On the significant enhancement of the continuum-collision induced absorption in H2O+CO2 mixtures

doi:10.1016/j.jqsrt.2016.02.017

Journal of Quantitative Spectroscopy and Radiative Transfer

Volume 175, May 2016, Pages 100-106

https://doi.org/10.1016/j.jqsrt.2016.02.017 Get rights and content

Highlights

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Infrared spectra of H₂O+CO₂ mixtures were recorded and processed.
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The binary absorption coefficients for mixed gases were obtained.
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The results show significantly enhanced absorptions in the regions around 9 and 4 μm.
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The data are discussed in regard a molecular band shape formation.

Abstract

The IR spectra of water vapor–carbon dioxide mixtures as well as the spectra of pure gas samples have been recorded using a Fourier-transform infrared spectrometer at a resolution of 0.1 cm⁻¹ in order to explore the effect of colliding CO₂ and H₂O molecules on their continuum absorptions. The sample temperatures were 294, 311, 325 and 339 K. Measurements have been conducted at several different water vapor partial pressures depending on the cell temperature. Carbon dioxide pressures were kept close to the three values of 103, 207 and 311 kPa (1.02, 2.04 and 3.07 atm). The path length used in the study was 100 m. It was established that, in the region around 1100 cm⁻¹, the continuum absorption coefficient $C_{H_{2} O + C O_{2}}$ is about 20 times stronger than the water–nitrogen continuum absorption coefficient $C_{H_{2} O + N_{2}}$ . On the other hand, in the far wing region (2500 cm⁻¹) of the ν₃ CO₂ fundamental band, the binary absorption coefficient $C_{C O_{2} + H_{2} O}$ appears to be about one order of magnitude stronger than the absorption coefficient $C_{C O_{2} + C O_{2}}$ in pure carbon dioxide. The continuum interpretation and the main problem of molecular band shape formation are discussed in light of these experimental facts.

Introduction

The water-vapor continuum is a significant contributor to the total absorption of IR-radiation in atmospheric windows, and plays an important role in the radiative-transfer processes of the Earth׳s atmosphere [1]. However, this absorption is still not well studied and understood [2], [3]. Carbon dioxide weak continuum and collision induced absorptions in window and micro-window regions are also important in many applications such as Earth remote sensing or radiative transfer in planetary atmospheres [4], [5].

There is the additional motivation for the presented study given below. Controversial explanations of the continuum as absorption by “far line wings” or absorption by water dimers are the subject of discussion during many past decades. Recently the water-vapor continuum alternate interpretation in terms of collision induced absorption (CIA) processes [3] has been proposed and discussed.

It is well known that in binary mixture of gases “a” and “b” (oxygen and nitrogen, for example) the collision induced absorption coefficient K_CIA(ν,Θ) can be expressed as: $K_{C I A} (ν, Θ) = - \ln T (ν, Θ) L^{- 1} = C_{a} (ν, Θ) ρ_{a}^{2} + [C_{a b} (ν, Θ) + C_{b a} (ν, Θ)] ρ_{a} ρ_{b} + C_{b} (ν, Θ) ρ_{b}^{2},$ where ν is the wavenumber, Θ is the temperature, T is the transmittance, L is the path length, $ρ_{a}$ and $ρ_{b}$ are the partial densities of the two gases. C_a and C_b are the binary absorption coefficients of the pure gases whereas C_ab and C_ba stand for the mutual influence of gases on absorption in regions of their CIA bands. The first letter in the subscripts denotes an absorbing molecule in a colliding pair.

The idea of this current research is based on two well-established experimental facts. The first one is the significant absorption enhancement in the oxygen fundamental collision-induced band after the addition of carbon dioxide to a sample of pure O₂ [6]. The absolute integrated band intensity $S_{O_{2} + O_{2}}^{1 \leftarrow 0} (Θ) = \int_{1175}^{1925} C_{O_{2} + C O_{2}} (v, Θ) d v$ was found to be 5 times stronger than the band intensity $S_{O_{2} + O_{2}}^{1 \leftarrow 0}$ in pure oxygen at Θ=270 K. A second similar fact is that in a N₂+H₂O mixture the band integrated intensity $S_{N_{2} + H_{2} O}^{1 \leftarrow 0}$ [7] is about 14 times stronger than the intensity $S_{N_{2} + N_{2}}^{1 \leftarrow 0}$ in pure nitrogen [8]. Because of these facts and the proposed continuum interpretation [3], one can expect that there will be a significant influence of carbon dioxide on the water vapor continuum absorption. Mixed H₂O+CO₂ spectra at comparatively high gas densities and long optical paths will be saturated in many broad spectral regions. Nevertheless, micro- and macro-windows of the water vapor spectrum in the regions around 1150 cm⁻¹ and 2700 cm⁻¹, where carbon dioxide absorption is weak, are expected to be suitable for measurements of the continuum enhancement due to the presence of CO₂.

On the other hand, the region beyond the ν₃ CO₂ band “head” (2400 cm⁻¹≤ν≤2600 cm⁻¹) is perfect for measurements of additional absorption induced by collisions of carbon dioxide and water molecules. The nature of this absorption is still not clear despite numerous experimental and theoretical studies. Usually it was interpreted in terms of “far wings of lines” (or “mixed lines”) composing the band. This interpretation contradicts the fact that the ν₃ CO₂ weak collision-induced “sub-band” exists as its natural component and this component should not be related to lines of allowed transitions [3]. Thus it was interesting to explore the influence of water vapor on weak continuous absorption in carbon dioxide. The expected experimental results could be significant in a discussion on the continuum origin and on physical mechanisms of molecular band profile formation.

It should be noted that the contribution by local permitted lines to the measured transmittance values may be significant in many micro-windows. Of course, this contribution must be estimated and subtracted from the experimental results in the same way as it was in our previous works [3], [9].

Section snippets

Experimental

Measurements were made at NIST using a BOMEM DA3−002 FTIR spectrometer² with a 2 m base-length multi-pass cell, which has been described

Results

Fig. 1 shows an example of the spectra of pure water vapor (1), pure CO₂ (2) and a mixture of both species (3) over two spectral regions around 1200 cm⁻¹ and in the vicinity of 2550 cm⁻¹.

The upper panel of Fig. 1 clearly shows that the presence of CO₂ significantly increases the absorption in the micro windows between 1100 and 1200 cm⁻¹ where the absorption from CO₂ is quite small. But a far more interesting fact is observed in the lower panel where significant enhancement of the absorption of the

Analysis and discussion

Fig. 4 shows the spectral behavior of the water–carbon dioxide continuum in the 1200 cm⁻¹ region. This graph presents the binary absorption coefficients, averaged over all four temperatures since there was no clear and distinct temperature dependence detected, as is seen in the insert. An additional reason for averaging was a desire to get more realistic and representative spectral behavior of the absorption coefficients. These results agree with our expectations. The binary absorption

Conclusions

The data presented in this study confirm our expectations of significant mutual enhancement of continuous absorption in H₂O+CO₂ mixtures. The effect of carbon dioxide on the water-vapor continuum in the micro- and macro-windows in 1200 and 2800 cm⁻¹ regions is 20 and 15 times stronger than that produced by nitrogen. Similarly, we found a significant influence of water vapor on absorption in the high-frequency far wing of the allowed ν₃ CO₂ fundamental band. The binary absorption coefficients $C_{c o_{2}}$

Acknowledgments

The author acknowledges a support from the Upper Atmospheric Research Program of NASA, and partial financial support from the Russian Foundation for Basic Researches through Grants 15-05-00736 and 13-05-00751. He would like to thank the NIST Physics Laboratory Management for the opportunity to carry out the reported research. The author acknowledges Dr. W.J. Lafferty for revision of the manuscript and for many suggestions and corrections. The author also addresses many special thanks to Dr.

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Citation Excerpt :
The first is a purely theoretical study in which direct predictions were made using a quasistatic approach [5] but could not be tested due to the lack, at that time, of any experimental results. In the second, much more recent, measurements were made around 1300 cm-1 [6] but no comparison with predictions of the former model was made. We here present the results of measurements of the transmission by H2O+CO2 mixtures made using two different setups altogether covering the spectral range from 40 to 4800 cm-1.
Transmission spectra of H₂O+CO₂ mixtures have been recorded, at 296, 325 and 366 K, for various pressures and mixture compositions using two experimental setups. Their analysis enables to retrieve values of the “continuum” absorption by the CO₂-broadened H₂O line wings between 100 and 1500 cm^-1. The results are in good agreement with those, around 1300 cm^-1, of the single previous experimental study available. Comparisons are also made with direct predictions based on line-shape correction factors χ calculated, almost thirty years ago, using a quasistatic approach and an input H₂OCO₂ intermolecular potential. They show that this model quite nicely predicts, with slightly overestimated values, the continuum over a spectral range where it varies by more than three orders of magnitude. An empirical correction is proposed, based on the experimental data, which should be useful for radiative transfer and climate studies in CO₂ rich planetary atmospheres.

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¹: Current address: Institute of Experimental Meteorology, SPA “Typhoon”, 4 Pobedy St., Obninsk, Kaluga Reg., 249020, Russia.

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On the significant enhancement of the continuum-collision induced absorption in H2O+CO2 mixtures

Highlights

Abstract

Introduction

Section snippets

Experimental

Results

Analysis and discussion

Conclusions

Acknowledgments

J Quant Spectrosc Radiat Transf

J Quant Spectrosc Radiat Transf

Icarus

J Mol Spectrosc

J Mol Spectrosc

J Quant Spectr Radiat Transf

J Quant Spectrosc Radiat Transf

J Quant Spectrosc Radiat Transf

J Mol Spectrosc

J Quant Spectrosc Radiat Transf

J Quant Spectrosc Radiat Transf

On the significant enhancement of the continuum-collision induced absorption in H₂O+CO₂ mixtures