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Evidence shows that atmospheric water vapor has been increasing globally, both over oceans and land surfaces. Long-term datasets in the Upper Midwest indicate higher amounts of water vapor in the surface layer, which is likely to have significant consequences. Understanding the recent trends in increased atmospheric water vapor is extremely complex and there is a fair amount of uncertainty related to the climate feedback processes. Generally, increased atmospheric water vapor is expected to be driven by surface warming, but changes in land use, atmospheric circulation, and human water use may also be contributing factors. In addition to being a major greenhouse gas, water vapor also has a large influence on atmospheric processes including stability, convection, cloud formation, and the development of storm systems.
The isotopic composition of atmospheric water vapor is a forensic tracer that can be used to better understand the physical and biophysical processes involved in land-atmosphere transport and recycling of water. Although there is considerable research of water isotopes in the condensed phase, little research exists on the isotopic composition of water in the vapor phase.
Our goal is to provide new information on how the land surface and atmospheric dynamics influence the isotopic composition of water vapor within a region. By identifying the dominant forcing mechanisms and their influences on the isotopic composition of the atmosphere, we can better utilize water isotopes as tracers of global and regional climate change.
Sampling & Measurement TechniquesWe are archiving precipitation, groundwater, plant (leaf and stem) water, soil water, and surface water. Through isotopic analysis of water at these stages as it moves through the biosphere and atmosphere, we will be able to develop an understanding on how local land-atmosphere processes influence the fractionation of water isotopes.
A TDL system (TGA200A, Campbell Scientific, Inc.) is used to measure isotopic water vapor mixing ratios and fluxes in a flux-gradient and eddy covariance mode.
Leaf, stem, and soil water are extracted using a custom glass vacuum line.
Samples are loaded into the vacuum line, frozen with liquid nitrogen and then opened to the vacuum and pumped down to a pressure of ~10 millitorr (mTorr). They are then heated using a water bath, and the liquid nitrogen is used to cool a collection tube. This temperature gradient drives the water from the heated plant and soil samples to the frozen collection tubes. The samples are heated for at least 1.5 hours to ensure the plant and soil samples are completely dehydrated and all of the water is collected. The water samples are then transferred to 1.5 mL vials, wrapped in parafilm, and refrigerated.
The extracted water from plant and soil samples as well as precipitation, surface water, and groundwater samples are analyzed for isotopic composition using an off-axis laser spectroscopy system (DLT-100, Los Gatos Research). Typical precision for this system is 0.2 and 1.0 per mil for delta O18 and delta D, respectively.
Precipitation samples have been collected from 2006-2011 at Rosemount, MN. Groundwater samples have been obtained both from the Rosemount research site and from domestic wells within a certain vicinity of the research site. Surface water samples have been collected from lakes and rivers within a 15 km radius of Rosemount and the St. Paul UMN campus. Soil samples have been collected from the Rosemount site from 2006-2011. These samples have been taken ~10 cm from the surface. These data have been shared with the MIBA (Moisture Isotopes in the Biosphere and Atmosphere) program.
In 2010, we deployed a tunable diode laser at the tall tower trace gas observatory (TGO) to obtain new isotopic water vapor data and to provide a better understanding of the water vapor transport and fractionation processes in the atmospheric boundary layer. Preliminary results are provided below in a recent poster that was presented at the American Geophysical Union. Other related publications are also listed below.
"Identification and correction of spectral contamination in 2H/1H and 18O/16O measured in leaf, stem, and soil water", N.M. Schultz, T.J. Griffis, X. Lee, and J.M. Baker (Rapid Communications in Mass Spectrometry, 2011, 25, 3360-3368)
