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CN-122016740-A - High-sensitivity detection system and method for measuring atmospheric nitrogen oxides

CN122016740ACN 122016740 ACN122016740 ACN 122016740ACN-122016740-A

Abstract

The invention discloses a high-sensitivity detection system and method for measuring atmospheric nitrogen oxides, and relates to the technical field of atmospheric detection. The system comprises a laser generation module, a sampling and gas processing module and a fluorescence detection and data processing module, wherein the laser generation module generates ultraviolet laser for exciting nitric oxide fluorescence, the atmosphere to be detected or sample gas provided by a standard gas cylinder enters an NO detection cavity after flow proportioning and control, the second path of gas enters the NOx detection cavity after photolysis conversion by an LED photolysis converter, the ultraviolet laser sequentially passes through the NOx detection cavity and the NO detection cavity, a fluorescence collection system is arranged in each detection cavity, laser energy after passing through the two detection cavities is monitored in real time by a laser energy meter, and the concentration on-line monitoring of NO and NO 2 is realized according to fluorescence signal intensity and laser energy. The system has the characteristics of compact structure, high light path stability and high detection precision.

Inventors

  • HU RENZHI
  • LI YUNLONG
  • CAI HAOTIAN

Assignees

  • 中国科学院合肥物质科学研究院

Dates

Publication Date
20260512
Application Date
20260107

Claims (10)

  1. 1. The high-sensitivity detection system for measuring the nitrogen oxides in the atmosphere is characterized by comprising 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 by the laser is subjected to frequency multiplication by a first frequency multiplication crystal to generate first frequency multiplication laser, the first frequency multiplication laser is divided into two beams by a beam splitting device, the first beam laser is used as pumping light of an optical parametric oscillator to generate near infrared light continuously adjustable within a set wavelength range, and the second beam laser is subjected to frequency multiplication again by a second frequency multiplication crystal to generate second frequency multiplication laser; In the gas sampling and processing module, the flow ratio and control of the atmosphere to be detected or the sample gas provided by the standard gas cylinder are carried out through a mass flow controller to obtain two paths of gases, wherein the first path of gas enters the NO detection cavity, and the second path of gas enters the NOx detection cavity after being subjected to photolytic conversion through an LED photolytic converter, and the LED photolytic converter is used for photolytically converting NO 2 into NO; In the fluorescence detection and data processing module, ultraviolet laser generated by the laser generation module sequentially passes through the NOx detection cavity and the NO detection cavity, a fluorescence collection system for detecting fluorescence signal intensity is arranged in each detection cavity, laser energy after passing through the two detection cavities is monitored in real time by a laser energy meter, and on-line concentration monitoring of NO and NO 2 is realized according to the detected fluorescence signal intensity and the monitored laser energy.
  2. 2. The high-sensitivity detection system for measuring atmospheric nitrogen oxides according to claim 1 is characterized in that in a laser generation module, an Nd-YAG laser is used as a fundamental frequency light source to output 1064nm fundamental frequency laser, a first frequency doubling crystal is used for generating 532nm first frequency doubling laser through frequency doubling of the first frequency doubling crystal, then the first frequency doubling laser is filtered by a dichroic mirror to convert residual fundamental frequency laser and is absorbed by a waste light collector, the pure first frequency doubling laser is split into two beams by a beam splitter, the first beam laser is injected into an optical parametric oscillator through a focusing lens to serve as pumping light, the optical parametric oscillator adopts a narrow-linewidth cavity structure to output near infrared light with the wavelength being continuously adjustable in the range of 1081nm to 1135nm, a second frequency doubling crystal is used for generating 266nm second frequency doubling laser through frequency doubling of the second frequency doubling crystal, then the second frequency doubling laser is used for adjusting the polarization state through a half-wave plate and shaping lens to enable the light spot characteristics to be matched with light output by an optical parametric oscillator, the near infrared light is used for combining the second frequency doubling laser with the second frequency doubling laser to be used for generating ultraviolet laser light which is used for generating ultraviolet excitation by the ultraviolet laser which is combined with the ultraviolet laser light which is used for generating ultraviolet radiation which is used for ultraviolet excitation by the ultraviolet radiation which is generated by the ultraviolet radiation which is used for the ultraviolet radiation.
  3. 3. A high sensitivity detection system for atmospheric nitrogen oxide measurement according to claim 1, wherein the wavelength tuning step size of the optical parametric oscillator is better than 0.026nm.
  4. 4. The system of claim 1, wherein the sum frequency crystal is mounted on an electric rotary displacement stage, and the output wavelength of the optical parametric oscillator and the rotation angle of the electric rotary displacement stage are synchronously controlled according to a predetermined phase matching mapping model during the wavelength scanning performed by the laser generating module, wherein the phase matching mapping model is the output wavelength of the optical parametric oscillator Optimum phase matching angle with sum frequency crystal The function relation between the two is used for compensating the fluctuation of the refractive index of the crystal caused by the change of the wavelength, so that the conversion efficiency of the ultraviolet laser is maintained to be maximized in the tuning range.
  5. 5. A high sensitivity detection system for atmospheric nitrogen oxide measurement according to claim 1 wherein in the gas sampling and gas processing module, the standard gas cylinder comprises NO 2 standard gas cylinder, NO standard gas cylinder, and synthetic air gas cylinder.
  6. 6. The high-sensitivity detection system for measuring the atmospheric nitrogen oxides according to claim 1, wherein the LED photolytic converter comprises an LED photolytic converter air inlet, an LED photolytic converter air outlet and an LED light source, an ultraviolet-level fused quartz window sheet is arranged between the LED light source and a gas flow channel and used for physically isolating the LED light source from corrosive sample gas, a spiral stepping light shield is arranged on the periphery of the LED light source and used for adjusting the photolytic conversion efficiency, and a light shield limit screw is arranged at a spiral position of the light shield.
  7. 7. The high sensitivity detection system for atmospheric nitrogen oxide measurement according to claim 1, wherein the detection of fluorescence signal intensity is performed by a fluorescence collection system, wherein the fluorescence collection system is as follows: the sampling airflow is sampled from a sampling port and enters a fluorescent cavity, ultraviolet laser is incident from the other surface perpendicular to the airflow and the photomultiplier, NO molecules in the airflow are excited, and after the NO molecules are excited, fluorescent light is radiated to the periphery; the light path back side is provided with a concave reflector for reflecting and recovering the back fluorescence into a light collecting path, the forward fluorescence is focused on the photocathode of the photomultiplier through a lens group consisting of a collimating lens, a bandpass filter and a focusing lens in sequence, and the front of the photomultiplier is provided with a filter for filtering Rayleigh scattered light and background stray light.
  8. 8. A high sensitivity detection method for atmospheric nitrogen oxide measurement, characterized in that a high sensitivity detection system for atmospheric nitrogen oxide measurement according to any one of the preceding claims 1-7 is employed, the method being as follows: Step one, wavelength scanning and locking The optical parametric oscillator scans the wavelength between 1081nm and 1135nm, the phase matching angle of the sum frequency crystal changes synchronously during scanning, and the output wavelength of the sum frequency crystal is converted to 213.5nm to 215.5nm based on the mapping relation of the sum frequency conversion so as to excite NO gas to be detected and obtain a corresponding excitation spectrum; Identifying the characteristic peak position corresponding to the maximum absorption section in the excitation spectrum of the NO standard gas, and defining the wavelength corresponding to the characteristic peak position as the resonance excitation wavelength The corresponding wavelength in the excitation spectrum when NO is not excited is defined as the non-resonant excitation wavelength ; Step two, resonance/non-resonance state measurement switching Controlling an optical parametric oscillator in And (3) with Periodically switching between two wavelengths; Step three, double-channel differential detection and background subtraction The atmosphere to be detected is sampled and then divided into two paths, one path directly enters the NO detection cavity, the other path converts NO 2 into NO through the LED photolysis converter and then enters the NOx detection cavity, and the two detection cavities are synchronously collected in the following steps And (3) with The fluorescence signal intensity under the state records the NO detection cavity and the NOx detection cavity in Intensity of fluorescent signal collected under state 、 And in Intensity of fluorescent signal collected under state 、 Subtracting the background signal intensity by using a difference method to obtain the net fluorescence signal intensity in the NO detection cavity and the NOx detection cavity 、 Step four, concentration inversion calculation And calculating the concentration of NO and NO 2 in the atmosphere to be detected according to the net fluorescence signal intensity in the two detection cavities.
  9. 9. The method of claim 8, wherein in step four, the concentration inversion calculation is specifically as follows: (1) Calculation of the concentration of NO in the atmosphere to be measured Based on the net fluorescence signal intensity in the NO detection chamber Obtaining the concentration of NO in the atmosphere to be tested The method comprises the following steps: Wherein, the For the laser energy monitored by the laser energy meter, A system sensitivity constant of the NO detection cavity obtained by calibrating the NO standard gas with known concentration; (2) Calculation of the concentration of NO 2 in the atmosphere to be measured Based on net fluorescence signal intensity in NOx detection chamber Obtaining the concentration of total NO The method comprises the following steps: Wherein, the The system sensitivity constant of the NOx detection cavity is obtained through calibration of the NO with known concentration and NO 2 standard gas, wherein the NO in the atmosphere to be detected and the NO converted by the NO 2 in the atmosphere to be detected form total NO; then according to the concentration of total NO And the concentration of NO in the atmosphere to be measured Combining the conversion efficiency of the LED photolytic converter to obtain the atmosphere to be detected Concentration of (2) The method comprises the following steps: Wherein, the The conversion efficiency of the LED photolytic converter is achieved.
  10. 10. A computer program product comprising computer program/instructions which, when executed by a processor, implements a highly sensitive detection method for atmospheric nitrogen oxide measurement as claimed in any one of claims 8 to 9.

Description

High-sensitivity detection system and method for measuring atmospheric nitrogen oxides Technical Field The invention relates to the technical field of atmospheric detection, in particular to a high-sensitivity detection system and method for measuring atmospheric nitrogen oxides. Background Nitrogen oxides act as a core driver for the atmospheric reaction of the troposphere, controlling the formation of ozone and fine particulate matter and determining the atmospheric oxidation capacity. Despite the higher urban area concentrations, in the background atmosphere, NOx (nitrogen oxides, collectively) concentrations are typically below a critical threshold of 10-30 ppt. In order to accurately analyze the atmospheric oxidability and verify the chemical transmission model, the monitoring instrument needs to have detection capacity of 1-10ppt level. However, the prior mainstream technology has obvious limitations that the chemiluminescence method is influenced by the water vapor quenching effect, and the molybdenum reformer cannot eliminate the interference of nitrogen-containing compounds such as PANs and the like, resulting in NThe differential absorption spectrometry relies on long optical path, has low spatial resolution and is easily affected by aerosol extinction, and the cavity-based absorption spectrometry is subjected to high-reflection mirror pollution and uncertainty of NO indirect measurement, so that the high-precision requirement of trace background monitoring is difficult to meet. Laser-induced fluorescence technology is a preferred means of detecting trace amounts of active intermediates by virtue of extremely high sensitivity and selectivity, but its engineering application is subject to long-term technical bottlenecks of deep ultraviolet excitation light sources. The strong absorption band of NO molecules is located at 214nm or 226nm, the traditional frequency doubling dye laser has complex system, difficult maintenance and toxic dye, and the excitation efficiency of Nd-YAG five-time frequency scheme is low and nonlinear transformation is unstable. More critical is that the light source must have a deep ultraviolet wavelength control accuracy better than 0.001nm in order to subtract background scattering noise through the differential measurement strategy. The conventional crystal angle or temperature tuning technology is difficult to maintain wavelength stability under high-frequency modulation, and drift is very easy to occur. Therefore, the deep ultraviolet all-solid-state laser light source technology with high spectral resolution and long-term engineering stability is required to be broken through, so as to solve the contradiction between scientific research requirements and instrument performance. Disclosure of Invention In order to overcome the defects in the prior art, the invention provides a high-sensitivity detection system and a method for measuring the nitrogen oxides in the atmosphere. In order to achieve the above purpose, the present invention adopts the following technical scheme, including: A high-sensitivity detection system for measuring atmospheric nitrogen oxides comprises 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 by the laser is subjected to frequency multiplication by a first frequency multiplication crystal to generate first frequency multiplication laser, the first frequency multiplication laser is divided into two beams by a beam splitting device, the first beam laser is used as pumping light of an optical parametric oscillator to generate near infrared light continuously adjustable within a set wavelength range, and the second beam laser is subjected to frequency multiplication again by a second frequency multiplication crystal to generate second frequency multiplication laser; In the gas sampling and processing module, the flow ratio and control of the atmosphere to be detected or the sample gas provided by the standard gas cylinder are carried out through a mass flow controller to obtain two paths of gases, wherein the first path of gas enters the NO detection cavity, and the second path of gas enters the NOx detection cavity after being subjected to photolytic conversion through an LED photolytic converter, and the LED photolytic converter is used for photolytically converting NO 2 into NO; In the fluorescence detection and data processing module, ultraviolet laser generated by the laser generation module sequentially passes through the NOx detection cavity and the NO detection cavity, a fluorescence collection system for detecting fluorescence signal intensity is arranged in each detection cavity, laser energy after passing through the two detection cavities is monitored in real time by a laser energy meter, and on-line concentration monitoring of NO and NO 2 is realized according to the detected fluorescence signal i