A Comparison of Water Vapor Line Parameters for Modeling the Venus Deep Atmosphere

TL;DR

This paper compares various water vapor spectral line databases to improve the accuracy of water abundance measurements in Venus's deep atmosphere, highlighting discrepancies and recommending the most reliable line list.

Contribution

It provides a detailed comparison of H2O line lists for Venus atmosphere modeling and recommends the BT2 list as the most accurate for high-temperature conditions.

Findings

Discrepancies exist between different H2O line lists.

Correcting line intensities reduces estimated water vapor abundance.

BT2 line list is the most complete and accurate for Venus conditions.

Abstract

The discovery of the near infrared windows into the Venus deep atmosphere has enabled the use of remote sensing techniques to study the composition of the Venus atmosphere below the clouds. In particular, water vapor absorption lines can be observed in a number of the near-infrared windows allowing measurement of the H2O abundance at several different levels in the lower atmosphere. Accurate determination of the abundance requires a good database of spectral line parameters for the H2O absorption lines at the high temperatures (up to ~700 K) encountered in the Venus deep atmosphere. This paper presents a comparison of a number of H2O line lists that have been, or that could potentially be used, to analyze Venus deep atmosphere water abundances and shows that there are substantial discrepancies between them. For example, the early high-temperature list used by Meadows and Crisp (1996) had large systematic errors in line intensities. When these are corrected for using the more recent high-temperature BT2 list of Barber et al. (2006) their value of 45+/-10 ppm for the water vapor mixing ratio reduces to 27+/-6 ppm. The HITRAN and GEISA lists used for most other studies of Venus are deficient in "hot" lines that become important in the Venus deep atmosphere and also show evidence of systematic errors in line intensities, particularly for the 8000 to 9500 cm-1 region that includes the 1.18 um window. Water vapor mixing ratios derived from these lists may also be somewhat overestimated. The BT2 line list is recommended as being the most complete and accurate current representation of the H2O spectrum at Venus temperatures.