1.3. Sampling and analyses of ground-based vapor
In the context of isotope analysis instrumentation, there are two
well-known options, namely, (1) mass spectrometry and (2) laser
absorption-based spectrometry for measuring isotopic composition of
water and vapor (Costinel, Grecu, Vremera, & Cuna, 2009; Gupta, Noone,
Galewsky, Sweeney, & Vaughn, 2009). Among these two instruments, the
CRDS (Cavity Ring Down Spectrometer) is field deployable and portable
instrument, which can measure both concentration and isotopic
composition of ambient water vapor online. However, it is not cheap
enough to be deployed at large number of locations for wider spatial
coverage with high sampling density. In the context of sampling method,
cryogenic trapping of water vapor from ambient air stream pumped at low
flow rate is the standard method of sampling vapor for isotopic analyses
(R. D. Deshpande et al., 2013; Purushothaman et al., 2014). However,
pre-requisite of lower flow rate to avoid fractionation during sampling
necessitates longer sampling duration (typically 4-5 hours) for
collecting about 15 ml of liquified vapor, and availability of electric
power. The sampling duration is even longer in arid regions where
absolute moisture content in the air is very low. Similarly,
pre-requisite of optimally lower temperature (~ -78 °C)
to ensure complete cryogenic trapping necessitates availability of
cooling options such as liquid nitrogen-alcohol mixture or dry ice.
Thus, power supply, liquid nitrogen/dry ice and the long time are the
essential pre-requisites for conventional sampling procedures which
restrict the sampling of atmospheric water vapor only where these
facilities can be made available.
To overcome these instrumentation and sampling limitations, it was felt
necessary particularly in the developing countries, to develop a novel
method of sampling ambient water vapor for its isotopic composition
which can be replicated at numerous locations across different
geographical regions with minimal logistic requirement.
Towards this, a novel, simple and cost-effective sampling method was
developed as part of a National Programme for Isotope Fingerprinting of
Waters of India (IWIN National Programme) (R. Deshpande & Gupta, 2008,
2012). In this method, ambient water vapor in condensed rapidly on an
ice-cooled metallic surface (R. D. Deshpande et al., 2013). However,
condensation of water vapor from ambient air on metallic surface at 0°C
involves kinetic fractionation due to differential diffusivity of
isotopic molecular species through supersaturated boundary layer.
Therefore, the liquid condensate from ambient vapor is isotopically
different from vapor. The isotopic difference between the vapor and
liquid condensate has been explained by R. D. Deshpande et al. (2013) in
terms of kinetic fractionation under supersaturated environment, similar
to that explained by Jouzel and Merlivat (1984) for solid condensation.
However, the isotopic composition of vapor cannot be computed from the
liquid (or solid) condensate using these models because of uncertainties
in estimating effective degree of supersaturation and diffusion
coefficient for isotopic molecular species. Ignoring these limitations
and in spite of the fact that isotopic composition of liquid condensate
does not reflect true isotopic composition of ambient vapor some of the
recent studies have used isotopic composition of liquid condensate to
infer hydrological processes (Krishan, Rao, & Kumar, 2012; Saranya et
al., 2018). Some studies have used simple linear regression to estimate
isotopic composition of vapor from that of liquid condensate, though
with large uncertainty (Purushothaman et al., 2014).
With a view to advance the usability of isotopic composition of liquid
condensation as a close approximation of isotopic composition of ambient
vapor, we present a new non-linear regression approach to compute the
true δ18O value of ground level vapor using measured
δ18O value of liquid condensate. Significance of this
approach lies in the fact that, in contemporary hydrology, it is very
important to monitor ground level water vapor in order to track the
transport of water mass from surface or subsurface to atmosphere in
different geographic areas differing in their weather, water resources,
lithology, ecology and microclimate. This approach can be easily
employed to monitor ground level water vapor in any remote area with
minimal resources and infrastructure. This approach will therefore be
very useful for coordinated network programs in developing countries.