rus
Issue #2/2015
A. Aliyeva
Method of High Accuracy Trace Gases Concentration Measurement Concentration Using Hyperspectrial Sun Photometers
Method of High Accuracy Trace Gases Concentration Measurement Concentration Using Hyperspectrial Sun Photometers
Method of High Accuracy Trace Gases Concentration Measurement Concentration Using Hyperspectrial Sun Photometers
Теги: gas analyzers sun photometers troposphere pollution газоанализаторы загрязнение тропосферы солнечные фотометры
Dynamics of changes of these small components of atmosphere is often interconnected. For example, as it is specified in [1] the analysis of daily variation of concentrations shows that minimum ozone content in the surface layer of atmosphere coincides with the maximum concentrations of NO and , and during the night time (from 11 pm to 7 am) when the concentrations of nitrogen oxides are minimal, the level of ozone content reaches maximum value (Fig. 1).
The time diagram of nitrogen oxides and indicates that photochemical processes proceed intensively in the city atmosphere. And daily variation of nitrogen dioxide is mainly determined by the dynamics of and level of ozone content in air. Thus, during the day ozone concentration is low and at night it grows by 2–3 times. It is stipulated by its settlement from air upper layers to the atmosphere surface layer, when the levels of nitrogen oxide content in it are minimal due to the absence of motor transport on roads and it fully corresponds to the existing conceptions on photochemical processes in the polluted atmosphere [1].
According to the paper [2], interconnection of average monthly concentrations of ozone and nitrogen dioxide is almost identical in all cities of Europe (Fig. 2). As it is seen from the graph shown in Fig. 2, the increase of concentration is accompanied by the decrease of concentration and vice versa, which indicates the major role of the mechanism of formation on the basis of the reaction:
NO + O3 = NO2 + O2.
It should be noted that antiphase character of above mentioned parameters is also attributable to the absorption coefficients of the specified trace gases. In Fig. 3 the graphs of absorption cross-section of several atmosphere trace gases are shown. As it is seen from the graphs shown in Fig. 3, absorption cross-sections of and within the range of wavelengths 250–650 nm have clearly antiphase character, in other words the growth of one of them is accompanied by the decrease of the other one [3].
Above mentioned properties of concentrations and absorption cross-sections of and suggest that the optical thicknesses of these gases also have the property of antiphasicity. This circumstance allows us to offer the new high accuracy method of aggregate measurement of concentrations or quantity of these gases in atmosphere. Let us explain the suggested method at the example of the illustration of gas optical thicknesses (Fig. 4).
Let us assume that at the wavelengths and the spectral measurements of and are accomplished. We think that the concentrations of other trace gases influencing on the measurement results at the wavelengths and are known and can be taken into account during the processing of these measurements. In general case, on the basis of Bouguer-Beer law we will obtain the following
, (1)
, (2)
where is the signal at spectrometer exit; is the intensity of extraterrestrial radiation; is the optical air mass; is the total optical thickness of the rest of gases. In order to simplify the mathematical calculations, we assume that the instrument function is equal to one.
Expressions (1) and (2) can be easily formulated as follows:
, (3)
, (4)
where .
In general case optical thicknesses of trace gases can be determined as follows
, (5)
where is the coefficient of trace gas absorption; is the total (column) quantity of the tested atmosphere trace gases.
Taking into account the expressions (5), the equations (3) and (4) can be formulated in the following manner:
, (6)
. (7)
Solution of the combined equations (6), (7) allows determining the parameters and .
In addition, it is obvious that the results of performed measurements will contain the following characteristic types of errors which are attributable to the atmospheric spectrometric measurements:
•Noises due to the influence of atmospheric aerosol;
•Noises of spectrometer optoelectronic tract.
It should be shown that the variation character of and in time and absorption coefficients by the wavelength make it possible to decrease partially the aerosol error in the suggested measurement method. On the basis of algorithm, the main point of the suggested method can be set forth in the following manner:
1.Two-wavelength spectral measurements at the wavelengths and . Determination of and with the errors and respectively where
, (8)
, (9)
where the digital indices indicate the aerosol or instrument origin.
1.Since in the point (Fig. 4) we obtain , the total optical thickness of measured gases in the point can be determined as
. (10)
1.The calculated graphs and are plotted and the continuous special measurements within the range and are performed (Fig. 5).
2.The point of intersection of the graphs and is determined. Abscissa of this point is designated as .
3.Difference between the ordinate of the point and ordinate of the point P of intersection of the function and line determines the total error and which is determined as , where is the aerosol error in the point ; refer to the noises of optoelectronic tract.
4.Estimation of allows evaluating the result of compatible measurements as follows
. (1)
Here, the error of determination of is calculated as , where is the aerosol error in the point refer to the noises of optoelectronic tract.
1.Benefit in the decrease of aerosol error in the point is determined as
. (12)
1.The optical thicknesses and are determined as
. (13)
1.Column values and are determined as
. (14)
In the most general case, the point is located symmetrically relative to the points and . In addition, the dependence of the optical thickness of atmospheric aerosol according to Angstrom formula is clearly linear. It results in the fact that with the decrease of wavelength the share of aerosol error of measurements in the total error grows. Thus, the significant nonlinearity of aerosol optical thickness by the wavelength is nonlinear and this fact stipulates the benefit in the accuracy of the suggested measurement method.
Depending on wavelength, the nonlinear character of aerosol noises allows increasing the measurement accuracy using the references in the detected point instead of the noisy value with the reference in . It should be noted here that aerosol noises in the point influencing on the calculation results (6), (7) in equal manner and calculated values and turn out to be noisy in equal degree.
In summary we will formulate the main conclusions and provisions of performed studies:
1.The antiphase character of the main parameters of optical thickness of and has been analyzed.
2.The method of high accuracy measurement of trace gases which have antiphase properties of the main parameters has been suggested.
The time diagram of nitrogen oxides and indicates that photochemical processes proceed intensively in the city atmosphere. And daily variation of nitrogen dioxide is mainly determined by the dynamics of and level of ozone content in air. Thus, during the day ozone concentration is low and at night it grows by 2–3 times. It is stipulated by its settlement from air upper layers to the atmosphere surface layer, when the levels of nitrogen oxide content in it are minimal due to the absence of motor transport on roads and it fully corresponds to the existing conceptions on photochemical processes in the polluted atmosphere [1].
According to the paper [2], interconnection of average monthly concentrations of ozone and nitrogen dioxide is almost identical in all cities of Europe (Fig. 2). As it is seen from the graph shown in Fig. 2, the increase of concentration is accompanied by the decrease of concentration and vice versa, which indicates the major role of the mechanism of formation on the basis of the reaction:
NO + O3 = NO2 + O2.
It should be noted that antiphase character of above mentioned parameters is also attributable to the absorption coefficients of the specified trace gases. In Fig. 3 the graphs of absorption cross-section of several atmosphere trace gases are shown. As it is seen from the graphs shown in Fig. 3, absorption cross-sections of and within the range of wavelengths 250–650 nm have clearly antiphase character, in other words the growth of one of them is accompanied by the decrease of the other one [3].
Above mentioned properties of concentrations and absorption cross-sections of and suggest that the optical thicknesses of these gases also have the property of antiphasicity. This circumstance allows us to offer the new high accuracy method of aggregate measurement of concentrations or quantity of these gases in atmosphere. Let us explain the suggested method at the example of the illustration of gas optical thicknesses (Fig. 4).
Let us assume that at the wavelengths and the spectral measurements of and are accomplished. We think that the concentrations of other trace gases influencing on the measurement results at the wavelengths and are known and can be taken into account during the processing of these measurements. In general case, on the basis of Bouguer-Beer law we will obtain the following
, (1)
, (2)
where is the signal at spectrometer exit; is the intensity of extraterrestrial radiation; is the optical air mass; is the total optical thickness of the rest of gases. In order to simplify the mathematical calculations, we assume that the instrument function is equal to one.
Expressions (1) and (2) can be easily formulated as follows:
, (3)
, (4)
where .
In general case optical thicknesses of trace gases can be determined as follows
, (5)
where is the coefficient of trace gas absorption; is the total (column) quantity of the tested atmosphere trace gases.
Taking into account the expressions (5), the equations (3) and (4) can be formulated in the following manner:
, (6)
. (7)
Solution of the combined equations (6), (7) allows determining the parameters and .
In addition, it is obvious that the results of performed measurements will contain the following characteristic types of errors which are attributable to the atmospheric spectrometric measurements:
•Noises due to the influence of atmospheric aerosol;
•Noises of spectrometer optoelectronic tract.
It should be shown that the variation character of and in time and absorption coefficients by the wavelength make it possible to decrease partially the aerosol error in the suggested measurement method. On the basis of algorithm, the main point of the suggested method can be set forth in the following manner:
1.Two-wavelength spectral measurements at the wavelengths and . Determination of and with the errors and respectively where
, (8)
, (9)
where the digital indices indicate the aerosol or instrument origin.
1.Since in the point (Fig. 4) we obtain , the total optical thickness of measured gases in the point can be determined as
. (10)
1.The calculated graphs and are plotted and the continuous special measurements within the range and are performed (Fig. 5).
2.The point of intersection of the graphs and is determined. Abscissa of this point is designated as .
3.Difference between the ordinate of the point and ordinate of the point P of intersection of the function and line determines the total error and which is determined as , where is the aerosol error in the point ; refer to the noises of optoelectronic tract.
4.Estimation of allows evaluating the result of compatible measurements as follows
. (1)
Here, the error of determination of is calculated as , where is the aerosol error in the point refer to the noises of optoelectronic tract.
1.Benefit in the decrease of aerosol error in the point is determined as
. (12)
1.The optical thicknesses and are determined as
. (13)
1.Column values and are determined as
. (14)
In the most general case, the point is located symmetrically relative to the points and . In addition, the dependence of the optical thickness of atmospheric aerosol according to Angstrom formula is clearly linear. It results in the fact that with the decrease of wavelength the share of aerosol error of measurements in the total error grows. Thus, the significant nonlinearity of aerosol optical thickness by the wavelength is nonlinear and this fact stipulates the benefit in the accuracy of the suggested measurement method.
Depending on wavelength, the nonlinear character of aerosol noises allows increasing the measurement accuracy using the references in the detected point instead of the noisy value with the reference in . It should be noted here that aerosol noises in the point influencing on the calculation results (6), (7) in equal manner and calculated values and turn out to be noisy in equal degree.
In summary we will formulate the main conclusions and provisions of performed studies:
1.The antiphase character of the main parameters of optical thickness of and has been analyzed.
2.The method of high accuracy measurement of trace gases which have antiphase properties of the main parameters has been suggested.
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