WO2016197259A1 - Method and device for detecting aerosol particles - Google Patents

Abstract

In accordance with the invention, a method is provided for measuring a property of particles of an aerosol suspended in a carrier gas, in which primary electromagnetic radiation with a defined wavelength or a plurality of defined wavelengths is radiated onto the carrier gas, a measurement signal is generated from elastically scattered components of the primary radiation and the property is established from the measurement signal. The method is distinguished by virtue of the defined wavelength, or one of the defined wavelengths, being matched to a characteristic wavelength of at least one particle type potentially occurring in the aerosol.

Description

 METHOD AND DEVICE FOR DETECTING

AEROSOL PARTICLES

The invention relates to a method and a device for the selective determination of a certain class of particles suspended in a carrier gas (aerosols).

Aerosols play an essential role in the atmosphere, e.g. because of their negative health effects, as they can cause damage to buildings, cloud visibility, influence climate change and, particularly in the case of particles resulting from volcanic eruptions, can endanger air traffic.

At the altitudes where air traffic takes place, there are often water droplets or ice crystals that can be of the same size as volcanic particles. A method for the detection and quantitative measurement of volcanic particles must be able to distinguish these from water or ice particles. A similar need exists in connection with other aerosols, for example in the measurement of particularly health-endangering respirable particles. There too, one must be able to distinguish the harmful particles from harmless water or salt particles. There are already devices that are trying to achieve this goal based on light scattering. These exploit the fact that polarization is retained in the case of light scattering on spherical particles, but is completely or partially lost in other geometries (for example, H. Kobayashi et al., Developement of a polarization optica! Particle counter ..., Atmospheric Environment 97, 486 -492, 2014). Accordingly, when polarized light is irradiated, a certain selection can be made from the measurement of the degree of polarization of the scattered light because the water droplets are spherical, but the volcanic particles are not always. This method is problematic in the discrimination of ice, since these crystals are usually not spherical. This concept is described in US 2014/0330459 AI and US 8,724,099 B2. While in the first case the measurement is made at a very short distance, in the second a Doppler light detection and ranging (LID AR) system is shown, which also determines the depolarization in addition to the usual signal evaluation of these systems

Another way to obtain particle-specific information is to measure not only the elastically scattered light, but also fluorescent light. This is mainly done for the detection of bioaerosols, e.g. in WO 2005/029046 A2. There, a combination of elastic scattered light measurement, fluorescence measurement and time of flight measurement is described, which allows a fairly broad characterization of aerosol particles, including measurement of mass, density, etc. However, such a method is limited in its applications to certain types of particles.

DE 102007021 452 A1 describes a method and a device for detecting particles of specific particle size. For this purpose, an aerosol to be examined is irradiated with a first wavelength, which is smaller than the size of the particle to be detected, and a second wavelength, which is greater than the size of the particles to be detected. The light scattered by the aerosol is detected for the two wavelengths and from detected signal differences, it is concluded that there are precursors of particles of the specific particle size. The method described uses the dependence of the scattering properties on the particle size and only works if the refractive index does not change between the different particles examined.

The measuring device shown in DE 102009046279 A1 is based on a similar functional principle which is dependent on the particle size for determining the particle mass concentration in a sample gas. The measuring device has a measuring gas channel for receiving the measuring gas, radiation sources for generating light beams of different wavelengths, detection means for determining the portions of the light beams of different wavelengths scattered in the measuring gas channel and a computing unit for determining the particle mass concentration from the determined portions of the scattered light beams. The wavelengths of the light rays are determined on the basis of the size of the types of particles to be determined, and it is assumed that the particle types to be measured are known, so that characteristic curves can be generated and stored in the measuring device.

US 2015/0120092 A1 shows a system for detecting water and / or ice, or methods for distinguishing ice and water droplets. It exploits the fact that the absorption behavior of liquid water is reversed with respect to that of ice at a wavelength of 2.15 μm by illuminating a measuring area with light in the wavelength range between 2.05-2.3 μm, the radiation or reflection from the measuring area in the wavelength ranges 2.05-. 2.15 μαι and 2.15-2.3 microns is measured and a ratio between the two signals thus generated is formed. With this method it is possible, if appropriate with the determination and inclusion of further quantities (temperature, reflection coefficient, radius of water drops), to distinguish ice particles, liquid water drops and supercooled water drops. A Identification or differentiation of further particle types is not possible.

No. 8666 S70 Bl shows methods for the detection of suspended particles, in particular volcanic ash, outside an aircraft. In these methods, an area is illuminated with light and the intensity of the backscattered light as well as the rate of change of the intensity is determined. On this basis, and possibly by including further parameters such as flight altitude, weather forecasts, etc., it is estimated whether the aircraft enters a cloud of volcanic ash. Thus, these methods are designed for marginal regions in clouds of volcanic ash or water.

It is an object of the present invention to provide a measuring method and a corresponding device which overcome the disadvantages and limitations of the known prior art and which are particularly suitable, rapidly, especially in real time, volcanic ash or potentially harmful aerosol particles of water or ice particles to distinguish.

This object is achieved by the invention as defined in the patent claims.

According to one of the underlying concept of the invention, the finding is exploited that the light scattering capacity of a particle is significantly dependent on the wavelength of the incident light and on the particle properties, even with elastic light scattering. In the case of elastic light scattering, the wavelength of the emitted light is the same as that of the incident light. There are many approaches in the literature which are based on spectroscopic or similar methods and characterize matter - also in particulate form - on the basis of absorption or emission properties. These approaches are based on the fact that the interaction between matter and absorbed or emitted radiation is wavelength-dependent due to the quantization of the states.

Wavelength dependent measurements in the transmitted light method (i.e., the extinction principle) and on samples with defined geometric properties are known. With elastic light scattering on aerosol particles, these approaches do not work a priori, since the transmitted light method does not produce satisfactory results and backscattered radiation does not directly derive from a transition between different excited states of matter. For spherical particles, the elastic scattering properties as a function of the wavelength are described by Mie theory, which takes into account the particle size and the wavelength-dependent complex refractive index. In particular, the particle size, unknown in individual cases and varying above an aerosol, is critical.

It is a finding of the invention that wavelength-dependent measurements are nevertheless also of interest for aerosols when the radiation elastically scattered on particles is used - even though the light scattering critically depends on the particle size and particle shape and this particle size and particle shape varies greatly in an aerosol and is unknown a priori.

A method for measuring a property of particles of an aerosol suspended in a carrier gas comprises the following steps:

- Injecting electromagnetic primary radiation of a defined wavelength or of a plurality of defined wavelengths on the carrier gas; Generating a measurement signal by measuring proportions of primary radiation elastically scattered on the particles;

Determining the property from the measurement signal;

Wherein the defined wavelength or one of the defined wavelengths is tuned to a characteristic wavelength of at least one particle type potentially occurring in the aerosol, in particular by at least one of the following measures being taken:

A. The defined wavelength (eg 2750 nm) or one of the defined wavelengths is chosen so that the light scattering capacity of a particular type of particle is particularly deep or particularly high compared to another type of particle, for example with a standard particle size of 4 μm diameter (eg with an apparatus according to the standard ISO 21501-1: 2009 measured) and an angle of 150 ° between the primary light beam direction and the direction of the backscattered light (scattering angle) μm at least a factor of 5 lower or at least a factor of 5 higher than an average over another Wavelength range, eg between 400 nm and 800 nm.

B. The defined wavelength or one of the defined wavelengths is 2.75 μm ± 0.3 μm, 1.9 μm ± 0.2 μm or 5.9 μm ± 0.4 μm (at these wavelengths the light scattering capacity of water and ice particles is particularly deep).

C. The primary radiation contains portions of at least two wavelengths, and determining the characteristic from the measurement signal includes a comparison of the light scattering intensities for the two For example, forming a ratio of the light scattering intensities at both wavelengths.

"In particular, the wavelengths can be selected so that the light scattering intensities differ with a standard particle size of 4 microns diameter and a scattering angle of 150 ° at least one particle species by at least a factor of 5, at least a factor of 10 or even at least a factor of 20.

In particular, the said characteristic wavelength can be a characteristic wavelength of a material from which particles of this (first) particle type are largely, essentially, for example, except for impurities, also in the form of crystallization nuclei. In other words, the at least one particle type potentially occurring in the aerosol, which defines the characteristic wavelength, comprises a material, wherein the characteristic wavelength is a characteristic wavelength of this material.

This material may in particular be water (in solid and / or liquid form).

Optionally, a threshold can be defined and the characteristic wavelength can be distinguished by the fact that the measurement signal (scatter intensity or size derived therefrom, eg according to measure C a ratio of scattering intensities) of the at least one particle species potentially occurring in the aerosol, which via its material characteristic wavelength is within a range of particle sizes regardless of the particle size below or above the threshold Such a size range may, for example, be a range between 1 and 10 μm, or between 2 and 20 μm.

According to embodiments, in particular such a threshold can be determined, and the determination of the property can be based on the fact that the measurement signal, which of a particle of the second potentially occurring particle type within a range of particle sizes (eg ash or smoke particles), regardless of the particle size above the threshold is when the corresponding measurement signal of a particle of the first potentially occurring particle type (water or ice particles) is below the detection threshold.

For single particle count measurements, the threshold can optionally be used as a discrimination threshold, i. Scattered light pulses whose intensity (or ratio of intensities) are below the measurement threshold are discarded.

Additionally or alternatively, the characteristic wavelength in Ausffihrungsformen can be characterized by at least one of the following properties:

- The real part of the refractive index of said material or the

Deriving the real part of the refractive index of said material by wavelength shows a local extremum at the characteristic wavelength;

- The imaginary part of the refractive index of said material or the derivative of the imaginary part of the refractive index of said Material by wavelength shows a local extremum at the characteristic wavelength.

In applications, volcanic, harmful and / or heat or fire particles are distinguished from harmless water / ice particles by the procedure according to the invention. said material of the first particle type is water in solid and / or liquid form.

In contrast to the prior art, the approach according to the invention is therefore based on the fact that the wavelength dependence of the elastic light scattering capacity of particles is used, not as known from the prior art for determining the particle size (it is logical that the particle size is due to physical laws critical in the light scattering ability), but it is used in generally unknown Orössenverteilung the quite surprising property that even with elastic scattering, the material composition, so ultimately the chemical properties, critical into the Lichtstreuvermögen - although by elastic scattering by definition, no transitions in the scattering material be generated.

Measure A mentioned above may mean that a search for a range of maximum or minimum light scattering capacity is carried out for a particle of interest - either of particular interest in the measurement or, on the contrary, to be hidden. This can be done purely experimentally by tuning a wavelength range using a standard sample and / or computationally and / or in combination. For example, after minima or maxima of the real part n of the refractive index or the ratio (nl) / k, where k is the imaginary part of the Refractive indices is to be searched. For very small particles, the size ((n ² -l) / (n ² + 2)) ² determining Rayleigh scattering may also be of interest. If one of these magnitudes is maximal or minimal, the light scattering power of the particles of interest at this wavelength can be experimentally compared with an average but the said wavelength range.

In the case of measure B, the wavelength can be in the range 2.75 μm ± 0.3 μm, in particular 2.75 μm ± 0.2 μm, especially 2.75 μm ± 0.1 μm, or in the range 1.9 μm ± 0.2 μm, in particular 1.9 μm ± 0.1 μm, or in the range 5.9 μm ± 0.4 microns, in particular 5.9 microns ± 0.2 microns, in reality, the light as known from monochromatic radiation, of course, has a finite line width, ie covering a narrow wavelength range compared to "white" light, for example, the line width may be at most 10% or 8% or 5% or 3% of the central wavelength.

In a combination of measure B. with the measure C., for example, one of the defined wavelengths 2.75 microns ± 0.3 microns, 1.9 microns ± 0.2 microns or 5.9 microns ± 0.4 microns, and the other between 0.4 .mu.m and 1 .mu.m, ie in near infrared or in the visible range. At 2.75 μm ± 0.3 μm, 1.9 μm ± 0.2 μm or 5.9 μm ± 0.4 μm, the light scattering capacity of volcanic ash and other typical particles is average to large and the light scattering capacity of water and ice particles is particularly low. In the range between 0.4 .mu.m and 1 .mu.m, the light scattering capacity of water and ice particles, however, is relatively large, so that a very high ratio results, which will be illustrated concretely and quantitatively below with reference to FIG. In general, for measurements of the type discussed here, the light scattering angle will be unequal to 0 (ie, it will not be measured in the transmitted light method), and preferably it will be greater than 90 °, ie, the backscattering will be measured.

Measuring methods that work with several wavelengths are not new. They are used in the already mentioned above application WO 2005/029046 A2; It is also known to use corresponding measurements to obtain information on the refractive index. According to this approach of the invention, however, the two measured quantities are now compared quantitatively - for example by ratio formation - in order to use targeted differences in the wavelength dependence of the light scattering capacity of different particles.

The measured property - this concerns the invention in general and potentially all possible measures discussed here - may include:

A measure of the concentration of selective particles, for example the.

Number or mass concentration; o Depending on the application, a measure of the concentration is sufficient, which may depend both on the mass and on the number, and from which a precise numerical determination of the number concentration or total mass is not necessarily possible). o The process is selective, taking into account certain types of particles more than others; In particular, on the one hand particles of volcanic origin and on the other water / ice particles can be considered differently, or the method can be particularly selective for particles containing a particular element or mineral. - A measure of the particle size or size distribution.

The method can include providing a detection volume and conveying the aerosol to be characterized by the detection volume, for example by active pumping means or by utilizing a flow, for example the travel wind in the aircraft or a vehicle.

In this embodiment, according to a first option, the size of the detection volume and the flow conditions may be chosen such that the method is based on single particle counting, i. with each particle flying through the detection volume, a scattered light measurement is performed (each particle generates a scattered light pulse).

Optionally, the absolute intensity at one wavelength can also be used to determine the particle size and, if appropriate, the mass, as made standard in optical particle counters or particle spectrometers. The mass concentration can then be determined from the measurement results of many particles.

According to a second option, the detection volume can be significantly increased, so that no single pulses, but a continuous or only slightly pulsating signal is generated. Then, the integral scattered light intensity can be measured, i. the measurement is summed over many particles.

As an example, the detection volume for the first option may be about 1 mm ³ , and for the second option about 1 cm ³ . The optimum depends on the aerosol concentration and can be determined in the experiment if necessary, by adaptation until a desired Lichtstreuintensitätsverlauf results

In this second option, the absolute intensity at one wavelength can optionally be used to obtain additional information about the absolute concentration (eg mass concentration).

The adjustment of the measuring volume according to the first or second option can be done with known means (focusing, etc.) and adaptation until the desired result (individual pulses or continuous signal) is reached.

In both options it is possible to measure the angular dependence of the scattered light, from which, for example, size information can be obtained, in addition to the concentration measured primarily at first.

On the other hand, the method can also be carried out as a LIDAR measurement, ie the primary radiation is directed to the aerosol, which is located in a not necessarily precisely defined, even more remote distance. Among other things, the LIDAR measurement may differ from that with a defined local detection volume in that the light beam of the primary radiation is parallel (eg collinear) for detecting scattered radiation. The scattering angle is thus in essence 180 °. The method then works even with distant particles; the particles do not have to be in a defined measuring volume. In a LIDAR measurement, the distance of the light-scattering particles from the measuring arrangement can optionally also be determined by methods known per se; From this, together with information about the light scattering intensity, it is also possible to make a statement about particle (measurement) concentrations.

An apparatus for measuring a property of aerosol particles suspended in a carrier gas comprises:

A radiation source of electromagnetic primary radiation of a defined wavelength or of several defined wavelengths;

A detection unit for generating a measurement signal by measuring proportions of the primary radiation elastically scattered on the particles;

An evaluation unit for determining the property from the measurement signal;

- Wherein the defined wavelength or one of the defined wavelengths is tuned to a characteristic wavelength of at least one potentially occurring in the aerosol particle type.

In addition, the apparatus may include means for performing any of the embodiments of the method described herein.

Applications of the inventive method include in particular the selective detection of volcanic ash and their distinction of water and ice particles, for example. At high altitude, in particular in or on airplanes on a weather balloon or in a measuring station in the mountains, for example. Of at least 3000 m.ü .M. For aircraft, both an apparatus with a defined detection volume to which an air flow is supplied and a LIDAR apparatus are conceivable. For example, the device may be positioned to measure a property of particles of an aerosol suspended within a carrier gas within the aircraft, for example within the fuselage or in a wing, the measurement being through a window, such as sapphire glass. In this case, the device can be set up for single particle counting or single particle analysis, and / or for the use of a method which is based on an integral scattered light intensity. Furthermore, the device may be arranged such that measurements on one or more aerosols are possible in the near or far region of the device.

For single particle counting, or single particle analysis, on a outside of the aircraft (and also outside the nacelle described below) volume said volume defined for example by the beam path and / or the beam characteristic of the radiated primary radiation and in its volume for the individual particle count, or Single particle analysis, be set up.

Alternatively, the device positioned within the aircraft may be used in combination with a local, flow-through detection volume located within the aircraft, particularly in the vicinity of the device. This allows the use of a method which is based on single particle counting, or single particle analysis, in particular in the vicinity of the radiation source. However, the use of a method which is based on an integral scattered light intensity is possible even if there is a local, throughflowed detection volume. It is also possible to mount the device outside the aircraft, for example in a nacelle. The nacelle may for example be mounted below a wing. In this embodiment as well, the device can be set up for single particle counting, or for single particle analysis, and / or for the use of a method which is based on an integral scattered light intensity. The detection volume can in turn be located in the outer area of the nacelle, or of the aircraft, or in a local, traversed detection volume. By integrating a detection volume of the type mentioned in the nacelle, a method can be used which is based on single particle counting or single particle analysis, in particular in the vicinity of the radiation source. However, even with an existing detection volume of the type mentioned, it is possible to use a method based on an integral scattered light intensity.

Irrespective of whether the device is mounted in the aircraft, for example in the fuselage or in a wing, or outside the aircraft, for example in the previously described nacelle, the device may be arranged such that measurements on an aerosol in the near or far region of the aircraft Device are possible.

Selective suppression of water droplets could also be of interest as a further application for smoke detectors because these water droplets are often the cause of false alarms.

By choosing different wavelengths, other materials can also be selectively detected. It is also not limited to two wavelengths, the use of more than two wavelengths can further increase the selectivity or simultaneously several different substances can be detected. It is It is conceivable that different types of mineral dust can be distinguished: hematite-containing particles scatter red light more strongly than limonite or quartzite-containing particles.

Hereinafter, embodiments of the invention will be described in detail with reference to drawings. Show it:

1 shows the real part (upper curve, n) and the imaginary part (lower curve, k) of the refractive index of water in function of the wavelength; in this text, wavelengths are generally indicated as vacuum wavelengths;

FIG. 2 shows the ratio of scattering intensities at 660 nm and 2750 nm wavelength as a function of the particle size for different materials; FIG.

FIG. 3 shows a construction of a device according to the invention for carrying out the method according to the invention with a defined measuring volume; FIG.

4 shows a structure of a device according to the invention for carrying out the method according to the invention in the LIDAR method; and

5 shows a further construction of a device according to the invention for carrying out the method according to the invention with a defined measuring volume. Fig. 1 shows as an example the complex refractive index of water in function of the wavelength λ. In the lower part of the figure, the imaginary part k is logarithmically plotted, in the upper part the real part n on a linear scale. At wavelengths just below 3 microns, the complex refractive index changes greatly. Since the intensity of the scattered light depends on the refractive index, the ratio of the scattering intensity at this wavelength to another is a specific feature for water or ice.

Fig. 2 shows the ratio of scattering intensities at 0.66 and 2.75 microns wavelength as a function of particle size for water and various mineral particles. The curves represent: 101 water, 102 African mineral dust, 103: andesite, 104: basalt, 105 volcanic ash, 106: sand, 107: pumice stone.

The small real part in the refractive index of water at 2.75 micron wavelength leads to a small scattering intensity and thus to a high intensity ratio. In application of the inventive approach, it can be seen that water can be distinguished from the other particles by this method. It is not necessary to know exactly the refractive indices of the other materials.

A possible construction for a measuring apparatus is shown in FIG. The beams of an IR laser 3 and a visible laser 1 are focused by two mirrors 9 on the same detection volume 6 (typically a few mm ³ , ie, for example, between 1 mm ³ and 100 mm ³ ). The optics of two detectors (7 visible) and 8 for IR light) is also directed to this volume. The detectors are wavelength-selective, ie they contain, for example, a bandpass filter which is adapted in each case to the wavelength of the primary radiation. An airflow 5 passes through this Volume. A particle that flies through the volume with the air stream 5 simultaneously generates a scattered light pulse in both detectors. The measurement signals are fed to an evaluation unit 20 and evaluated there, by deducing from the measurement signals at least one property of the aerosol. The evaluation unit may include a dedicated control electronics of the detectors and / or generic data processing means (computer). Not all elements of the evaluation unit must physically be present in the same location, and also approaches with arranged at different locations, via signal and / or data processing connections interconnected subunits are conceivable.

For each such pair of pulses, the intensity ratio is determined and used to assign the associated particle to a category (water or mineral).

The absolute intensity at one wavelength can then optionally be used to determine the particle size and optionally the mass, as is done by default in optical particle counters or particle spectrometers. The mass concentration can then be determined from the measurement results of many particles.

Instead of analyzing individual pulses as described above (single particle analysis), the detection volume can also be significantly increased, so that at the same time there is a larger number of particles which generate a continuous or only slightly pulsating scattered light signal, which is then evaluated. The selection is made again from the intensity ratio, the particle size is no longer detectable. The integral intensity can also be used to estimate the mass concentration. In addition to the wavelength, the scattering angle (angle between incident light and light detected by the detector, in the example shown is α for the visible light and β for the IR light, the scattering angle is defined in this text so that a scattering angle of 180 ° of a backscatter back in the direction from which the primary radiation comes) corresponds to a parameter that can be optimized so that the desired selectivity is as large as possible.

Since the angle dependence of the scattered light is different depending on the particle size, an optional multi-angle measurement can additionally provide size information.

In further variants of the structure according to FIG. 3, only one light source and only one detector are used, for example at a wavelength of 2.75 μm, which results in a construction which is comparatively less sensitive to water / ice particles.

Another possible embodiment, which allows remote measurement as LIDAR, Fig.4. Here, the beams of the IR laser 3 and the visible laser 1 are brought into coincidence via a semitransparent mirror 12, so that a parallel beam with both wavelengths is formed. The light backscattered by a particle cloud 14 is fed via a further semitransparent mirror 13 and the two mirrors 10 and 11 to the two detectors 7 and 8. The lasers are pulsed in this case, so that the distance of the particle cloud or its density profile can be determined via a transit time measurement.

The embodiment according to FIG. 4 can alternatively be designed only with a light source and a detector and measure, for example, at a wavelength of 2.75 μm. FIG. 5 shows a further embodiment of a device according to the invention which is similar to the embodiment shown in FIG. 3 and which is particularly suitable for measurements through a window 13. The window 13 is integrated, for example, in the aircraft skin of an aircraft, for example in the region of the aircraft fuselage or in the region of a wing. However, the window 13 can also be part of a gondola of the type described above, for example.

In the embodiment shown in FIG. 5, the beams of an IR laser 3 and a visible laser 1 are combined into a beam via a first dielectric mirror 9.1 and by means of a mirror 10 onto a common detection volume 6 (typically a few mm ³ , ie, for example 1 mm ³ and 100 mm ³ ) Focused The optics of two detectors (7 for visible and 8 for IR light) is also directed to this volume The detectors are again wavelength-selective, ie they contain, for example, one on the wavelength of the primary radiation adapted bandpass filter. The light scattered by the particles is directed by means of a parabolic mirror 11 onto a second dielectric mirror 9.2, which transmits the visible light and reflects the IR radiation. Analogously to FIG. 3, a particle which flies with the air flow 5 through the detection volume 6 simultaneously generates a scattered light pulse in both detectors. The mirrors, lasers and detectors may be separated from the airflow by a wall 12 and the light rays are guided through sapphire glass optical windows 13. The measurement signals are in turn supplied to an evaluation unit 20 and evaluated there, by inferring from the measurement signals on at least one property of the aerosol. The evaluation unit may include a dedicated control electronics of the detectors and / or generic data processing means (computer). Not all elements of the evaluation unit must physically be present in the same location, and also approaches with arranged at different locations, via signal and / or data processing connections interconnected subunits are conceivable. In turn, besides the wavelength, the scattering angle (angle between incident light and light detected by the detectors, in the example shown, α for the visible light and the IR light, the scattering angle is defined in this text such that a scattering angle of 180 ° of a backscatter back in the direction from which the primary radiation comes) corresponds to a parameter that can be optimized so that the desired selectivity is as large as possible.

Claims

1. A method for measuring a property of particles of an aerosol suspended in a carrier gas comprises the following steps: Irradiating electromagnetic primary radiation of a defined wavelength or of a plurality of defined wavelengths onto the carrier gas; Generating a measurement signal by measuring proportions of primary radiation elastically scattered on the particles; • determining the property from the measurement signal; • Wherein the defined wavelength or one of the defined wavelengths is tuned to a characteristic wavelength of at least one potentially occurring in the aerosol particle type. 2. The method of claim 1, wherein the particle type is a first particle type, wherein the characteristic wavelength is a characteristic wavelength of a material of the first particle type. 3. The method of claim, wherein the characteristic wavelength is characterized by at least one of the following properties: The real part of the refractive index of said material or the derivative of the real part of the refractive index of said material after the wavelength shows a local extremum at the characteristic wavelength, The imaginary part of the refractive index of said material or the derivative of the imaginary part of the refractive index of said material by wavelength exhibits a local extremum at characteristic wavelength. 4. The method of claim 2 or 3, wherein the material is water in any state of aggregation. 5. The method of claim 2, wherein the aerosol comprises a second potential particle type with a second material, and wherein the characteristic wavelength is selected such that the light scattering power of particles of the first particle type for each particle size is smaller than the light scattering power of Particles of the second particle type. 6. The method of claim 1, further comprising establishing a threshold and determining the property that the measurement signal of a particle of a second potentially occurring particle type within a range of particle sizes is above the threshold independent of the particle size, and the corresponding measurement signal of a particle of a first potentially occurring particle type is below the threshold. 7. The method according to any one of the preceding claims, wherein the defined wavelength or one of the defined wavelength is selected so that the Lichttreuvermögen a particular particle type is particularly low or special high. 8. The method of claim 7, wherein the Lichttreuvermögen a particular type of particle is particularly deep or special high by a standard particle size of 4 microns diameter and a scattering angle of 150 ° by at least a factor of 5 deeper or at least a factor of 5 higher than an average over wavelengths between 400 nm and 800 nm. 9. The method according to any one of the preceding claims, wherein the defined wavelength or one of the defined wavelengths is 2.75 microns ± 0.3 microns or 1.9 microns ± 0.2 microns or 5.9 μπι ± 0.4 microns The method of any one of the preceding claims, wherein the primary radiation includes portions of at least two wavelengths, and wherein determining the characteristic from the measurement signal includes comparing the light scattering intensities at the two wavelengths. The method of claim 10, wherein the comparison includes forming a ratio of the light scattering intensities at both wavelengths. A method according to claim 10 or 11, wherein the wavelengths are selected such that the light scattering intensities differ by at least a factor of 5 for a standard particle size of 4 μm diameter and a scattering angle of 150 ° for at least one particle type. 13. The method according to any one of the preceding claims, wherein the steps of "irradiation of electromagnetic primary radiation" and "generating a Measuring signal "be carried out at an altitude of over 3000 m above sea level. 14. The method according to any one of the preceding claims, wherein a detection volume is provided and the aerosol to be characterized by the detection volume is promoted, on which the primary electromagnetic radiation is irradiated. 15. The method of claim 14, wherein the detection volume is selected so that the measurement signal comprises individual measurement pulses per particle. 16. The method of claim IS, wherein an absolute intensity of the measuring pulses is measured. 17. The method of claim 14, wherein the scattered light intensity is summed over many particles. 18. A method according to any one of claims 1-13, wherein the light beam of the primary radiation is parallel to the direction of the detected scattered radiation. 19. The method according to any one of the preceding claims, wherein the scattering angle is at least 90 °. 20. Device for measuring a property of suspended in a carrier gas particles of an aerosol, in particular with a method according to one of the preceding claims, comprising: - A radiation source (1, 3) of electromagnetic primary radiation of a defined wavelength or of a plurality of defined wavelengths; A detection unit (7, 8) for generating a measurement signal by measuring proportions of the primary radiation elastically scattered on the particles; - An evaluation unit (20) for determining the property of the measurement signal; - Wherein the defined wavelength or one of the defined wavelengths is tuned to a characteristic wavelength of at least one potentially occurring in the aerosol particle type 21. An aircraft equipped with an apparatus according to claim 20. 22. With a device according to claim 20 equipped system for the detection of smoke.