Aerosol measuring Device and Method
 The present application claims priority from Swiss patent application CH01 559/12 filed on August 30, 2012, the contents whereof are hereby incorporated by reference. Field of the invention
 The present invention relates to a method of measuring aerosols without direct contact by detection of an induced electric current.
Description of related art
 Aerosols are relevant for global climate and, depending on their nature, can also represent a biological hazard to the persons who inhale them. Their measurement is therefore of high practical importance.
 Many kinds of aerosol detectors are known in the art. In particular it is known to charge electrically the particles in suspension in the aerosol, and subsequently measure the charge of the particles in order to draw conclusions about the aerosol. With such instruments it is possible to measure or estimate, for example, the number of the suspended particles, their size distribution, average diameter and lung-deposited surface area.
 In a known class of aerosol measuring devices, particles are charged in unipolar chargers, which generate ions of a given polarity and then mix these ions with the particles. Excess ions are captured in an ion trap in which charged particles are selectively retained by means of a weak electrical field. The detection of the particles is then performed in insulated filters, porous bodies, impactors, or by deposition on insulated
measurement electrodes, to which they are electrically attracted.
 This means that the particles are collected in the instrument, which needs to be cleaned on a regular schedule. The detection
conventionally ensues by collecting the particles in order to measure their charges as a weak electrical current. The currents on the filters, porous bodies, impactors or measurement electrodes have to be measured with extremely sensitive current amplifiers, named electrometers, with
sensitivities of the order of 1 f A or 1 pA. Electrometers, being affected by a number of offset and drift errors that, as a rule, are temperature and age- dependent, require periodical calibration and careful offset compensation.  In several known instruments in which the particles are unipolarly and continuously charged by diffusion, and exemplified by US7812306, the charge taken up by the aerosol's particles is roughly proportional to the lung-deposited surface area. More complex instruments are also known, in which two or more measurement electrodes and electrometers are used to measure the charge of the particles. This, in combination with a size- selective deposition stage allows in some cases the determination of the average particle size as well as the particle number concentration. Examples of such devices can be found in EP1681 550 and EP1655595. EP1 1 56320 further includes multiple detection stages that allow the determination of the particle size distribution.
 Documents GB2374671 and EP1681 550 show how concentration variations of charged particles will cause perturbations by induced currents in instruments using multiple electrometer detection stages, and propose suitable methods for correcting these perturbations. According to these documents, aerosol particles pass through additional detection stages, which are constructed as Faraday cages. A Faraday cage consists e.g. of a conductive tube with meshes at its ends. In contrast to the standard methods, the particles are not deposited in the Faraday cage. In the patents mentioned, the signal of the additional Faraday cage detection electrodes is only used to correct the signal measured on the remaining particle- collecting electrodes.
 Document EP1655595 shows a method in which a periodically switched electrostatic deposition of particles induces a periodic
concentration change, which can be measured directly with a Faraday cage. According to this patent, the amplitude of this electrometer signal is proportional to the particle number concentration. In this method, the particles are retained in the instrument: not in the electrometer stage itself, but before it.
Brief summary of the invention
 It is an object of the present invention to propose an aerosol measuring device that provides precise measurements and is more accurate and reliable than the known devices.
 Further, it is an object of the present invention to propose an aerosol measuring device that requires less calibration and cleaning than known devices.  According to the invention, these aims are achieved by a measuring device in which the aerosols are charged by a pulsed ion source and detected synchronously with the ion source and in particular, as it is the object of the appended claims, by a device for the measurement of aerosol comprising: a channel along which an aerosol is caused to flow, a pulsed charger on the channel, arranged to produce a time-variable quantity of positive and/or negative ions in the channel, whereby part of said charge is transferred to aerosol particles, an electrometer, connected to a measurement electrode placed along the channel, downstream of the pulsed charger, for detecting a time-variable signal indicative of the number of aerosol particles that have been charged.
 Further advantageous, but not essential, variants of the invention are the object of the dependent claims.
Brief Description of the Drawings
 The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which: · Figure 1 shows schematically a measurement device according to the present invention to measure the lung-deposited surface area with pulsed aerosol charging.
• Figure 2 illustrates in a diagram the temporal variation of the charge on the aerosol in a charger operated in pulsed unipolar mode.
• Figure 3 shows the temporal variation of the charge on the
aerosol in a charger operated in pulsed bipolar mode.
• Figures 4a to 4e present different variants of the Faraday cage
(sensing electrode) of the invention. · Figure 5 shows a variant of the inventive device set up to
determine the optimal coverage of a microscope probe with nanoparticles.
• Figure 6 shows schematically a variant of the inventive device for determining the particle size distribution with pulsed charging of the aerosol.
• Figure 7 illustrates the temporal variation of the electrometer signal at a Faraday cage when the aerosol is charged according to the invention.
• Figure 8 shows the characteristic positive-negative double pulse that is produced by a single cloud of charged aerosol passing through a Faraday cage.
Detailed Description of possible embodiments of the Invention
 Figure 1 illustrates schematically an instrument according to an aspect of the present invention to measure the lung-deposited surface area of particles by a non-contact method. The aerosol enters the instrument at an inlet 1 and moves along the instrument under the action of a suitable pump or ventilator, for example placed downstream of the outlet 7, and not represented. Preferably, the flow speed is chosen in relation to the transverse dimension of the aerosol-carrying passage, such that the flow is laminar.  The aerosol flows first in a charger that consists for example of a corona wire 2, an electrostatic screen or grid 3, and collecting counter- electrode. At the corona wire, a unipolar or bipolar pulsed high voltage is applied, for example according to a periodical function, such that a corona discharge occurs at the wire, which produces ions of either one (unipolar) or both (bipolar) polarities. The ions can cross the electrostatic screen 3 and the flow of the aerosols while they drift towards the counter-electrode 4.
 Preferably, the ionic current flowing from the corona wire 2 to the counter-electrode 4 is monitored at either or both of these electrodes, by suitable electronic means, and its value is set and/or controlled in order to achieve a determined charging level of the aerosol as it flows in the space between the screen 3 and the counter-electrode 4. It is moreover advantageous, even if not absolutely indispensable, that the counter electrode 4 extend a space downstream of the charger, in such a manner as to trap all the ions that have not become attached to an aerosol particle.  The ion source of the invention could generate either positive or negative ions, or both of them in an alternate fashion. A positive corona wire is however preferable since it produces much less ozone than a negative one.
 Figures 2 and 3 represent the charge density as a function of time of the aerosol at a position downstream of the charger according to the
invention, in the case of a unipolar, respectively bipolar charger. It can be appreciated that the pulsed charger of the invention produces clouds of charged and discharged aerosol, or clouds of charged aerosol with alternate polarities.  Reverting to figure 1 , the instrument of the invention comprises, downstream of the charger, a Faraday cage 5 that is connected to a sensitive electrometer 6. The structure of the Faraday cage can vary according to the circumstances and the construction of the device, but it is preferably such as to guarantee that, as it is traversed by the charged aerosol particles, these induce an opposite charge on the Faraday cage as close as possible in magnitude to their net charge. It is also preferred that the Faraday cage allow unimpeded fluid flow such that the aerosol particles can traverse it with as little loss as possible.
 Figures 4a-4e shows different variations of the Faraday cage 5 to which the electrometer 6 is connected. The Faraday cage may encircle completely the channel in which the aerosol flows. In figure 4a, for example, the Faraday cage is constituted by a metallic or conductive tube 10 with conductive grids 61 at both ends. In this case the induction is complete: the charge induced on the cage equates the opposite of the net sum of the charge on the aerosols contained therein. Figure 4b shows an example of a Faraday cage constituted by an open conductive or metallic tube 10 encircling the channel in which the aerosol flows, with open ends. In this variant, the induction can be nearly complete dependent from the aspect ratio of the tube 10. Despite a minor loss of induced signal, this variant may be attractive because the fine meshes 61 of figure 4a could break, clog, and are an obstacle to the fluid flow.
 In both the realizations of figures 4a and 4b, the Faraday cage can be realized as an insulated section of the channel along which the aerosol flows, downstream of the charger. Importantly, in this and in the following embodiments, the Faraday cage is insulated with respect to the rest of the instrument by means of high-quality insulator materials, to minimize leakage currents, and connected to the input of the electrometer,
which constitutes a virtual ground point. It is important that the insulating material be chosen as to reduce as much as possible leakage, piezoelectric, and microphonic currents that may alter the measurement. The instrument may also include suitable (not illustrated) electrostatic screen and guard electrodes, kept to the same potential as the measurement electrode, to limit such interferences.
 Referring now to figure 4c, the Faraday cage need not be a solid conductive tube but could consist in insulated electrode part 12 on a tube 65. Preferably, the tube 65 is in this case at least in part dielectric and serves also to insulate the electrode 12, which could be realized by metallic foil or paint on a lateral surface of the dielectric tube 65. The electrode 12 could cover all the lateral surface, in which case the induced signal would be essentially the same as in the open tube of figure 4b, or a fraction, in which case the Faraday cage would only partially encircle the channel in which the aerosol flows. This variant has the advantage that the charge-sensitive electrode 12 is not in direct contact with the aerosol, and is protected from dust and other contaminations.
 Figure 4d illustrates another variant of the Faraday cage of the invention that comprises an insulated metal plane 14 on a printed circuit board or equivalent flat surface 13 which is screwed or otherwise mounted on a grounded flow channel 1 5. In this case, the induction electrode does not encircle the channel, but is adjacent thereto. This variation allows particularly simple designs of aerosol detectors.
 Figure 4e illustrates schematically a further variant of the Faraday cage of the invention that includes an induction electrode 62 buried inside a dielectric wall 68 of a channel that delimitates the flow of the aerosol 80. In this variant the electrode 62 is fully protected from contamination.
 The measured signal of the electrometer in the pulsed unipolar mode is shown in figure 7. Since the charge generated by the charger is variable with time, the aerosol flow will consist of a succession of charged and neutral clouds of particles, or clouds of opposite polarities in the case
of a bipolar realization. These clouds will flow in the Faraday cage, or along the measurement electrode, and impress on it a time-variable signal. When a charged aerosol cloud flows through the cage, a compensation current 40 flows on the Faraday cage, which makes certain that the entire cage with its contents is electrically neutral. This compensation current is equal to the current that would be measured in a particle filter that captures all particles, and therefore, according to US patent 7,912,306, it is proportional to the lung-deposited surface area of the particles. As soon as the charged aerosol cloud leaves the Faraday cage again, the compensation current flows in the opposite direction but with the same magnitude 41 . The amplitude of the measured signal is therefore twice as large as that in traditional instruments which impart a constant charge on the aerosol and measure the aerosol by capturing it in a particle filter connected to an electrometer. If the instrument is built with a pulsed bipolar charger, the amplitude of the signal doubles once more, and is then four times higher than in traditional instruments.
 The measuring device described above has the following advantages compared to other known instruments: a. The measured signal amplitude is twice as high as in traditional instruments in the pulsed unipolar mode and four times higher in the pulsed bipolar mode. b. The non-contact measurement of the lung-deposited surface area makes filter exchanges superfluous, thereby reducing
maintenance effort. c. The absence of the particle filter leads to a lower pressure drop over the instrument, which thereby needs much less power to generate the gas flow in the instrument, thus allowing instruments with very low power requirements to be built.
When the zero offset of the electrometer drifts, this is
automatically compensated for, as only the amplitude, that is to say the AC component, of the signal is evaluated.
The DC component of the electrometer's output can be used to monitor the leakage currents on the Faraday cage, which can be taken as an indirect indication of the contamination state of the instrument.
The signal at the output of the electrometer is correlated in frequency and phase with pulses of the ion source. Hence, it can be detected synchronously, for example by a lock-in amplifier or any suitable synchronous detector. This brings a very large improvement in signal-to-noise ratio and a superior immunity to vibrations and temperature drifts compared to conventional instruments.  Figure 5 shows an instrument for the optimal coverage of a microscope sample with nanoparticles. This embodiment includes a microscope sample 20, for example a grid for a transmission electron microscope, mounted on a sample holder 21 , where the aerosol flows past, before it leaves the instrument through the outlet 22. An electric field deposits the charged particles on the sample. This field can be produced by applying an electrical voltage to the sample holder. For optimal results, and to ensure an optimal use of the microscopic analysis, which is costly, particles should be collected until there is a sufficient coverage of the sample, but no longer - i.e. there should be neither too few nor too many particles on the sample. Since the particle concentration is measured continuously during the sampling process by the measurement electrode, it is possible to judge when enough particles have been sampled. The sampling process can then be stopped automatically. The instrument can also be operated in a mode, where it only samples particles when the particle concentration exceeds a given threshold level. The sampling of particles on the microscope sample 20 can be switched on or off by
diverting the aerosol flow, or acting on the collecting field, or by any other suitable means.
 Standard methods for sampling particles for microscopy are the direct aspiration of particles on porous filters, the deposition by
thermophoresis or by electrical charging followed by the deposition in an electrical field. In the cases of aspiration on filters as well as
thermophoresis, the necessary sampling time can only be guessed at by experience. In the case of electrical charging followed by electrostatic deposition, it is possible to measure the current that flows onto the sample, but only with a lot of effort, since the sample is at a high electric potential which makes the current measurement much more complicated.
Advantages compared to the state of the art are therefore: a. Integration of the measurement of the lung-deposited surface area in the instrument, so that the instrument can at first be used for the detection of particle sources without sampling. b. Simple measurement of the necessary sampling time. c. Sampling only when a threshold value is exceeded.
 Figure 6 shows a variant of the inventive instrument for non- contact measurement of particle number and average particle diameter, or the particle size distribution. In this embodiment, a plurality of Faraday cages and electrometers 31 are used in a cascade, and the ionized aerosol traverses the Faraday cages 31 in succession. Between the cages, a fraction of the particles is deposited by an electric field that is applied by deposition electrodes (30). In the electrical field, small particles are deposited
preferentially, as they have a higher electrical mobility than larger particles. Therefore, the first cage measures all particles, and then the first deposition electrode removes the smallest particles. In the second cage, only the remaining particles are measured, the difference between the signals of the first two cages therefore corresponds to the smallest particles. On further deposition electrodes, larger particles can be removed by means of
higher electrical fields or longer drift times, thereby more size classes can be resolved. Every single cloud of charged aerosol then produces the characteristic double pulse (shown in Figure 8), consisting of a positive pulse 50 and a negative pulse 51 ) (or vice versa, depending on the polarity of the charger) in each of the cages sequentially. In the first cage, the largest pulse amplitude is observed, and in each further cage the pulse amplitude decreases, since a fraction of the particles is removed between the cages. The instrument may have two Faraday cages, whereby it is possible to measure the average particle diameter and the particle number concentration, or multiple additional cages.
 This embodiment again has several advantages compared to the state of the art: a. Only the amplitude of the AC component of the measured
signals has to be detected. As in embodiment 1 , this is higher than in traditional instruments. b. The electrometer zero offset is again corrected for automatically c. The measured signals can all be assigned to the same aerosol cloud, therefore with N Faraday cages, N amplitudes can be measured which all belong to the same aerosol cloud. In known documents, a similar technique is described, but there, the particle deposition by the electrical field is sequential in time. When the aerosol concentration changes rapidly, an error occurs, since the signals which are compared correspond to different aerosol concentrations, which leads to meaningless results. d. The Faraday cages measure only induced currents. In the known devices, the measured current is a combination of induced currents and currents due to particle capture. The induced currents have to be corrected for with complex methods, which is not the case in our invention. Furthermore, in the devices of the prior art, particles are collected in porous bodies, which have to
be cleaned periodically, as their particle size dependent collection efficiency changes with dirt loading. This problem does not occur in the case of collection by an electrical field.
 Reference numbers used in the drawings
2 corona wire
5 Faraday cage
10 conductive tube
1 1 conductive mesh
1 1 electrode
13 flat surface
1 5 channel
20 microscope sample
21 sample holder
30 deposition electrodes
31 electrometers and Faraday cages
62 buried electrode
65 insulating tube