Abstract
Oxygen and hydrogen isotopic composition is a powerful tool to trace
origin, movement and mixing of atmospheric water vapor and study
contemporary hydrometeorological processes affecting spatio-temporal
distribution of water in different hydro-climatic regions. Isotopic
composition is an important input parameter for isotope mass balance and
isotope enabled atmospheric circulation models. Although laser-based
absorption spectrometers can conveniently measure the isotopic
composition of water vapor it is not cheap enough to be deployed at
large number of locations which is necessary to understand spatial
variation in vapor flux. Therefore, a novel, simple and cost-effective
method for collecting ambient water vapor for estimating its isotopic
composition is developed. This method involves liquid condensation of
ambient water vapor on an ice-cooled (0 °C) metallic surface under the
supersaturated environment which involves kinetic fractionation due to
diffusion of isotopic water molecules through supersaturated boundary
layer at the metallic surface (R. D. Deshpande, Maurya, Kumar, Sarkar,
& Gupta, 2013). The true isotopic composition of ambient water vapor is
estimated from measured isotopic composition of liquid condensate after
correcting for the kinetic fractionation using a non-linear regression
model. For this, isotopic composition of liquid condensate is compared
with the actual vapor collected by complete cryogenic trapping at -78°C.
The oxygen isotopic composition of ambient water vapor estimated from
liquid condensation is accurate within ±1.8‰ at Ahmedabad compared to
the vapor sampled by complete cryogenic trapping. This simple method can
be employed for isotope tagging of water vapor at large number of
locations in remote areas with minimal resources, particularly in
developing countries.