CN115656058B - A method for measuring NO2 concentration based on DOAS - Google Patents
A method for measuring NO2 concentration based on DOAS Download PDFInfo
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Abstract
本发明公开了一种基于DOAS的NO2浓度测量方法，该方法包括以下步骤：S1、获取不同功率强度的原始光谱数据进行预处理；S2、计算光谱校正系数并进行验证；S3、采集不同浓度的NO2的透射光谱进行处理，得到NO2光谱数据；S4、通过计算每个光谱点与浓度之间的关系，计算最优差分吸收度位置数据集和最优差分吸收截面；S5、通过实际测得气体的差分吸收度、最优差分吸收度位置数据集和最优差分吸收截面建立方程，求得NO2浓度值；S6、利用最小二乘法对所述NO2浓度值进行修正。通过指数光谱修正法对光谱数据进行修正，克服氙灯光源稳定性和重复性差的缺陷，从而大幅提高光谱数据的精确度；提高NO2测量精度，满足实际应用需求。
The present invention discloses a method for measuring NO2 concentration based on DOAS, which includes the following steps: S1, obtaining original spectral data of different power intensities for preprocessing; S2, calculating the spectral correction coefficient and verifying it; S3, collecting the transmission spectra of NO2 of different concentrations for processing to obtain NO2 spectral data; S4, calculating the relationship between each spectral point and the concentration, calculating the optimal differential absorbance position data set and the optimal differential absorption cross section; S5, establishing an equation by actually measuring the differential absorbance of the gas, the optimal differential absorbance position data set and the optimal differential absorption cross section, and obtaining the NO2 concentration value; S6, correcting the NO2 concentration value using the least squares method. The spectral data is corrected by the exponential spectral correction method to overcome the defects of poor stability and repeatability of the xenon lamp light source, thereby greatly improving the accuracy of the spectral data; improving the NO2 measurement accuracy and meeting the actual application requirements.
Description
Technical Field
The invention relates to the technical field of gas concentration measurement, in particular to a DOAS-based NO 2 concentration measurement method.
Background
Based on a differential absorption spectrometry (DOAS) (DIFFERENTIAL OPTICAL ABSORPTION SPECTROSCOPY), the method can detect SO 2、NOx multi-component gas with low concentration and high sensitivity, is less influenced by cross interference of moisture and other gases, is widely and widely studied at home and abroad in recent years, and is successfully applied to places with concentrated emission of pollution sources such as coal-fired power plants, paper factories, cement industry and the like.
Along with the increasingly strict national requirements on environmental protection, the emission concentration of standard particulate matters, sulfur dioxide and nitrogen oxides in the flue gas of a power plant needs to be stabilized to be less than 10 mg/cubic meter, 35 mg/cubic meter and 50 mg/cubic meter, SO that the problem of the measurement accuracy of the emission concentration of SO 2 and NO x is urgently needed in the current industrial field.
In the analyzer based on DOAS, pulse xenon lamp light source is adopted, so that the analyzer has the characteristics of long service life, high power and no need of preheating, but the defect of poor stability and repeatability of the xenon lamp is always a difficult problem to be overcome in DOAS technology. In addition, the absorption cross section of NO 2 in the ultraviolet band is far smaller than that of NO and SO 2, which also causes that nearly half of analyzers in the market at present adopt a NO x converter to convert NO 2 into NO for measurement, and the use of the NO x converter causes the complex structure and high cost of the instrument on one hand, and causes the loss of NO 2 in the conversion process on the other hand, SO that the actual emission concentration of NO 2 cannot be accurately measured.
Disclosure of Invention
Aiming at the problems in the related art, the invention provides a DOAS-based NO 2 concentration measuring method, so as to overcome the technical problems in the prior related art.
For this purpose, the invention adopts the following specific technical scheme:
A method for measuring concentration of NO 2 based on DOAS, the method comprising the steps of:
S1, acquiring original spectrum data with different power intensities for preprocessing;
s2, calculating a spectrum correction coefficient and verifying;
S3, collecting transmission spectrums of NO 2 with different concentrations, and processing to obtain NO 2 spectrum data;
s4, calculating an optimal differential absorbance position data set and an optimal differential absorbance section by calculating the relation between each spectrum point and the concentration;
s5, establishing an equation through actually measured differential absorbance of the gas, an optimal differential absorbance position data set and an optimal differential absorption section to obtain a concentration value of NO 2;
S6, correcting the concentration value of the NO 2 by using a least square method.
Further, the preprocessing of the raw spectrum data with different power intensities comprises the following steps:
S11, selecting a wavelet basis function and a decomposition layer number N, and performing N-layer multi-scale decomposition on the signal;
s12, reconstructing the low-frequency coefficient of the Nth layer to obtain a reconstructed spectrum;
S13, adjusting the power of a xenon lamp light source, introducing pure nitrogen into the air chamber, and collecting a plurality of groups of original spectrum data with different power intensities;
s14, filtering each piece of original spectrum data by utilizing wavelet transformation;
S15, averaging the spectrum data with different intensities after filtering to obtain the preprocessed spectrum data.
Further, the wavelet transformation is an integral transformation of the signal f (t) ∈l 2 (R), and the operation process and expression include:
wherein a and b represent the telescoping and translating factors of ψ (t), respectively;
t represents the time of the signal and, T is E (- ≡infinity), ++ infinity a) is provided;
Is the result of translation and scaling by ψ (t);
is the complex conjugate of ψ ab (t);
The inverse transformation process expression is:
Wherein, Is an allowable condition.
Further, the calculation and verification of the spectral correction coefficient comprise the following steps:
S21, selecting at least one wave band in a wave band which is not absorbed by SO 2, NO and NO 2;
S22, recording the preprocessed spectrum data of the light source powers 800, 775 and 750 as lamp_800, lamp_775 and lamp_750 respectively, and calculating correction coefficients by taking lamp_750 as a reference spectrum, wherein the expression is as follows:
R1=log(Lamp_750/Lamp_775)
Wherein R1 is the ratio logarithm of two kinds of power spectrum data, and represents the change of spectrum data of other power spectrum data relative to fixed power;
K represents a correction coefficient;
Lamp_775 represents spectral data at a light source power of 775;
lamp_750 represents spectral data when the light source power is 750;
253.6-256 represent wave bands which are not absorbed by SO 2, NO and NO 2;
s23, calibrating the optical data by using the correction coefficient, wherein the expression is as follows:
R2=log(Lamp_750/Lamp_800)
NEW_Lamp_800=exp(K*mean(R2(253.6~256)))*Lamp_800
wherein R2 represents a change in spectral data of other power relative to the spectral data of fixed power;
Lamp_800 represents spectral data at a light source power of 800;
New_Lamp_800 represents the spectrum data corrected by lamp_800.
Further, the method for acquiring and processing the transmission spectra of NO 2 with different concentrations to obtain NO 2 spectrum data comprises the following steps:
S31, keeping the power of the light source unchanged, and respectively collecting a plurality of groups of NO 2 spectral data with different concentrations;
s32, performing filtering processing on the spectrum data of each concentration by utilizing wavelet transformation;
s33, calculating the average value of the plurality of groups of spectrum data after filtering;
And S34, calibrating the average value by using the correction coefficient to obtain calibrated NO 2 spectrum data.
Further, the calculating of the optimal differential absorbance position data set and the optimal differential absorbance cross section by calculating the relation between each spectral point and the concentration comprises the following steps:
S41, selecting a spectrum in a range of 390 nm-415 nm as a wave band for calculating the concentration of NO 2 according to the spectrum range acquired by the spectrometer and the absorption cross sections of SO 2, NO and NO 2;
S42, according to a differential absorption spectrum technology, obtaining differential absorbance and differential absorption cross sections of NO 2 at the concentration of [0,20,40,60,80,100] ppm;
S43, calculating a correlation coefficient between the concentration value [0,20,40,60,80,100] ppm and the differential absorbance of each wavelength point in the range of 390 nm-415 nm, and selecting the wavelength point corresponding to the correlation coefficient greater than 0.99 to form an optimal differential absorbance position data set;
S44, selecting the differential absorption section to be positioned at the position corresponding to the optimal differential absorption position data set to form an optimal differential absorption section.
Further, the operational expression of the differential absorption spectrum technology includes:
σi(λ)＝σi,slow(λ)+σi,rapid(λ)
Wherein I 0 (λ) represents the light source intensity of the incident light at the wavelength λ;
I (λ) represents the light source intensity of the outgoing light at the wavelength λ;
L represents an optical path;
a (λ) represents a transfer function of the system;
c i represents the i-th gas concentration;
σ i (λ) represents the absorption cross section of the i-th gas;
Epsilon R (lambda) and epsilon M (lambda) represent the extinction coefficients of Rayleigh and Mie scattering, respectively;
d (λ) represents an optical thickness of the substance;
σ i,slow (λ) represents a portion that slowly varies with wavelength;
σ i,rapid (λ) represents the portion that changes rapidly with wavelength, i.e. the differential absorption cross section of the gas.
Further, the equation is established by actually measuring the differential absorbance of the gas, the position data set of the optimal differential absorbance and the optimal differential absorbance section, and the expression is as follows:
C=(P′·P)-1·P′·D
Wherein, C represents NO 2 concentration value;
d represents the gas differential absorbance after selection by using the optimal differential absorbance position data set;
p represents the optimal differential absorption cross-section and P' represents the transpose of P.
Further, the correcting the concentration value of the NO 2 by using the least square method includes the following steps:
s61, a relation between the NO 2 concentration value and the actual concentration value is obtained by fitting a cubic polynomial through a least square method, and a cubic polynomial relation is obtained;
S62, substituting the NO 2 concentration value into the cubic polynomial relation to obtain a corrected concentration value.
Further, the expression of the third order polynomial is:
Fit_c=polyfit(C,y,3)
con=polyval(Fit_c,C)
wherein, polyfit () is a polynomial fitting function in Matlab;
C represents NO 2 concentration value;
y represents the corresponding actual concentration value;
3 is the polynomial fitting degree;
Fit_c represents the fitted cubic polynomial coefficient;
polyval () represents a polynomial evaluation function;
con represents the corrected concentration value.
The method has the advantages that the spectrum data is corrected by an exponential spectrum correction method, the defect of poor stability and repeatability of a xenon lamp light source is overcome, the accuracy of the spectrum data is greatly improved, a correlation coefficient method is utilized to calculate the correlation coefficient between concentration and differential absorbance, an optimal differential absorbance position data set and an optimal differential absorbance section are further constructed, the NO2 measurement accuracy is improved, and the practical application requirements are met.
In addition, in order to meet the practical application, the chip calculation amount is reduced, the reaction time is improved, wavelet transformation is adopted for removing high-frequency noise of spectrum data and solving a slow-change part, and only a corresponding low-frequency part is reconstructed according to the spectrum absorption frequency range.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a flow chart of a DOAS-based NO 2 concentration measurement method according to an embodiment of the present invention;
FIG. 2 is a graph of spectral pretreatment data in a DOAS-based NO 2 concentration measurement method according to an embodiment of the present invention;
FIG. 3 is a graph of two different spectral calibration methods in a DOAS-based NO 2 concentration measurement method according to an embodiment of the present invention;
FIG. 4 is a graph of differential absorption cross-section determination variation in a DOAS-based NO 2 concentration measurement method according to an embodiment of the present invention;
Fig. 5 is a graph of correlation coefficient between differential absorbance and concentration in a DOAS-based NO 2 concentration measurement method according to an embodiment of the present invention.
Detailed Description
According to an embodiment of the present invention, there is provided a DOAS-based NO 2 concentration measurement method.
The invention will now be further described with reference to the accompanying drawings and detailed description, as shown in fig. 1, a method for measuring concentration of NO 2 based on DOAS according to an embodiment of the invention, the method comprising the steps of:
S1, acquiring original spectrum data with different power intensities for preprocessing, wherein the method comprises the following steps of:
s11, selecting a wavelet basis function and a decomposition layer number N, and performing N-layer multi-scale decomposition on the signal (the selected basis function is bior < 2.8 > and the decomposition layer number is 2 layers through testing in the invention);
s12, reconstructing the low-frequency coefficient of the N layer (the high-frequency part is verified to basically not contain useful information, so that in order to reduce the calculation amount, the high-frequency coefficient is not subjected to threshold quantization processing) to obtain a reconstructed spectrum;
S13, adjusting the power of a xenon lamp light source, introducing pure nitrogen into the air chamber, and collecting a plurality of groups of original spectrum data with different power intensities;
in the invention, under the conditions that the power of a xenon lamp light source is respectively 800, 775 and 750, pure nitrogen is introduced into a gas chamber, the temperature, the pressure and the like are the same, 10 pieces of spectrum data are collected for each power, and after each spectrum is subjected to wavelet transformation treatment in the steps (a) and (b), the average value of 10 spectrums is calculated, so that the preprocessed spectrum data are obtained, as shown in figure 2;
s14, filtering each piece of original spectrum data by utilizing wavelet transformation;
S15, averaging the spectrum data with different intensities after filtering to obtain the preprocessed spectrum data.
The existing common spectral data denoising method mainly comprises principal component analysis, a low-pass filtering method, a Kalman filtering method, fourier transformation and the like, but the principal component analysis is overlarge in calculation amount, the Fourier transformation lacks the analysis capability on local signals, an absorption wave band in the spectral data is a partial wave band in the whole acquisition range, and finally, the spectral data is denoised by evaluating and selecting wavelet transformation and is simulated by Matlab.
Wherein the wavelet transformation is integral transformation of the signal f (t) epsilon L 2 (R), and the operation process and the expression comprise:
wherein a and b represent the telescoping and translating factors of ψ (t), respectively;
t represents the time of the signal and, t is E (- ≡infinity), ++ infinity a) is provided;
Is the result of translation and scaling by ψ (t);
is the complex conjugate of ψ ab (t);
The inverse transformation process expression is:
Wherein, Is an allowable condition.
In practical applications, the continuous variables a and b in ψ ab (t) are taken as integer discrete forms, and ψ ab (t) is expressed as:
The corresponding wavelet transform is denoted as discrete wavelet transform:
Wf(j,k)＝(f(t),Ψj,k(t))
s2, calculating a spectrum correction coefficient and verifying, wherein the method comprises the following steps of:
S21, selecting at least one wave band (253.3-256 nm wave band is selected) from the wave bands which are not absorbed by SO2, NO and NO 2;
S22, recording the preprocessed spectrum data of the light source powers 800, 775 and 750 as lamp_800, lamp_775 and lamp_750 respectively, and calculating correction coefficients by taking lamp_750 as a reference spectrum, wherein the expression is as follows:
R1=log(Lamp_750/Lamp_775)
wherein, R1 is the ratio logarithm of the two kinds of power spectrum data, and represents the change of the spectrum data of other powers relative to the spectrum data of fixed power (the spectrum data of other powers can be selected);
K represents a correction coefficient;
Lamp_775 represents spectral data at a light source power of 775;
lamp_750 represents spectral data when the light source power is 750;
253.6-256 represent wave bands which are not absorbed by SO 2, NO and NO 2;
s23, calibrating the optical data by using the correction coefficient, wherein the expression is as follows:
R2=log(Lamp_750/Lamp_800)
NEW_Lamp_800=exp(K*mean(R2(253.6~256)))*Lamp_800
wherein R2 represents a change in spectral data of other power relative to the spectral data of fixed power;
Lamp_800 represents spectral data at a light source power of 800;
New_Lamp_800 represents the spectrum data corrected by lamp_800, namely, the spectrum data to be calibrated is needed.
As shown in fig. 3, the left graph is a graph of the index calibration method proposed in the present invention, which uses a conventional method, i.e. a non-absorption band is used to obtain a scaling factor, and then the scaling factor is directly multiplied by the spectrum to be calibrated, and the right graph is a graph of the index calibration method proposed in the present invention, and it is obvious that the effect of the index calibration method is better, because the light intensity variation of different bands is not an absolute linear relationship with the variation of the light source power.
S3, collecting and processing transmission spectrums of NO 2 with different concentrations to obtain NO 2 spectrum data, wherein the method comprises the following steps of:
S31, keeping the power of the light source unchanged, and respectively collecting a plurality of groups of NO 2 spectral data with different concentrations;
In the embodiment of the invention, the power of the xenon lamp light source is kept to be 750, NO2 spectrum data with the concentration of [0,20,40,60,80,100] ppm are respectively collected,
S32, performing filtering processing on the spectrum data of each concentration by utilizing wavelet transformation;
s33, calculating the average value of the plurality of groups of spectrum data after filtering;
And S34, calibrating the average value by using the correction coefficient to obtain calibrated NO 2 spectrum data.
S4, calculating an optimal differential absorbance position data set and an optimal differential absorbance section by calculating the relation between each spectrum point and the concentration, wherein the method comprises the following steps of:
S41, selecting a spectrum in the range of 390 nm-415 nm as a wave band for calculating the concentration of NO 2 according to the spectrum range acquired by the spectrometer and the absorption cross sections of SO 2, NO and NO 2 (provided by the HITRAN database);
S42, according to a differential absorption spectrum technology (DOAS), obtaining differential absorbance and differential absorption cross sections of NO2 at the concentration of [0,20,40,60,80,100] ppm;
S43, calculating a correlation coefficient between a concentration value [0,20,40,60,80,100] and the differential absorbance of each wavelength point in the range of 390 nm-415 nm, and selecting a wavelength point corresponding to the correlation coefficient greater than 0.99 to form an optimal differential absorbance position dataset;
S44, selecting the differential absorption section to be positioned at the position corresponding to the optimal differential absorption position data set to form an optimal differential absorption section.
Wherein the differential absorption spectroscopy (DOAS) is based on the detection of narrowband absorption characteristics of trace gas molecules, the intensity of the absorbed light obeys the Lambert Beer absorption law, and when Rayleigh scattering (Rayleigh), mie scattering (Mie) and other molecular absorption are considered, the operational expression comprises:
the spectroscopic detection technique applies this law to measure the average concentration of trace gas along the optical path. General definition The optical thickness of a substance, expressed as D (λ), is expressed as:
In order to eliminate the influence of Rayleigh scattering, mie scattering and the like, filtering technology is generally used mathematically to separate out the spectral changes caused by molecular absorption contained in the atmospheric absorption spectrum, and the absorption cross section of a gas can be generally regarded as being composed of two parts, expressed as:
σi(λ)＝σi,slow(λ)+σi,rapid(λ)
wherein I 0 (λ) represents the light intensity of the incident light at the wavelength λ;
i (λ) represents the light intensity of the outgoing light at the wavelength λ;
L represents an optical path;
a (λ) represents a transfer function of the system;
c i represents the i-th gas concentration;
σ i (λ) represents the absorption cross section of the i-th gas;
Epsilon R (lambda) and epsilon M (lambda) represent the extinction coefficients of Rayleigh and Mie scattering, respectively;
d (λ) represents an optical thickness of the substance;
σ i,slow (λ) represents a portion that slowly varies with wavelength;
σ i,rapid (λ) represents the portion that changes rapidly with wavelength, i.e. the differential absorption cross section of the gas.
The total absorption cross section σ i (λ) minus the calculated slow variation σ i,slow (λ) is the differential absorption cross section of the gas σ i,rapid (λ), where the slow variation σ i,slow (λ) can be obtained by reconstructing the low frequency part of the wavelet multi-scale decomposition, for example the slow variation part of SO 2 can be obtained by reconstructing the low frequency part of the wavelet 5-layer decomposition, as shown in fig. 4.
S5, establishing an equation through actually measured differential absorbance of the gas, an optimal differential absorbance position data set and an optimal differential absorption section to obtain a concentration value of NO 2;
The equation is established through the actual measured differential absorbance of the gas, the optimal gas differential absorbance position data set and the optimal differential absorbance section, and the expression is as follows:
C=(P′·P)-1·P′·D
Wherein, C represents NO 2 concentration value;
d represents the gas differential absorbance after selection by using the optimal differential absorbance position data set;
p represents the optimal differential absorption cross-section and P' is the transpose of P.
S6, correcting the concentration value of the NO 2 by using a least square method, wherein the method comprises the following steps of:
s61, a relation between the NO 2 concentration value and the actual concentration value is obtained by fitting a cubic polynomial through a least square method, and a cubic polynomial relation is obtained;
S62, substituting the NO 2 concentration value into the cubic polynomial relation to obtain a corrected concentration value.
Wherein, the expression of the cubic polynomial is:
Fit_c=polyfit(C,y,3)
con=polyval(Fit_c,C)
wherein, polyfit () is a polynomial fitting function in Matlab;
C represents the concentration value of NO 2;
y represents the corresponding actual concentration value;
3 is the polynomial fitting degree;
Fit_c represents the fitted cubic polynomial coefficient;
polyval () represents a polynomial evaluation function;
con represents the corrected concentration value.
In summary, by means of the technical scheme, the spectrum data is corrected by the index spectrum correction method, the defect of poor stability and repeatability of a xenon lamp light source is overcome, the accuracy of the spectrum data is greatly improved, the correlation coefficient between concentration and differential absorbance is obtained by the correlation coefficient method, an optimal differential absorbance position data set and an optimal differential absorbance section are further constructed, the NO2 measurement accuracy is improved, and the practical application requirements are met.
In addition, in order to meet the practical application, the chip calculation amount is reduced, the reaction time is improved, wavelet transformation is adopted for removing high-frequency noise of spectrum data and solving a slow-change part, and only a corresponding low-frequency part is reconstructed according to the spectrum absorption frequency range.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.
Claims (8)
1. A method for measuring concentration of NO 2 based on DOAS, comprising the steps of:
S1, acquiring original spectrum data with different power intensities for preprocessing;
s2, calculating a spectrum correction coefficient and verifying;
S3, collecting transmission spectrums of NO 2 with different concentrations, and processing to obtain NO 2 spectrum data;
s4, calculating an optimal differential absorbance position data set and an optimal differential absorbance section by calculating the relation between each spectrum point and the concentration;
s5, establishing an equation through actually measured differential absorbance of the gas, an optimal differential absorbance position data set and an optimal differential absorption section to obtain a concentration value of NO 2;
S6, correcting the concentration value of the NO 2 by using a least square method;
the method for calculating and verifying the spectrum correction coefficient comprises the following steps:
S21, selecting at least one wave band in a wave band which is not absorbed by SO 2, NO and NO 2;
S22, recording the preprocessed spectrum data of the light source powers 800, 775 and 750 as lamp_800, lamp_775 and lamp_750 respectively, and calculating correction coefficients by taking lamp_750 as a reference spectrum, wherein the expression is as follows:
R1=log(Lamp_750/Lamp_775)
Wherein R1 is the ratio logarithm of two kinds of power spectrum data, and represents the change of spectrum data of other power spectrum data relative to fixed power;
K represents a correction coefficient;
Lamp_775 represents spectral data at a light source power of 775;
lamp_750 represents spectral data when the light source power is 750;
253.6-256 represent wave bands which are not absorbed by SO 2, NO and NO 2;
s23, calibrating the optical data by using the correction coefficient, wherein the expression is as follows:
R2=log(Lamp_750/Lamp_800)
New_lamp_800=exp (k×mean (R2 (253.6 to 256))) lamp_800, where R2 represents a change in spectral data of other power relative to the spectral data of fixed power;
Lamp_800 represents spectral data at a light source power of 800;
New_Lamp_800 represents the spectrum data corrected by lamp_800;
The method for calculating the optimal differential absorbance position data set and the optimal differential absorbance section by calculating the relation between each spectrum point and the concentration comprises the following steps:
S41, selecting a spectrum in a range of 390 nm-415 nm as a wave band for calculating the concentration of NO 2 according to the spectrum range acquired by the spectrometer and the absorption cross sections of SO 2, NO and NO 2;
S42, according to a differential absorption spectrum technology, obtaining differential absorption degree and differential absorption cross section of NO 2 at the concentration of [0,20,40,60,80,100] ppm;
S43, calculating a correlation coefficient between the concentration value [0,20,40,60,80,100] ppm and the differential absorbance of each wavelength point in the range of 390 nm-415 nm, and selecting the wavelength point corresponding to the correlation coefficient greater than 0.99 to form an optimal differential absorbance position data set;
S44, selecting the differential absorption section to be positioned at the position corresponding to the optimal differential absorption position data set to form an optimal differential absorption section.
2. The method for measuring the concentration of NO 2 based on DOAS according to claim 1, wherein the step of obtaining raw spectral data of different power intensities for preprocessing includes the steps of:
S11, selecting a wavelet basis function and a decomposition layer number N, and performing N-layer multi-scale decomposition on the signal;
s12, reconstructing the low-frequency coefficient of the Nth layer to obtain a reconstructed spectrum;
S13, adjusting the power of a xenon lamp light source, introducing pure nitrogen into the air chamber, and collecting a plurality of groups of original spectrum data with different power intensities;
s14, filtering each piece of original spectrum data by utilizing wavelet transformation;
S15, averaging the spectrum data with different intensities after filtering to obtain the preprocessed spectrum data.
3. A method for measuring concentration of NO 2 based on DOAS as in claim 2, wherein the wavelet transform is an integral transform of the signal f (t) ∈L 2 (R), and the operation and expression include:
wherein a and b represent the telescoping and translating factors of ψ (t), respectively;
t represents the time of the signal and, t is E (- ≡infinity), ++ infinity a) is provided;
Is the result of translation and scaling by ψ (t);
is the complex conjugate of ψ ab (t);
The inverse transformation process expression is:
Wherein, Is an allowable condition.
4. The method for measuring the concentration of NO 2 based on DOAS according to claim 1, wherein the steps of collecting and processing the transmission spectra of NO 2 with different concentrations to obtain NO 2 spectrum data include the following steps:
S31, keeping the power of the light source unchanged, and respectively collecting a plurality of groups of NO 2 spectral data with different concentrations;
s32, performing filtering processing on the spectrum data of each concentration by utilizing wavelet transformation;
s33, calculating the average value of the plurality of groups of spectrum data after filtering;
And S34, calibrating the average value by using the correction coefficient to obtain calibrated NO 2 spectrum data.
5. The method for measuring the concentration of NO 2 based on DOAS as claimed in claim 4, wherein the operation expression of the differential absorption spectrum technology comprises:
σi(λ)=σi,slow(λ)+σi,rapid(λ)
wherein I 0 (λ) represents the light intensity of the incident light at the wavelength λ;
I (λ) represents the light intensity of the transmitted light at the wavelength λ;
L represents an optical path;
a (λ) represents a transfer function of the system;
c i represents the i-th gas concentration;
σ i (λ) represents the absorption cross section of the i-th gas;
Epsilon R (lambda) and epsilon M (lambda) represent the extinction coefficients of Rayleigh and Mie scattering, respectively;
d (λ) represents an optical thickness of the substance;
σi, slow (λ) represents the portion that varies slowly with wavelength;
σ i,rapid (λ) represents the portion that changes rapidly with wavelength, i.e. the differential absorption cross section of the gas.
6. The method for measuring the concentration of NO 2 based on DOAS according to claim 1, wherein the equation is established by actually measuring the differential absorbance of the gas, the position data set of the optimal differential absorbance and the optimal differential absorbance section, and the expression is:
C=(P′·P)-1·P′·D
Wherein, C represents NO 2 concentration value;
d represents the gas differential absorbance after selection by using the optimal differential absorbance position data set;
p represents the optimal differential absorption cross-section and P' represents the transpose of P.
7. A method for measuring concentration of NO 2 based on DOAS as claimed in claim 1, wherein said correcting said concentration value of NO 2 by least square method comprises the steps of:
s61, a relation between the NO 2 concentration value and the actual concentration value is obtained by fitting a cubic polynomial through a least square method, and a cubic polynomial relation is obtained;
S62, substituting the NO 2 concentration value into the cubic polynomial relation to obtain a corrected concentration value.
8. The method for measuring the concentration of NO 2 based on DOAS as claimed in claim 7, wherein the expression of the cubic polynomial is:
Fit_c=polyfit(C,y,3)
con=polyval(Fit_c,C)
wherein, polyfit () is a polynomial fitting function in Matlab;
C represents NO 2 concentration value;
y represents the corresponding actual concentration value;
3 is the polynomial fitting degree;
Fit_c represents the fitted cubic polynomial coefficient;
polyval () represents a polynomial evaluation function;
con represents the corrected concentration value.
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