Ultralow-concentration NO detection method based on ultraviolet differential optical absorption spectrum technology

Abstract

The invention relates to an ultra-low concentration NO detection method based on an ultraviolet differential optical absorption spectrum technology, and belongs to the field of gas detection. According to the method, on the basis of the classic DOAS technology, the process of calculating the differential absorption cross section of the detected gas is omitted, and a mathematical model of the differential optical density and concentration of the detected gas is constructed; when the gas is measured, the difference optical density of the gas to be measured only needs to be calculated and is substituted into the mathematical model, and the concentration value of the gas to be measured is calculated; and correcting the calculated gas concentration by using a temperature compensation method so as to obtain the accurate concentration of the gas to be detected. According to the invention, the detection of pollution gas in ultra-low emission flue gas is realized, and the requirements of new ultra-low emission on a monitoring system are met.

Description

Technical Field

[0001] The invention belongs to the field of gas detection and relates to an ultra-low concentration NO detection method based on ultraviolet differential optical absorption spectroscopy technology. Background Art

[0002] Chemical plants, coal-fired boilers, metallurgical plants, coal-fired power plants, cement kilns and other industries burn large amounts of coal and emit large amounts of flue gas and particulate matter. These flue gases contain a large amount of SO 2 、NO X If the SO in flue gas cannot be reduced 2 、NO X The content of SO will interfere with the ecological environment and even cause permanent damage. In order to control environmental pollution, the environmental protection department has issued ultra-low emission standards for flue gas emissions from coal-fired boilers. 2 ,NO,NO 2 The concentration of the emission is not higher than 10, 35, 50 mg / m 3 At present, the main methods for monitoring the concentration of these pollutants are DOAS (differential optical absorption spectroscopy), NDIR (non-dispersive infrared), LIF (laser induced fluorescence), COSPEC (correlation spectroscopy), DIAL (differential absorption lidar) and TDLAS (tunable diode laser absorption spectroscopy). Among these technologies, DOAS technology has high reliability and high detection accuracy, and can simultaneously detect SO 2 and NO X And it is widely used. The traditional DOAS method has achieved great results in long optical path (greater than 50 meters) and high concentration gas measurement (this method mainly uses the standard differential absorption cross section of NO combined with the least squares method for concentration inversion, but it is difficult to obtain the standard differential absorption cross section of NO in the laboratory). However, the ultra-low emission standards have brought new challenges to DOAS technology. For example, the detection limit does not meet the requirements of ultra-low emissions, and the relative error of detecting the concentration of ultra-low emission pollutants is large. The new emission regulations require the monitoring system to have a lower detection limit, higher resolution, and higher accuracy.

[0003] Therefore, a new concentration gas detection method is urgently needed to detect pollutant gases in ultra-low emission flue gas. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide an ultra-low concentration NO detection method based on ultraviolet differential optical absorption spectroscopy technology. Aiming at the new ultra-low emission requirements for the monitoring system, some improvements are made on the basis of the classic DOAS technology to realize the detection of pollutant gases in ultra-low emission flue gas.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] An ultra-low concentration NO detection method based on ultraviolet differential optical absorption spectroscopy technology, based on the classic DOAS technology, omits the process of calculating the differential absorption cross-section of the measured gas, and constructs a mathematical model of the differential optical density and concentration of the measured gas; when measuring the gas, it is only necessary to calculate the differential optical density of the measured gas and bring it into the mathematical model to calculate the concentration value of the measured gas; then the calculated gas concentration is corrected using the temperature compensation method to obtain the accurate concentration of the measured gas.

[0007] Furthermore, the mathematical model of the differential optical density and concentration is specifically: within the system measurement concentration range, a high-precision gas distribution system is used to obtain the differential optical density of each concentration or a certain concentration value interval, and the differential optical density is summed. Each concentration value corresponds to a sum of differential optical densities, and finally a polynomial fitting is performed on the points corresponding to each concentration value.

[0008] Furthermore, for NO detection, in the 200-230 nm band, the differential optical density of NO has three absorption peaks. The differential optical densities of the three absorption peaks are summed up and the relationship between them and the concentration is fitted using a quadratic polynomial.

[0009] The final concentration value C of NO is expressed by the average value of the fitting function calculation results corresponding to the three absorption peaks, as shown in the following formula:

[0010] C=(y 1 +y 2 +y 3 ) / 3

[0011] Among them, y i is the NO concentration corresponding to the absorption peak i (×10 -6 ), i=1,2,3.

[0012] Furthermore, the temperature compensation method specifically includes: the relationship between the true value and the measured value of the gas to be measured and the temperature is expressed by the following formula:

[0013] C 修 =C*P(T)

[0014] Among them, P(T) is the compensation function related to temperature, C 修 is the corrected concentration value of the gas to be measured (×10 -6 ), C is the gas concentration value calculated using the mathematical model (×10 -6 );

[0015] The value of the compensation function is linearly related to the temperature, and the expression is:

[0016] P(T)=0.90834+0.00281*T

[0017] Wherein, T represents temperature.

[0018] Furthermore, this method is applicable to ultra-low concentration SO 2 and NO 2 Detection.

[0019] The beneficial effects of the present invention are as follows: in view of the new ultra-low emission requirements for monitoring systems, the present invention designs a mathematical model of gas differential optical density and concentration based on the classic DOAS technology, thereby realizing the detection of pollutant gases in ultra-low emission flue gas. At the same time, by studying the influence of temperature on the differential optical density and concentration inversion of NO, the present invention designs a temperature compensation method to correct the gas concentration calculated by the model, thereby obtaining an accurate concentration of the gas to be measured. Compared with the traditional DOAS method, the improved DOAS method of the present invention has the advantages of not requiring the standard differential absorption cross section of NO, small amount of calculation, low detection limit, and high detection accuracy.

[0020] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0022] Figure 1 Schematic diagram of absorption spectrum detection;

[0023] Figure 2 is differential optical density, optical density, and slow change;

[0024] Figure 3 is the relationship between the integral value of NO differential optical density and its concentration;

[0025] Figure 4 Schematic diagram of the experimental system;

[0026] Figure 5 It is a gas absorption cell and an internal optical path;

[0027] Figure 6 Analysis of NO test results;

[0028] Figure 7 For single measurement uncertainty test;

[0029] Figure 8 is the comparison of the differential optical density of NO at 30℃ and 50℃;

[0030] Fig. 9 30×10 -6 NO temperature experiment;

[0031] Fig.10 is the relationship between temperature and concentration;

[0032] Fig.11 30×10 -6 The error of the test results varies with temperature;

[0033] Fig.12 is the fitting result of P(T) function;

[0034] Fig.13 NO 25×10 -6 and 3×10 -6 Comparison of relative errors before and after temperature compensation. DETAILED DESCRIPTION

[0035] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0036] See also Figures 1 to 13 The present invention provides an ultra-low concentration NO detection method based on ultraviolet differential optical absorption spectroscopy technology. On the basis of the classic DOAS technology, the process of calculating the differential absorption cross section of the measured gas is omitted, and a mathematical model of the differential optical density and concentration of the measured gas is constructed; when measuring the gas, it is only necessary to calculate the differential optical density of the measured gas and bring it into the mathematical model to calculate the concentration value of the measured gas; then the calculated gas concentration is corrected using a temperature compensation method to obtain an accurate concentration of the measured gas.

[0037] 1. Classic DOAS principle

[0038] DOAS is based on the Lambert-Beer law. When a beam of light passes through a gas medium, the medium molecules absorb the light. The amount of absorption is related to the gas concentration and the optical path. Figure 1 It is a schematic diagram of measuring the absorbance of a medium.

[0039] A beam with intensity I 0After the light passes through the gas absorption cell, its intensity will be attenuated due to the absorption of the gas. The light intensity measured at the receiving end is I. Under ideal conditions, I and I 0 The relationship is given by the Lambert-Beer law:

[0040] I(λ)=I 0 (λ)exp[-L(Cσ(λ)] (1)

[0041] Among them, I 0 (λ) is the original light intensity (counts), I(λ) is the outgoing light intensity after passing through the measured gas (counts), L is the optical path length of the measured gas (cm), and C is the concentration of the measured gas (molecule / cm 3 ), σ(λ) is the differential absorption cross section of the measured gas (cm 2 / mol);

[0042] However, in the actual measurement process, there are many factors that cause light intensity attenuation, such as Mie scattering ε M , Rayleigh scattering ε R Rayleigh scattering and Mie scattering cause light scattering deviation, and the scattered light cannot be received by the detector. This light that is not received by the detector can be regarded as the process of molecular "absorption" in the DOAS technology processing process, so it is necessary to correct equation (1). The corrected Lambert-Beer law is shown in equation (2):

[0043] I(λ)=I 0 (λ)exp[-L(Cσ(λ)+ε R +ε M )] (2)

[0044] Formula (2) can be further transformed into formula (3):

[0045]

[0046] make It is called optical density. As can be seen from formula (3), the optical density includes gas molecule absorption components and Rayleigh scattering and Mie scattering components. The light intensity attenuation caused by molecular absorption and the attenuation caused by Rayleigh scattering or Mie scattering are expressed differently in the spectrum. The change in optical density caused by gas molecule absorption changes rapidly with the change of wavelength, which is called fast change. The change in optical density caused by Rayleigh scattering and Mie scattering changes slowly with the change of wavelength, which is called slow change. In DOAS technology, the key technology is to extract the fast-changing part of the optical density that represents the characteristic absorption of the gas. Currently, polynomial fitting or Savitzky-Golay method is generally used to obtain the slow change. In the present invention, the slow-changing part is expressed by ODS Indicates. After obtaining OD S Then, the fast change caused by gas molecule absorption can be calculated using formula (4), which is also called differential optical density. In the present invention, the fast change part is represented by DOD.

[0047] DOD=OD-OD S (4)

[0048] In this embodiment, taking NO as an example, the differential optical density, slow-changing part, and optical density of NO are as follows: Figure 2 shown.

[0049] After obtaining the differential optical density, when the absorption cross section is known, the formula can be modified as shown in formula (5):

[0050] DOD=LCσ(λ) (5)

[0051] Where DOD is the differential optical density of the gas being measured, L is the optical path length of the gas absorption cell (cm), and C is the concentration of the gas being measured (molecule / cm 3 ), σ(λ) is the differential absorption cross section of the measured gas (cm 2 / mol).

[0052] 2. Improved DOAS

[0053] The principle of the improved DOAS technology is still based on the classic DOAS technology, omitting the process of calculating the differential absorption cross section of the measured gas and adding a mathematical model between the integral value of the measured gas differential optical density and the concentration. This process requires the system to have a high-precision gas distribution system. In our experimental system, the GW5000 high-precision gas distribution system is used, which can dilute the high-concentration source gas into the gas of the required concentration. Within the system measurement concentration range, the high-precision gas distribution system is used to obtain the differential optical density of each concentration or a certain concentration value, and the differential optical density is summed. Each concentration value corresponds to a sum of differential optical densities. Finally, a polynomial fitting is performed on this series of points. In this way, the mathematical model of differential optical density and concentration can be obtained.

[0054] When measuring gas, you only need to calculate the differential optical density of the gas and bring it into its mathematical model to calculate the concentration value of the measured gas. Taking NO as an example, in the 200-230nm band, the differential optical density of NO has three absorption peaks, such as Figure 3 As shown in the figure, the relationship between the differential optical density integral value and concentration of these three peaks is shown in Figure 3 As shown in (a), (b) and (c).

[0055] The differential optical densities of the three absorption peaks are summed up and the relationship between them and the concentration is fitted using a quadratic polynomial. The results are shown in equations (6), (7), and (8).

[0056] The relationship between peak 1 differential optical density and concentration is as shown in formula (6):

[0057] y 1 =0.2941+0.56289*x 1 +0.03527*x 1 2 ,R 2 =0.9998 (6)

[0058] The relationship between peak 2 differential optical density and concentration is as shown in formula (7):

[0059] y 2 =0.28736+0.57541*x 2 +0.04076*x 2 2 ,R 2 =0.998 (7)

[0060] The relationship between peak 3 differential optical density and concentration is as shown in formula (8):

[0061] y 3 =-0.16129+0.67487*x 3 +0.02811*x 3 2 ,R 2 =0.9994 (8)

[0062] Among them, y i is the NO concentration value (×10 -6 ), x i is the differential optical density integral value of NO absorption peak i, R 2 is the coefficient of determination of mathematical regression.

[0063] From R 2 From the values, the fitting degree of these three curves is very good. In the experiment, the final concentration value of NO is expressed by the average value of the fitting function calculation results corresponding to the three absorption peaks, as shown in formula (9):

[0064] C=(y 1 +y 2 +y 3 ) / 3 (9)

[0065] The parameters in equations (6), (7), and (8) are not universal. These parameters are closely related to the optical devices used in the system and need to be measured in combination with the characteristics of each system.

[0066] 3. Experiment

[0067] 1) Experimental system

[0068] The whole experimental system consists of the following equipment: light source, gas chamber, spectrometer, optical fiber, gas distribution instrument and standard gas. Figure 4 shown.

[0069] A—Spectrometer, using 16-bit AD converter, USB2.0 and RS232 communication, spectral range 185~415nm, signal-to-noise ratio 450:1.

[0070] B—light source, wavelength range 190~1100nm.

[0071] C—Air chamber temperature controller uses PID control principle to control the temperature of the air chamber.

[0072] D—air chamber, adopts White multiple return structure, optical path is 7 meters, internal optical components adopt high temperature doping coating process, ensure higher reflectivity and more corrosion resistance. Figure 5 shown.

[0073] H—Standard gas cylinder.

[0074] I—High-precision gas distribution system uses high-concentration source gas and high-precision mass flow meter to prepare standard gases of different concentrations.

[0075] J—Waste gas treatment device, absorbs the waste gas generated by the experiment.

[0076] 2) Experimental results and analysis

[0077] To verify whether this method is feasible, the experiment is designed as follows: the concentration value is 100×10 -6 The NO source gas is measured dynamically through a high-precision gas distribution system to obtain the concentration values ​​shown in Table 1. The concentration value changes from small to large, and then changes from large to small. After one concentration value is measured, the gas distribution system parameters are changed to measure the next concentration value. The experimental environment temperature is 25℃, 1atm, the relative humidity is 65%, and the gas chamber temperature is stable at 35℃±0.5℃. Prior to this, a mathematical model between differential optical density and concentration has been obtained. Within the measurement range of this device, an in-and-out experiment was conducted, and the experimental results are shown in Table 1.

[0078] Table 1 NO 0.6~30×10-6 inbound and return measurement results

[0079]

[0080] The forward and reverse curves are similar to the true values. Figure 6As shown in (a), the Pearson coefficient of the forward and reverse processes is 0.99997, and the error distribution is as follows: Figure 6 (b) as shown.

[0081] When the standard concentration is 0.6×10 -6 When the relative error of the system is the largest, it reaches 8.33%, and the standard concentration is between 1 and 12×10 -6 The relative error is less than 3.00% when the standard concentration is between 15 and 30×10 -6 When the relative error is less than 0.80%, the relative error is reduced by 1 / 2 compared with the most recent research results.

[0082] To verify the repeatability of the system, the experiment was designed as follows: 1×10 -6 After the system is stable, record the experimental data for 5 minutes. The results are as follows: Figure 7 After calculation, the arithmetic mean of this measurement is 1.0041×10 -6 The standard deviation of a single measurement is 0.0017779. The experimental standard deviation of the average value can be calculated according to formula (10)

[0083]

[0084] According to the calculation method of Type A uncertainty, the Type A uncertainty can be obtained to be 0.15%.

[0085] The detection limit of the instrument is also a very important indicator, and its calculation formula can be expressed as follows:

[0086]

[0087] Among them, C L The detection limit (×10 -6 ), SNR is the signal-to-noise ratio, which is generally set to 3, S 0 is the standard deviation of the measurement (×10 -6 ), C 真 is the true concentration value (×10 -6 ), C 测 is the average value of this measurement (×10 -6 ).

[0088] Substituting the experimental data, the final detection limit of this method for NO is 0.005×10 -6 , the detection limit has been significantly improved.

[0089] At room temperature, NO molecules are mainly distributed in the ground state vibration energy level. As the temperature increases, the number of molecules in the ground vibration energy level decreases, causing the absorption peak to decrease. As the temperature increases, intermolecular collisions intensify, and the collision broadens and increases. At the same time, collisions lead to the decoupling of the effects between different electronic states, and the corresponding absorption is weakened. This will inevitably have a certain impact on the experimental results, especially when the measured concentration is very low, the impact of temperature on the experimental results is more worthy of study. Figure 8 is the difference optical density of NO at 30°C (curve d) and 50°C (curve e) for the same concentration. Figure 8 Figures a, b, and c are enlarged views of the peak points 1, 2, and 3 of the three absorption peaks of NO. It can be clearly seen from the local enlarged view that compared with 30°C, the differential optical density corresponding to the three absorption peaks is reduced to varying degrees. Therefore, when measuring low-concentration gases, temperature compensation is required for the measurement results.

[0090] In order to study the influence of the measured gas concentration on temperature, two sets of experiments were designed in this invention. The standard gas concentration used in the experiments was 30×10 -6 The temperature range is 30℃~50℃, and the temperature increase step is 2℃. Figure 8 It is found that as the temperature increases, the differential optical density decreases. Theoretically, when the temperature is lower than the temperature at which the mathematical model of differential optical density and concentration is obtained, the concentration value is greater than the true value. When the temperature is higher than this temperature, the measured concentration value is less than the true value. In the present invention, the temperature of the gas when obtaining the relationship between differential optical density and concentration is defined as T B During the experiment, the changes in concentration and temperature were continuously recorded, such as Fig. 9 As shown. The initial temperature of the gas under test in the experiment is 29.1℃, and the temperature on the left side of the straight line C in the figure is lower than T B ,from Fig. 9 It can be seen that the concentration is greater than 30×10 -6 , which is consistent with the theory. The temperature between lines C and D is around 33°C. From the concentration values ​​between lines EF, it can be seen that the calculated concentration values ​​are between 29.95 and 30.03×10 -6 When the temperature exceeds 33.3℃, ​​the concentration value continues to decrease. Record the stable temperature and concentration values ​​at each stage of temperature rise. Fig.10 As shown. Fig.10 It can be seen from the figure that the fitting result shows that the concentration is a linear monotonically decreasing function with temperature, and R 2 The error of the test result varies with temperature. Fig.11 shown.

[0091] The initial temperature of the gas under test in the experiment was 29°C, which is lower than T B , the relative error of the test result is positive. As the temperature increases, the error decreases. When the temperature reaches T BWhen the temperature is higher than T B When the temperature is too low, the measured result is less than the true value, and the error increases with the increase of temperature. Therefore, when measuring gas concentration, it is particularly important to perform temperature compensation on the measured result.

[0092] 4. Temperature compensation of measurement results

[0093] The relationship between the true value and the measured value and temperature can be expressed by equation (12):

[0094] C 真 =C 测 *P(T) (12)

[0095] Among them, P(T) is a function related to temperature, C 真 is the actual concentration (×10 -6 ), C 测 To measure the concentration (×10 -6 ), the function value of P(T) can be expressed by formula (13):

[0096] P(T)=C 真 / C 测 (13)

[0097] In the experiment, 30×10 -6 The temperature of NO standard gas varied from 29.05 to 47.87 °C. The temperature was increased in steps of 2 °C. The concentration value at each stage when the temperature was stable was recorded, and then the value of P(T) was obtained. The concentration values ​​and the function P(T) at different temperatures are shown in Table 2.

[0098] Table 2 Measured concentration values ​​when temperature changes (×10 -6 ) and P(T) function values

[0099]

[0100] Fig.12 is the relationship between the value of the function P(T) and temperature, from Fig.12 It can be seen that the value of the compensation function is linearly related to temperature, R 2 is 0.995, and the relationship between P(T) and temperature is shown in formula (14):

[0101] P(T)=0.90834+0.00281*T (14)

[0102] After obtaining the temperature compensation function, the following two sets of experiments were conducted to verify the performance of the compensation function. The NO concentrations were 25×10 -6 and 3×10 -6And it remains unchanged, the temperature is increased in steps of 2°C, and the relative errors before and after temperature compensation are shown in Table 3 and Table 4.

[0103] Table 3 25×10 -6 NO concentration value and relative error before and after temperature compensation

[0104]

[0105]

[0106] Table 4 Concentration values ​​and error values ​​of 3×10-6NO before and after temperature compensation

[0107]

[0108] Fig.13 (a) and (b) are 25×10 -6 and 3×10 -6 The relative error histograms before and after compensation are shown in Table 3, Table 4 and Fig.13 It can be seen that when the concentration is 25×10 -6 Without temperature compensation, the measured concentration value decreases with the increase of temperature, and the relative error increases with the increase of temperature, with the maximum relative error reaching -4.76%. After the experimental results are temperature compensated, the relative error is between 0.08% and 0.60%, which shows that the experimental results are significantly improved after temperature compensation. -6 When the measured concentration value and relative error change trend are similar to 25×10 -6 The change trend is the same as that of 25×10 -6 The compensation effect is not as significant.

[0109] In summary, it can be seen from the results of the temperature experiment that when the temperature is greater than T B When the temperature is lower than T B When the temperature is equal to T B When , the measured concentration value is closest to the true concentration value; the relative error of the measured value increases with the increase of temperature, and the influence of temperature on the experimental results can be significantly compensated by the compensation method in this paper.

[0110] 5. Summary

[0111] Based on the traditional DOAS, this paper proposes an improved DOAS technology. Through experimental verification, this method can detect 0~30×10 -6 When the standard concentration is 0.6×10 -6When the standard concentration is between 1 and 15×10 -6 When the standard concentration is between 15 and 30×10 -6 The relative error is less than 0.8% when the improved DOAS technology is used, and the detection limit of the system is 0.005×10 -6 , the uncertainty of a single measurement is 0.15%. At the same time, this paper studies the influence of temperature on the experimental results and the compensation method. The experimental results show that this method has a good compensation effect on the error caused by temperature. The concentration calculation method and temperature compensation method adopted by the present invention are also effective for other gases such as SO 2 and NO 2 Also works.

[0112] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A method for detecting ultra-low concentration NO based on ultraviolet differential optical absorption spectroscopy, characterized in that: Based on the classic DOAS technology, the process of calculating the differential absorption cross section of the measured gas is omitted, and a mathematical model of the differential optical density and concentration of the measured gas is constructed; when measuring the gas, it is only necessary to calculate the differential optical density of the measured gas and bring it into the mathematical model to calculate the concentration value of the measured gas; then the calculated gas concentration is corrected using the temperature compensation method to obtain the accurate concentration of the measured gas.

2. The ultra-low concentration NO detection method according to claim 1, characterized in that: The mathematical model of differential optical density and concentration is specifically as follows: within the concentration range measured by the system, a high-precision gas distribution system is used to obtain the differential optical density of each concentration or a certain concentration value interval, and the differential optical densities are summed. Each concentration value corresponds to a sum of differential optical densities, and finally a polynomial fitting is performed on the points corresponding to each concentration value.

3. The ultra-low concentration NO detection method according to claim 1 or 2, characterized in that: For NO detection, in the 200-230nm band, the differential optical density of NO has three absorption peaks. The differential optical densities of the three absorption peaks are summed up and the relationship between them and the concentration is fitted using a quadratic polynomial. The final concentration value C of NO is expressed by the average value of the fitting function calculation results corresponding to the three absorption peaks, as shown in the following formula: C=(y1+y2+y3) / 3 Among them, y i is the NO concentration value corresponding to the absorption peak i, i=1,2,3.

4. The ultra-low concentration NO detection method according to claim 3, characterized in that: The temperature compensation method specifically includes: the relationship between the true value and the measured value of the gas to be measured and the temperature is expressed by the following formula: C 修 =C*P(T) Among them, P(T) is the compensation function related to temperature, C 修 is the corrected concentration value of the gas to be measured, and C is the gas concentration value calculated using the mathematical model; The value of the compensation function is linearly related to the temperature, and the expression is: P(T)=0.90834+0.00281*T Wherein, T represents temperature.

5. The ultra-low concentration NO detection method according to claim 1, characterized in that: This method is suitable for the detection of ultra-low concentrations of SO2 and NO2.

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