CN113295642A - Mid-infrared spectrum measurement system and method for ammonia molecule absorption line parameters - Google Patents

Mid-infrared spectrum measurement system and method for ammonia molecule absorption line parameters Download PDF

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CN113295642A
CN113295642A CN202110534283.8A CN202110534283A CN113295642A CN 113295642 A CN113295642 A CN 113295642A CN 202110534283 A CN202110534283 A CN 202110534283A CN 113295642 A CN113295642 A CN 113295642A
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sample cell
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王薇
谢宇
单昌功
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Hefei Institutes of Physical Science of CAS
Hefei University
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Hefei University
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    • GPHYSICS
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    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
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Abstract

The invention discloses a mid-infrared spectrum measurement system and method for ammonia molecule absorption line parameters. The measuring system mainly adopts a light source, a high-resolution Fourier Transform Infrared (FTIR) interferometer, a gas sample cell, an optical filter, a detector and a computer. According to the invention, ammonia molecule absorption line parameters are obtained according to a high-resolution mid-infrared spectrum of ammonia standard gas with fixed temperature and pressure measured by a high-resolution FTIR interferometer and a detector.

Description

Mid-infrared spectrum measurement system and method for ammonia molecule absorption line parameters
Technical Field
The invention relates to the field of molecular line parameter optical detection technology and method for environmental remote sensing monitoring and gas absorption spectral lines, in particular to a mid-infrared spectrum measurement system and method for ammonia molecular absorption line parameters.
Background
In recent years, ammonia (NH) in the atmosphere of China is discharged due to volatilization of agricultural fertilizers, livestock and poultry excrement in animal husbandry, biomass combustion, motor vehicle discharge, industrial production and discharge of other artificial activities3) The emission of (2) rises sharply. The ammonia sedimentation is the main reason for acidification and eutrophication of soil and water body and threatens the survivalThe biodiversity of the state system is used as the alkaline gas with the maximum content in the troposphere, and the ammonia gas is an important precursor of secondary particles, is closely related to the formation of haze and causes harm to human health. Therefore, there is a need for extensive long-term monitoring of ammonia in regional atmosphere, knowledge of its temporal and spatial distribution characteristics and trends, and determination of regional emissions and source/sink distributions.
In recent years, ground-based remote sensing based on infrared spectroscopy technology becomes a high-precision technology for monitoring the concentration and vertical profile of ammonia in the atmosphere. In the infrared remote sensing inversion of the foundation, the inversion gas concentration vertical profile is generally subjected to spectral absorption line fitting based on a molecular absorption line parameter database (such as a HITRAN database), and the fitted spectrum and the residual error of the observed spectrum are minimized by continuously updating the inversion parameters, so that the concentration information with different heights is obtained. Therefore, the inversion accuracy of the atmospheric ammonia gas profile depends on the accuracy of the molecular line parameters. However, the strong uncertainty of the ammonia molecule absorption line in the mid-infrared band in the HITRAN database is about 20%, the uncertainty of the broadening coefficient is 10%, and the uncertainty of the large molecular line parameter influences the accuracy of the gas profile inversion. Therefore, in order to accurately invert the ammonia gas concentration at different heights of the atmosphere, the ammonia gas molecular line parameters under different pressures and temperatures need to be researched, and particularly, an accurate molecular absorption line is required to be established.
In the process of analyzing and fitting the actually measured spectrum by ground-based remote sensing, the absorption line type of the spectrum directly influences the spectrum fitting result, and the absorption line type of the spectrum changes along with the change of gas pressure and temperature, so that different broadening physical mechanisms and coupling thereof generate complex spectrum absorption line types in the pressure range of actual atmosphere. It has been found that various gas molecules such as carbon dioxide (CO)2) Water vapor (H)2O), Hydrogen Cyanide (HCN), Hydrogen Fluoride (HF) and the like, and the simple Voigt line type is replaced by a complex molecular absorption line type such as a speed-dependent Voigt line type or a Galatry line type considering the molecular collision narrowing effect in the simulation spectrum calculation, so that the spectrum fitting residual error can be reduced, and the accuracy of molecular line parameters can be improved. However, at present, no spectral line model can be applied to all atmospheric pressure ranges, and therefore needs to be establishedMore experiments are carried out by using the measuring system and the measuring method, and the influence of different spectral line types on spectral parameters and the applicability of the spectral line types in different pressure ranges are verified.
Commonly used infrared spectroscopy techniques for measuring molecular absorption line parameters are fourier transform infrared spectroscopy (FTIR), Tuned Diode Laser Absorption Spectroscopy (TDLAS), and cavity ring-down spectroscopy (CRDS). The existing infrared spectrum molecular absorption line detection system generally has the problems of low spectral resolution and large system noise, so that the high-resolution infrared spectrum detection system and the spectral fitting method need to be established to research the ammonia molecular absorption line parameters in the infrared band.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the system and the method overcome the defects of the prior art, provide the mid-infrared spectrum measurement system and the method for ammonia gas molecular absorption line parameters, make up the defects of the traditional measurement technical means for the atmospheric trace gas absorption line parameters, can realize high-precision and high-accuracy measurement of the ammonia gas absorption line parameters, and can realize the applicability research of a complex molecular absorption line model under different atmospheric pressures.
In order to achieve the purpose, the invention adopts the technical scheme that: the mid-infrared spectrum measuring system for ammonia molecule absorption line parameters comprises a mid-infrared light source 1, a focusing parabolic mirror a2, a diaphragm 3, a collimating parabolic mirror 4, a plane mirror a5, a high-resolution FTIR interferometer 6, a plane mirror b7, a focusing parabolic mirror b8, a gas sample cell 9, a filter 10, a Mercury Cadmium Telluride (MCT) detector 11, a gas inlet 12, a gas mass flow controller 13, a suction pump 14, a water cooling system 15, a liquid nitrogen tank 16, a seal box 17, a vacuum pump 18 and a computer 19; the light source 1 provides light in a middle infrared band; the focusing parabolic mirror a2 focuses the light beam emitted by the light source 1; the diaphragm 3 is used for controlling the luminous flux entering the measuring system and preventing a Mercury Cadmium Telluride (MCT) detector 11 from being saturated; the collimating paraboloidal mirror 4 changes the divergent light emitted by the diaphragm 3 into parallel light; the plane mirror a5 reflects the parallel light reflected by the collimating parabolic mirror 4 into the high resolution FTIR interferometer 6; the high-resolution FTIR interferometer 6 changes the incident parallel light into coherent parallel light and emits the coherent parallel light; the plane mirror b7 reflects the parallel light emitted by the high-resolution FTIR interferometer 6 to a focusing parabolic mirror b 8; the focusing parabolic mirror b8 focuses the parallel light emitted by the FTIR interferometer 6 to the gas sample cell 9; the filter 10 makes the light of the middle infrared band after passing through the gas sample cell 9 reach a Mercury Cadmium Telluride (MCT) detector 11; the Mercury Cadmium Telluride (MCT) detector 11 responds to light in a middle infrared band to generate an interference signal; a gas sample cell 9 is filled with dry nitrogen or ammonia standard gas from a gas inlet 12, a gas outlet of the gas sample cell 9 is connected with a gas mass flow controller 13 and a gas pump 14, the gas pump 14 pumps the gas to the gas sample cell 9, and the gas mass flow controller 13 is used for controlling the gas flow; the water cooling system 15 is used for cooling the mid-infrared light source 1; the liquid nitrogen in the liquid nitrogen tank 16 is used for refrigerating a Mercury Cadmium Telluride (MCT) detector 11 so as to reduce the noise of the detector; the mid-infrared light source 1, the focusing parabolic mirror a2, the diaphragm 3, the collimating parabolic mirror 4, the plane mirror a5, the high-resolution FTIR interferometer 6, the plane mirror b7, the focusing parabolic mirror b8, the gas sample cell 9, the optical filter 10 and the Mercury Cadmium Telluride (MCT) detector 11 are jointly arranged in a sealed box 17; the vacuum pump 18 is arranged near the seal box 17 and used for pumping the gas in the seal box 17 to enable the gas to be in a near vacuum state (the gas pressure is less than 100Pa), so that the interference of the gas in the seal box 17 on measurement is avoided; the gas transmitted by the gas inlet 12 enters the gas sample cell 9 through a gas transmission gas path under the action of a gas mass flow controller 13 and a suction pump 14, and an infrared interference pattern of a coherent light beam emitted by the FTIR interferometer 6 after passing through the gas sample cell 9 is collected by a Mercury Cadmium Telluride (MCT) detector 11; the diaphragm 3, the high-resolution FTIR interferometer 6 and the vacuum pump 18 are all controlled by the computer 19, an interferogram collected by the HgCdTe detector 11 is input into the computer 19, a spectrogram is obtained through a fast Fourier transform algorithm, and the obtained spectrogram is analyzed by the computer (19) to obtain molecular line parameters of the ammonia gas.
The light source 1 is a mid-infrared light source, and the wavelength coverage range is 350-8000cm-1
The diaphragm 3 is a diaphragm group which is formed by installing a group of diaphragms with fixed apertures with the aperture sizes of 0.5mm, 1.0mm, 1.5mm, 1.7mm and 2.0mm on the rotating wheel. In the actual measurement, a proper diaphragm is selected according to the signal intensity of the mercury cadmium telluride detector 11.
The spectral resolution of the high resolution FTIR interferometer 6 is 0.005cm-1The beam splitter is a potassium bromide (KBr) beam splitter, and the coverage wavelength range is 450--1
The gas sample cell 9 is a multi-reflection sample cell, the cell wall is made of stainless steel, the interior of the cell is electropolished and plated with gold, the length of the sample cell substrate is 20cm, and the optical path length is 72 m.
The gas path pipe between the gas inlet 12, the gas sample cell 9, the gas mass flow controller 13 and the air pump 14 is stainless steel, and the inside is electropolished and plated with gold.
The gas mass flow controller 13 controls the flow rate of the gas to be in the range of 0.5 to 1.5Lmin-1
The flow rate of the air pump 14 is 3Lmin-1
The filter 10 is a narrow-band filter, and the wavelength range of the transmitted light is 500-1500cm-1
The wavelength response range of the Mercury Cadmium Telluride (MCT) detector 11 is 600-10000cm-1
The seal box 17 is made of organic glass, and the vacuum pump 18 vacuumizes the seal box 17 through an air suction port of the seal box 17.
A mid-infrared spectrum measuring method of ammonia molecule absorption line parameters comprises the following steps: firstly, pumping the pressure in a sealed box 17 to be below 100Pa by using a vacuum pump 18, then pumping a gas sample cell 9 to be a fixed low pressure (lower than 1000Pa) by using a suction pump 14, pumping high-purity nitrogen (the volume percentage is 99.999%) into the gas sample cell 9, and after the high-purity nitrogen in the gas sample cell 9 is kept at a constant pressure and a constant temperature (the pressure range is 0.6kPa to 100kPa, and the temperature range is 20 ℃ to 25 ℃) for 10 minutes, collecting an interferogram of the high-purity nitrogen as a background interferogram by using a high-resolution FTIR interferometer 6 and a tellurium-cadmium-Mercury (MCT) detector 11; after the background interferogram is collected, the gas sample cell 9 is pumped to a fixed low pressure (below 1000Pa), and then NH is filled into the gas sample cell 93Standard gas in the gas-waiting sample cell 9Keeping the pressure and the temperature at constant pressure and constant temperature (the pressure range is 0.6kPa to 100kPa, the temperature range is 20 ℃ to 25 ℃) for 5 minutes, keeping the constant pressure and the temperature consistent with the constant pressure and the temperature when the background interferogram is collected in the previous step, and collecting the interferogram of the standard sample gas as a sample interferogram by using a high-resolution FTIR interferometer 6 and a Mercury Cadmium Telluride (MCT) detector 11; respectively calculating the acquired background interference pattern and the acquired sample interference pattern by using a computer 19 by adopting a fast Fourier transform algorithm to obtain a background spectrogram and a sample spectrogram, and then calculating to obtain a high-resolution transmittance spectrum of the sample; the temperature is kept unchanged, the pressure of the gas sample in the gas sample pool 9 is changed, the above operation steps are repeated, and the background spectrogram and the sample spectrogram under 900Pa, 3000Pa, 8000Pa, 40000Pa and 90000Pa5 different constant pressures are acquired. And obtaining ammonia gas molecular line parameters including absorption line position, line intensity and line type parameters based on a HITRAN database and an inversion algorithm.
Compared with the prior art, the invention has the following advantages:
(1) the invention can realize the measurement of ammonia molecule absorption line parameters with high precision and high accuracy.
(2) The invention can realize the applicability research of different molecular absorption linear models under different atmospheric pressures between 0.9kPa and 90 kPa.
Drawings
FIG. 1 is a structural schematic diagram of a mid-infrared spectrum measurement system for ammonia gas molecular absorption line parameters.
Note: in the figure, the solid line represents the light path, the dotted line represents the data line, the dotted line represents the gas transmission gas path, and the dotted line represents the cooling water or liquid nitrogen transmission pipeline.
In the figure, 1, an infrared light source, 2 focusing parabolic mirrors a, 3 diaphragms, 4 collimating parabolic mirrors, 5 flat mirrors a, 6 high-resolution FTIR interferometers, 7 flat mirrors b, 8 focusing parabolic mirrors b, 9 gas sample cells, 10 optical filters, 11 tellurium-cadmium-Mercury (MCT) detectors, 12 gas inlets, 13 mass flow controllers, 14 air pumps, 15 water cooling systems, 16 liquid nitrogen tanks, 17 seal boxes, 18 vacuum pumps and 19 computers.
Detailed Description
The invention is described in detail below with reference to the figures and the embodiments. The following examples are only for explaining the present invention, the scope of the present invention shall include the full contents of the claims, and the full contents of the claims of the present invention can be fully realized by those skilled in the art through the following examples.
As shown in fig. 1, the mid-infrared spectrum measurement system for ammonia gas molecular absorption line parameters of the present invention includes a mid-infrared light source 1, a focusing parabolic mirror a2, a diaphragm 3, a collimating parabolic mirror 4, a plane mirror a5, a high resolution FTIR interferometer 6, a plane mirror b7, a focusing parabolic mirror b8, a gas sample cell 9, a filter 10, a Mercury Cadmium Telluride (MCT) detector 11, a gas inlet 12, a gas mass flow controller 13, a gas extraction pump 14, a water cooling system 15, a liquid nitrogen tank 16, a seal box 17, a vacuum pump 18, and a computer 19; the light source 1 provides light in a middle infrared band; the focusing parabolic mirror a2 focuses the light beam emitted from the light source 1; the diaphragm 3 is used for controlling the luminous flux entering the measuring system and preventing a Mercury Cadmium Telluride (MCT) detector 11 from being saturated; the collimating paraboloidal mirror 4 changes the divergent light emitted from the diaphragm 3 into parallel light; the plane mirror a5 reflects the parallel light reflected by the collimating parabolic mirror 4 into the high resolution FTIR interferometer 6; the high-resolution FTIR interferometer 6 changes the incident parallel light into coherent parallel light and emits the coherent parallel light; the plane mirror b7 reflects the parallel light emitted by the high-resolution FTIR interferometer 6 to the focusing parabolic mirror b 8; focusing parabolic mirror b8 focuses the parallel light exiting FTIR interferometer 6 onto gas sample cell 9; the filter 10 makes the light of the specific mid-infrared band after passing through the gas sample cell 9 reach a Mercury Cadmium Telluride (MCT) detector 11; a Mercury Cadmium Telluride (MCT) detector 11 responds to light in a middle infrared band to generate an interference signal; a gas sample cell 9 is filled with dry nitrogen or ammonia standard gas from a gas inlet 12, a gas outlet of the gas sample cell 9 is connected with a gas mass flow controller 13 and a gas pump 14, the gas pump 14 pumps the gas to the gas sample cell 9, and the gas mass flow controller 13 is used for controlling the gas flow; the water cooling system 15 is used for cooling the mid-infrared light source 1; liquid nitrogen in the liquid nitrogen tank 16 is used for refrigerating the mercury cadmium telluride detector 11, so that the noise of the detector is reduced; the mid-infrared light source 1, the focusing parabolic mirror a2, the diaphragm 3, the collimating parabolic mirror 4, the plane mirror a5, the high-resolution FTIR interferometer 6, the plane mirror b7, the focusing parabolic mirror b8, the gas sample cell 9, the optical filter 10 and the Mercury Cadmium Telluride (MCT) detector 11 are jointly arranged in a sealed box 17; a vacuum pump 18 is arranged near the seal box 17 and used for pumping the gas in the seal box 17 to be in a near vacuum state (the gas pressure is less than 100 Pa); gas transmitted by a gas inlet 12 enters a gas sample cell 9 through a gas transmission gas path under the action of a gas mass flow controller 13 and a suction pump 14, and an infrared interference pattern of coherent light beams emitted by an FTIR interferometer 6 after passing through the gas sample cell 9 is collected by a Mercury Cadmium Telluride (MCT) detector 11; the diaphragm 3, the high-resolution FTIR interferometer 6 and the vacuum pump 18 are all controlled by the computer 19, an interference pattern collected by a Mercury Cadmium Telluride (MCT) detector 11 is input into the computer 19, a spectrogram is obtained through a fast Fourier transform algorithm, and the obtained spectrogram is analyzed by the computer 19 to obtain molecular line parameters of ammonia.
In this embodiment, the light source 1 is a mid-infrared light source, and the wavelength coverage range is 350-8000cm-1. The diaphragm 3 is a diaphragm group which is formed by installing a group of diaphragms with fixed apertures with the aperture sizes of 0.5mm, 1.0mm, 1.5mm, 1.7mm and 2.0mm on the rotating wheel; in actual measurement, a proper diaphragm is selected according to the signal intensity of a Mercury Cadmium Telluride (MCT) detector 11. The spectral resolution of the high resolution FTIR interferometer 6 is 0.005cm-1The beam splitter is a potassium bromide (KBr) beam splitter, and the coverage wavelength range is 450--1. The gas sample cell 9 is a multi-reflection sample cell, the cell wall is made of stainless steel, the interior of the cell is electropolished and plated with gold, the length of the sample cell substrate is 20cm, and the optical path length is 72 m. The gas path pipe between the gas inlet 12, the gas sample cell 9, the gas mass flow controller 13 and the air pump 14 is stainless steel, and the inside is electropolished and plated with gold. The gas mass flow controller 13 controls the flow rate of the gas to be in the range of 0.5 to 1.5Lmin-1. The flow rate of the air pump 14 is 3Lmin-1. The filter 10 is a narrow-band filter, and the wavelength range of the transmitted light is 500-1500cm-1. The wavelength response range of the Mercury Cadmium Telluride (MCT) detector 11 is 600-10000cm-1. The seal box 17 is made of organic glass, and the vacuum pump 18 vacuumizes the seal box 17 through an air suction port of the seal box 17。
When the ammonia molecular absorption line parameter mid-infrared spectrum measuring system is used, firstly, a vacuum pump 18 is used for pumping the pressure in a sealing box 17 to be lower than 100Pa, then, a gas sample pool 9 is pumped to be a fixed low pressure (lower than 1000Pa) by a suction pump 14, then, high-purity nitrogen (the volume percentage is 99.999%) is pumped into the gas sample pool 9, the high-purity nitrogen in the gas sample pool 9 is kept at a constant pressure and a constant temperature (the pressure range is 0.6kPa to 100kPa, and the temperature range is 20 ℃ to 25 ℃) for 10 minutes, and then, a high-resolution FTIR interferometer 6 and a tellurium cadmium Mercury (MCT) detector 11 are used for collecting an interferogram of the high-purity nitrogen as a background interferogram; after the background interferogram is collected, the gas sample cell 9 is pumped to a fixed low pressure (below 1000Pa), and then NH is filled into the gas sample cell 93When the standard gas in the gas sample cell 9 is kept at a constant pressure and a constant temperature (the pressure range is 0.6kPa to 100kPa, the temperature range is 20 ℃ to 25 ℃) for 5 minutes, the constant pressure and the temperature are kept consistent with the constant pressure and the temperature when the background interferogram is collected in the previous step, and the interferogram of the standard gas is collected by a high-resolution FTIR interferometer 6 and a Mercury Cadmium Telluride (MCT) detector 11 to be used as a sample interferogram; respectively calculating the acquired background interference pattern and the acquired sample interference pattern by using a computer 19 by adopting a fast Fourier transform algorithm to obtain a background spectrogram and a sample spectrogram, and then calculating to obtain a high-resolution transmittance spectrum of the sample; keeping the temperature unchanged, changing the pressure of the gas sample in the gas sample pool 9, repeating the above operation steps, and collecting a background spectrogram and a sample spectrogram under 5 different constant pressures, such as 900Pa, 3000Pa, 8000Pa, 40000Pa, 90000Pa and the like; and obtaining ammonia gas molecular line parameters based on a HITRAN database and an inversion algorithm, wherein the ammonia gas molecular line parameters comprise the central position of an absorption line, spectral line pressure displacement, spectral line intensity and spectral line broadening parameters.
The transmittance spectra measured at gas temperature T (in K) and pressure p (in atm) are:
Figure BDA0003069005390000061
where v isFrequency (in cm)-1),IsampleAnd I0Respectively, an ammonia standard gas spectrum and a pure nitrogen background spectrum.
In addition, the ammonia gas absorption cross section sigma (nu, p, T) (unit is cm)2·molecule-1) The absorption path length l (in m) and the ammonia gas concentration c (in nmol mol)-1) Calculating a simulated transmittance spectrum:
Figure BDA0003069005390000062
here, the absorption cross section σ (v, p, T) is the molecular absorption line intensity S (v, T) (unit is cm. molecule)-1) Is combined with the molecular absorption line profile f (v, p, T), where γ is the line width and ILS (v) is a linear function of the FTIR instrument.
The ammonia molecular absorption line profiles are described herein in terms of a simple Voigt molecular absorption line profile and a complex Galatry molecular absorption line profile, respectively:
the Voigt profile is:
Figure BDA0003069005390000063
the Galatry line is:
Figure BDA0003069005390000064
where x' is x-s, x is (v-v)0)/αD,y=αlD,z=β/αDS is the pressure displacement (dimensionless), s is δ p/αDWhere δ is the pressure displacement coefficient (in cm)-1·atm);ν0Is the absorption line center frequency (in cm)-1) And v is the frequency (in cm)-1),αlAnd alphaDHalf-widths (in cm) of the Lorentz and Doppler line types, respectively-1) And beta is the optical diffusion rate (in sec)-1)。
Then applying the non-linear maximumSmall-two times spectrum fitting algorithm searching gammasimulatedOptimum parameters v, S (v, T), delta, alpha in (v, p, T)l、αDAnd β, by making the objective function χ2(v, p, T) minimization seeks an optimal solution.
Figure BDA0003069005390000071
Where i corresponds to N data points in the spectrum Γ and the linewidth γ is a broadening parameter of the spectral line including αl、αDAnd β. Finally, the applicability of different molecular absorption line types under different atmospheric pressures is researched by comparing spectrum fitting residuals under different gas pressures.
In a word, the invention develops a set of mid-infrared spectrum measuring system for ammonia molecular absorption line parameters based on FTIR technology for measuring ammonia molecular absorption line parameters, can realize high-precision and high-accuracy measurement of ammonia absorption line parameters, and can realize applicability research of a complex molecular absorption line model under different atmospheric pressures. The mid-infrared spectrum measuring system for ammonia molecule absorption line parameters can effectively make up for the defects of the traditional measuring technology.
The invention has not been described in detail and is part of the common general knowledge of a person skilled in the art.
The above description is only an embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can understand that the modifications or substitutions within the technical scope of the present invention are included in the scope of the present invention, and therefore, the scope of the present invention should be subject to the protection scope of the claims.

Claims (10)

1. The utility model provides an ammonia molecule absorbs mid infrared spectroscopy measurement system of line parameter which characterized in that includes: the device comprises a medium infrared light source (1), a focusing parabolic mirror a (2), a diaphragm (3), a collimating parabolic mirror (4), a plane mirror a (5), a high-resolution FTIR interferometer (6), a plane mirror b (7), a focusing parabolic mirror b (8), a gas sample cell (9), a light filter (10), a Mercury Cadmium Telluride (MCT) detector (11), a gas inlet (12), a gas mass flow controller (13), an air pump (14), a water cooling system (15), a liquid nitrogen tank (16), a sealing box (17), a vacuum pump (18) and a computer (19); the light source (1) provides light in the mid-infrared band; the focusing parabolic mirror a (2) focuses the light beam emitted by the light source (1); the diaphragm (3) is used for controlling the luminous flux entering the measuring system and preventing a Mercury Cadmium Telluride (MCT) detector (11) from being saturated; the collimating paraboloidal mirror (4) changes the divergent light emitted by the diaphragm (3) into parallel light; the plane mirror a (5) reflects the parallel light reflected by the collimating paraboloidal mirror (4) into the high-resolution FTIR interferometer (6); the high-resolution FTIR interferometer (6) changes the incident parallel light into coherent parallel light and emits the coherent parallel light; the plane mirror b (7) reflects the parallel light emitted by the high-resolution FTIR interferometer (6) to the focusing parabolic mirror b (8); the focusing parabolic mirror b (8) focuses the parallel light emitted by the FTIR interferometer (6) to a gas sample cell (9); the filter (10) enables the light of the middle infrared band passing through the gas sample cell (9) to reach a Mercury Cadmium Telluride (MCT) detector (11); the Mercury Cadmium Telluride (MCT) detector (11) responds to light in a middle infrared band to generate interference signals; dry nitrogen or ammonia standard gas is introduced into the gas sample cell (9) from a gas inlet (12), a gas outlet of the gas sample cell (9) is connected with a gas mass flow controller (13) and a gas extraction pump (14), the gas extraction pump (14) extracts gas to the gas sample cell (9), and the gas mass flow controller (13) is used for controlling the gas flow; the water cooling system (15) is used for cooling the mid-infrared light source (1); the liquid nitrogen in the liquid nitrogen tank (16) is used for refrigerating a Mercury Cadmium Telluride (MCT) detector (11) and reducing the noise of the detector; the middle infrared light source (1), the focusing parabolic mirror a (2), the diaphragm (3), the collimating parabolic mirror (4), the plane mirror a (5), the high-resolution FTIR interferometer (6), the plane mirror b (7), the focusing parabolic mirror b (8), the gas sample cell (9), the optical filter (10) and the Mercury Cadmium Telluride (MCT) detector (11) are jointly arranged in a sealed box (17); the vacuum pump (18) is arranged near the seal box (17) and used for pumping gas in the seal box (17) to enable the gas to be in a near vacuum state, and interference of the gas in the seal box (17) on measurement is avoided, wherein the near vacuum refers to the gas pressure being less than 100 Pa; gas transmitted by the gas inlet (12) enters the gas sample cell (9) through a gas transmission gas path under the action of a gas mass flow controller (13) and a suction pump (14), and an infrared interference pattern of a coherent light beam emitted by the FTIR interferometer (6) after passing through the gas sample cell (9) is collected by a tellurium-cadmium-Mercury (MCT) detector (11); the diaphragm (3), the high-resolution FTIR interferometer (6) and the vacuum pump (18) are all controlled by the computer (19), an interference pattern collected by a Mercury Cadmium Telluride (MCT) detector (11) is input into the computer (19), a spectrogram is obtained through a fast Fourier transform algorithm, and the obtained spectrogram is analyzed by the computer (19) to obtain molecular line parameters of the ammonia gas.
2. The ammonia gas molecular absorption line parameter mid-infrared spectroscopy measurement system of claim 1, wherein: the light source (1) is a mid-infrared light source, and the wavelength coverage range is 350--1(ii) a The diaphragm (3) is a diaphragm group which is formed by installing a group of diaphragms with fixed apertures with the aperture sizes of 0.5mm, 1.0mm, 1.5mm, 1.7mm and 2.0mm on the rotating wheel.
3. The ammonia gas molecular absorption line parameter mid-infrared spectroscopy measurement system of claim 1, wherein: the high resolution FTIR interferometer (6) has a spectral resolution of 0.005cm-1The beam splitter is a potassium bromide beam splitter, and the coverage wavelength range is 450--1
4. The ammonia gas molecular absorption line parameter mid-infrared spectroscopy measurement system of claim 1, wherein: the gas sample cell (9) is a multi-reflection sample cell, the cell wall is made of stainless steel, the interior of the cell is electropolished and plated with gold, the length of a sample cell substrate is 20cm, and the optical path length is 72 m; and the gas path pipeline among the gas inlet (12), the gas sample cell (9), the gas mass flow controller (13) and the air pump (14) is made of stainless steel, and the interior of the gas path pipeline is electropolished and plated with gold.
5. The ammonia gas molecular absorption line parameter mid-infrared spectroscopy measurement system of claim 1, wherein: the gas mass flow controller (13) controls the flow rate of the gas to be in the range of 0.5-1.5L min-1
6. The ammonia gas molecular absorption line parameter mid-infrared spectroscopy measurement system of claim 1, wherein: the flow rate of the air suction pump (14) is 3L min-1
7. The ammonia gas molecular line parameter mid-infrared spectroscopy measurement system of claim 1, characterized in that: the filter (10) is a narrow-band filter, and the wavelength range of the transmitted light is 500-1500cm-1
8. The ammonia gas molecular line parameter mid-infrared spectroscopy measurement system of claim 1, characterized in that: the wavelength response range of the mercury cadmium telluride detector (11) is 600-10000cm-1
9. The ammonia gas molecular line parameter mid-infrared spectroscopy measurement system of claim 1, characterized in that: the seal box (17) is made of organic glass, and the vacuum pump (18) vacuumizes the seal box (17) through an air suction port of the seal box (17).
10. A mid-infrared spectrum measuring method of ammonia molecular line parameters is characterized in that: use of a mid-infrared spectrometric system according to any one of claims 1-9, first pumping the pressure inside the sealed box (17) to below 100Pa with a vacuum pump (18), then pumping the gas sample cell (9) to a fixed low pressure with a suction pump (14), said low pressure being lower than 1000Pa, then pumping high purity nitrogen gas (99.999% by volume) into the gas sample cell (9), said constant pressure ranging from 0.6kPa to 100kPa, 10 minutes after maintaining the high purity nitrogen gas in the gas sample cell (9) at a constant pressure and at a constant temperature ranging from 20 ℃ to 25 ℃, collecting the interferogram of high purity nitrogen gas as background interferogram using a high resolution FTIR interferometer (6) and a mercury telluride detector (11); after the background interferogram is collected, the gas sample cell (9) is pumped to a fixed low pressure, the low pressure being lower than 1000Pa, and then NH is filled into the gas sample cell (9)3When the standard sample gas in the gas sample cell (9) is kept at a constant pressure and a constant temperature for 5 minutes, the constant pressure range is 0.6kPa to 100kPa, the constant temperature range is 20 ℃ to 25 ℃, the constant pressure and the temperature are kept consistent with the constant pressure and the temperature when the background interferogram is collected in the previous step, and a high-resolution FTIR interferometer (6) and a tellurium-cadmium-mercury detector (11) are used for collecting the interferogram of the standard sample gas as a sample interferogram; the computer (19) adopts a fast Fourier transform algorithm to respectively calculate the acquired background interference pattern and the acquired sample interference pattern to obtain a background spectrogram and a sample spectrogram, and then calculates to obtain a high-resolution transmittance spectrum of the sample; keeping the temperature unchanged, changing the pressure of the gas sample in the gas sample pool, repeating the above operation steps, and collecting 900Pa, 3000Pa, 8000Pa, 40000Pa and 90000Pa5 background spectrograms and sample spectrograms under different constant pressures; and obtaining ammonia gas molecular line parameters including absorption line position, line intensity and line type parameters based on a HITRAN database and an inversion algorithm.
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