RU2510054C1 - Method of determining vertical profile of concentration of gases in atmosphere - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method involves emitting a set of electromagnetic waves at different frequencies in the vicinity of a line of absorption of a measured gas; detecting radiation passing through the atmosphere using a receiver; measuring overall attenuation of radiation that has passed through the atmosphere at the emitted frequencies; comparing values of the measured radiation attenuation with design values of overall radiation attenuation obtained based on a priori or standard data on vertical profiles of temperature, atmospheric pressure and concentration of the measured gas; wherein to obtain gas concentration values at a given height, measurements are taken at two pairs of frequencies lying at different slopes of the line of absorption of the measured gas which corresponds to the given height. A linear combination of attenuations at different frequencies is used.

EFFECT: high measurement accuracy.

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Description

The invention relates to remote measurement of the concentration of various gases in the atmosphere by electromagnetic transillumination of the atmosphere in the absorption band of the measured gas, for example, measuring the altitude profile of water vapor from radio transmission from the satellite to the Earth's surface in the water vapor absorption band.

A known method for determining the profile of ozone concentration based on measuring the shape of the line of absorption of ozone during registration from the satellite of the radiation of the Sun during sunset or sunrise [1].

The disadvantage of this method is the low measurement accuracy due to the assumption of a model of a layered uniform distribution of gas concentration, which is not always the case. Another disadvantage is the averaging over a large horizontal region.

Another way is to register microwave radiation in the absorption line of a gas and restore the vertical concentration profile of a given gas by the difference in brightness temperature at different frequencies in the vicinity of the absorption line [2].

The disadvantage of this method is the low accuracy of the measurements due to the strong change (3-4 orders of magnitude) of the measured value with height, which leads to the masking of even strong changes in concentration at high altitudes against the background of small relative changes in concentration at low altitudes.

Another analogue is the radiometric spectral method for measuring humidity in the stratosphere [3]. At the same time, a radiometer is installed on the ground or on the satellite, operating at different frequencies in the vicinity of the absorption line of the measured gas. The radio brightness temperature is measured, which is sensitive to the profile of the measured gas, it is compared with calculated values obtained on the basis of a priori or standard data on the vertical profiles of temperature, atmospheric pressure and concentration of the measured gas, and the desired profile of the measured gas is calculated based on the differences between these values.

However, this method has low accuracy, because it does not allow to take into account the influence of clouds and precipitation on the measurement results without accurate data on the properties of clouds and precipitation.

The closest analogue is the satellite spectral method for measuring humidity in the stratosphere based on the Doppler effect [4]. In this case, a microwave emitter is installed on the satellite, operating in the vicinity of the absorption line of the measured gas at one or two frequencies, and on the surface of the Earth, a receiver that records the total absorption of radiation transmitted through the atmosphere. The change in radiation frequencies occurs due to the Doppler effect caused by the movement of the satellite. In this case, the total absorption of the atmosphere at various frequencies is measured and compared.

However, this method also has low accuracy, since it does not allow taking into account the influence of clouds and precipitation on the measurement results without accurate data on the properties of clouds and precipitation.

The technical result of the proposed method is to increase the accuracy of measurements and a sharp decrease in the influence of other gases, clouds and precipitation due to the fact that to obtain a gas concentration at a given height, measurements are carried out on two pairs of frequencies located on different slopes of the absorption line of the measured gas, which corresponds to a given altitude, and use a linear combination of attenuation at these frequencies.

Qualitatively higher measurement accuracy arises when using not extra-atmospheric radiation sources (for example, the Sun), but stable radiation sources installed on the satellite, or when the atmosphere is illuminated by reflecting from the satellite a set of electromagnetic waves emitted from the Earth and registering the radiation twice transmitted through the atmosphere by the receiver on the ground.

On figa presents a diagram of atmospheric radio transmission from the satellite to the Earth, in which a transmitter emitting a given set of frequencies is installed on satellite 1, and a receiving antenna 2 is located on the Earth. The satellite’s orbit height is indicated by N. Figure 1b shows another atmospheric radio transmission scheme the Earth-satellite-Earth path, in which a given set of frequencies is emitted by the receiver-transmitter 2 from the Earth towards the satellite 1, on which the reflector is mounted, part of the radiation is reflected in the opposite direction and is registered by the same transmitter-receiver 2.

Figure 2 shows a qualitative view of the frequency dependence of attenuation of radiation by atmospheric gases α _st (H, v) for the upper, middle and lower layers of the atmosphere, curves 1, 2 and 3, respectively, as well as the frequency dependence of the attenuation of clouds and precipitation - curve 4. Graphs performed on a semi-logarithmic scale. If we conditionally divide the entire atmosphere into three layers plus clouds and precipitation, then we can assume that the total differential absorption of the entire atmosphere along the radiation propagation path is the sum of the difference in the absorption coefficients at frequencies vl, v2 in all four graphs. Figure 2 (together with the example in figure 3) qualitatively explain how a linear combination of two differential absorption [α _article (H, v2) -α _article (H, v1)] + C · [α _article (H, v3) -α _st (H, v4)] on different slopes of the absorption line of the measured gas, it is possible to sharply reduce the contribution of the lower layers of the atmosphere, to distinguish the contribution of the desired middle layer, and also to subtract the effect of clouds and precipitation. The constant C is selected based on the condition of the best compensation for the contribution of the lower layers. The graphs show that for the upper layer, curve 1, the difference in the attenuation coefficients α _st (H, v2) -α _st (H, v1) is small, therefore, this layer makes a small contribution to the total differential attenuation. The greatest difference in attenuation occurs for the desired middle layer (curve 2), and the contributions of the lower layer 3 and the cloud layer 4 are subtracted by adding the differential attenuations on different slopes of the line.

Figure 3 calculates the curves for the differential absorption of the standard atmosphere in the vicinity of the 22.235 GHz water vapor absorption band, and shows an example of how using a linear combination of two frequency pairs on different slopes of the absorption line can drastically reduce the effect of the lower layers on the nuclei obtained for the upper atmosphere . Curve 1 is the differential absorption [α _st (H, v2) -α _st (H, v1)] at frequencies v1 = 22.221 GHz, v2 = 22.23 GHz. This curve shows a significant contribution of the lower layers to the area under the curve. Curve 2 is the differential absorption [α _st (H, v3) -α _st (H, v4)] on the other slope of the line at v3 = 22.24 and v4 = 22.249. The contribution of the lower layers has the opposite sign. Curve 3 - a linear combination of the first and second graph almost doubles the contribution in the desired height and mutually subtracts the contributions of the lower gas layer. The contributions of clouds and precipitation are deducted in the same way. As a result, we get a clear maximum at the required height.

An example of determining the gas concentration profile up to a height of 90 km is given for water vapor in the absorption band of 22.235 GHz. If monochromatic radiation with known intensities I ₀₁ and I _{02 is} emitted from a satellite at a given pair of frequencies v1 and v2, then signals with intensities I ₁ and I ₂ will be recorded at a receiver installed on the Earth’s surface in accordance with the formulas

$$I_{\mathrm{one}}=I_{01}\exp \left(-{\displaystyle \int \alpha \left(r,v\mathrm{one}\right)dr}\right),{\mathrm{<figure-callout\; id="2"\; label="I"\; filenames="00000009.png,00000010.png"\; state="\{\{state\}\}">I</figure-callout>}}_2=I_{02}\exp \left(-{\displaystyle \int \alpha \left(r,v2\right)dr}\right),(\mathrm{one})$$

where α (r, v1,) and α (r, v2) are the profiles of the linear attenuation coefficient of the signals along the radiation propagation path at frequencies v1 and v2, respectively, and the integration is carried out along the entire length of the path. The quantities I ₁ and I _{2 are} recorded by which the total atmospheric attenuation τ (v1), τ (v2) is calculated, which, due to the relatively weak scattering at these frequencies, is determined by the profile of the linear radiation absorption coefficient α (r, v) at given frequencies vl and v2

$$\tau \left(v\mathrm{one}\right)=\ln \left(I_{01}/I_{\mathrm{one}}\right)={\displaystyle \int \alpha \left(r,v\mathrm{one}\right)dr}\tau \left(v2\right)=\ln \left(I_{02}/I_2\right)={\displaystyle \int \alpha \left(r,v2\right)dr}(2)$$

When registering the differential transmission of the atmosphere at two frequencies, the difference Δτ (v1, v2) is recorded, which is equal to the logarithm of the ratio of the signal intensities I ₁ and I ₂ at the corresponding frequencies v1 and v2 (for the same values of the radiated intensity I ₀₁ and I ₀₂ )

$$\Delta \tau \left(v\mathrm{one,}v2\right)={\displaystyle \int \left[\alpha \left(r,v2\right)-\alpha \left(r,v\mathrm{one}\right)\right]dr=\ln \left(I_{\mathrm{one}}/I_2\right)}(3)$$

Since the linear absorption coefficient α (r, v) is proportional to the concentration of the desired gas N (r) [2], it is convenient to express α (r, v) as the product of N (r) and some factor α (r, v), which is concentration N (r) is independent, but depends on the temperature profiles T (r) and atmospheric pressure P (g). That is, α (r, v) = α (r, v) N (r). In this case, relation (3) can be expressed more clearly:

$$\Delta \tau \left(v\mathrm{one,}v2\right)={\displaystyle \int \left[a\left(r,v2\right)-\alpha \left(r,v\mathrm{one}\right)\right]N\left(r\right)=\ln \left(I_{\mathrm{one}}/I_2\right)}(\mathrm{four})$$

The resulting equation is a Fredholm integral equation of the first kind, in which the unknown quantity is the concentration of the desired gas N (r), and the expression in parentheses is the weight function or core of the integral equation. The disadvantage of this approach is that the concentration of the desired gas along the measurement path usually changes by 3-4 orders of magnitude. Therefore, the apparent decrease in the weight function at low altitude does not say anything about the quality of the restoration, since the contribution of this function at these heights should be multiplied by a value of the order of 10 000z due to the high gas concentration at lower heights.

It is more productive to use a priori (standard, previously accumulated or known from independent measurements) data on the temperature profiles T (r), atmospheric pressure P (r) and the concentration of the measured gas N _st (r), and to choose the relative value of the measured gas concentration from a priori or standard profile, namely, the quantity n (r) = [N (r) -N _st (r)] / N _st (r). This value along the measurement path varies by a value comparable to unity, therefore, the integrated cores in the proposed method will clearly demonstrate the sensitivity of the method to changes in the concentration of the desired gas in the atmosphere. In addition, a priori data more accurately calculate the coefficients in the integral equations, which increases the accuracy of solving the problem.

For a standard atmosphere, equation (4) takes the form:

$$\Delta {\tau }_{\mathrm{from}t}\left(v\mathrm{one,}v2\right)={\displaystyle \int \left[a\left(r,v2\right)-a\left(r,v\mathrm{one}\right)\right]}{N}_{\mathrm{from}t}\left(r\right)dr,(5)$$

From equations (4) and (5), it is easy to obtain an expression for the deviation of the measured quantity Δτ (v1, v2) from the standard Δτ _article (v1, v2). We denote this value by Dif τ (v1, v2)

Difτ (v1, v2) = Δτ (v1, v2) -Δτ _article (v1, v2) = ∫ [a (r, v2) -a (r, v1)] N _article (r)] [N (r) - N _st (r)] / N _st (r) dr,

Considering that n (r) = [N (r) -N _st (r)] / N _st (r) is the relative deviation of the concentration of the measured gas from the standard profile, we obtain the integral equation for measuring the value of n (r):

$$Dif\tau \left(v\mathrm{one,}v2\right)={\displaystyle \int \left[{\alpha }_{\mathrm{from}t}\left(r,v2\right)-{\alpha }_{\mathrm{from}t}\left(r,v\mathrm{one}\right)\right]n\left(r\right)dr,(6)}$$

where the values of α _st (r, v2) = a (r, v2) N _st (r) and α _st (r, v1)) = a (r, v1)] N _st (r) are the dependences of the linear gas absorption coefficient for standard concentration profile along the path at frequencies v2 and v1, respectively. The weight function or kernel of the integral equation in this case is the quantity W (r, v1, v2) = α _st (r, v2) -α _st (r, v1).

To obtain Difτ (v1, v2) through the measured values of the intensity of the transmitted radiation, the ratio is used:

$$Dif\tau \left(v\mathrm{one,}v2\right)=\ln \left[I_{\mathrm{one}}\left(v\mathrm{one}/I_2\left(v2\right)\right)\right]-\ln {\left(I_{\mathrm{one}}\left(v\mathrm{one}\right)/I_2\left(v2\right)\right)}_{\mathrm{from}t}(7)$$

where I ₁ (v1) / I ₂ (v2) is the ratio of the measured intensities of the transmitted radiation at the receiver, provided that the emitted intensities of the transmitter are equal to I ₁₀ (v1) / I ₂₀ (v2), and the quantity (I ₁ (v1) / I ₂ (v2)) _st is the calculated ratio of intensities for the standard profile of the measured gas. Using a priori data on the temperature profiles T (r), atmospheric pressure P (r) and the concentration of the measured gas N _st (r) allows you to more accurately calculate the coefficients in the integral equations, which increases the accuracy of solving the problem.

Using a linear combination of differential absorption on the two slopes of the absorption line can improve the accuracy of measurements, improve the shape of the nuclei (weight functions) and, most importantly, dramatically reduce the influence of clouds, precipitation and other gases. Consider this method in more detail on the example of the absorption line of water vapor γ = 22.235 GHz.

A specific example of the efficiency of using a linear combination of differential absorption is explained in Figure 3. If one pair of frequencies v1, v2 is used on the low-frequency layer of the absorption line (see also figure 2), then the kernel of the integral equation (6) has the form shown by curve 1 in figure 3. According to the shape of this core, in addition to the maximum at an altitude of 41 kilometers, the integrand weight function has a high sensitivity to water vapor and near the ground, in a layer of 0 ... 5 kilometers. However, a similar pair of frequencies v3, v4 on the other slope of the absorption line (see figure 2) forms almost the same maximum in the core at 41 kilometers, but in the layer 0 ... 5 kilometers the core becomes negative (curve 2 in figure 4) repeating curve 1 in almost symmetrical form. A linear combination of differential absorption, consisting of Difτ (v1, v2) + 0.95 · Difτ (v3, v4), gives the resulting core, represented by curve 3 in figure 4. As you can see, this core has clearly pronounced maximum sensitivity at 41 kilometers and almost does not feel the contribution of the lower atmosphere. In a similar way, the influence of any other mechanisms of attenuation of radiation (scattering and absorption by clouds, precipitation, other gases) is subtracted if the spectral characteristic of these attenuation mechanisms does not have a resonant shape on the frequency range used (v1 ... v4) but changes smoothly on the used interval.

From the magnitude of the change in concentration relative to the a priori profile, it is not difficult to calculate the profile of the absolute concentration of water vapor at various heights:

N (r) = n (r) N _article (r) + N _article (r)].

The inventive step of the proposed technical solution is confirmed by the distinctive part of the claims.

Literature

1. Krueger A.J., Guenther B., Fleig A.J. et al. Satellite ozone measurements // Phil. Trans. Roy. Soc. London - 1980. - V. A296. - No. 1. - P. 191-204.

2. Gorelik A.G., Knyazev L.V., Prozorovsky A.Yu. Ultimate sensitivity of spectrometric measurements of humidity in the stratosphere and mesosphere in the absorption line of water vapor λ = 1.35 cm. Proceedings of the All-Union Symposium on radiophysical methods for studying the atmosphere. L-d Hydrometeoizdat. 1977, p. 223-228.

3. Rodgers C.D. Inverse Methods for Atmospheric Soundings: Theory and Practice. 2000.253 pp.

4. Khachatryan J.B. Satellite spectrometric method for determining humidity in the stratosphere using the Doppler effect. Zeitschrift fur Meteorologie, Band 38, Heft 4. (1988). 206-211.

Claims (3)

1. A method for determining the vertical profile of gas concentration in the atmosphere, based on the emission of a set of electromagnetic waves of different frequencies in the vicinity of the absorption line of the measured gas, registration of the radiation transmitted through the atmosphere by the receiver, measurement of the total attenuation of radiation transmitted through the atmosphere at the radiated frequencies, and its comparison with the calculated values of the total attenuation of radiation obtained on the basis of a priori or standard data on vertical profiles of temperature, atmospheric pressure and concentration from measured gas, characterized in that to obtain a gas concentration at a given height, the measurements are carried out on two pairs of frequencies located on different slopes of the absorption line of the measured gas, which corresponds to a given height, and use a linear combination of attenuation at these frequencies. 2. The method according to claim 1, characterized in that the radiation of a set of electromagnetic waves of different frequencies is produced from a satellite, and the radiation transmitted through the atmosphere is recorded by a receiver on Earth. 3. The method according to claim 1, characterized in that the radiation of a set of electromagnetic waves of different frequencies is formed by reflection from a satellite of a set of electromagnetic waves emitted from the Earth, and the radiation transmitted through the atmosphere is recorded by a receiver on Earth.

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