JP3636829B2

JP3636829B2 - Gas concentration measurement method using multi-wavelength light

Info

Publication number: JP3636829B2
Application number: JP17870596A
Authority: JP
Inventors: 直彦後藤
Original assignee: Central Research Institute of Electric Power Industry
Current assignee: Central Research Institute of Electric Power Industry
Priority date: 1996-06-20
Filing date: 1996-06-20
Publication date: 2005-04-06
Anticipated expiration: 2016-06-20
Also published as: JPH1010044A

Description

【０００１】
【発明の属する技術分野】
本発明は、多波長光により大気中のガス（原子、分子、エアロゾル等）濃度を計測する方法に関するものである。
【０００２】
【従来の技術】
大気中のガス濃度を測定する方法として、例えば図に示すように、単体又は複数の多波長光照射装置１から多波長の光２を順次に計測対象ガス３に照射し、その多波長の光を受光装置４により受光し、データ解析装置５により計測対象ガス３の構成成分の濃度計測を行う方法がある。この多波長光を用いる従来の計測方法であるレーザレーダは、計測対象ガスにより吸収され易い（吸収の大きい）波長と、吸収され難い（吸収の小さい）波長の２波長を用いた差分吸収レーザレーダ（ＤＩＡＬ）計測法が用いられている。また、さらに精度を高めるために、３つ以上の多波長を用いる測定方法がある。
この従来の２波長を用いたＤＩＡＬ計測法における受信光子数は、式１で現される。
【０００３】
【数１】

ここで、Ｎr は受信光子数，Ｎt は照射光子数，ηは光学系全効率，ΔＲは距離分解能，βb は後方散乱係数，Ａは受信面積，Ｒは測定距離，βa は消散係数である。
このＤＩＡＬ計測法では、式１を２波長について記述し、その差を求めることにより表される。そこで、測定したいガス濃度ｎ（Ｒ）及び吸収断面積σにより、βa ＝σｎ（Ｒ）と表され、２つの波長に対してσのみが異なる場合、一般的なＤＩＡＬ計測法の式として、次の式２で表される。
【０００４】
【数２】

σonとσoff は、それぞれ、吸収の大きい波長と吸収の小さい波長に対する吸収断面積である。
【０００５】
【発明が解決しようとする課題】
しかしながら、この式２の場合は、前提条件として測定対象ガス以外のガスによる吸収や散乱の影響が、２つの波長に対して同じであると見做せる場合に限られるものである。即ち、２波長での消散係数βa が測定対象ガスのみで異なる場合に成り立つものであり、他のガスの影響がある場合は成り立たないのである。そして、測定対象ガス以外のガスに対して、消散係数βa が同じ２波長を選ぶことは困難な場合が多く、２波長を用いるＤＩＡＬ計測法では、測定対象となるガス以外の消散係数（吸収断面積）が等しくない場合は、誤差があるため測定精度が悪いという問題がある。
また、３つ以上の多波長を用いる測定は、用いる波長の数に応じた回数の測定を必要とし、計測に長時間を必要とする問題があった。
【０００６】
【課題を解決するための手段】
本発明による多波長光によるガス濃度計測方法は、大気中の各種測定対象ガスに、吸収され易い波長の光と吸収され難い波長の光を照射し、それぞれの照射光の大気からの後方散乱光（反射光）を望遠鏡で受光して測定する差分吸収レーザーレーダー（ＤＩＡＬ）計測法を用いるガス濃度計測方法において、測定対象ガスに対して、吸収され易い異なる波長の２波長と、吸収され難い異なる波長の２波長であって、かつ前記吸収され易い２波長の測定誤差対象ガスに対する消散係数の和と前記吸収され難い２波長の測定誤差対象ガスに対する消散係数の和とが等しくなるように４波長をそれぞれ選定し、前記吸収され易い２波長の光を１組とし、前記吸収され難い２波長の光を１組としてそれぞれ前記測定対象ガスに照射し、該照射された２組の反射光をそれぞれ受光し、該反射光の光子数に基づいてガス濃度を計測することを特徴とするものである。
【０００７】
【発明の実施の形態】
本発明を、４波長を用いた例について説明する。この４波長のうち２波長は測定対象ガスに吸収され易い波長で、他の２波長は測定対象ガスに吸収され難い波長を選定する。また、式１において、消散係数βa のみが４波長で異なるものとする。そして、測定対象ガスに吸収され易い２波長での消散係数をそれぞれβon1 ，βon2 、また測定対象ガスに吸収され難い２波長での消散係数をそれぞれβoff1，βoff2とする。そして、測定対象ガスをｘ，誤差となるガスを０とすると、各４波長の消散係数は式３で表される。
【０００８】
【数３】

σi はそれぞれの波長に対する吸収断面積である。それぞれの波長での式１を記述して、よく吸収する２波長（on）での２式から、吸収しにくい２波長（off ）の２式を引くと、次の式４が導かれる。
【０００９】
【数４】

ここで、式５の
【数５】

であるような波長を選択すると、式４は次の式６
【数６】

となり、誤差となる測定対象ガス以外のガスの影響が含まれない式となる。
【００１０】
ここで、
【数７】

とすると、式６の項
【数８】

は、
【００１１】
【数９】

となる。
【００１２】
同様に、式６の項の
【数１０】

は、次の式１３に変換できる。
【数１１】

【００１３】
ただし、前述の式１１及び式１３は、次の式１４又は式１５を条件とする。
【数１２】

よって、式６はさらに次の式１６のように変形できる。
【数１３】

即ち、測定は、４波長すべてを計測する必要がなく、on（on1 ，on2 ）とoff （off1，off2）の２波長づつまとめて計測することができるので、２波長を用いたＤＩＡＬ計測法と同じ時間で計測することが可能である。
なお、前述の式１４と式１５は、式１６に示すガス濃度ｎ（Ｒ）の精度を高める条件を示すもので、式１４と式１５の左辺が小さくなるように、波長に対する照射光の強度を調節することにより、測定精度を向上させることが可能である。
【００１４】
【実施例】
測定対象ガスをＳＯ₂（二酸化硫黄）として、前記計算式１６に、図１に示す公知の特性データに基づく表１に示した数値（各波長におけるＳＯ₂の吸収断面積）と、表１のデータにもとづいて算出された表２に示した受信光子数を、式６および式１６に当てはめて計算した例を示す。
なお、エアロゾルの消散係数を、300nm で 2×10^-4ｍ^-1とし、エアロゾルによる光の消散がレイリー散乱によると仮定し、 1／λ⁴に比例するとする。
ＳＯ₂の密度は、図１に示した吸収断面積の特性、及び温度 15℃， 1atm ，単位 1ppb では、ｎ＝2.55×10¹⁶ｍ^-3であることにより、吸収の大きい波長を300.0nm （on1 ）と298.6nm （on2 ）、吸収の小さい波長を299.3nm （off1,off2）の３つの波長で計算を行う。
ここで、
【数１４】

と仮定する。（光出力10J 、受光望遠鏡の直径50cm、後方散乱係数 1.0×10^-5 m^-1光学系全効率0.01に相当）
また、測定距離Ｒを 3km、距離分解能ΔＲを100mとする。
【００１５】
【表１】

このときの受信される光子数を表２に示す。
【００１６】
【表２】

以上により、式１１の左辺と右辺はともに有効数字が３桁以内で 1.237となり、また式１３の左辺と右辺はともに有効数字が３桁以内で 0.808となり一致する。
この条件で、式６の従来の多波長によるＤＩＡＬ計測法で全４波長を計測し、密度を算出すると2.35×10¹⁶cm^-3となるが、本発明の方法で密度を算出した場合においても、その計測値は2.34×10¹⁶cm^-3となり、２桁の精度で正しく、誤差は無視できる程度のものである。
なお、この実施例では便宜上吸収され難い波長は１波長として示したが、前記の式５の条件等を満たす波長の異なる２波長を用いて、全４波長で計測できることは当然である。また、吸収され難い波長を２波長とし、吸収され易い波長を１波長として３波長を用いて計測してもよい。
また、式８に表２の計算データを入れてΔＮ_onを求めると、ΔＮ_on＝Ｎ_on1−Ｎ_on／2 ＝67.5となるが、298.6 nmの照射強度を300.0 nmのそれより、9843／9708＝1.0139倍にすることにより、Ｎ_on1とＮ_on2がほとんど等しくなり、ΔＮ_onはほとんど０となり、計算されるガス濃度ｎ（Ｒ）は、近似式を用いるために生じる誤差はなくなる。
【００１７】
【発明の効果】
以上詳細に説明したように、本発明は所要の条件に適合する異なる波長の３波長又は４波長を、測定対象ガスに吸収され易い波長と吸収され難い波長の２組に分けて順次に測定対象ガスに照射して，その複数又は単数の波長の反射信号をそれぞれ受信してそのデータに基づき演算処理するものであるため、特に測定対象ガス周辺の誤差となるガスによる光の吸収が大きい場合には、従来の２波長によるＤＩＡＬ計測法に比較して、測定精度の向上を図ることができる。また、従来の精度向上を図るための３波長以上の多波長によるＤＩＡＬ計測法に比較して、測定時間を短縮することができ、過渡現象の測定も可能となるなどの効果を奏するものである。
【図面の簡単な説明】
【図１】本発明の一実施例の測定対象ガスＳＯ₂の波長／吸収断面積の特性図である。
【図２】本発明の対象とするＤＩＡＬ計測方法の測定構成概略図である。
【符号の説明】
１多波長光照射装置
２多波長の光
３計測対象ガス
４受光装置
５データ解析装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for measuring the concentration of gas (atom, molecule, aerosol, etc.) in the atmosphere using multi-wavelength light.
[0002]
[Prior art]
As a method for measuring the gas concentration in the atmosphere, for example, as shown in the figure, the multi-wavelength light 2 is sequentially irradiated to the measurement target gas 3 from a single or a plurality of multi-wavelength light irradiation apparatuses 1 and the multi-wavelength light is irradiated. Is received by the light receiving device 4, and the concentration of the constituent components of the measurement target gas 3 is measured by the data analyzing device 5. A laser radar that is a conventional measurement method using this multi-wavelength light is a differential absorption laser radar that uses two wavelengths, a wavelength that is easily absorbed (largely absorbed) and a wavelength that is difficult to be absorbed (smallly absorbed). The (DIAL) measurement method is used. In order to further improve accuracy, there is a measurement method using three or more multiple wavelengths.
The number of received photons in the conventional DIAL measurement method using two wavelengths is expressed by Equation 1.
[0003]
[Expression 1]

Here, Nr is the number of received photons, Nt is the number of irradiated photons, η is the total efficiency of the optical system, ΔR is the distance resolution, βb is the backscattering coefficient, A is the receiving area, R is the measurement distance, and βa is the extinction coefficient.
This DIAL measurement method is expressed by describing Equation 1 for two wavelengths and calculating the difference between them. Therefore, when the gas concentration n (R) to be measured and the absorption cross-sectional area σ are expressed as βa = σn (R) and only σ is different for the two wavelengths, the following formula is used as a general DIAL measurement method. It is represented by the following formula 2.
[0004]
[Expression 2]

[sigma] on and [sigma] off are absorption cross sections for a wavelength having a large absorption and a wavelength having a small absorption, respectively.
[0005]
[Problems to be solved by the invention]
However, in the case of Equation 2, the precondition is limited to the case where the influence of absorption or scattering by a gas other than the measurement target gas can be considered to be the same for the two wavelengths. That is, this is true when the extinction coefficient βa at the two wavelengths is different only for the measurement target gas, and is not true when there is an influence of other gases. In many cases, it is difficult to select two wavelengths having the same extinction coefficient βa for gases other than the gas to be measured. In the DIAL measurement method using two wavelengths, the extinction coefficient (absorption cutoff) other than the gas to be measured is used. If the (area) is not equal, there is a problem that measurement accuracy is poor due to an error.
In addition, the measurement using three or more multi-wavelengths requires a number of measurements corresponding to the number of wavelengths used, and has a problem of requiring a long time for measurement.
[0006]
[Means for Solving the Problems]
The gas concentration measurement method using multi-wavelength light according to the present invention irradiates various measurement target gases in the atmosphere with light having a wavelength that is easily absorbed and light having a wavelength that is difficult to absorb, and backscattered light from the atmosphere of each irradiation light. In a gas concentration measurement method using a differential absorption laser radar (DIAL) measurement method in which (reflected light) is received by a telescope and measured, two wavelengths of different wavelengths that are easily absorbed differ from a measurement target gas and are difficult to be absorbed. 4 wavelengths so that the sum of the extinction coefficient for the two-wavelength measurement error target gas that is two wavelengths and is easily absorbed is equal to the sum of the extinction coefficient for the two-wavelength measurement error target gas that is difficult to absorb. Are selected, and the measurement target gas is irradiated with the two wavelengths of light that are easily absorbed as one set, and the two wavelengths of light that are not easily absorbed as one set. Receiving the reflected light, respectively, it is characterized in that to measure the gas concentration on the basis of the number of photons reflected light.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described using an example using four wavelengths. Of these four wavelengths, two wavelengths are wavelengths that are easily absorbed by the measurement target gas, and the other two wavelengths are wavelengths that are difficult to be absorbed by the measurement target gas. In Equation 1, only the extinction coefficient βa is different at four wavelengths. The extinction coefficients at two wavelengths that are easily absorbed by the measurement target gas are βon1 and βon2, respectively, and the extinction coefficients at the two wavelengths that are difficult to be absorbed by the measurement target gas are βoff1 and βoff2, respectively. Then, assuming that the measurement target gas is x and the error gas is 0, the extinction coefficients for each of the four wavelengths are expressed by Equation 3.
[0008]
[Equation 3]

σi is the absorption cross section for each wavelength. When Equation 1 at each wavelength is described, and Equation 2 at 2 wavelengths (off) that are difficult to absorb is subtracted from Equation 2 at 2 wavelengths (on) that absorbs well, Equation 4 below is derived.
[0009]
[Expression 4]

Here, Equation 5 in Equation 5

If we select a wavelength such that
[Formula 6]

Thus, the equation does not include the influence of gases other than the measurement target gas that are errors.
[0010]
here,
[Expression 7]

Then, the term of Equation 6

Is
[0011]
[Equation 9]

It becomes.
[0012]
Similarly, in the term of Equation 6,

Can be converted into the following equation (13).
[Expression 11]

[0013]
However, the above-mentioned formula 11 and formula 13 are conditional on the following formula 14 or formula 15.
[Expression 12]

Therefore, Expression 6 can be further transformed as Expression 16 below.
[Formula 13]

In other words, it is not necessary to measure all four wavelengths, and it is possible to measure two wavelengths, on (on1, on2) and off (off1, off2), so that the DIAL measurement method using two wavelengths It is possible to measure at the same time.
In addition, the above-mentioned formula 14 and formula 15 show the conditions for increasing the accuracy of the gas concentration n (R) shown in formula 16, and the intensity of the irradiation light with respect to the wavelength so that the left side of formula 14 and formula 15 becomes small. It is possible to improve measurement accuracy by adjusting.
[0014]
【Example】
The measurement target gas is SO ₂ (sulfur dioxide), and the numerical value shown in Table 1 (SO ₂ absorption cross section at each wavelength) based on the known characteristic data shown in FIG. An example is shown in which the number of received photons shown in Table 2 calculated based on the data is calculated by applying Equation 6 and Equation 16.
It is assumed that the aerosol extinction coefficient is 2 × 10 ⁻⁴

m

^{−1 at} 300 nm and that the light dissipation by the aerosol is due to Rayleigh scattering and is proportional to 1 / λ ⁴ .
The density of SO ₂ is the absorption cross section characteristic shown in FIG. 1, and the temperature is 15 ° C., 1 atm, and the unit is 1 ppb, so that n = 2.55 × 10 ¹⁶ m ^−3. On1) and 298.6nm (on2), and the wavelength with small absorption is calculated at three wavelengths of 299.3nm (off1, off2).
here,
[Expression 14]

Assume that (Equivalent to an optical output of 10J, a receiving telescope diameter of 50 cm, and a backscattering coefficient of 1.0 × 10 ^-5 m ^-1 optical system total efficiency of 0.01)
The measurement distance R is 3 km, and the distance resolution ΔR is 100 m.
[0015]
[Table 1]

Table 2 shows the number of photons received at this time.
[0016]
[Table 2]

From the above, the left and right sides of Equation 11 are 1.237 with 3 significant digits, and the left and right sides of Equation 13 are 0.808 with 3 significant digits.
Under this condition, when all four wavelengths are measured by the conventional multi-wavelength DIAL measurement method of Equation 6 and the density is calculated to be 2.35 × 10 ¹⁶ cm ⁻³ , even when the density is calculated by the method of the present invention. The measured value is 2.34 × 10 ¹⁶ cm ⁻³ , which is accurate with two digits of accuracy and the error is negligible.
In this embodiment, the wavelength that is difficult to be absorbed for convenience is shown as one wavelength, but it is natural that measurement can be performed with all four wavelengths using two different wavelengths satisfying the condition of the above-described formula 5 and the like. Alternatively, the measurement may be performed using three wavelengths, with two wavelengths that are difficult to be absorbed and one wavelength that is easily absorbed.
Further, when ΔN _on is calculated by putting the calculation data of Table 2 into Equation 8, ΔN _on = N _on1 −N _on /2=67.5, but the irradiation intensity at 298.6 nm is 9843/9708 than that at 300.0 nm. = by the 1.0139-fold, most equalizes the n _on1 and n _on2, .DELTA.N _on most zero, calculated as gas concentration n (R), the error will not occur in order to use an approximate expression.
[0017]
【The invention's effect】
As described above in detail, the present invention divides three or four different wavelengths that meet the required conditions into two sets of wavelengths that are easily absorbed by the gas to be measured and wavelengths that are difficult to be absorbed. Since the gas is irradiated and the reflected signals of the plurality or single wavelengths are received and processed based on the data, especially when the light absorption by the gas that causes an error around the measurement target gas is large. Compared with the conventional DIAL measurement method using two wavelengths, the measurement accuracy can be improved. In addition, the measurement time can be shortened and transient phenomena can be measured as compared with the conventional DIAL measurement method using three or more wavelengths for improving accuracy. .
[Brief description of the drawings]
FIG. 1 is a characteristic diagram of wavelength / absorption cross section of a measurement object gas SO _{2 according to} one embodiment of the present invention.
FIG. 2 is a schematic diagram of a measurement configuration of a DIAL measurement method that is an object of the present invention.
[Explanation of symbols]
1 Multi-wavelength light irradiation device 2 Multi-wavelength light 3 Gas to be measured 4 Light-receiving device 5 Data analysis device

Claims

The difference between the measurement target gas in the atmosphere, which is irradiated with light with a wavelength that is easily absorbed and light with a wavelength that is difficult to absorb, and the backscattered light (reflected light) from the atmosphere of each irradiated light is received by a telescope and measured. In gas concentration measurement method using absorption laser radar (DIAL) measurement method,
For the measurement target gas, two wavelengths of different wavelengths that are easily absorbed and two wavelengths of different wavelengths that are difficult to be absorbed, and the sum of the extinction coefficients for the measurement error target gases of the two wavelengths that are easily absorbed and the absorption. 4 wavelengths are selected so that the sum of the extinction coefficients for the difficult measurement error target gases of 2 wavelengths are equal.
Irradiate the measurement object gas with a set of two wavelengths of light that are easily absorbed and a set of two wavelengths of light that are difficult to absorb,
A gas concentration measurement method using multi-wavelength light, which receives each of the two sets of reflected light irradiated and measures the gas concentration based on the number of photons of the reflected light.

Either one of the two wavelengths of the different wavelengths that are easily absorbed or the two wavelengths of the different wavelengths that are difficult to be absorbed is defined as one wavelength, and is processed as data for two wavelengths by doubling when calculating the data of the wavelength. The gas concentration measuring method using multiwavelength light according to claim 1.

2. The gas concentration measurement method using multi-wavelength light according to claim 1, wherein data analysis processing is performed with the wavelength that is easily absorbed and the wavelength that is not easily absorbed as three or more different wavelengths.

4. The multi-wavelength light according to claim 1, wherein the irradiation intensity of each wavelength of the multi-wavelength that is easily absorbed or multi-wavelength that is difficult to be absorbed is adjusted based on a ratio of the number of received photons of each wavelength. Gas concentration measurement method.