As a manuscript

DOLGI SERGEI IVANOVICH

LASER GAS ANALYZERS BASED ON THE DIFFERENTIAL ABSORPTION METHOD

01.04.01 - Devices and methods of experimental physics

dissertation for the degree of candidate of physical and mathematical sciences

Barnaul - 2004

The work was carried out at the Institute of Atmospheric Optics, Siberian Branch of the Russian Academy of Sciences

Scientific advisers: - Doctor of Physical and Mathematical Sciences

Professor, Corresponding Member of the Russian Academy of Sciences Zuev Vladimir Vladimirovich

Official opponents: - Doctor of Physical and Mathematical Sciences

Professor Sutorikhin Igor Anatolyevich. - Candidate of Physics and Mathematics, Senior Researcher Vladimir Egorovich Prokopiev.

Lead organization: Tomsk Polytechnic University

The defense will take place on December 15, 2004. at 14:00 at a meeting of the dissertation council D 212.005.03 at Altai State University at the address: 656049, Barnaul, Lenin Ave., 61

The dissertation can be found in the library of Altai State University.

Scientific Secretary

dissertation council Ph.D.

D.D. Ruder

Relevance of the topic. The environment undergoes changes under the influence of various factors. The rapid development of industry, energy, agriculture and transport leads to an increase in anthropogenic impact on the environment. A number of harmful by-products in the form of aerosols, gases, domestic and industrial waste waters, oil products, etc., enter the atmosphere, hydrosphere and lithosphere, which negatively affect the living conditions of man and the biosphere as a whole. Therefore, environmental control is an urgent problem of our time.

At present, chemical, thermal, electrical, chromatographic, mass spectral and optical gas analyzers are used to monitor the state of the atmosphere. Moreover, only the latter are non-contact, they do not require sampling, which introduces additional errors in the measured value. A special place among the optical methods of gas analysis belongs to laser methods, which are characterized by: high concentration sensitivity of measurements and spatial resolution, distance and speed. First of all, this concerns laser gas analyzers operating on the resonant absorption effect, which has the largest cross section for the interaction of optical radiation with the medium under study, providing the maximum sensitivity. Such gas analyzers implement, as a rule, a differential absorption scheme. With the development of laser technology in our country and abroad, optical-acoustic (for local gas analysis) and path (giving integral values ​​of the concentration of the gas under study) laser gas analyzers, as well as lidars (LIDAR, an abbreviation of the English words Light Detection and Ranging), have been developed. information on the concentration of gases in the atmosphere with spatial resolution. But at the beginning of work on the dissertation, with rare exceptions, all of them were laboratory models designed to measure one, maximum two gas components, while environmental monitoring requires a multicomponent gas analysis.

All gas constituents of the Earth's atmosphere, except for the main ones: nitrogen, oxygen, and argon, are usually referred to as the so-called minor gas constituents (MGS). The percentage of IGMs in the atmosphere is small, but the increase in their content due to the anthropogenic factor has a significant impact on many processes occurring in the atmosphere.

As is clear from the literature, the mid-IR region of the spectrum is most suitable for the purposes of laser gas analysis of the MGS. The main vibrational-rotational bands of most IGMs, which have allowed structures, are located here. High-energy molecular lasers, including reliable and efficient CO and CO2 lasers, emit in this region. For these lasers, highly efficient parametric frequency converters (PFCs) have been developed, which make it possible to quite tightly overlap the radiation lines with

the buoyant spectral int of the transparency of the atmosphere

SIMIOTEKA i

spheres. Another informative spectral range for laser gas analysis is the UV region. There are strong electronic bands of many polluting gases here. In contrast to the mid-IR region of the spectrum, the UV absorption bands are nonselective and overlapped. The greatest development in this area was obtained by the ozonometric method due to the presence here of the Hartley-Huggins ozone absorption band.

Purpose of work. Development of gas analyzers based on the differential absorption method for detecting and measuring MGM concentrations and determining their space-time distribution in the atmosphere.

In the course of the work, the following tasks were performed:

Creation of a channel for sensing the vertical distribution of ozone (VRO) in the stratosphere (based on the receiving mirror 0 0.5 m) at the Siberian lidar station (SLS);

Monitoring the state of the ozonosphere in the routine measurement mode;

Study of climatology of the ozonosphere, assessment of trends in stratospheric ozone.

The following are submitted for defense:

2. Developed models of laser gas analyzers of the TRAL series, in the mid-IR range of the spectrum, allowing to quickly measure the concentrations of more than 12 gases at and below the MPC on paths up to 2 km long using a mirror or topographic retro-reflector.

3. The UV ozone lidar created by the author based on the excimer XeQ laser, which provided uninterrupted long-term sounding of the ozonosphere over Tomsk at the Siberian lidar station in the altitude range of 13-45 km with a maximum vertical resolution of 100 m.

Scientific novelty of the work:

For the first time, informative wavelengths of sounding of the IGM atmosphere using IR molecular lasers and PPCs were selected and experimentally tested;

A number of unique mobile and stationary route gas analyzers have been created, which make it possible to quickly carry out multicomponent analysis of the gas composition of the atmosphere;

Measurements of daily variations in the concentration of MGM (such as C2H4, NH3, H2O, CO2, CO, Oz, N0, etc.) in ecologically clean regions of the country subject to significant anthropogenic load have been carried out;

Using the results of the work. The data obtained using gas analyzers were presented to the USSR Olympic Committee in 1979-1980. in Moscow, as well as to environmental organizations in the city of Tomsk, Kemerovo, Sofia (NRB), were included in the final reports of the IAO SB RAS on various RFBR grants, agreements, contracts and programs, for example TOR (tropospheric ozone research), SATOR (stratospheric and tropospheric ozone research) and others.

The practical value of the work is as follows: - an optical-acoustic gas analyzer has been developed, which allows to measure with high accuracy the concentration of both the sum of hydrocarbons of the methane group and separately methane and heavier hydrocarbons in a mixture of natural and associated petroleum gases. With the help of this gas analyzer, it is possible to search for oil and gas by gas halos of gases coming out to the surface of the earth over hydrocarbon fields;

The developed route gas analyzers make it possible to measure MGM concentrations at and below the MPC from a wide list of priority polluting gases;

Create a channel for sensing the vertical distribution of ozone SLS based on a 0 0.5 m mirror, which will allow obtaining reliable VOD profiles in the altitude range of 13-45 km with a maximum resolution of 100 m.

The reliability of the work results is ensured by: - ​​good agreement between the experimental data obtained using the developed gas analyzers, and the data obtained simultaneously by other methods, as well as; data obtained by other authors in similar climatic and ecological conditions;

Good coincidence of the VOD profiles in the stratosphere, measured by the lidar, ozonosondes data, as well as satellite measurements within the error of the devices used.

Approbation of work. The main results on the topic of the dissertation, obtained by the author, were published in 11 articles in Russian scientific peer-reviewed journals, reported at: VI, VII and XI All-Union Symposia on Laser and Acoustic Sounding (Tomsk, 1980, 1982, 1992); VI All-Union Symposium on the Propagation of Laser Radiation in the Atmosphere (Tomsk, 1881); XII All-Union Conference on Coherent and Nonlinear Optics (Moscow, 1985); V International School-Seminar on Quantum Electronics. Lasers and their application (NRB, Sunny Beach, 1988); 5th Scientific Assembly of the International Association for Atmospheric Physics and Meteorology (Reading, Great Britain, 1989); XI Symposium on Laser and Acoustic Sounding (Tomsk, 1992); And, III, IV and VI Inter-republican symposia "Optics of the atmosphere and ocean" (Tomsk, 1995, 1996, 1997 and 1999); III Siberian meeting on climate and ecological monitoring (Tomsk, 1999); I Interregional meeting "Ecology of Siberian rivers and the Arctic" (Tomsk 1999); VII International Symposium on Atmospheric and Ocean Optics (Tomsk 2000); VIII and IX International Symposia on Atmospheric and Ocean Optics and Atmospheric Physics (Tomsk 2001 and 2002); 11 Workshop on Atmospheric Radiation Measurements (Atlanta, USA 2001); IX Working Group "Aerosols of Siberia" (Tomsk 2002); 21 and 22 International Laser Conference (Quebec, Canada, 2002, Matera, Italy 2004); II International conference "Environment and ecology of Siberia, the Far East and the Arctic" (Tomsk 2003). International Conference on Optical Technologies for Atmospheric, Oceanic and Environmental Research (Beijing, China 2004).

Personal contribution. The work uses the results obtained either personally by the author or with his direct participation. This is the author's participation in the development of both general schemes for the construction of gas analyzers, and their individual optical-mechanical and electronic assemblies and blocks; installation and commissioning works. The development of measurement techniques, test and expeditionary and field tests of the created gas analyzers, also presented in the work, were carried out with the direct participation of the author. Since 1996, practically all observations of the state of the ozonosphere on the SLS were carried out with the active participation of the author. He created an improved channel for sensing the vertical distribution of ozone SLS based on a XeQ laser and a 0 0.5 m receiving mirror.

Development of infrared gas analyzers "LAG-1" and "Resonance-3" was carried out jointly with Ph.D. G.S. Khmelnitsky, the rest of the results were obtained under the guidance of Corresponding Member. RAS, Doctor of Physical and Mathematical Sciences V.V. Zuev with the participation of employees of his laboratory at different stages of work.

In the introduction, the relevance of the topic is substantiated, the goals and objectives of the study are formulated, the scientific novelty and practical significance are emphasized, and the main provisions for defense are given.

The first chapter describes the optical-acoustic method, a block diagram of an optical-acoustic gas analyzer designed for separate measurement of the concentrations of methane and other saturated hydrocarbons in air samples.

Numerous studies have shown the presence of increased concentrations of hydrocarbons (HC) in the atmosphere and soil air samples over the areas of oil and gas fields. The authors expressed the opinion that this is due to the release of hydrocarbons from the reservoir to the day surface. Geochemical methods of prospecting for oil and gas fields are based on these facts. According to the data, the percentage (by volume) composition of natural gases from deposits of the former USSR: methane 85-95%; ethane up to 7%; propane up to 5%; butane up to 2%; pentane and heavier hydrocarbons up to 0.4%. Composition of associated petroleum gases of oil and gas fields: methane up to 80%; ethane up to 20%; propane up to 16%; isobutane + n-butane up to 6%; pentane and heavier hydrocarbons up to 0.9%. Thus, pentane and heavier hydrocarbons contribute insignificantly to the gas halo content over oil and gas fields.

Rice. 1. Block diagram of a gas analyzer 1- 2-CO g laser with a diffraction grating; 4, 5 - He-Ne laser; 7, 9, 10-pulse shapers; 8-modulator; 11- modulator control unit; 12-camera spectrophone; 13-microphone; 14-selective amplifier; 15- ADC !; 16-frequency counter; 17 attenuator; 18-receiver; 19-digital clock; 20-ADC2; 21- control unit; 22 microcomputer; 23-digit printing.

When searching for oil and gas fields along gas halos of hydrocarbons emerging above the fields on the earth's surface, it is of great importance to separately measure the concentration of methane and heavier hydrocarbons, since methane can be a product not only of deep structures, but also of the upper biologically active layers and is not always a harbinger of a field. ... This is typical, for example, for Za-

Western Siberia, where methane can be generated in large quantities by swamps located on its territory, while heavy hydrocarbons are not generated in the upper layers of the earth's crust. The paper analyzes the possibility of such a separate measurement, provided that the methane content in the mixtures is no more than 100 times higher than the content of other hydrocarbons.

The developed highly sensitive optical-acoustic gas analyzer "LAG-1" makes it possible to register the concentration of hydrocarbons with any ratio of a mixture of methane and other HCs. The block diagram of the gas analyzer is shown in Fig. 1.

The gas pressure in the chamber of a cylindrical spectrophone (optical-acoustic detector) when modulated laser radiation passes through it at the modulation frequency of radiation ω, depends on the laser radiation power and, the absorption coefficient of the gas under study aop and the Q-factor of the acoustic resonator at the modulation frequency Q (ω) as:

5zhg02 [co2 + t1) "

where £) is the diameter of the cylinder; тг is the temperature relaxation time of the spectrophone.

The pressure pulsations are converted into an electrical signal by a condenser microphone type MKD / MV 101 (13). Further, the signal is amplified by a selective amplifier of the U2-8 type (14), digitized by ADC1 (15) and enters the results processing system. The laser radiation passed through the spectrophone camera is attenuated by an attenuator (17), hits a thermoelectric receiver (18), is digitized by ADC2 (20) and also enters the results processing system.

The system calculates the absorption coefficients:

and gas concentration in the case of prevailing absorption in a single line:

/ = /, 2, 3 ... n,

where l is the calibration factor of the spectrophone; n is the number of measurements; £ / s / -signal from the microphone; -signal proportional to the power of laser radiation; - the background signal of the spectrophone; mass absorption coefficient of the test gas. The result of the calculation, together with the wavelength code and time, is displayed for digital printing.

In the tuning range of the III-N-laser, the emission line at a wavelength of 1.15 μm coincides with the absorption line of atmospheric water vapor, and the 3.39 μm line coincides with the absorption bands of methane group hydrocarbons, starting with methane itself. In the range of CO2 laser wavelength tuning (9.1-10.8 mm), there are absorption bands of shock waves, starting from

ethane, thus, by measuring the concentrations of the sum of hydrocarbons and separately ethane, propane and butane, it becomes possible to determine the concentration of methane. Table 1 presents a list of these gaseous constituents, their absorption coefficients at the corresponding radiation wavelengths and CO2 lasers:

Table 1

Gas He-Me X. = 3.39 μm a, cm "1 atm" 1 CO2

A, μm a, cm "1 atm" 1

Methane 9.0 - -

Ethane 4.1 10.8847 0.5

Propane 9.0 10.8352 0.45-0.5

N-butane 12.6 10.4 762 0.9

Isobutane 13 10.8598 0.4

Due to the fact that the CO2 laser has a wide tuning range, it is possible to separately measure ethane, propane, n-butane, isobutane, ethylene and benzene and other gaseous components. It can be seen from the same table that the absorption coefficients of CO2-laser radiation by hydrocarbons are 10-20 times less than the absorption coefficients of the radiation of the III-N-laser. But for a resonant spectrophone, the sensitivity is proportional to the power of the laser radiation passing through it (formula 1), and then with the power of an LG-126 type laser at a length

wavelength 3.39 μm 8 mW, and a CO2 laser 10 W, this gas analyzer has a sensitivity 100 times higher for heavy shock waves.

Figure 2 shows the results of comparative measurements of HC obtained during one of the expeditions along the Ob River by several different gas analyzers: LAG-1 (both the sum of HCs with methane and separately heavier HCs were measured), Iskatel (the sum of HCs with methane ) and a SKR lidar (the amount of HC was measured without methane). The data obtained by all these devices indicate a sharp increase in the hydrocarbon content in the atmosphere over oil and gas fields.

Distance hmm

Rice. 2. Concentrations of hydrocarbons as measured by different gas analyzers

Away from deposits, concentrations of ethane, propane and butane are not

exceeded 0.02 million "1, methane - 1.7-2 million" 1, but as we approached the explored deposits, the concentration of heavier hydrocarbons increased significantly. So, for example, in the area of ​​the oil field in the lower reaches of the Vakh River (point 650 km in Fig. 2), the following concentrations were measured: the amount of HC 5.1 mln "1, ethane - 1.0 mln" 1, propane - 1.7 mln "1, butane - 0.3 mln" 1, with a methane concentration of 2.1 mln "1. Thus, it can be seen that with relatively small variations in the concentration of methane in the atmosphere (1.5-2.0 mln" 1), large values ​​of the amount of hydrocarbons over oil and gas fields are due to increased concentrations of heavy hydrocarbons.

The tests carried out have shown good performance characteristics of the LAG-1 gas analyzer in field conditions. The results obtained with its help are in good agreement with the results obtained on other measuring systems in the course of joint measurements, show their reliability. The use of two laser sources (He-N and CO2) and a spectrophone in the complex makes it possible to measure the concentration of a wide range of both atmospheric and polluting gases. Most importantly, it is possible to separately measure the methane fraction and heavier hydrocarbons in a mixture of natural and associated petroleum gases. This allows us to hope for the use of the proposed gas analyzer for the search for oil and gas deposits by gas halos of hydrocarbons coming out to the surface of the earth, as well as for the operational analysis of the gas fraction of cores during exploratory well drilling.

The second chapter describes a number of route gas analyzers "Resonance-3", "TRAL", "TRAL-3", "TRAL-ZM", "TRAL-4" operating on the basis of the differential absorption (DP) method. The method itself is briefly described.

The power of the optical signal received at time I, with the DP tracing method for one wavelength X, can be written as:

where Р- is the transmitted optical power (W),

g - distance (cm), s - speed of light - 3 x 1010 cm / s,

P, (r) ~ total optical efficiency of the transceiver,

<т,- поперечное сечение поглощения (см2),

A - receiving aperture (cm2),

a (g) - attenuation coefficient (cm "1),

I, is the solid angle of backward scattering of the target (cf "1),

/ "is the index of the wavelength, / = / and 2, for the wavelengths at the maximum and minimum absorption, respectively, N0 is the gas concentration (cm" 3).

For two close wavelengths, it is true:

Then the average gas concentration in the investigated volume can be expressed as follows, as well as lidars (LIDAR - an abbreviation of the English words Light Detection and Ranging), which provide information with a space-time resolution for studying the concentration of MGM in the atmosphere. But at the beginning of work on the dissertation, with rare exceptions, all of them were designed to measure one, maximum two gas components, or were laboratory models, while environmental monitoring requires a multicomponent gas analysis on fairly long routes (along city highways, territory large industrial enterprises).

As is clear from the literature, the mid-IR region of the spectrum is most suitable for the purposes of laser gas analysis of the MGS. The main vibrational-rotational bands of most IGMs are located here. There are allowed structures and individual absorption lines of almost all atmospheric gases with the exception of simple ones, such as N2, O2, H2.

As is known, high-performance molecular lasers such as CO, CO2, NH3, HF, DF and others emit in the mid-IR range of the spectrum. Of these, the most reliable and acceptable for the purpose of gas analysis are highly efficient CO2 lasers. In these lasers, in addition to the traditional 9.6 and 10.6 µm bands, sequential bands can be generated that are displaced relative to the traditional ones by about 1 cm "1, as well as the main 4.3 µm band and hot emission lines. and CO2 isotopes to obtain an additional set of shifted lasing lines, then we obtain a rich set of emission lines for this laser source.

Recently developed highly efficient parametric frequency converters based on nonlinear crystals ZnGeP2, CdGeAs2, TlAsSe3, AgGaSe2, etc. have made it possible to obtain the second, third and fourth harmonics of CO2 laser radiation, as well as the total difference frequencies of two CO2 and other lasers, such as CO2 , NH3, Erbium, etc. For laser sounding of atmospheric IGMs, it is important that most of these emission lines, including transformed ones, fall into the spectral transparency windows of the atmosphere.

Thus, a low-pressure molecular CO2 laser equipped with a set of nonthreshold parametric frequency converters made of ZnGeP2, CdGeAs2, TlAsSe3, and AgGaSe2 meets most of the following requirements. The distance between adjacent lines of such lasers is approximately 1.5-2 cm "1, which simplifies the problem of spectral selection and their frequency tuning. Applying a two-stage conversion, for example, of a CO2 laser or the sum-difference frequencies of two CO2, or CO2 and CO2 lasers and their harmonics, it is possible to very densely, with a step up to 10 ^ cm "1, cover the range from 2 to 17 microns. The position of the centers of the emission lines of the pump lasers and the rather narrow spectral width (2x 10 "3 cm" 1) are provided by the physical parameters of the active medium. The position of the centers of the lines, and, consequently, the position of the emission lines of the converted frequencies are known with a very high accuracy, which removes the problem of monitoring the spectral characteristics. The efficiency of such converters is quite high and ranges from tenths to tens of percent, which makes it possible to create route gas analyzers using topographic objects and atmospheric aerosols as reflectors.

Another informative spectral range for laser gas analysis is the UV region. There are strong electronic bands of many polluting gases here. In contrast to the mid-IR region of the spectrum, the UV absorption bands are nonselective and overlapped. The greatest development in this area was obtained by the ozonometric method due to the presence here of the Hartley-Huggins ozone absorption band.

The ability to perform spatially resolved measurements of atmospheric ozone with a lidar was first shown in 1977 (Meger et al). And since the second half of the 1980s, laser sounding of the ozonosphere has become a regular feature at a number of observatories. It provides information on the vertical distribution of ozone (VOD), successfully complementing similar information obtained by the contact method using ozonesondes and rockets, especially above 30 km, where ozonosondes data become unrepresentative.

The Siberian Lidar Station has been monitoring the ozonosphere since December 1988. During this period, the lidar technology was constantly improved, the measurement and data processing technique was developed and improved, software for controlling the measurement process, new software packages for processing the results obtained were created.

Purpose of work. Development of gas analyzers based on the differential absorption method for detecting and measuring the concentration of MGM and determining their space-time distribution in the atmosphere.

In the course of the work, the following tasks were performed;

Development of an optical-acoustic gas analyzer for local gas analysis and the study using it of the spatial distribution of hydrocarbons and other gas mixtures;

Development and creation of path laser gas analyzers for studying the gas composition of the atmosphere;

Development of methods for measuring IGM in the atmosphere;

Full-scale tests of the developed devices based on the developed measurement techniques;

Study of the temporal dynamics of the IGM in ecologically clean regions of the country subject to significant anthropogenic load;

Creation of a channel for sounding the vertical distribution of ozone (VOD) in the stratosphere (based on the receiving mirror 0 0.5 m) CJIC;

Monitoring the state of the ozonosphere in the routine measurement mode; - study of climatology of the ozonosphere, assessment of trends in stratospheric ozone.

The following are submitted for defense:

1. The developed laser optical-acoustic gas analyzer "LAG-1", which allows, on the basis of the developed technique, to separately measure the concentration of methane and heavier hydrocarbons in air mixtures of natural and associated oil gases with any ratio of components in the mixture.

2. Developed layouts of laser gas analyzers of the TRAL series, in the mid-IR range of the spectrum, allowing to quickly measure the concentration of more than 12 gases at and below the MPC on paths up to 2 km long using a mirror or topographic retroreflector.

3. The UV ozone lidar created by the author based on the excimer XeC1 laser, which provided uninterrupted long-term sounding of the ozonosphere over Tomsk at the Siberian lidar station in the altitude range of 13-45 km with a maximum vertical resolution of 100 m.

Scientific novelty of the work.

For the first time, the informative wavelengths of the IGM sounding of the atmosphere were selected and experimentally tested;

A number of unique mobile and stationary path-line gas analyzers based on tunable molecular lasers with radiation frequency converters have been created, which make it possible to quickly carry out multicomponent analysis of the gas composition of the atmosphere;

Measurements of the daily variations in the concentration of MGM (such as C2H4, NH3, H2O, CO2, CO, O3, N0, etc.) in ecologically clean regions of the country subject to significant anthropogenic load have been carried out;

For the first time, the climatological features of the ozonosphere over Tomsk were determined on the basis of regular and long-term measurements of the profiles of the vertical distribution of ozone;

Using the results of the work. The data obtained using gas analyzers were presented to the USSR Olympic Committee in 1979-1980. in Moscow, as well as to environmental organizations in the city of Tomsk, Kemerovo, Sofia (NRB). They were included in the final reports of the IAO SB RAS on various RFBR grants, agreements, contracts and programs, for example, "TOR" (tropospheric ozone research), "SATOR" (stratospheric and tropospheric ozone research) and others.

The practical value of the work is as follows:

An optical-acoustic gas analyzer has been developed, which makes it possible to measure with high accuracy the concentration of both the sum of hydrocarbons of the methane group and separately methane and heavier hydrocarbons in a mixture of natural and associated petroleum gases. With the help of this gas analyzer, it is possible to search for oil and gas by gas halos of gases coming out to the surface of the earth over hydrocarbon fields;

The developed route gas analyzers make it possible to measure MGM concentrations at and below the MPC from a wide list of priority polluting gases;

A channel for probing the vertical distribution of ozone CJIC has been created on the basis of a 0 0.5 m receiving mirror, which makes it possible to obtain reliable VOD profiles in the altitude range of 13-45 km with a maximum resolution of 100 m.

The reliability of the work results is ensured by: - ​​good agreement between the experimental data obtained using the developed gas analyzers, and the data obtained simultaneously by other methods, as well as; data; obtained by other authors in similar climatic and ecological conditions;

Good coincidence of the VOD profiles in the stratosphere, measured by the lidar, ozonosondes data, as well as satellite measurements within the error of the devices used | (15 %).

Personal contribution. The work uses the results obtained either personally by the author or with his direct participation. This is the author's participation in the development of both general schemes for the construction of gas analyzers, and their individual optical-mechanical and electronic assemblies and blocks, in carrying out installation and commissioning works. The development of measurement techniques, test and expeditionary ^ and field tests of the created gas analyzers, also presented in the work, took place with the direct participation of the author. Since 1996, practically all observations of the state of the ozonosphere at the CJIC were carried out with the active participation of the author. He created an improved CJIC channel for sensing the vertical distribution of ozone based on a XeC1 laser and a 0 0.5 m receiving mirror. The reanalysis of the RFO data carried out by the author made it possible to determine the peculiarities of the climatology of the ozonosphere over Tomsk.

The development process of gas analyzers, their test tests, processing of the results obtained during expeditionary work, the long-term accumulation of such a large amount of empirical information on BPO and its analysis could not have been carried out without the active participation of a whole team, without which this dissertation work would not have taken place. The statement of the problem and scientific leadership at different stages were carried out by Corresponding Member. RAS Zuev V.V. and Ph.D. Khmelnitsky G.S. The development of gas analyzers and their test and field tests were carried out jointly with the doctor of physical and mathematical sciences. Andreev Yu.M., Doctor of Physics and Mathematics Geiko P.P., researcher Shubin S.F. Theoretical work on the search for informative wavelengths was carried out by Ph.D. Mitselem A.A., Doctor of Physics and Mathematics Kataev M.Yu., Candidate of Physics and Mathematics Ptashnikom I.V., Ph.D. Romanovsky O.A. Lidar VOD measurements were carried out jointly with senior researcher A.V. Nevzorov, Ph.D. Burlakov V.D. and d.ph.-m.s. Marichev V.N., and processing of sounding data together with Ph.D. Bondarenko SL. and d.ph.-m.s. Elnikov A.V.

Approbation of work. The main results on the topic of the dissertation, obtained by the author, were published in 11 articles in Russian scientific peer-reviewed journals, reported at: VI, VII and XI All-Union Symposia on Laser and Acoustic Sounding (Tomsk, 1980, 1982, 1992); VI All-Union Symposium on the Propagation of Laser Radiation in the Atmosphere (Tomsk, 1881); XII All-Union Conference on Coherent and Nonlinear Optics (Moscow, 1985); V International Schools: I Seminar on Quantum Electronics. Lasers and their application (NRB, Sunny Beach, 1988); 5th Scientific Assembly of the International Association for Atmospheric Physics and Meteorology (Reading, Great Britain, 1989); XI Symposium on Laser and Acoustic Sounding (Tomsk, 1992); And, III, IV and VI Inter-republican symposia "Optics of the atmosphere and ocean" (Tomsk, 1995, 1996, 1997 and 1999); III Siberian meeting on climate and ecological monitoring (Tomsk, 1999); I Interregional meeting "Ecology of Siberian rivers and the Arctic" (Tomsk 1999); VII International Symposium on Atmospheric and Ocean Optics (Tomsk 2000); VIII and IX International Symposia on Atmospheric and Ocean Optics and Atmospheric Physics (Tomsk 2001 and 2002); 11 Workshop on Atmospheric Radiation Measurements (Atlanta, USA 2001); IX Working Group "Aerosols of Siberia" (Tomsk 2002); 21 and 22 International Laser Conference (Quebec, Canada, 2002, Matera, Italy 2004); II International conference "Environment and ecology of Siberia, the Far East and the Arctic" (Tomsk 2003); International Conference on Optical Technologies for Atmospheric, Oceanic and Environmental Research (Beijing, China 2004).

The structure and scope of the thesis. The dissertation work consists of an introduction, three chapters and a conclusion. The volume of the thesis is 116 pages, it contains 36 figures, 12 tables. The list of used literature contains 118 titles.

Conclusion of the thesis on the topic "Instruments and Methods of Experimental Physics"

Conclusion

In the course of the dissertation work, the author as part of the team did the following:

An optical-acoustic gas analyzer for local gas analysis was developed, with its help a study of the spatial distribution of -hydrocarbons (in the course of several expeditions on a motor ship) in areas where oil fields are located was carried out. The measured increase in the content of hydrocarbons in air samples in the area of ​​oil fields confirmed the hypothesis of the presence of gas halos over hydrocarbon fields and the prospects of using this gas analyzer for searching for oil and gas fields;

A complex of path laser gas analyzers operating in the IR region of the spectrum by the method of differential absorption and allowing to measure the concentration of more than 12 gases at and below the MPC has been developed and created;

The technique of measuring the IGM in the atmosphere has been worked out;

Full-scale tests of the developed devices were carried out;

The pairs of informative wavelengths were experimentally tested and conclusions were drawn about their suitability for the purposes of gas analysis according to MIS;

Studies of the temporal dynamics of the IGM in ecologically clean regions of the country subject to significant anthropogenic load have been carried out;

Comparative measurements of MGM concentrations were carried out by the developed laser gas analyzers and devices operating on the basis of standard methods, which showed good agreement of the results obtained;

A channel for probing the vertical ozone distribution (VOD) in the stratosphere (based on the 0 0.5 m receiving mirror) CJIC was created, which provided reliable VOD profiles over Tomsk for many years, which were confirmed well in agreement with satellite and ozone probe data. This made it possible to carry out climatological studies and assess the trends of stratospheric ozone, which showed that in the lower stratosphere at altitudes below 26 km, intra-annual changes in ozone concentrations are characterized by a maximum in spring and a minimum in autumn, and at altitudes above 26 km, the maximum shifts to the summer, and the minimum to winter. ... At an altitude of 26 km, in the area of ​​which the cycle break is located, the ozonosphere is divided into two parts: at the bottom, its behavior is determined mainly by dynamic processes, and at the top, by photochemical ones. A more detailed consideration of the intra-annual variations in VOD makes it possible to single out the following points: a) at an altitude of 14 km, where, apparently, the influence of fluctuations in the height of the tropopause is still significant, a localized maximum is not observed; b) in the range up to 18 km inclusive, the maximum seasonal fluctuations occur in February, and in the range of 20-26 km - in March; The greatest correspondence of the intra-annual variations in the SSO with the annual TOC variation is observed in the altitude range of 20-24 km, especially at an altitude of 22 km. c) at all heights, the BPO trends turned out to be statistically insignificant. At the same time, in the lower part of the ozonosphere, they are characterized by weakly negative values, and in the upper part, by weakly positive ones. In the area of ​​localization of the stratospheric ozone maximum 20 km), the values ​​of negative trends are small (-0.32% per year). These results are consistent with the insignificant statistically insignificant TO trend (0.01 + 0.026% per year) over the same six-year period.

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Laser gas analyzer SITRASN SL is designed for automatic measurement of the volume fraction of oxygen or carbon monoxide in process and flue gas streams.

Description

The principle of operation of the gas analyzer is photometric.

The gas analyzer is a continuous-flow device operating on the principle of single-line molecular absorption spectroscopy.

The SITRANS SL gas analyzer consists of a pair of cross-channel sensors with transmitter and receiver blocks. The transmitter unit is equipped with a laser, the beam of which propagates to the receiver along the measurement path. The receiver unit contains a photodetector with an electronic device. The receiver unit is connected to the transmitter with a sensor connecting cable. The receiver connecting cable is used for connecting the power supply and communication interfaces. The receiver housing houses a local user interface with an LCD display that can be read through a window in the lid. In standard conditions, it is controlled by a remote control. Structurally, the gas analyzer is made in the form of two units - a receiver and a transmitter.

The transmitter diode laser emits an infrared beam that passes through the sample gas and is detected by the receiver unit. The wavelength of the output signal of the diode laser corresponds to the absorption line of the detected gas. A laser continuously scans this absorption line with high spectral resolution. Measurements are not affected by any interference, since quasi-monochromatic laser radiation is absorbed extremely selectively at a specific wavelength in the scanned spectral range. The optical path length is from 0.3 to 8.0 m. Depending on the laser wavelength, the gas analyzer measures the concentration of oxygen or carbon monoxide.

On the front panel of the gas analyzer there is a display for displaying measurement results, as well as a menu for setting the device parameters.

The external view of the device is shown in Fig. 1.

Fig. 1. Gas analyzer appearance

Software

The gas analyzer has built-in software developed by the manufacturer specifically for solving problems of measuring the volume fraction of oxygen and carbon monoxide in gas samples. The software provides the output of concentration readings on the instrument display, instrument control and data transmission.

The software is identified at the user's request through the service menu of the gas analyzer by displaying the software version on the screen.

The software identification data is shown in Table 1.

Table 1.

sheet No. 3 total sheets 5

The level of software protection against unintentional and deliberate changes corresponds to level "C" according to MI 3286-2010.

The influence of the software on the metrological characteristics was taken into account when standardizing the metrological characteristics.

Specifications

1. The ranges of measurements of the volume fraction of the determined components, the limits of the basic permissible error of the gas analyzer and the unit price of the smallest category are given in Tables 2 and 3 (with an optical path length of 1 m).

table 2

Table 3

2. Time of establishment of indications (time of data recording depending on the measured concentration): from 2 to 10 s.

3. Limit of permissible variation of readings, Ld, in fractions of the limit of permissible basic error: 0.3

4. Additional error from the influence of changes in the ambient temperature in the operating temperature range for every 10 ° С deviation from the nominal temperature value of 20 ° С, in fractions of the maximum permissible basic error: 0.5.

5. Power supply is provided by direct current voltage of 24 V.

6. Power consumption, VA, no more: 10.

7. Overall dimensions, mm, no more: receiver and emitter - diameter 165, length 357.

8. Weight, kg, no more:

Receiver 6.0;

Emitter 5.2.

9. Full average service life, years: 3

10. MTBF, h not less: 25000

11. Operating conditions of the analyzer:

Ambient temperature range from minus 20 to 55 ° С;

Relative humidity of the ambient air up to 95% at a temperature of 30 ° C;

Atmospheric pressure range from 80 to 110.0 kPa (630 - 820 mm Hg).

12. Parameters of the analyzed gas at the inlet to the analyzer:

Temperature range from minus 20 to 70 ° С

Type approval mark

is applied in a typographic way on the title page of the operating manual and on the rear panel of the gas analyzer in the form of a sticker.

Completeness

The analyzer delivery set includes:

Laser gas analyzer SITRANS SL (receiver) 1;

Laser gas analyzer SITRANS SL (transmitter) 1;

Remote control 1:

Operation manual, copies: 1;

Verification method No. MP-242-1232-2011, copy. 1.

Verification

carried out according to document MP-242-1232-2011 "Laser gas analyzer SITRANS SL. Verification Methodology ", approved by the State Research Center of SI FSUE" VNIIM im. DI. Mendeleev "in September 2011

Basic means of verification:

Standard samples of composition: gas mixtures 02 / N2 GSO 3720-87 and GSO 3729-87;

Standard samples of composition: gas mixtures CO / N2 GSO 3806-87 and GSO 3816-87.

Calibration zero gas - nitrogen of high purity according to GOST 9293-74.

Information about measurement methods

Methods of measurements in gas streams are given in the document “Laser gas analyzer SITRANS SL. Manual".

Regulatory and technical documents establishing the requirements for the laser gas analyzer SITRANS SL

1 GOST 8.578-2008 GSI. State verification scheme for measuring instruments for the content of components in gaseous media.

2 GOST 13320-81 Industrial automatic gas analyzers. General technical conditions.

3 Technical documentation from Siemens AG, a division of Siemens S.A.S, France.

Usage: control of harmful substances in the air. The essence of the invention: the device contains a laser gas discharge tube, a beam forming unit made in the form of a diffraction grating on a piezo corrector, which are located in a tangential unit associated with a stepping motor, an optoacoustic cell, a reference cell, a measuring and background microphone, and two pyroelectric sensors, connected through an analog-to-digital converter and an interface unit to the input of a personal computer. 1 ill.

The proposed invention relates to measuring technology and is intended for monitoring harmful substances in the air. Lists of harmful substances in the air of the working or living area have hundreds of substances that affect the human body. Many devices are known, for example, serving to control the composition of air using various measurement methods: chemical-analytical, chromatographic, coulometric, etc. One of the most suitable for performing operational measurements with the ability to control a large number of harmful substances is the method using the absorption of infrared radiation. Known gas analyzers of the GIAM type are designed to register one of the following gases: CO, CO 2, CH 4, SO 2, NO. Incandescent filaments (lamps) with a continuous spectrum of radiation are used as sources of infrared radiation. To select the spectral range corresponding to the absorption spectrum of the test substance, light filters are used. Measurements are made using a reference cell with a reference gas. The intermittent luminous flux is alternately directed to the working and comparative cuvettes, passing through which it (the luminous flux) is recorded by an optoacoustic detector filled with the measured gas. The difference in signals from the detectors is used to determine the concentration of the test substance in the air. Devices of this type, possessing good efficiency (the time for establishing readings is about 10 s), do not allow simultaneous (in one sample) registration of more than one component of pollutants. Known universal gas monitor 1302 from Brüel & Kjr, which allows simultaneous registration of up to five impurities in one air sample. The device uses a filament as a source of infrared radiation. The change in the spectrum of infrared radiation falling into the sensitive volume of the optical-acoustic cell is given automatically in the course of measurements using a set of narrow-band light filters installed on a rotating disk. The air sample fills the volume of the optical-acoustic cell. For the duration of the measurement, the inlet and outlet of the cell is blocked from the outside air. Microphones are used to measure the amplitude of pressure fluctuations arising in the cell when the intermittent light flux is absorbed by the sample under study. Measurements are made for each filter. The total measurement time for one sample is approximately 2 minutes. Based on the measurement results, the concentration of up to five impurities in one sample is determined. The control of the device operation and processing of the measurement results is carried out using the built-in processor. The separately supplied set of two 2 replaceable narrow-band light filters allows the registration of a large number of impurities that absorb infrared radiation. However, the device makes it possible to carry out measurements only with a priori known composition of pollutants. Otherwise, the overlap of absorption bands of various substances does not allow obtaining adequate information on the composition of harmful substances in the air. The closest to the proposed solution is a laser gas analyzer described in and containing a laser gas-discharge tube to which a high-voltage source and a cooling unit are connected, located on one optical axis, a beam forming unit and an optical-acoustic cell, to which an air intake unit is connected, measuring a microphone and a pyroelectric sensor, an analog-to-digital converter connected through an interface unit and a data input and output unit with the input of a personal electronic computer. The output of which through the interface unit is connected to the input of the control unit. The use of a laser source of infrared radiation allows realizing a high spectral resolution of approximately (10-20 nm) in the device. The registration of absorption in the test gas is carried out using an optoacoustic cell. The gas analyzer consists of three main parts: a source of tunable infrared radiation, an optical-acoustic cell (OAP), a system for recording and processing information. In the device, the beam forming unit is made in the form of an optically coupled modulator, shaper, mirror, focusing lens and diffraction grating. The selected in the device method of tuning the wavelength of laser radiation using a diffraction grating and a rotating mirror allows the selection of 36 radiation lines. The identification of emission lines is carried out only when setting up the device. When radiation is absorbed in the test gas filling the OAP, an acoustic wave is formed in it, which is recorded by a condenser microphone. Signals from a microphone and a pyroelectric radiation detector, which records the power of laser radiation, are fed to the input of a two-channel registration system consisting of two synchronous detectors. An analog recording of the registered signals is carried out using a recorder. The information can be read using a digital voltmeter and a computer. The disadvantages of the prototype are the limited number of radiation lines, which affects the multicomponent in one air sample, and the lack of control over the radiation wavelength. The objective of the invention is to provide an express multicomponent analysis of the composition of the air for harmful substances with high accuracy. This task is in a device containing a laser gas analyzer containing a laser gas-discharge tube to which a high-voltage source and a cooling unit are connected, a beam-forming unit, made in the form of a diffraction grating on a piezoelectric corrector, and an optical-acoustic cell to which are connected an air intake unit and a measuring microphone, a pyroelectric sensor connected through a series-connected analog-to-digital converter and an interface unit to the PC input, is solved due to the fact that the gas analyzer additionally contains a background microphone, a reference cell located on one optical axis and an additional pyroelectric sensor connected similar to the main pyroelectric sensor, as well as a differential amplifier, in the beam-forming unit a diffraction grating and a piezo-corrector are located in a tangential unit connected to a stepping motor, and the outputs of the measuring and background microphones connected through a differential amplifier to the ADC, the outputs of the control unit are connected to the corresponding inputs of the piezoelectric corrector and the stepper motor of the beam forming unit, the output of the personal computer through the interface unit is connected to the control unit. The essence of the invention lies in the fact that the proposed implementation of the beam forming unit allows you to have a large (up to 70 lines of IR radiation) set of wavelengths with a fixed and controlled wavelength (multicomponent and accuracy); the software and the data bank used in the PC and its communication through the interface unit and the control unit with all the sensors of the gas analyzer, ensures the promptness of the correction of the parameter drift and the processing of information. The drawing shows a block diagram of the gas analyzer. It contains a laser gas-discharge tube LGRT 1 (CO 2 -laser), a high-voltage power supply unit 2 LGRT, a cooling unit 3 serves to cool the LGRT, aperture 4 regulates the radiation power, a diffraction grating 5, the rotation of which changes the radiation wavelength, piezoelectric corrector 6 compensates for temperature instability , tangential block 7, the longitudinal movement of which by 20 mm leads to a rotation of the diffraction grating 5 by 14 o, the stepper motor 8 moves the tangential block 7, mirrors 9, directing IR radiation to the input window of the AOC, elements 4, 5, 6, 7 , 8 and 9 constitute a beam forming unit 26, a pyroelectric sensor 10, which receives IR radiation partially reflected from the input window of the OAP, a pyroelectric sensor 11, which records IR radiation that has passed through the OAP through a reference cuvette, a background microphone 12, not " seeing the "sensitive volume of the OAP, a measuring microphone 13, which records a periodic change in pressure in the OAP due to absorption of n intermittent luminous flux, optical-acoustic cell ОАЯ 14, a sensitive element of the gas analyzer, a reference cuvette 15 with a known filling, used to control the radiation wavelength, a blower 16 supplying the test air to the ОАЯ, solenoid valves 17, 18 and 19, which regulate the flow of the test air, air intake (tube) 20, obturator 21 serving to periodically interrupt the radiation flow, filter 22, temperature sensor 23 in the cooling system, pressure sensor 24 in the cooling system, pressure sensor 25 in the air intake circuit, PC 27 controls the operation and collects measurement results, the interface unit 28 is connected by a bus line with a PC 27, with a control unit 29, an analog-to-digital converter ADC 30, a PC 27 of the IBM PC type is provided with software 31 and a data bank 32 (shown conditionally). Signals 13 and 12 are subtracted from one another, the difference is normalized to the readings of the pyroelectric sensor 10. The measurements are carried out at wavelengths set from the PC 27 (each wavelength corresponds to a certain step of the stepper motor 8). The interface unit 28 serves to interface the PC 27 and the executive-recording part of the gas analyzer with the ADC 30, which converts the signals from the pyroelectric sensors 10, 11 and from the differential amplifier 33 into a digital code. The control unit 29 carries out the operation of the actuators of the blower 16, the piezo corrector 6, the stepper motor 8, the solenoid valves 17, 18 and 19. The control unit 29 also monitors the pressure and temperature in the cooling circuit of the LGRT 1 and monitors the pressure in the air intake system. The sample is taken in the OAO 14 through the air intake pipe 20, filter 22. The air moves through the pipe 20 under the action of the blower 16. The flow direction is adjusted by valves 17, 18, 19. The pressure sensor 25 serves to check the serviceability of the air intake system. In the measurement mode, part of the radiation is absorbed by the gas under study in the OAD 14, causing periodic pressure fluctuations with a frequency equal to the frequency of interruption of the radiation beam by the obturator 21, which are recorded by the microphone 13. Part of the radiation, passing through the exit window of the OAO 14, enters the reference cell 15, and then to the pyroelectric sensor 11. During processing, the signals from the differential amplifier 33 (whose inputs are connected to microphones 12 and 13) and the pyroelectric sensor 11, normalized to the readings of the sensor 10, are used. using specially developed software 31 and databank 32. The operation of the gas analyzer, the operator and the functional purpose of the programs is described below. Working with the gas analyzer begins with connecting the PC 27 to the network and downloading the SCO 2 software, which contains the following programs: 1. CONTROL; 2. TEST 3. TEST LINE; 4. SPECTRA; 5. CALCULATION; 6. RESULT; 7. BANK. After loading SCO 2, the message "ON THE GAS ANALYZER" appears on the display screen of the PC 27, the "CONTROL" program is turned on, providing a check of the functioning of the gas analyzer before starting measurements. The obturator 21, the supercharger 16, the valves 17, 18 and 19 are checked. Then, in accordance with the "CONTROL" program, the display shows the query "TEST MEASUREMENT". If a test measurement is required, confirmed by pressing the D key, the operator performs the work according to the "TEST" program. The message "FILL OAU WITH ZERO GAS" appears on the display screen. "READY", after filling with the D key, the measurement program starts: measurements are taken of the signals of microphones 12 and 13, of the pyroelectric power sensor 10 at different values ​​of the radiation lines (i.e., at different values ​​of the step number of the stepper motor 8. The results are entered into the PC memory 27 for use in the program "CALCULATION"). After that the message "TAKE ZERO MEASUREMENT" appears on the screen. If measurements are not carried out using the "TEST" program, this message appears immediately. Measurements are carried out using the "TEST LINE" program by pressing the D key. Air is pumped through OAJ 14 by blower 16, valves 18 and 19 are closed, valve 17 is opened, after which blower 16 is turned off and signals are measured from microphones 12 and 13 connected to a differential amplifier 33, and pyroelectric sensors 10 and 11 with different numbers of steps of the stepper motor 8. The measurement results after the ADC 30 are normalized to the readings of the pyroelectric power sensor 10. If the measurements were not carried out using the "TEST" program, then the signals of the microphones 12 and 13 are written to a file for processing in the "CALCULATION" program, otherwise they are not used further; the signal from the sensor 11 is entered into the file for the program of basic measurements "SPECTRA". At the end of the program, the prompt "SCAN OPERATION MODE" appears on the display screen. When you press the D key, the work will be carried out according to the "SPECTRA" program with the measurement of the signals of microphones 12 and 13 and of the pyroelectric sensor 10 in the entire radiation range, at each group of steps corresponding to the presence of radiation. In this case, to control the radiation spectrum, the measurement results are compared with the measurements from the sensor 11 and the data on the absorption spectrum of the gas in the reference cuvette 15 entered into the data bank during the calibration of the gas analyzer. If necessary, an amendment is made to the numbering of steps specified by the "SPECTRA" program. The measurement results are entered into a file for the "CALCULATION" program. If you refuse to work in the scanning mode (pressing the "H" key), the message "INPUT THE NAMES OF CONTAMINANTS FROM THE NAMED LIST" appears, the work continues according to the "SPECTRA" program. A list of contaminants appears on the screen. After selecting the pollutants, the message "OPERATING MODE ONE TIME" appears. When the D key is pressed, a single measurement is carried out: an air sample OAYA 14 is collected, signals from microphones 12 and 13 and from sensor 10 are measured on the absorption lines of the sought substances, determined by the step number of the stepper motor 8, taking into account the zero measurement. The measurement results are entered into a file for processing using the "CALCULATION" program. In case of refusal from a single measurement (pressing the H key), the message "SET THE MEASUREMENT TIME IN HOURS" appears, after which continuous measurements are carried out using the "SPECTRA" program for the specified time. The interval between individual measurements is 5 min. The measurement results are entered into a file for processing using the "CALCULATION" program. The processing of the measurement results is carried out according to the "CALCULATION" program at the end of the measurements (single mode), between separate measurements (continuous mode). Processing is carried out using a databank (BANK program), which contains the instrumental absorption spectra of gases, the sensitivity to each individual gas, the minimum detectable quantities, the absorption spectrum of the reference cell gas, the maximum permissible concentration of DNA gases for the air of the working and living areas. The results are displayed on the screen in the form of a table (single measurements) or a graph (continuous measurements) in comparison with the MPC. In case of uncertainty in the processing results (for example, coincident absorption spectra), a message is displayed about the inadequacy of the measurements. Thus, the proposed gas analyzer provides technical means for the rapid determination of the absorption peaks of various air impurities (up to 60 components in one sample), the impurity concentration is determined by the magnitude of the absorption peak, which favorably distinguishes it from analogues and the prototype.

CLAIM

A laser gas analyzer containing a laser gas-discharge tube to which a high-voltage voltage source and a cooling unit are connected, a beam forming unit, made in the form of a diffraction grating on a piezoelectric corrector, located on the same optical axis with the laser gas-discharge tube, and an optoacoustic cell (OAP), to which the unit is connected air intake and a measuring microphone, a pyroelectric sensor connected through a series-connected analog-to-digital converter (ADC) and an interface unit to the input of a personal computer, characterized in that the gas analyzer additionally contains a background microphone, located on the same optical axis with an optical-acoustic cell, a reference cuvette and an additional pyroelectric sensor, connected in the same way as the main pyroelectric sensor, as well as a differential amplifier, in the beam-forming unit, the diffraction grating and the piezoelectric corrector are located in the tangential unit associated with the stepping motor, for that A rotary mirror is installed in the generic unit, which directs radiation to the input window of the OAD, and the outputs of the measuring and background microphones are connected to the ADC through a differential amplifier, the outputs of the control unit are connected to the corresponding inputs of the piezo corrector and the stepper motor of the beam forming unit, the output of the personal computer through the interface unit is connected to the unit management.

The highly sensitive laser gas analyzer is designed to analyze the content of impurity gases in air samples. The main elements of the gas analyzer: a waveguide CO 2 -laser, a resonant optical-acoustic cell, and a computer, the library of which contains information about the absorption lines of 37 gases. Information on the limits of gas detection by the developed gas analyzer is presented. The detection limit for ammonia with an error of 15% is 0.015 ppb.

The need for constant monitoring of the content in the air of a large number of pollutants in large areas at a reasonable cost of funds and labor sets the task of equipping the environmental control service with gas analyzers that meet the following requirements: 1) the detection threshold at the level of maximum permissible concentrations of analyzed substances; 2) high selectivity in relation to foreign substances; 3) multi-component analysis; 4) high speed (short measurement cycle time when taking one sample), which provides the ability to work in motion and a relatively quick response to exceeding a given concentration level; 5) continuous measurements for 2-4 hours to determine the size of the contaminated area.

The existing methods for detecting gases can be conditionally divided into traditional (non-spectroscopic) and optical (spectroscopic) ones. The paper lists the advantages and disadvantages of the main traditional methods from the point of view of their application for the analysis of gas impurities of a complex composition in air.

Spectroscopic methods, the rapid development of which is determined by the unique characteristics of lasers, make it possible to eliminate the main disadvantages of traditional devices and provide the necessary speed, sensitivity, selectivity, and continuity of analysis. In most cases, to detect air pollution by spectroscopic methods, the middle IR spectral region is used, where the main vibrational bands of the overwhelming majority of molecules are concentrated. The visible and UV regions are less informative in this respect.

A special place in the family of IR laser gas analyzers is occupied by devices with CO 2 lasers. These lasers are durable, reliable and easy to use and can detect over 100 gases.

A gas analyzer (prototype) that meets the above requirements is described below. A waveguide CO 2 laser is used as a radiation source, and a resonant optoacoustic cell (RSA) is a sensitive element. The optical-acoustic method is based on the registration of a sound wave excited in a gas upon absorption of an amplitude-modulated laser radiation in the ROA. The sound pressure, which is proportional to the specific absorbed power, is recorded by the microphone. The block diagram of the gas analyzer is shown in Fig. 3.1. The modulated radiation of the CO 2 laser hits the wavelength tuning unit. This unit is a diffraction grating that allows you to tune the radiation wavelength in the range of 9.22-10.76 microns and obtain 84 laser lines. Further, the radiation is directed through the system of mirrors into the sensitive volume of the ROA, where the gases that absorb the radiation entering it are recorded. The energy of the absorbed radiation increases the temperature of the gas. The heat released on the cell axis is transferred mainly by convection to the cell walls. The modulated radiation causes a corresponding change in gas temperature and pressure. The change in pressure is perceived by the membrane of the capacitive microphone, which leads to the appearance of a periodic electrical signal, the frequency of which is equal to the modulation frequency of the radiation.

Figure 3.1. Gas analyzer block diagram

Fig. 3, 2 shows a sketch of the internal cavity of the r.o.a.y. It is formed by three cylindrical active volumes: symmetrically located volumes 1 and 2 with a diameter of 20 mm and an internal volume 3 with a diameter of 10 mm. The inlet 4 and outlet 5 windows are made of BaF 2 material. The microphone is installed at the bottom of the cell and is connected to the active volume by a hole 6 with a diameter of 24 mm.

Figure 3.2 The inner cavity of the resonant optical-acoustic cell. 1, 2 - external volumes, 3 - internal volume. 4, 5 - input and output windows, 6 - microphone hole

Optical resonance "caused by the absorption of laser radiation by a gas, under normal conditions arises at a modulation frequency of 3.4 kHz, and the background signal due to absorption of radiation by the ROA windows is maximum at a frequency of 3.0 kHz. The Q factor in both cases is> 20 Such a design of the ROA provides a high sensitivity of the gas analyzer and makes it possible to suppress the contribution of the background signal using a frequency- and phase-selective amplifier. At the same time, the ROA is insensitive to external acoustic noise. electrical signal when measuring concentration is determined by the formula

where K is the cell constant, is the laser radiation power, b is the absorption coefficient of radiation by the gas, and C is the gas concentration.

Before measurements, the gas analyzer is calibrated using a span gas (CO2) with a known concentration.

The amplitude is measured using an ADC board included in the Advantech computer. The same computer is used to control the wavelength tuning unit and calculate the concentrations of the measured gases.

The developed information processing program is intended for the qualitative and quantitative analysis of the gas mixture by the absorption spectrum of the laser radiation of the CO 2 laser. The initial information for the program is the measured absorption spectrum of the analyzed gas mixture. An example of an absorption spectrum of nitrogen, plotted in units of optical thickness, is shown in Fig. 3.3a, and Fig. 3.3b shows an example of an absorption spectrum with a small addition of ammonia.

Figure 3.3 Absorption spectra: a - nitrogen at normal atmospheric pressure, b - nitrogen-ammonia mixture.

Optical thickness, where

Cm -1 atm -1 - absorption coefficient of the j-th gas on the i-th laser line, С i, atm - concentration of the j-th gas, i

The library of possible components contains the values ​​of the absorption coefficients and is a matrix of dimensions (N x m). The number of gases presented in the library is m = 37, the maximum number of analyzed laser lines is N - 84 (21 lines in each branch of the CO 2 laser).

In the process of analyzing the spectrum of a gas mixture formed by overlapping absorption lines of gases included in the mixture, the program selects from the library those components that allow the best description of the mixture spectrum. One of the main criteria for searching for the best set of components is the value of the root-mean-square deviation between the experimental and the absorption spectrum found as a result of iterations:

The algorithm for solving the inverse problem - searching for concentrations from the known absorption spectrum - was constructed using the Gaussian elimination method and the Tikhonov regularization method, and the main difficulties in its implementation are associated with the estimation of the stability of the solution (the elements of the absorption coefficient matrix, as well as the free terms, are known only approximately ), choosing the regularization parameter and finding criteria for terminating the iterative process.

The table contains calculated information about the detection limits of some gases described by the gas analyzer:

	Detection limit, ppb		Detection limit, ppb
Acrolein		Monomethyl hydrazine
		Perchlorethylene
t-butanol		Propanol
Vinyl chloride
Sulfur hexafluoride		Trichlorethylene
Hexachlorobutadiene
Hydrazine
Dimethylhydrazine
1.1-difluoroethylene
Isopropane
Methyl chloroform		Ethyl acetate
Methyl ethyl ketone

Main operating characteristics of the gas analyzer: the number of simultaneously measured gases - up to 6; measurement time 2 min; detection limit for carbon dioxide 0.3 ppm: detection limit for ammonia 0.015 ppb: measurement range for carbon dioxide 1 ppm -10%; measurement range for ammonia 0.05 ppb-5 ppm; measurement error 15%; supply voltage 220V ± 10%. [ 1]