The authors took the data from the 2004 observations and fitted them with spectra simulated using VSTAR (Versatile Software for Transfer of Atmospheric Radiation). They find a best fit water vapor abundance of 31 parts per million by volume (-6 + 9 ppmv), which is in agreement with recent results by Bézard et al. 2011 using the SPICAV instrument aboard the Venus Express spacecraft. This is also consistent with the current consensus that water vapor abundance is approximately 30 ppmv below 30km altitude.
So why all the attention to water vapor in the troposphere of Venus? Well,
1. Water vapor is an important chemical reactant in the lower atmosphere as it is the major reservoir of hydrogen, which is hypothesized to buffer or regulate the atmospheric abundances of HCL and HF,
2. Water vapor is important to the formation of the H2SO4 clouds that enshroud the planet, and
3. combining these with the loss of water vapor over long timescales through oxidation reactions with iron minerals at the surface and through photo-disassociation in the upper atmosphere, studies of the abundance distributions and profiles of water vapor in the lower troposphere help to constrain the chemistry and evolution of the near-surface environment.
Bézard, B., Fedorova, A., Bertaux, J., Rodin, A., & Korablev, O. (2011). The 1.10- and 1.18-μm nightside windows of Venus observed by SPICAV-IR aboard Venus Express Icarus, 216 (1), 173-183 DOI: 10.1016/j.icarus.2011.08.025
Chamberlain, S., Bailey, J., Crisp, D., & Meadows, V. (2013). Ground-based near-infrared observations of water vapour in the Venus troposphere Icarus, 222 (1), 364-378 DOI: 10.1016/j.icarus.2012.11.014