol generated for laboratory research in which the aerosol size distribution is to be measured by DMS. The aerosol can also be generated by a specific process or for a specific purpose, in an industrial setting, where knowledge about the aerosol size distribution is important. In all cases, aerosol size distribution analysis by DMS can be made with the system shown in <FIG>.

Aerosol from source <NUM> in <FIG> first flows through electrical ionizer <NUM> at a specific rate of flow, Q1, and under conditions that will insure a charge distribution similar to that of a Boltzmann distribution will develop. A measuring instrument <NUM> for size distribution analysis by DMS then samples the aerosol at a rate of flow, Q2, required for the measurement. If Q1 is larger than Q2, the excess, Q3=Q1 - Q2, will be discharged as waste as shown. If Q1 is smaller than Q2, additional clean gas from source <NUM> can be introduced at a rate of flow Q4 to mix with the aerosol to provide a mixture having a total flow rate Q1+Q4 that is equal to or larger than the flow rate Q2. The excess, Q3=Q1 + Q4 - Q2, if any, can be discharged as waste as shown in <FIG>.

The operating principle of the electrical ionizer <NUM> is explained with the aid of <FIG>. With the exposed high voltage electrode tip <NUM> being placed in a position near the surface <NUM> of one wall of the enclosure, an electric field will develop in the chamber. The electric field can be depicted by electric field lines, some of which are shown in <FIG>. Electric field lines, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, emanate from the tip <NUM> and terminate on the nearby wall surface <NUM>, whereas field lines <NUM>, <NUM>, <NUM>, <NUM> and <NUM> also emanate from the same high-voltage tip <NUM> but terminate on wall surfaces that are farther away. The depicted field lines, including lines <NUM>, <NUM>, <NUM>, <NUM>, <NUM><NUM>, <NUM>, <NUM>, <NUM> and <NUM>, all begin with an electrical potential equal to the potential, i.e. the voltage on the high-voltage electrode <NUM> at the tip <NUM>, and end on the same grounded surface of the enclosure which is grounded and at a potential of <NUM> volt. The potential gradient, which is the rate of change of the electrical potential per unit length along an electric field line, is thus higher along the shorter electric field lines, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, than along the longer electric field lines, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. The electric field intensity is the gradient of the electrical potential. Therefore, it is higher along the shorter electric field lines that terminate on the near-wall surface <NUM> adjacent to the electrode tip <NUM>, than along those that terminate on surfaces of enclosure <NUM> that are farther away.

When gaseous ions are generated by a corona discharge in the region of space adjacent to the electrode tip <NUM>, the ions will flow along the electric field lines at a velocity in proportion to the electric field intensity. The velocity of ions, which is referred to as the drift velocity, will thus be higher along the electric field lines with a high electric field intensity, and lower along the longer electric field lines with a lower electric field intensity. By creating two separate streams of ions, one flowing at a high velocity toward the near wall, i.e. wall near the high voltage electrode, and another flowing at a lower velocity toward the far wall, we have created two separate regions of space in the ionization chamber, in which one region on the average has a lower electric field intensity than the other. The region with a high electric field, i.e. electric field intensity, is the region of space below the boundary electric field lines <NUM> and <NUM>, while the low electric field region is the region above these boundary field lines <NUM> and <NUM>.

When using the ionization chamber depicted in <FIG> for charge conditioning of aerosol particles for DMS, it is important that charge-conditioning takes place mainly in the low field region, above the region of space in the vicinity of electric field lines <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> where the electric field is high. When the aerosol is introduced into the ionization chamber through inlet <NUM>, its nominal flow path is the line <NUM> connecting the inlet <NUM> with the outlet <NUM>. It is important that this flow path does not pass through the high field region surrounding the electrode tip and adjacent to surface <NUM>, but mainly through the low field region above it.

The ionization chamber depicted in <FIG> and <FIG> can be a rectangular chamber with opposite walls that are parallel to each other. It can also depict a cylindrical chamber with parallel walls on the top and bottom separated by a circular cylinder or a cylinder with other cross sectional shapes such as the shape of an ellipse, a polygon, among others. The design approach described above in this disclosure is suitable for all of these ionization chamber cross-sectional shapes. In addition, the ionization chamber can have a curved wall such as the curved surface formed by a sphere, as depicted in <FIG>. In <FIG> like reference characters are used to refer to like elements of the system as in <FIG> and <FIG>, including the electrical ionizer, <NUM>, the inlet <NUM> and the outlet <NUM> for the ionization chamber <NUM>, the aerosol flow path <NUM>, the high voltage electrode <NUM>, its tip <NUM>, and the insulating support <NUM>. The outer most electric field lines <NUM> and <NUM> in the vicinity of the electrode <NUM> are also similarly identified. All of these parts have a similar function and are similarly identified for simplicity and clarity, in spite of the different ionization chamber shape used, which is a sphere in <FIG> and a rectangular or a cylindrical shaped chamber in <FIG> and <FIG>.

<FIG> shows another approach of electrode design. Again like parts are like identified. In this design, electrode <NUM> is held in an insulating support <NUM>, topped by a tip <NUM> in the form of small wire loop, or a short length of a wire held mechanically at the top of electrode <NUM>. The radius of curvature of the needle electrode shown in <FIG> and the radius of the wire electrode tip <NUM> have a substantial influence on the voltage needed to start and sustain a corona discharge from the electrode. Generally, the smaller the radius of curvature of the tip or the radius of the wire, the lower will be the voltage needed to start and maintain a stable corona discharge. A sharp pointed or tapered tip is preferred. High discharge voltage will cause a greater rate of erosion by ion bombardment of the electrode tip, thus making it necessary to replace the electrode at frequent intervals. For practical purposes, a voltage in the range between <NUM> to <NUM> volts is considered the most appropriate for operating a high voltage electrode in the present disclosure. The voltage can be a DC voltage in this range, or an rms, i.e. root-mean-square AC voltage in this range.

The corona current flow from the high voltage electrode to the grounded electrode nearby also has an effect on the performance of the electrical ionizer. A high ionization current will generally lead to more rapid charge conditioning, and a low ionization current will require keeping the aerosol flow through the ionizer at a sufficiently low flow rate to insure the desired charge distribution similar to that of a Boltzmann distribution will indeed develop. This, however, can lead to greater undesired particle loss by electrostatic deposition in the ionization chamber. Such a loss is undesirable because it would lead to reduced measurement accuracy for DMS measurement. The most appropriate range of corona ion flow is in the range between <NUM> and <NUM> mA in rms AC current flow for proper charge conditioning for DMS aerosol measurement. In the case where a DC corona discharge is needed for creating a unipolar charge, i.e. charge of the same electrical polarity, either a positive or a negative polarity, the required DC current is also generally in the same <NUM> to 5mA range.

In practical applications, it is also important to design the electrical ionizer suitable for different rate of aerosol flow needed by different applications. Too high of an aerosol flow rate or too small an ionization chamber volume will lead to incomplete charging or charge conditioning of the aerosol. Too large an ionization chamber or too low an aerosol flow rate through the ionization chamber will lead to greater losses of particles in the chamber, which is also undesirable. For the purpose of this disclosure, the most appropriate range is a range where the nominal residence time of the aerosol flow in the chamber, which is the ratio of chamber volume to the volumetric aerosol flow rate is within an appropriate range. An aerosol flow rate of <NUM> cubic centimeter per second through an ionization chamber having a <NUM> cubic centimeter volume will result in a nominal residence time of <NUM>/<NUM> = <NUM> seconds. The most appropriate residence time for designing ionization chambers is between <NUM> and <NUM> seconds.

Different application also requires different ionization chamber sizes. For charging and charge conditioning purposes for DMS, the ionization chamber is generally in the range between <NUM> to <NUM> cubic centimeters in total volume.

The above approach to designing electrical ionizers for aerosol charging and charge conditioning for laboratory aerosol studies and aerosol size analysis by DMS is generally adequate for most applications. A similar approach can be used to design electrical ionizers for other applications where accuracy of charging and charge conditioning are important, and where the loss of particles in flowing through the ionizer is also an important consideration. For those skilled in the art of aerosol measurement, it will become clear that the specific approach described in this disclosure can also be used to design electrical ionizers for those applications where the end applications may be different, but the factors affecting the acceptance of the electrical ionizer are similar or substantially the same. Further discussion of the application of the approach described in this disclosure for other applications, therefore, will not be made further.

<FIG> compare laboratory measurement made on two aerosols by differential mobility spectrometry (DMS): (<NUM>) a laboratory room air aerosol and (<NUM>) aerosol with uniform sized particles of polystyrene latex sphere of <NUM> generated in the laboratory. In both cases the measurement was made with an electrical ionizer operating at a residence time of <NUM> seconds and a residence time of <NUM>. Similar measurement was also made with a conventional Po <NUM> radioactive ionizer.

Results in <FIG> and <FIG> show that qualitative agreement is achieved with an electric ionizer operating at <NUM> second with that measured with a Po <NUM> radio-active ionizer, while a quantitative agreement is achieved when the electrical ionizer is operated at a residence time of <NUM> seconds. Similar results are achieved as shown <FIG> and <FIG> for the case of polystyrene latex (PSL) aerosol containing uniform sized <NUM> PSL spheres.

These and other laboratory studies we have made show that the specific approach described in this disclosure to designing electrical ionizer for aerosol charge conditioning and measurement by DMS will lead to accurate measurement results. The approach can also be used for other applications where charging or charge-conditioning of aerosols are important.

Claim 1:
An apparatus (<NUM>) for exposing particles in a gas to ions in order to cause a charge on said particles to change, said apparatus comprising:
a chamber (<NUM>) with an inlet (<NUM>) for said gas to enter and an outlet (<NUM>) for said gas to exit, said chamber (<NUM>) being surrounded by an enclosure (<NUM>) with a conductive wall (<NUM>), said conductive wall (<NUM>) being held at a ground potential, wherein the enclosure (<NUM>) includes a first wall surface, a second wall surface opposite to and facing the first wall surface, and chamber walls that space the first wall surface from the second wall surface;
a nominal gas flow path (<NUM>) along a line connecting the centre of said inlet (<NUM>) with the centre of said outlet (<NUM>) and passing between the first wall surface and the second wall surface;
an electrode (<NUM>), with an exposed tip (<NUM>) in contact with said gas in said chamber (<NUM>) and located adjacent to the first wall surface, wherein said electrode (<NUM>) is held at a different potential than said conductive wall (<NUM>) and an electric field is developed in said chamber (<NUM>), said electrode (<NUM>) being connected to a source of voltage sufficient to cause a corona discharge to occur forming ions in said chamber (<NUM>);
said electric field has a potential gradient having an electric field intensity in a second region of said chamber (<NUM>) adjacent the second wall surface, which is lower than in a first region of said chamber (<NUM>) adjacent said exposed tip (<NUM>) and said first wall surface; and
characterized in that:
said nominal gas flow path (<NUM>) is closer to said second wall surface than said first wall surface and passes through said second region between said exposed tip (<NUM>) and said second wall surface.