High-sensitivity detection system and method for measuring nitrogen oxide in atmosphere

Abstract

The invention discloses a high-sensitivity detection system and method for measuring nitrogen oxide in atmosphere, and relates to the technical field of atmosphere detection. The system comprises a laser generation module, a sampling and gas processing module and a fluorescence detection and data processing module, the laser generation module generates ultraviolet laser for exciting nitric oxide fluorescence, after to-be-detected atmosphere or sample gas provided by a standard gas cylinder is subjected to flow proportioning and control, a first path of gas enters the NO detection cavity, a second path of gas enters the NOx detection cavity after being subjected to photolysis conversion through the LED photolysis converter, the ultraviolet laser sequentially penetrates through the NOx detection cavity and the NO detection cavity, and the NOx detection cavity and the NO detection cavity are communicated with the NOx detection cavity. A fluorescence collection system is arranged in each detection cavity, the laser energy meter monitors the energy of laser penetrating through the two detection cavities in real time, and online monitoring of the concentration of NO and NO2 is achieved according to the fluorescence signal intensity and the laser energy. The system has the characteristics of compact structure, high optical path stability and high detection precision.

Description

Technical Field

[0001] This invention relates to the field of atmospheric detection technology, and in particular to a highly sensitive detection system and method for measuring atmospheric nitrogen oxides. Background Technology

[0002] Nitrogen oxides (NOx), as a core driving factor of tropospheric atmospheric chemical reactions, control the formation of ozone and fine particulate matter and determine atmospheric oxidation capacity. Although concentrations are high in urban areas, NOx concentrations in the background atmosphere are typically below the critical threshold of 10-30 ppt. To accurately analyze atmospheric oxidation and validate chemical transport models, monitoring instruments need to have a detection capability of 1-10 ppt. However, existing mainstream technologies have significant limitations: chemiluminescence methods are affected by water vapor quenching effects, and molybdenum conversion furnaces cannot eliminate interference from nitrogen-containing compounds such as PANs, leading to increased NOx concentrations. The measured values ​​are often inflated and the detection limit is insufficient; differential absorption spectroscopy relies on long optical paths, has low spatial resolution, and is easily affected by aerosol extinction; cavity-based absorption spectroscopy faces the challenges of high-reflectivity contamination and uncertainties in indirect NO measurement, making it difficult to meet the high-precision requirements for trace background monitoring.

[0003] Laser-induced fluorescence (LAFI) technology, with its extremely high sensitivity and selectivity, is a preferred method for detecting trace amounts of reactive intermediates. However, its engineering application has long been constrained by the technological bottleneck of deep-ultraviolet (DUV) excitation sources. The strong absorption band of NO molecules is located at 214 nm or 226 nm. Traditional frequency-doubled dye laser systems are complex, difficult to maintain, and the dyes are toxic. Furthermore, the Nd:YAG fifth-harmonic generation scheme suffers from low excitation efficiency and unstable nonlinear transformations. More critically, to subtract background scattering noise through differential measurement strategies, the light source must possess deep-ultraviolet wavelength control precision better than 0.001 nm. Existing conventional crystal angle or temperature tuning techniques struggle to maintain wavelength stability under high-frequency modulation and are prone to drift. Therefore, a breakthrough is needed in developing deep-ultraviolet all-solid-state laser source technology that combines high spectral resolution with long-term engineering stability to resolve the contradiction between scientific research needs and instrument performance. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the present invention provides a highly sensitive detection system and method for measuring atmospheric nitrogen oxides.

[0005] To achieve the above objectives, the present invention adopts the following technical solution, including: A highly sensitive detection system for measuring atmospheric nitrogen oxides includes a laser generation module, a sampling and gas processing module, and a fluorescence detection and data processing module. In the laser generation module, the fundamental frequency laser output from the laser is frequency-doubled by a first frequency-doubled crystal to generate a first frequency-doubled laser. The first frequency-doubled laser is split into two beams by a beam splitter. The first laser beam serves as the pump light for an optical parametric oscillator, generating continuously tunable near-infrared light within a set wavelength range. The second laser beam is frequency-doubled again by a second frequency-doubled crystal to generate a second frequency-doubled laser. After collinearly combining the near-infrared light and the second frequency-doubled laser, they are injected into a sum-frequency crystal to generate a sum-frequency effect, producing an ultraviolet laser for exciting nitric oxide fluorescence. The wavelength scanning of the ultraviolet laser is achieved by adjusting the wavelength of the near-infrared light. In the gas sampling and gas processing module, the atmospheric sample or the sample gas provided by the standard gas cylinder is mixed and controlled by the mass flow controller to obtain two gas streams; the first gas stream enters the NO detection chamber, and the second gas stream first undergoes photolysis conversion through the LED photolysis converter before entering the NOx detection chamber; the LED photolysis converter is used to photolyze NO2 into NO. In the fluorescence detection and data processing module, the ultraviolet laser generated by the laser generation module passes sequentially through the NOx detection cavity and the NO detection cavity; each detection cavity is equipped with a fluorescence collection system for detecting the intensity of the fluorescence signal; the laser energy meter monitors the laser energy after passing through the two detection cavities in real time; and the concentrations of NO and NO2 are monitored online based on the detected fluorescence signal intensity and the monitored laser energy.

[0006] Preferably, in the laser generation module, an Nd:YAG laser is used as the fundamental frequency source, outputting a 1064nm fundamental frequency laser. The first frequency doubling crystal is a KTP crystal, which doubles the fundamental frequency laser to generate a 532nm first frequency doubling laser. Subsequently, the first frequency doubling laser is filtered by a dichroic mirror to remove residual fundamental frequency laser light, which is then absorbed by a waste light collector. The purified first frequency doubling laser is split into two beams by a beam splitter. The first beam is focused by a focusing lens and injected into an optical parametric oscillator as pump light. The optical parametric oscillator adopts a narrow linewidth cavity structure, and its output wavelength is continuously adjustable in the range of 1081nm to 1135nm. The near-infrared light is generated; the second frequency-doubled crystal is a BBO crystal, and the second laser beam is frequency-doubled again by the second frequency-doubled crystal to generate a 266nm second frequency-doubled laser. Then, the polarization state of the second frequency-doubled laser is adjusted by a half-wave plate and shaped by a shaping lens to match its spot characteristics with the light output from the optical parametric oscillator; the near-infrared light and the second frequency-doubled laser are collinearly combined by a beam combiner, and then focused by a focusing lens group and injected into the sum-frequency crystal; the sum-frequency crystal is an LBO crystal, which generates a 214nm ultraviolet laser; the ultraviolet laser generated by the sum-frequency crystal is filtered by a dichroic mirror to output a pure ultraviolet laser, which is used as the ultraviolet laser to excite NO.

[0007] Preferably, the wavelength adjustment step size of the optical parametric oscillator is better than 0.026 nm.

[0008] Preferably, the sum-frequency crystal is mounted on an electrically driven rotary stage. During wavelength scanning by the laser generation module, the output wavelength of the optical parametric oscillator and the rotation angle of the electrically driven rotary stage are synchronously controlled according to a preset phase-matching mapping model. The phase-matching mapping model is the output wavelength of the optical parametric oscillator. Optimal phase matching angle with sum-frequency crystal The functional relationship between them is used to compensate for the fluctuation of crystal refractive index caused by wavelength change, so that the ultraviolet laser can maintain the maximum conversion efficiency within the tuning range.

[0009] Preferably, in the gas sampling and gas processing module, the standard gas cylinders include: NO2 standard gas cylinder, NO standard gas cylinder, and synthetic air cylinder.

[0010] Preferably, the LED photolysis converter includes an LED photolysis converter inlet, an LED photolysis converter outlet, and an LED light source; a UV-grade fused silica window is provided between the LED light source and the gas flow channel to physically isolate the LED light source from the corrosive sample gas; a spiral stepping light shield is provided around the LED light source to adjust the photolysis conversion efficiency; and a light shield limiting screw is provided at the spiral of the light shield.

[0011] Preferably, the fluorescence signal intensity is detected by a fluorescence collection system; the fluorescence collection system is shown below: The sampling gas flow enters the fluorescence cavity through the sampling port. The ultraviolet laser is incident from the opposite side, perpendicular to the gas flow and the photomultiplier tube, to excite the NO molecules in the gas flow. After the NO molecules are excited, they radiate fluorescence in all directions. A concave reflector is provided on the back side of the optical path to reflect the back fluorescence back into the collecting optical path; the forward fluorescence passes through a lens group consisting of a collimating lens, a bandpass filter and a focusing lens in sequence and is focused onto the photocathode of the photomultiplier tube; a filter is provided before the photomultiplier tube to filter out Rayleigh scattering light and background stray light.

[0012] This invention also provides a highly sensitive detection method for measuring atmospheric nitrogen oxides, characterized by employing the aforementioned highly sensitive detection system for measuring atmospheric nitrogen oxides, and the method is as follows: Step 1: Wavelength Scanning and Locking The optical parametric oscillator performs wavelength scanning between 1081 nm and 1135 nm. During the scanning process, the phase matching angle of the sum-frequency crystal changes synchronously. Based on the mapping relationship of sum-frequency conversion, the output wavelength of the sum-frequency crystal is converted to 213.5 nm to 215.5 nm to excite the NO gas to be measured and obtain the corresponding excitation spectrum. Identify the position of the characteristic peak corresponding to the maximum absorption cross section in the excitation spectrum of NO standard gas, and define the wavelength corresponding to this characteristic peak position as the resonant excitation wavelength. The wavelength corresponding to the unexcited NO in the excitation spectrum is defined as the non-resonant excitation wavelength. ; Step 2: Switching between resonance / non-resonance state measurements Controlling the optical parametric oscillator in and It switches periodically between two wavelengths; Step 3: Dual-channel differential detection and background subtraction After the atmospheric sample is taken, it is divided into two paths. One path goes directly into the NO detection chamber, and the other path passes through an LED photolysis converter to convert NO2 into NO before entering the NOx detection chamber. Data is simultaneously collected from both detection chambers. and The fluorescence signal intensity under the specified conditions was recorded in the NO detection chamber and the NOx detection chamber. Fluorescence signal intensity collected under the condition , and in Fluorescence signal intensity collected under the condition , The net fluorescence signal intensity in the NO detection chamber and NOx detection chamber was obtained by subtracting the background signal intensity using the differential method. ,

[0013] Step 4: Concentration Inversion Calculation

[0014] The concentrations of NO and NO2 in the atmosphere to be measured are calculated based on the net fluorescence signal intensity in the two detection chambers.

[0015] Preferably, in step four, the concentration inversion calculation is as follows: (1) Calculation of NO concentration in the atmosphere to be measured Based on the net fluorescence signal intensity in the NO detection chamber The concentration of NO in the atmosphere to be measured was obtained. for:

[0016] in, The laser energy monitored by the laser energy meter. The system sensitivity constant of the NO detection chamber is obtained by calibration with NO standard gas of known concentration; (2) Calculation of NO2 concentration in the atmosphere to be measured Based on the net fluorescence signal intensity in the NOx detection chamber The total NO concentration was obtained. for:

[0017] in, The system sensitivity constant of the NOx detection chamber is obtained by calibration with standard gases of known concentrations of NO and NO2; the total NO consists of NO in the atmosphere to be measured and NO converted from NO2 in the atmosphere to be measured. Then based on the total NO concentration and the concentration of NO in the atmosphere to be measured By combining the conversion efficiency of the LED photolysis converter, the concentration of PM2.5 in the atmosphere to be measured is obtained. concentration for:

[0018] in, The conversion efficiency of the LED photolysis converter.

[0019] The present invention also provides a computer program product, characterized in that it includes a computer program / instruction which, when executed by a processor, implements the above-described high-sensitivity detection method for measuring atmospheric nitrogen oxides.

[0020] The advantages of this invention are: (1) Achieving extremely high spectral resolution using step size mapping. This invention employs all-solid-state laser cascade frequency conversion technology. By adjusting the wavelength of the near-infrared optical parametric oscillator (OPO) (with a step size better than 0.026 nm), fine scanning in the deep ultraviolet band (0.001 nm resolution) is indirectly achieved using the sum-frequency mapping relationship. This strategy effectively avoids the inherent stability problem of direct mechanical tuning in the deep ultraviolet band, ensuring extremely high accuracy in spectral peak finding.

[0021] (2) The all-solid-state design significantly improves system stability and ease of use. Compared with traditional dye laser-induced fluorescence systems, this invention adopts an all-solid-state optical path design that combines Nd:YAG frequency doubling with OPO sum-frequency. This solution eliminates the need for frequent replacement of toxic and harmful organic dye solvents, and the system has a compact structure and high optical path stability, meeting the maintenance-free requirements for long-term online monitoring of the atmospheric environment.

[0022] (3) It possesses excellent laser energy fluctuation suppression and anti-interference capabilities. The system monitors and averages the energy of each pulse in real time through a laser energy meter, thereby achieving single-pulse energy normalization of the fluorescence signal. Combined with online / offline wavelength switching technology, it can accurately distinguish and subtract optical background signals (Rayleigh and Mie scattering) and system electronic noise, ensuring the accuracy of measurements in complex atmospheric environments.

[0023] (4) Achieving simultaneous detection of multiple components and extremely low detection limits. A dual-cavity parallel structure, combined with efficient LED photolysis conversion technology, was adopted to achieve simultaneous detection of NO and N in the atmosphere. Real-time synchronous monitoring. This differential detection mode not only eliminates the concentration fluctuation error caused by single-channel time-division measurement, but also achieves an extremely low detection limit at the ppt level with the help of highly sensitive LIF detection technology, enabling it to capture the minute evolution of trace nitrogen oxides in the atmosphere.

[0024] (5) Through dual-channel differential measurement, the present invention can eliminate laser energy fluctuations, aerosol scattering and background light interference in real time, and realize long-term, high-precision online monitoring of ppt-level NO and NO2 concentrations in the atmosphere. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall structure and optical path principle of the high-sensitivity detection system for atmospheric nitrogen oxide measurement of the present invention.

[0026] Figure 2 This is used in the embodiments of the present invention to transfer N A cross-sectional schematic diagram of the LED photolysis converter that converts NO to NO.

[0027] Figure 3 This is a schematic cross-sectional view of the fluorescence collection system (detection module) used to detect the intensity of fluorescence signals in an embodiment of the present invention.

[0028] Explanation of reference numerals in the attached figures: 1-Nd:YAG laser; 2-KTP crystal; 3-Waste light collector; 4-Dial mirror; 5-Beam splitter; 6-Reflector; 7-BBO crystal; 8-Focusing lens; 9-Optical parametric oscillator (OPO); 10-Half-wave plate; 11-Collimating lens; 12-Shaping lens; 13-Reflector; 14-Beam combiner; 15-First focusing lens; 16-Second focusing lens; 17-Sum crystal; 18-Dichroic mirror; 19-Reflector; 20-N 21-NO standard gas cylinder; 22-synthetic air cylinder; 23-mass flow controller; 24-LED photocatalytic converter; 25-NO detection chamber; 26-NOx detection chamber; 27-exhaust port; 28-UV anti-reflection window; 29-reflector; 30-laser energy meter; 31-computer; 32-LED photocatalytic converter inlet; 33-LED photocatalytic converter outlet; 34-LED light source; 35-light shield; 36-fused silica window; 37-light shield limiting screw; 38-collimating lens; 39-bandpass filter; 40-focusing lens; 41-window; 42-photomultiplier tube (PMT); 43-concave mirror; 44-sampling port. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] This embodiment provides a high-sensitivity detection system and method for measuring atmospheric nitrogen oxides. For example... Figure 1 As shown, the overall optical and gas path structure of the system mainly consists of three parts: a laser generation module, a gas sampling and gas processing module, and a fluorescence detection and data processing module.

[0031] The laser generation module employs all-solid-state laser cascade frequency conversion technology. The system uses a pulsed Nd:YAG laser 1 as the fundamental frequency source, emitting a 1064nm fundamental frequency laser. This laser first enters the first frequency doubling crystal, i.e., KTP crystal 2, where a 532nm laser wavelength (the first frequency-doubled laser wavelength) is generated through a nonlinear frequency doubling effect. The unconverted residual 1064nm fundamental frequency laser is filtered out by a dichroic mirror 4 and absorbed in a waste light collector 3. The purified 532nm laser then enters a beam splitter 5, where it is split into a first beam and a second beam.

[0032] The first beam (532nm), after being reflected by mirror 6, passes through focusing lens 8 and enters optical parametric oscillator 9 (OPO) as pump light. In this embodiment, the optical parametric oscillator 9 adopts a narrow linewidth cavity structure, containing a grating or etalon internally. Its output wavelength is continuously adjustable in the range of 1081nm to 1135nm, controlled by computer 31, and the wavelength adjustment step is better than 0.026nm. The light output from the optical parametric oscillator 9 is collimated by collimating lens 11 before use.

[0033] The second beam (532nm) directly enters the second frequency-doubled crystal, namely the BBO crystal 7 (barium borate BaB2O4, abbreviated as BBO), and is frequency-doubled again to generate a 266nm laser, which is the second frequency-doubled laser wavelength. The polarization state of this 266nm laser is adjusted by the half-wave plate 10 and shaped by the shaping lens 12 so that its beam characteristics match the phase of the light output by the optical parametric oscillator 9.

[0034] Subsequently, the near-infrared light generated by the optical parametric oscillator 9 is reflected by the reflector 13 and spatially combined with the 266nm laser at the beam combiner 14 (dichroic mirror), achieving collinear propagation. The two collinear beams are then focused by the focusing lens group (first focusing lens 15, second focusing lens 16) and enter the sum-frequency crystal 17. In this embodiment, the sum-frequency crystal 17 is preferably an LBO (lithium cesium borate) crystal. In the sum-frequency crystal 17, the 266nm photon and the near-infrared photon undergo a sum-frequency effect (SFG), generating a vacuum ultraviolet laser with a wavelength of approximately 214nm. The sum-frequency crystal 17 is mounted on an electrically driven rotary displacement stage, and its rotation angle is synchronously controlled by the computer 31 according to the wavelength change of the optical parametric oscillator 9 to meet the phase-matching condition, thereby utilizing the wide-range wavelength adjustment of near-infrared light to achieve narrow-range wavelength scanning of the ultraviolet laser.

[0035] The frequency matching process must adhere to the principles of energy conservation and momentum conservation (phase matching). Energy conservation requires that the output ultraviolet laser frequency equals the sum of the frequencies of the two incident photons, i.e.:

[0036] in, It is the second harmonic laser wavelength. The near-infrared light wavelength (1081nm to 1135nm) output by the optical parametric oscillator. The output ultraviolet laser wavelength is 214nm.

[0037] The phase-matching condition requires that the wave vectors of the three beams in the crystal satisfy the following:

[0038] in, The phase mismatch during the sum-frequency process should be close to zero to obtain the maximum conversion efficiency. , , These are the wave vectors of ultraviolet laser, second-harmonic laser, and near-infrared light in the nonlinear crystal, respectively. , , and represent the effective refractive indices of lasers of corresponding wavelengths in the crystal, where the effective refractive index of ultraviolet laser is... Phase matching angle of the sum-frequency crystal 17 Modulation; The phase matching angle of the sum-frequency crystal 17 (LBO crystal), i.e. the angle between the laser wave vector and the optical axis of the crystal, is adjusted by an electric rotary displacement stage.

[0039] Because LBO crystals exhibit dispersive properties, their refractive index is equal to the wavelength. The function follows the Sellmeier equation. In this embodiment, a Type I phase-matching method is used, and the effective refractive index of the ultraviolet laser is... Crystal cutting angle and phase matching angle The relationship is:

[0040] in, , These represent the principal refractive index values ​​of the LBO crystal in the principal axis coordinate system, expressed by the Sellmeier equation for the LBO crystal as a function of wavelength. Calculated in real time.

[0041] When infrared wavelength A step change of 0.026 will result in a phase mismatch. This leads to a significant decrease in ultraviolet laser energy. Computer 31 calculates the above equations in real time to obtain the optimal phase-matching angle. Correction amount :

[0042] in, This means that in order to maintain phase matching, the electric rotary stage needs to compensate for the small angle command value of rotation; The wavelength adjustment step size for the optical parametric oscillator (OPO) is better than 0.026 nm in this embodiment. Phase mismatch versus wavelength The rate of change (dispersion term). Refers to the phase mismatch amount relative to the phase matching angle The rate of change (angle sensitivity term).

[0043] The electric rotary stage is driven to perform compensated rotation, ensuring that the nonlinear conversion process remains within the maximum gain bandwidth throughout the entire spectral scanning range of 213.5nm to 215.5nm.

[0044] Subsequently, the mixed beam output from the sum-frequency crystal 17 is incident on a dichroic mirror 18, which is coated and designed to be highly reflective of 214nm laser light and highly transmittable of 266nm and near-infrared laser light. Therefore, the pure 214nm laser light is reflected and extracted, while the residual pump light is blocked after transmission, thus obtaining a high-purity 214nm ultraviolet laser source for exciting NO.

[0045] In the gas sampling and gas processing module, the target atmosphere (Air) or gas sampled from a standard gas cylinder (N) is used. The sample gas provided by the standard gas cylinder 20, NO standard gas cylinder 21, and synthetic air cylinder 22 is proportioned and controlled by the mass flow controller 23 (M) to obtain two gas streams. The gas pipeline is divided into two streams: the first stream of gas directly enters the NO detection chamber 25 through the NO detection chamber inlet to measure the NO concentration originally present in the environment; the second stream of gas first flows through the LED photolysis converter 24, which photolyzes the NO2 in the gas into NO, and then enters the NOx detection chamber 26 through the NOx detection chamber inlet.

[0046] like Figure 2 As shown, the LED photolysis converter 24 includes an LED photolysis converter inlet 32, an LED photolysis converter outlet 33, and a core LED light source 34. To physically isolate the LED light source 34 from the corrosive sample gas while ensuring photolysis efficiency, a UV-grade fused silica window 36 is placed between the LED light source 34 and the gas flow channel. A spirally stepped light shield 35 is arranged around the LED light source 34 to adjust the photolysis conversion efficiency. A light shield limiting screw 37 is provided at the spiral of the light shield 35 to ensure that the set position does not shift due to vibration or other factors.

[0047] In the fluorescence detection and data processing module, the 214nm laser generated by the laser generation module is reflected by mirror 19 and then passes sequentially through the NOx detection cavity and the NO detection cavity. Inside each detection cavity, such as... Figure 1 As shown, the laser beam passes through the fluorescence cavity (i.e., the detection cavity), exciting NO molecules in the gas flow to produce fluorescence. Each detection cavity is designed with a downward-facing vent 27 to maintain negative pressure and stable gas flow within the cavity. The two detection cavities are optically sealed together and at their ends by ultraviolet anti-reflection windows 28. The laser beam after passing through the two detection cavities is reflected by a mirror 29 to a laser energy meter 30 at the end. The laser energy after passing through the two detection cavities is monitored in real time by the laser energy meter 30 at the end, and the monitoring data is transmitted to a computer 31 for normalization calculations during concentration inversion.

[0048] The detection of fluorescence signal intensity is achieved by Figure 3 The fluorescence collection system shown is now complete. The sampling gas flow enters the fluorescence cavity through sampling port 44. A 214nm laser beam is incident from the opposite side, perpendicular to the gas flow and photomultiplier tube 42, exciting NO molecules in the gas flow. Once excited, the NO molecules radiate fluorescence in all directions. To improve collection efficiency, a concave reflector 43 is placed on the back side of the optical path to reflect the back-facing fluorescence back into the collection optical path. The forward-facing fluorescence passes sequentially through a lens group consisting of a collimating lens 38, a bandpass filter 39, and a focusing lens 40, and is focused onto the photocathode of the photomultiplier tube 42 (PMT). A filter 41 is placed before the photomultiplier tube 42 to filter out 214nm Rayleigh scattering light and other background stray light, allowing only the characteristic fluorescence band of NO to pass through.

[0049] Based on the above-described apparatus, the atmospheric nitrogen oxide measurement method of the present invention is as follows: Step 1: Wavelength Scanning and Locking The optical parametric oscillator (OPO) performs wavelength scanning between 1081 nm and 1135 nm, using the sum-frequency formula. ,in, The target wavelength is the ultraviolet laser wavelength used to excite the fluorescence of NO molecules. The near-infrared light wavelength output by the OPO. Between 1081 nm and 1135 nm; the adjustment step size of OPO is better than 0.026 nm. During the scanning process, the phase matching angle of the sum-frequency crystal changes synchronously. Specifically, the step size change of OPO in the near-infrared band of 0.026 nm corresponds to a step size change of about 0.001 nm in the deep ultraviolet band. Based on the mapping relationship of sum-frequency conversion, the ultraviolet laser wavelength output by the sum-frequency crystal is converted to between 213.5 nm and 215.5 nm, thereby generating an ultraviolet laser with a spectral scanning resolution of 0.001 nm, which is used to excite the NO gas to be measured and obtain its excitation spectrum. Identify the position of the characteristic peak corresponding to the maximum absorption cross section in the excitation spectrum of NO standard gas, and define the wavelength corresponding to this position as the resonant excitation wavelength. The wavelength corresponding to the unexcited NO in the excitation spectrum is defined as the non-resonant excitation wavelength. .

[0050] Step 2: Switching between online and offline measurements

[0051] Controlling the optical parametric oscillator in and It periodically switches between two wavelengths.

[0052] Step 3: Dual-channel differential detection and background subtraction

[0053] After the atmospheric sample is taken, it is divided into two paths. One path goes directly into the NO detection chamber, and the other path passes through an LED photolysis converter to convert NO2 into NO before entering the NOx detection chamber. Data is simultaneously collected from both detection chambers. and The corresponding fluorescence signal intensity in the state, where, Under the specified conditions, the fluorescence signal intensities collected by the photomultiplier tubes in the NO detection chamber and the NOx detection chamber are respectively... , ,exist The fluorescence signal intensity in the state is mainly the sum of the effective fluorescence signal intensity, the background scattering signal intensity, and the electronic noise signal intensity; The fluorescence signal intensities collected by the photomultiplier tubes in the NO detection chamber and NOx detection chamber under the given conditions were respectively , ,exist The fluorescence signal intensity under these conditions is mainly the sum of the background scattering signal intensity and the electronic noise signal intensity.

[0054] By subtracting the background signal intensity using the differential method, the net fluorescence signal intensities in the NO detection cavity and the NOx detection cavity were obtained as follows: , .

[0055] The fluorescence signal intensity is the signal intensity obtained by accumulating and averaging the fluorescence signals excited by multiple laser pulses within a set integration time, which is used to improve the signal-to-noise ratio of the system.

[0056] Step 4: Concentration Inversion Calculation

[0057] The concentrations of NO and NO2 in the atmosphere to be measured are calculated based on the net fluorescence signal intensity in the two detection chambers.

[0058] In step four, the concentration inversion calculation is as follows: (1) Calculation of NO concentration in the atmosphere to be measured Based on the net fluorescence signal intensity in the NO detection chamber The concentration of NO in the atmosphere to be measured was obtained. for:

[0059] in, The effective laser energy value is obtained by real-time monitoring via a laser energy meter and after averaging within the signal acquisition period. The system sensitivity constant of the NO detection chamber is obtained by calibration with a standard NO gas of known concentration.

[0060] System sensitivity constant Integrating absorption cross section Fluorescence quantum yield Collection efficiency Transmission efficiency and detector efficiency : ; in, For standard gas at wavelength The absorption cross section at the location; L is the effective length of the fluorescence collection region; The fluorescence quantum yield of a standard gas is defined as follows: =A / (A+Q), where A is the spontaneous emission rate and Q is the collision quenching rate; T represents the collection efficiency of the fluorescence collection system; T represents the transmission efficiency of the lens group in the fluorescence collection system. This represents the detection efficiency of the photomultiplier tube in the fluorescence collection system.

[0061] (2) Calculation of NO2 concentration in the atmosphere to be measured

[0062] Since the atmospheric sample is divided into two paths, one path directly enters the NO detection chamber, and the other path passes through an LED photolysis converter to convert NO2 into NO before entering the NOx detection chamber, the first path directly enters the NO detection chamber. The NO detection chamber measures the original NO concentration in the atmospheric sample. The second path, after passing through an LED photolysis converter to convert NO2 into NO, enters the NOx detection chamber. The gas in the NOx detection chamber contains both the original NO and the NO converted from NO2. The NOx detection chamber measures the total NO concentration. .

[0063] Based on the net fluorescence signal intensity in the NOx detection chamber The total NO concentration was obtained. for:

[0064] in, The system sensitivity constant of the NOx detection chamber is obtained by calibration with standard gases of known concentrations of NO and NO2; the total NO consists of NO in the atmosphere to be measured and NO converted from NO2 in the atmosphere to be measured. Then based on the total NO concentration and the concentration of NO in the atmosphere to be measured By combining the conversion efficiency of the LED photolysis converter, differential reduction is performed to obtain the atmospheric concentration in the target atmosphere. concentration for:

[0065] in, The conversion efficiency of the LED photolysis converter.

[0066] By using dual-channel differential measurement, this invention can eliminate laser energy fluctuations, aerosol scattering, and background light interference in real time, enabling long-term, high-precision online monitoring of ppt-level NO and NO2 concentrations in the atmosphere.

[0067] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A highly sensitive detection system for measuring atmospheric nitrogen oxides, characterized in that, It includes a laser generation module, a sampling and gas processing module, and a fluorescence detection and data processing module; In the laser generation module, the fundamental frequency laser output from the laser is frequency-doubled by a first frequency-doubled crystal to generate a first frequency-doubled laser. The first frequency-doubled laser is split into two beams by a beam splitter. The first laser beam serves as the pump light for an optical parametric oscillator, generating continuously tunable near-infrared light within a set wavelength range. The second laser beam is frequency-doubled again by a second frequency-doubled crystal to generate a second frequency-doubled laser. After collinearly combining the near-infrared light and the second frequency-doubled laser, they are injected into a sum-frequency crystal to generate a sum-frequency effect, producing an ultraviolet laser for exciting nitric oxide fluorescence. The wavelength scanning of the ultraviolet laser is achieved by adjusting the wavelength of the near-infrared light. In the gas sampling and gas processing module, the atmospheric sample or the sample gas provided by the standard gas cylinder is mixed and controlled by the mass flow controller to obtain two gas streams; the first gas stream enters the NO detection chamber, and the second gas stream first undergoes photolysis conversion through the LED photolysis converter before entering the NOx detection chamber; the LED photolysis converter is used to photolyze NO2 into NO. In the fluorescence detection and data processing module, the ultraviolet laser generated by the laser generation module passes sequentially through the NOx detection cavity and the NO detection cavity; each detection cavity is equipped with a fluorescence collection system for detecting the intensity of the fluorescence signal; the laser energy meter monitors the laser energy after passing through the two detection cavities in real time; and the concentrations of NO and NO2 are monitored online based on the detected fluorescence signal intensity and the monitored laser energy.

2. The high-sensitivity detection system for measuring atmospheric nitrogen oxides according to claim 1, characterized in that, In the laser generation module, an Nd:YAG laser is used as the fundamental frequency source, outputting a 1064nm fundamental frequency laser. A KTP crystal is used as the first frequency doubling crystal to generate a 532nm first frequency doubling laser. The first frequency doubling laser is then filtered by a dichroic mirror to remove residual fundamental frequency laser light, which is absorbed by a waste light collector. The purified first frequency doubling laser is split into two beams by a beam splitter. The first beam is focused by a focusing lens and fed into an optical parametric oscillator (OPO) as pump light. The OPO employs a narrow-linewidth cavity structure, and its output wavelength is continuously tunable within the range of 1081nm to 1135nm. Infrared light; the second frequency-harmonic crystal is a BBO crystal. The second laser beam is frequency-harmonicized again by the second frequency-harmonic crystal to generate a 266nm second frequency-harmonic laser. Then, the polarization state of the second frequency-harmonic laser is adjusted by a half-wave plate and shaped by a shaping lens to match its spot characteristics with the light output from the optical parametric oscillator. The near-infrared light and the second frequency-harmonic laser are collinearly combined using a beam combiner, and then focused by a focusing lens group before being injected into the sum-frequency crystal. The sum-frequency crystal is an LBO crystal, which generates a 214nm ultraviolet laser. The ultraviolet laser generated by the sum-frequency crystal is filtered by a dichroic mirror to output a pure ultraviolet laser, which is used as the ultraviolet laser to excite NO.

3. The high-sensitivity detection system for measuring atmospheric nitrogen oxides according to claim 1, characterized in that, The wavelength adjustment step of the optical parametric oscillator is better than 0.026 nm.

4. The high-sensitivity detection system for measuring atmospheric nitrogen oxides according to claim 1, characterized in that, The sum-frequency crystal is mounted on an electrically driven rotary stage. During wavelength scanning by the laser generation module, the output wavelength of the optical parametric oscillator is synchronously controlled with respect to the rotation angle of the electrically driven rotary stage according to a preset phase-matching mapping model. The phase-matching mapping model is the output wavelength of the optical parametric oscillator. Optimal phase matching angle with sum-frequency crystal The functional relationship between them is used to compensate for the fluctuation of crystal refractive index caused by wavelength change, so that the ultraviolet laser can maintain the maximum conversion efficiency within the tuning range.

5. A high-sensitivity detection system for measuring atmospheric nitrogen oxides according to claim 1, characterized in that, In the gas sampling and gas processing module, the standard gas cylinders include: NO2 standard gas cylinder, NO standard gas cylinder, and synthetic air cylinder.

6. A high-sensitivity detection system for measuring atmospheric nitrogen oxides according to claim 1, characterized in that, The LED photolysis converter includes an LED photolysis converter inlet, an LED photolysis converter outlet, and an LED light source; a UV-grade fused silica window is provided between the LED light source and the gas flow channel to physically isolate the LED light source from the corrosive sample gas; a spiral stepping light shield is provided around the LED light source to adjust the photolysis conversion efficiency; and a light shield limiting screw is provided at the spiral of the light shield.

7. A high-sensitivity detection system for measuring atmospheric nitrogen oxides according to claim 1, characterized in that, The fluorescence signal intensity is detected by a fluorescence collection system; the fluorescence collection system is shown below: The sampling gas flow enters the fluorescence cavity through the sampling port. The ultraviolet laser is incident from the opposite side, perpendicular to the gas flow and the photomultiplier tube, to excite the NO molecules in the gas flow. After the NO molecules are excited, they radiate fluorescence in all directions. A concave reflector is provided on the back side of the optical path to reflect the back fluorescence back into the collecting optical path; the forward fluorescence passes through a lens group consisting of a collimating lens, a bandpass filter and a focusing lens in sequence and is focused onto the photocathode of the photomultiplier tube; a filter is provided before the photomultiplier tube to filter out Rayleigh scattering light and background stray light.

8. A highly sensitive detection method for measuring atmospheric nitrogen oxides, characterized in that, The high-sensitivity detection system for measuring atmospheric nitrogen oxides according to any one of claims 1-7 is described below: Step 1: Wavelength Scanning and Locking The optical parametric oscillator performs wavelength scanning between 1081 nm and 1135 nm. During the scanning process, the phase matching angle of the sum-frequency crystal changes synchronously. Based on the mapping relationship of sum-frequency conversion, the output wavelength of the sum-frequency crystal is converted to 213.5 nm to 215.5 nm to excite the NO gas to be measured and obtain the corresponding excitation spectrum. Identify the position of the characteristic peak corresponding to the maximum absorption cross section in the excitation spectrum of NO standard gas, and define the wavelength corresponding to this characteristic peak position as the resonant excitation wavelength. The wavelength corresponding to the unexcited NO in the excitation spectrum is defined as the non-resonant excitation wavelength. ; Step 2: Switching between resonance / non-resonance state measurements Controlling the optical parametric oscillator in and It switches periodically between two wavelengths; Step 3: Dual-channel differential detection and background subtraction After the atmospheric sample is taken, it is divided into two paths. One path goes directly into the NO detection chamber, and the other path passes through an LED photolysis converter to convert NO2 into NO before entering the NOx detection chamber. Data is simultaneously collected from both detection chambers. and The fluorescence signal intensity under the specified conditions was recorded in the NO detection chamber and the NOx detection chamber. Fluorescence signal intensity collected under the condition , and in Fluorescence signal intensity collected under the condition , ; The net fluorescence signal intensity in the NO detection cavity and NOx detection cavity was obtained by subtracting the background signal intensity using the differential method. , Step 4: Concentration Inversion Calculation The concentrations of NO and NO2 in the atmosphere to be measured are calculated based on the net fluorescence signal intensity in the two detection chambers.

9. A highly sensitive detection method for measuring atmospheric nitrogen oxides according to claim 8, characterized in that, In step four, the concentration inversion calculation is as follows: (1) Calculation of NO concentration in the atmosphere to be measured Based on the net fluorescence signal intensity in the NO detection chamber The concentration of NO in the atmosphere to be measured was obtained. for: in, The laser energy monitored by the laser energy meter. The system sensitivity constant of the NO detection chamber is obtained by calibration with NO standard gas of known concentration; (2) Calculation of NO2 concentration in the atmosphere to be measured Based on the net fluorescence signal intensity in the NOx detection chamber The total NO concentration was obtained. for: in, The system sensitivity constant of the NOx detection chamber is obtained by calibration with standard gases of known concentrations of NO and NO2; the total NO consists of NO in the atmosphere to be measured and NO converted from NO2 in the atmosphere to be measured. Then based on the total NO concentration and the concentration of NO in the atmosphere to be measured By combining the conversion efficiency of the LED photolysis converter, the concentration of PM2.5 in the atmosphere to be measured is obtained. concentration for: in, The conversion efficiency of the LED photolysis converter.

10. A computer program product, characterized in that, It includes a computer program / instruction that, when executed by a processor, implements a highly sensitive detection method for measuring atmospheric nitrogen oxides as described in any one of claims 8 to 9.

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