WO2018083673A1

WO2018083673A1 - Apparatus for the physical characterization of tephra particles falling in the air

Info

Publication number: WO2018083673A1
Application number: PCT/IB2017/056908
Authority: WO
Inventors: Emanuele MARCHETTI; Dario DELLE DONNE; Giacomo Ulivieri
Original assignee: Item Srl; Universita' Degli Studi Di Firenze
Priority date: 2016-11-04
Filing date: 2017-11-04
Publication date: 2018-05-11
Also published as: IT201600111417A1

Abstract

An apparatus for the physical characterization of tephra particles comprises an optical barrier device consisting of a laser source that operates in ON / OFF mode at frequencies of the dozens of KHz and a central unit that acquires data related to the ON mode and the OFF mode to calculate at least the speed and size of the particles passing through the optical barrier and then transmit them to remote destinations in the form of aggregate information sets. Data acquisition, processing and transmission are done automatically and cyclically at regular intervals. The apparatus is advantageously completed by a second data acquisition unit which also allows estimates, by weight and level measurements, of the amount of tephra felt in a certain period of time.

Description

Title of Invention: APPARATUS FOR THE PHYSICAL CHARACTERIZATION OF TEPHRA

PARTICLES FALLING IN THE AIR

Technical Field

[1] The present invention relates to the sector of laser source optical barrier devices used for the physical characterization of small particles.

State of the art

[2] Physical characterization of bodies, especially of small and simple particles found in nature (rain, snow, dust, etc..) is primarily important for the development of calculation models at the base of meteorological studies. Having accurate and detailed information about the composition, weight, drop speed, finesse, etc. of rain drops, for example, makes it possible to provide a more accurate forecast of the development of weather conditions in a given place.

[3] To achieve these goals, they have been already developed rainfall physical characterization tools capable of surveying on the field the rainfall falling into a certain place for a period of time. One of these instruments is named disdrometer and is a tool designed to simultaneously output the measurement of the hydrometeors diameter and fall speeds passing through a laser matrix. It consists of a radiant laser source located in front of an optical sensor. When the laser bundle is interrupted by a hydrometeor, the instrument analyzes this interruption and determines the type of precipitation, speed and size. The tool measures hydrometeors with an equivalent diameter ranging from 0.3 to 30 mm and falling speeds up to 20 m/s. By representing the hydrometeors in a plane identified by the diameter and by the fall speed, it is possible to classify the typology in hail, rain, drizzle, graupel, or particles of accretion or melting.

[4] This instrument is already in use in the most advanced meteorological stations and especially in places where knowledge of the upcoming weather conditions is crucial, such as airports.

[5] An additional type of particles physical characterization is that one required for a better modeling of the atmospheric ash dispersion following volcanic eruption. This dispersion is strongly regulated by the granulometric distribution of the eruptured material, with the fine ash transported far into the plume and the largest tephra grain deposited shortly from the mouth. Consequently, if we want to get a more reliable estimate of ash concentration in the atmosphere, we need information on the grain size. The particle size of the ashes in suspension may be deduced from the particle size distribution of the relapse material, deposited at various distances from the eruptive mouth. In order to obtain useful information for the study of eruptive phenomena, it is necessary to have an instrument capable of discriminating and measuring extremely small particles, preferably up to a few tens of μηι, such that conventional disdrometers cannot be used in this field.

[6] In JP 2007 327889 A is described an apparatus for the detection of volcanic ash particles. Such apparatus is realized by means of an optical barrier device comprising: a source section provided with a laser emitter; a receiving section comprising photodetection organs and disposed facing the source section; an emission mask associated with the receiving section to determine the geometry of the laser beam reaching the photodetection organs; a central unit that receives and processes data from photodetection organs to calculate physical characterization parameters of tephra particles and to send control signals to an external unit. In the aforementioned apparatus, the laser emitter emits the laser beam according to a frequency modulation sinusoidal emission mode. In addition, in this apparatus an evaluation of the weather conditions is performed by the presence of two photodetection organs, one of which is associated with the receiving section and the other is associated with the source section for the detection of the scattered light. The apparatus described above is able to carry out an environmental light assessment only at the expense of a complex and expensive hardware structure. In addition, this apparatus cannot aggregate the information related to the physical characterization of the tephra particles to send them to remote destinations in realtime.

[7] For this reason, it is necessary to develop an instrument that can carry out this kind of analysis and that make it possible to better model the atmospheric evolution of volcanic ash. This would also improve the weather forecasts following a volcanic eruption and thus better manage the surrounding airspace and near volcanoes in similar situations.

Summary of the invention

[8] Object of the present invention is therefore to propose an instrument capable of performing a physical characterization of tephra particles even smaller than 100 μηη and in particular capable of calculating, based on the data obtained, physical properties such as size and falling speed of such particles.

[9] Another object of the present invention is to propose an instrument capable of performing such physical characterization on the field, i. e. located directly on the slopes or near the mouth of volcanoes, with the possibility of providing on remote locations the parameters of the particles characterization features.

[10] Another object of the present invention is to propose an instrument capable of performing calculations and processing data detected in "onboard" mode, that is directly in correspondence of the measuring station and which can send only the final results of that processing. [11] Another object of the present invention is to propose an instrument capable of transmitting the aforementioned results in wireless mode, i. e. with the aid of remote sending protocols, and that such transmission is carried out continuously at regular intervals.

[12] Another object of the present invention is to propose an instrument capable of also measuring and calculating the characteristic values of weight and volume deposition of the tephra particles.

[13] These and other objects are fully achieved by means of an apparatus for the physical characterization of tephra particles falling in the air comprising an optical barrier device, disposed on a self-supporting structure, and a central unit physically connected to the optical barrier device. Such an optical barrier device comprises a source section suitable for the emission of a laser beam and a receiving section comprising photodetection organs. The source section includes a laser emitter and a mask, integral with the emitter, to determine the geometry of the laser beam emitted from the source section. It is also suitable for emitting the laser beam through an ON mode and an OFF mode alternating at a certain working frequency. The mask is such that the laser beam emitted from the source section does not exceed 1 mm. The receiving section is arranged to detect data both during the ON mode and during the OFF mode of the source section. Both source and receiving sections are arranged facing each other at a certain distance. The central unit includes an acquisition unit for receiving data captured by the photodetection organs, a processing unit for processing data, and a communication unit capable of communicating remotely the parameters calculated by means of a wireless mode communication. In addition, the processing unit is suitable for processing the data captured by the photodetection means and for calculating the parameters constituted by the physical parameters of the tephra particles. These parameters include at least the granulometry and the falling speed of the particles in question.

[14] Advantageously, the laser beam has a substantially rectangular shape with a width of between 2 cm and 4 cm.

[15] Another advantage is when the central unit is located externally and in close proximity to the optical barrier device.

[16] Still advantageously, the receiving section includes a low pass filter for minimizing the environmental noise, a focal lens for focusing the laser beam toward the photodetection organs, and a signal amplifier.

[17] Another advantage is when the central unit includes a power supply unit to supply power to the same central unit.

[18] Advantageously, photodetection organs are constituted by a single photodiode. [19] Another advantage is when the source section and the receiving section are mounted on a self-supporting structure in oscillating mode independently of each other to allow alignment between them.

[20] Still advantageously, the central unit is capable of cyclically sending the calculated parameters according to a regular sending interval that is a time interval between the sending of calculated parameters and the sending of subsequent calculated parameters. The central unit is adapted to acquire from the photodetection organs the data used for the calculation of such subsequent calculated parameters according to an acquisition interval included in this transmission interval. Additionally, the sending interval includes the acquisition interval and a processing interval for calculating subsequent calculated parameters.

[21] Another advantage is when the device is suitable for operating together with a second apparatus suitable for performing a physical characterization of tephra particles falling in the air. This second apparatus comprises a self-supporting concave element suitable for keeping the concavity facing upwards, a concave collector, removably fixed inside the concave element, suitable for keeping the concavity facing upwards and for collecting inside the tephra particles falling in the air, a first sensitized element suitable for detecting the weight of tephra particles deposited inside the collector and a second sensitized element suitable for detecting the thickness of the deposit of tephra particles in the collector.

[22] Advantageously, the processing unit is suitable for processing data detected by the first and second sensitized element and for calculating calculated parameters consisting of the physical parameters of the tephra particles. These calculated parameters include at least the deposit weight and volume of the particles.

Brief Description of the Drawings

[23] The features of the invention and its advantages will be apparent from the following description of a preferred embodiment, with the aid of the accompanying drawings, in which:

• Figure 1 shows a schematic front view of an optical barrier device of the apparatus of the invention;

• Figure 2 shows a schematic sectional view of the source section of the optical barrier device of Figure 1 ;

• Figure 3 shows a schematic sectional view of the receiving section of the optical barrier device of Figure 1 ; • Figure 4 shows a schematic front view of an embodiment of an apparatus according to the present invention comprising the optical barrier device of Figure 1 ;

• Figure 5 shows a schematic diagram of the operation of the central unit of the apparatus of Figure 4;

• Fig. 6 shows a schematic diagram of a data transmission interval of the apparatus of Fig. 4.

• Fig. 7 shows a diagram illustrating a mode of operation of the optical barrier device of Fig. 1.

Description of the preferred embodiments

[24] Referring to the figures, a preferred embodiment of the apparatus according to the present invention is described.

[25] An apparatus according to the invention comprises an optical barrier device, 1 , as shown in FIG. 1 , comprising a source section 10 and a receiving section 1 1 mounted with oscillating pivots 13 and 14 so as to allow oscillation around a horizontal axis of the two sections, independently one from the other, on a self-supporting structure, 12. The self-supporting structure 12 comprises, in the embodiment shown, an upright 121 , provided with a pedestal 122, from which two

symmetrically arranged support arms 123, 124 branch off. Obviously, the supporting structure 12 may also have a very different shape, for example with a tripod bottom part, as long as it is configured to ensure stable ground support. The source section 10 and the receiving section 1 1 are mounted at the end of the support arm 123 and of the support arm 124, respectively, facing each other at a certain distance between them in the order of centimeters or dozens centimeters.

[26] The source section 10, as shown in detail in FIG. 2, comprises a laser emitter 101 , to which an emission mask 102 is joined together, with modeling function of the laser beam, L, emitted by said laser emitter 101 , with the laser emitter 101 and the emission mask 102 housed inside a support body 104, also including feed and control members, not represented. The emission mask 102 has an opening permitting the selective passage of the laser beam L. The opening of the mask 102 is advantageously of a rectangular shape with a width of about 3 cm and a height smaller than 1 mm. The laser emitter 101 is chosen so that the laser beam L is as homogeneous as possible across the entire area defined by the opening of the emission mask 102.

[27] Advantageously, the source section 10 comprises at least two emission masks 102 selectably positionable in front of the laser emitter 101 and having a different opening height. Indeed, as will be better illustrated in the following description of the invention apparatus operation, the calculation of the size and speed parameters of the tephra particles can be done by knowing also the height of the laser beam L and in order to discriminate particles of extremely small dimensions such as volcanic ash, it is essential that the laser beam height has values smaller than a millimeter.

Moreover, since an optimization of the speed calculation and an optimization of the particle size calculation require different heights of the laser beam L it is advantageous to vary the aforesaid height of the laser beam L.

[28] More generally, in embodiments of the present invention, the emission mask 102 is in each case included between the source section 10 and the receiving section 1 1 to determine the geometry of the laser beam.

[29] Facing the source section 10, at a certain distance, there is the receiving section 1 1 , as shown in FIG. 3, which comprises, arranged in the order from the closest to the farthest from the source section 10: a focal lens, 1 1 1 , for focusing the laser beam L, a filter, 1 12, for the environmental noise minimization, photodetection means, 1 13, and a signal amplifier, 114. Photodetection means 1 13 are advantageously constituted by a single photodiode, so as to significantly reduce the overall height of the sensitized area and thus avoid proper measurement to be altered by diffraction phenomena of the laser beam L which are likely due to the particles size to be detected.

[30] The mounting by means of the oscillating pivots 13 and 14, obviously provided with retaining members, allows to adjust and fix the horizontal inclination angle of the two sections 10 and 1 1 to allow the perfect alignment between the source section 10 and the receiving section 1 1 , so that the laser beam L emitted from the source section 10 can point correctly toward the photodetection means 1 13 of the receiving section 1 1. In the event that they are not perfectly aligned, the laser beam L would not be able to hit the photodetection means 1 13 by compromising the correct interpretation of the data captured by the optical barrier device. In fact, the laser beam L must be able to fully cross the whole receiving section 11 by passing through each single component (focal lens 1 1 1 and filter 1 12) in order to suitably allow the beam concentration toward the photodetection means 1 13. Since both the laser beam L and the photodetection means 113 have an extremely low height, to ensure the alignment of the laser beam L on the photodetection means 1 13, it is necessary to be able to precisely tuning the angle of inclination of each section with respect to the support 12.

[31] As can be seen in Fig. 4, the described embodiment of the apparatus of the invention also includes a control central unit 2, and a second data acquisition device, 3. [32] The control central unit 2 is integrated by means of physical connection organs, 4, of the wired type, to the optical barrier device 1 and is physically mounted under the second data acquisition device, 3.

[33] Referring to Figure 5, the central unit 2 is in turn composed of other specific and interoperable functional units. An acquisition unit 21 is adapted to receive the data collected from the three sensorial sources present on said devices 1 and 3, namely the photodetection means 113, said weight sensor 33, and said level sensor 34. The data collected by the acquisition unit 21 are then sent to a processing unit, 22, suitable for processing them to obtain certain calculated parameters that characterize the phenomenon of the tephra fall from a physical point of view, that is, particles granulometry, their falling speed, the weight and the volume of particles deposited per time unit. Once finished processing, the calculated parameters, appropriately aggregated, are passed to a third unit, which is a communication unit, 23, adapted to send them remotely via a wireless connection. Finally, there is a supply unit 24, which provides electrical power, preferably 12V DC, to the entire complex of said central unit 2.

[34] Further functional units are advantageously contained in the central unit 2. A time reference module (GPS), 25, is provided for timing synchronization of said processing unit 22. In this way, both raw and processed data, stored in an internal memory of the processing unit 22, are timely stamped. A local service interface, 26, allows local access and control, for example to allow remote system administration tasks that are not remotely accessible, even though the device can operate completely autonomously and be remotely controlled. A local storage interface, 27, allows to copy both raw and processed data to a portable memory (such as a portable storage device with USB interface). Battery power supply organs, 28, for overall power supply.

[35] The second data acquisition device 3 comprises a concave housing, 31 , of the box type open at the top, in which is fixed in a detachable mode a concave collector, 32, within which falling tephra samples are deposited. The concave collector 32 is firmly positioned on a metallic plate, 36, at the center of which a weight sensor is fixed, 33. Above the concave housing 31 there is a level sensor 34, preferably of the ultrasonic type, for measuring the level value of the material accumulated in the concave collector 32, a value that allows calculation of the volume of the deposited material in the second data acquisition device 3.

[36] The operating mode of the apparatus of the invention is dependent on the operating mode of the optical barrier device 1 , of said central unit 2 and of said second data acquisition device 3.

[37] In the optical barrier device 1 , the amount of light radiation affecting the photodetection means 1 13 is measured, in particular in the relative working frequency spectrum, which causes the latter to transmit proportional voltage signals amplified by the signal amplifier 1 14. When the source section 10 emits the laser beam L, the photodetection means 1 13 transmits a signal proportional to the luminous intensity of the laser beam L, which will be attenuated in the presence of falling particles passing through the laser beam L, shading the photodetection means 1 13 at a ratio proportional to the area of their projection on the photodetection means 113 itself. When the source section 10 does not emit the laser beam L, the photodetection means 1 13 transmits a much smaller signal as it is proportional to the ambient light affecting the detector, which also depends on the amount of particles present in the environment. Since the apparatus of the invention must be able to discriminate and measure particles of extremely small dimensions, the ambient light (noise) cannot be considered irrelevant and it is therefore necessary to measure and process the latter as well. For this purpose, the acquisition unit 21 is configured to command the laser emitter 101 to modulate the emission of the laser beam L in the form of a square wave with frequency of 30 KHz, resulting in an ON / OFF operation mode type. The photodetection means 1 13 operates in continuous mode so that if no particle obstructs the passage of the laser beam L, the photodetection means 1 13 will provide a square wave signal on its analog output. During the ON state of the source section 10, the presence of tephra particles (or other objects) in the laser beam L will produce a shadow effect on the photodetection means 1 13 that will provide a proportional decay of the output voltage. During the OFF state of the source section 10, the signal level of the photodetection means 1 13 will be proportional to the ambient light level in the spectrum frequency of the photodetection means 1 13. Even in the OFF mode of the source section 10, the presence of any particles near the photodetection means 1 13 will produce a small amplitude decay of the signal emitted by the photodetection means 1 13.

[38] The effect on the output signal from the photodetection means 1 13 produced by the passage of a single particle through the laser beam L is shown in Fig. 7, in which it is shown a Cartesian chart showing the amplitude of the output signal from the photodetection means 113 on the x- coordinate and the time corresponding to the passage of a particle in the y-coordinate. Here, the meaning of the labels shown in the figure and the following:

[39] - LH (high level) represents the amplitude of the output signal of the photodetection means 1 13 when the laser beam L is output (ON mode). Its maximum level (LHM), which is recorded when there are no particles passing through the laser beam L, as for the first bar and the last bar of the time series of fig. 7, depends on the characteristics of the photodetection means 1 13 and on the alignment of the source section 10 with the receiving section 1 1 , and is constant during the operation of the sensor. [40] - LL (low level) represents the amplitude of the output signal of the photodetection means 113 when the laser beam L is not output (OFF mode). As with LH, the maximum level (LLM) is recorded when there are no particles passing through the laser beam L, as for the second and the penultimate bar in the time series of FIG. 7. LLM depends on ambient light in the frequency spectrum of the photodetection means 1 13 and is so variable with the day time, the cloudiness and the thickness of the ash cloud in correspondence of the optical barrier device 1.

[41] - A1 (high level attenuation) represents the attenuation measured by the photodetection means 1 13 when the laser beam L is output and is obtained by the difference between LHM and LH. It is proportional to the size of the particles passing through the signal 103 at that specific time.

[42] - A2 (low level attenuation) represents the attenuation measured by the photodetection means 1 13 when the laser beam L is not emitted.

[43] The output signal amplitude decay, which is representative of the shadow entity produced by the particle passing through the laser beam L, is thus obtained from the difference between A1 and A2. This minimizes the effect of environmental noise.

[44] The typical high and low trend of the analog output signal of the photodetection means 1 13 shown in FIG. 7 is the result of the modulation of the laser beam L in the two modes described above of ON and OFF alternating with a frequency of 30 KHz. As explained above, to minimize the environmental noise, the acquisition unit 21 captures the output signal value of the photodetection means 1 13 for both the ON state and the OFF state of the laser beam L at the sampling speed of 30 KHz.

[45] Thanks to the above-described mode of operation, the apparatus of the invention is able to make environmental noise substantially non-influencing by determining the amount of ambient light and thereby estimating the atmospheric conditions without the need to provide further

photodetection meanss or other devices specially prepared for the environmental light detection.

[46] Now moving to the second data acquisition device 3, this is constituted as a self-standing collection unit , separated from the optical barrier device 1. The collector 32 is a cylindrical PVC collector (diameter 20 cm, height 22 cm) that is located inside the concave housing 31 for sampling of falling materials above the weight sensor 33. The collector 32 is advantageously fixed to the aluminum base 36 by means of magnets which allow an easy removal of the top part when required. A central hole permits the alignment of the collector 32 on the base 36 and the drainage of the water through a dedicated drainage system developed in the aluminum base 36 of the collector 32. [47] A loading cell appropriately chosen to have a resolution of the magnitude order of the gram or lower is used to obtain the weight of the ashes collected. Advantageously the operation of the weight sensor 33 is based on the use of four extensometers disposed in the Wheatstone bridge mode. To improve the reliability of the overall system, an analog-to-digital converter is integrated into the load cell sensor housing to provide a digital communication channel to the sensor electronics. In addition to the weight, the level of ashes within said collector 32 is measured with a level sensor 34 of the ultrasonic type, measuring the bidirectional distance between level sensor 34 and a reflecting surface in the range of 10-120 cm. The data acquired by the weight sensor 33 and the level sensor 34 are transmitted to the central unit 2.

[48] With reference to FIG. 6, the acquisition unit 21 captures the data of the optical barrier device 1 and of the second data acquisition device 3 for a time equal to an acquisition time interval, 51 , advantageously equal to a few seconds, preferably between 5 and 20 seconds. Preferably, with regard to the optical barrier device 1 , the acquisition takes place by passing from an AD converter with a suitable dynamic range through a two-channel system with a sampling rate equal to the working frequency of the laser emitter 101 , where a channel is devoted to the acquisition of the high level HL and the other channel is dedicated to the acquisition of the low level LL. An appropriate low pass filter is applied to clear the signal. The choice of the dynamic range of the AD converter and the sampling frequency of the acquisition channels affect the size and speed of the particles that the instrument can discriminate and are therefore suitably selected so as to allow particles size measurement even below 100 μηη.

[49] Upon completion of the acquisition time interval 51 , the data, transferred to the processing unit 22, are processed for a time equal to a processing interval, 52, to determine calculated parameters corresponding to the granulometry and the speed of the particles that have crossed the laser beam L during the acquisition interval 51 , and to the weight and level of the particles accumulated in the second data acquisition device 3. Once processing is complete, the calculated parameters are organized in a set of aggregated information able to provide a physical characterization of tephra fall, and then sent by the communication unit 23 remotely. Compared to the large amount of data acquired, the aggregate information set that is sent remotely can be contained in a data packet of a few Kbyte, making extremely easy and quick its transmission. For example, detected and measured particles are divided into dimensional classes (not more than a dozen classes) and speed classes (not more than a dozen) so that particles size and speed information can be aggregated into a matrix of dimensional classes and speed classes in which for each class is reported the number of detected particles. [50] From what above described it is easily deducible that the transmission of the aggregate information set obtained from the calculated parameters takes place cyclically according to a regular and peculiar timing. As can be seen from Fig. 6, the data sending interval, 5, that is, the time interval between sending an aggregate information set and sending the successive aggregate information set, includes the acquisition interval, 51 , and the processing interval, 52. For example, after a ten-second capture interval, a processing interval of fifteen seconds is required so that the sending interval 5, that is, the interval between two subsequent submissions, is about thirty seconds, which is more than enough with the current wireless transmission technology in order to send a data pack of a few Kbyte. Obviously, the acquisition 51 and the processing 52 intervals may also be very different and even at least partially superimposed as data processing may begin before the end of the acquisition interval 51. From the above it is concluded that the presence of the central unit 2 capable of processing the acquired data and compacting them into an aggregate information set is essential to allow the autonomous operation of the device in remote areas with limited bandwidth as it would be impossible, with the currently implemented data transmission technologies, to transmit remotely the enormous amount of raw data captured. In addition, the data acquisition is carried out for a time equal to about 1/3 of the apparatus overall work time for which you get an absolutely reliable estimate of the actual physical characteristics of the phenomenon of tephra fall analyzed. Additionally, the sending interval 5 of about 30 seconds allows to remotely obtain the physical characterization of the tephra fall phenomenon substantially in real time.

[51] The above features and advantages of the apparatus of the invention remain valid also in the presence of variants or modifications to the above described as identification.

[52] In a simpler and more economical embodiment , the apparatus of the invention does not provide the presence of the second data acquisition device 3 and only provides data on the size and speed of falling particles. In this case, the central unit 2 is physically associated with the supporting structure 12 of the optical barrier device 1 or is housed in a self-standing casing placed near the optical barrier device 1.

[53] Further embodiments of the apparatus of the invention may be realized, for example including modifications to the method of constraining the source and receiving sections 10 and 1 1 to the supporting structure 12, as well as modifications to the central unit 2 regarding the methods of acquisition and processing of the data.

[54] The above and further variations and modifications of practical application nature may of course be provided, while still remaining within the scope of protection defined by the claims below.

Claims

1. Apparatus for the physical characterization of tephra particles falling in the air, of the type comprising an optical barrier device (1), arranged on a supporting structure (12), said device (1) comprising:

= a source section (10) suitable for the emission of a laser beam (L) and comprising a laser emitter (101) and an emission mask (102), integral with said laser emitter (101) to determine the geometry of the laser beam (L) emitted from said source section (10), and

= a receiving section (11) comprising photodetection means (1 13), said source section (10) and said receiving section (1 1) being arranged facing each other with a certain distance between them, and a central unit (2), physically connected to said optical barrier device (1), comprising an acquisition unit (21) for receiving data detected by said photodetection means (1 13) and a processing unit (22) for processing said data, characterized in that said emission mask (102) is such that the laser beam (L) emitted from said source section (10) has a height not exceeding 1 mm, said source section (10) is suitable to emit said laser beam (L) through an ON mode and an OFF mode alternate according to a determined working frequency, said acquisition unit (21) is arranged to acquire data relating to output signals of said photodetection means (113) emitted both during said ON mode and during said OFF mode of said source section (10), said processing unit (22) is suitable to process said data acquired by said acquisition unit

(21) and calculate calculated parameters constituted of values of physical characterization of said tephra particles, said calculated parameters being organized by said processing unit

(22) in a set of aggregated information said central unit (2) includes a communication unit (23) adapted to communicate remotely said set of aggregated information via wireless communication mode.

2. Device according to claim 1 characterized in that said emission mask (102) has a substantially rectangular shape with a width between 2 cm and 4 cm.

3. Device according to claim 1 characterized in that said emission mask (102) is integral with said laser emitter (101).

4. Device according to claim 1 characterized in that said central unit (2) is located in the proximity of said optical barrier device (1).

5. Device according to claim 1 characterized in that said receiving section (1 1) comprises a low pass filter (1 12), a focal lens (1 1 1) for focusing said laser beam (L) towards said photodetection means (113) and a signal amplifier (1 14).

6. Device according to claim 1 characterized in that said photodetection means (1 13) are constituted by a single photodiode.

7. Device according to claim 1 characterized in that said source section (10) and said receiving section (1 1) are mounted on said supporting structure (12) in an oscillating mode independently one from the other to permit the alignment between them.

8. Device according to claim 1 characterized in that said central unit (2) is adapted to send cyclically said set of aggregated information according to a regular sending interval (5), said acquisition unit (21) being arranged to acquire data according to an acquisition interval (51) comprised in said sending interval (5), said sending interval (5) comprising said acquisition interval (51) and a processing interval (52) for the calculation of the calculated parameters and the organization of the same in said set of aggregated information.

9. Device according to claim 1 characterized in that said apparatus comprises a second data acquisition device (3) suitable to make a physical characterization of tephra particles falling in the air comprising: a concave collector (32), removably fixed inside a concave housing (31) of the box type open at the top, suitable to collect inside the particles of tephra falling free in the air, a weight sensor (33) suitable to detect the weight of tephra particles deposited inside said concave collector (32), a level sensor (34) suitable to detect the level reached by the mass of deposited particles in said concave collector (32), said acquisition unit (21) being arranged to acquire data from said weight sensor (33) and from said level sensor (34).