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

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 CO 2 and H 2 O 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 2 2 2 2  appears to be about one order of magnitude stronger than the absorption coefficient 2 2 CO CO C  in pure carbon dioxide. The continuum interpretation and the main problem of molecular band shape formation are discussed in light of these experimental facts.

The CO2–broadened H2O continuum in the 100–1500 cm-1 region: Measurements, predictions and empirical model

“…The results in Fig. 2 call for several remarks: (i) The first is that the various experimental determinations of the continuum are consistent if uncertainties are taken into account as shown by the present measurements and those of [6] around 1300 cm -1 . The second is that calculations using the line shape correction factors c of [5] lead, on overall, to very good results if one considers that it was predicted nearly three decades ago without use of any experimental information and that the continuum varies by three orders of magnitude within the considered spectral region.…”

“…All available experimental values of the H2O-CO2 continuum are displayed in Fig. 2 together with predictions made at 296 K despite the fact that the measurements of [6] and…”

“…The recorded data are analyzed in order to retrieve values of the CO2-broadened H2O continuum at some wave numbers between 100 and 1500 cm -1 . The results are compared with previous measurements [6] and direct calculations based on H2O line-shape correction factors c predicted [5] using a quasistatic model. For better agreement between measured and computed continua, empirical corrections are made to the c factors.…”

“…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.…”

Indirect Influence of Humidity on Atmospheric Spectra Near 4 μm

“…The errors in computed spectra introduced by assuming purely dry air broadenings and shiftings are thus small (≤2%) and hardly detectable with most Earth observation instruments. However, as shown by laboratory measurements (Baranov, 2011(Baranov, , 2016Baranov et al, 2012;Tran et al, 2018) and by calculations (Hartmann et al, 2018), there are two spectral features, both near 4 μm, where water vapor considerably enhances the absorption: The high frequency wing of the CO 2 ν 3 band and the fundamental collision-induced absorption (CIA) band of N 2 . With respect to dry air, H 2 O can here increase the absorption by more than one order of magnitude (see the supporting information file).…”