Temperature dependence of the water vapor self‐continuum by cavity ring‐down spectroscopy in the 1.6 µm transparency window

Abstract

The very weak water vapor self-continuum has been investigated by high-sensitivity cavity ring-down spectroscopy in the 1.6 μm window at five temperatures between 302 K and 340 K. The absorption cross sections, C s (ν, T), were retrieved for 10 selected wave numbers from a fit of the absorption coefficients measured in real time during pressure ramps, after subtraction of the contributions of the local water monomer lines and of water adsorbed on the cell mirrors. The values measured between 5875 and 6665 cm À1 range between 1.5 × 10 À25 and 2 × 10 À24 cm 2 mol À1 atm À1 with a minimum around 6300 cm À1 .At 302 K, an agreement within 50% is observed over the whole window with the cross sections provided by the MT_CKD V2.5 model. Nevertheless, while our measurements show that the C s (ν, T) decrease from 302 K to 340 K is no more than 50% for all our selected wave numbers, the MT_CKD V2.5 model predicts a much more pronounced temperature dependence in the center of the window, the agreement being better on the edges of the window. The obtained results are discussed in relation with theoretical modeling of the water vapor self-continuum as far wings of monomer lines or water dimer absorption. For potential atmospheric applications, cross sections are provided at each temperature with a sampling step of 10 cm À1for the entire 5850-6700 cm À1 range.

The self- and foreign-absorption continua of water vapor by cavity ring-down spectroscopy near 2.35 μm

“…The present work near 4250 cm -1 , together with our previous CRDS results at room temperature in the 1.6 µm window [18,19], are consistent with the MT_CKD 2.5 self-continuum values. This agreement provides a validation of this standard model widely used to implement the water continuum absorption in atmospheric radiative transfer codes.…”

“…Interestingly, the CAVIAR high temperature C S values extrapolated linearly to room temperature coincide much better to our room temperature value than to CAVIAR 293 K value. We noted the same kind of situation in the edges of the 1.6 µm window (see Figure 8 of Ref [19]). This may be due to the fact that optical depth of the high temperature measurements are much larger than those at room temperature, because of the larger amount of water vapor possible in the cell allowing for more accurate FTS measurements.…”

“…We nevertheless note that, in spite of their large error bars, the Bicknell's C S values at 4605 cm -1 are in disagreement with both the MT_CKD model and the data of Refs [3,17]. [13] (black solid line) and measured in Refs [3] (black circles), [17] (green triangles), [19] (orange stars), [14] (blue diamond's) and in this work (red stars). Note the logarithmic scale adopted for the ordinate axis.…”

“…[3] and [17], respectively, but are consistent within the 63 and 42 % error bars of the FTS measurements, respectively. These data together with our previous CRDS measurements in the 1.6 µm window [19] are gathered in Figure 7. A much better agreement is observed with the MT_CKD model with a difference less than 20%, our values being systematically higher.…”

“…The disagreement between various measurement data-sets motivated our investigation of the water self continuum in the 1.6 µm window using an alternative and more sensitive experimental approach [18,19]. Compared to FTS which requires longer acquisition times, laser techniques such as Cavity Ring Down Spectroscopy (CRDS) have the advantage of providing real time measurements at a chosen wavelength when experimental parameters (pressure, gas composition...) are varied.…”