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Ecosystem fluxes of hydrogen: a comparison of flux-gradient methods

Ecosystem fluxes of hydrogen: a comparison of flux-gradient methodsAtmospheric Measurement Techniques, 7, 2787-2805, 2014Author(s): L. K. Meredith, R. Commane, J. W. Munger, A. Dunn, J. Tang, S. C. Wofsy, and R. G. PrinnOur understanding of biosphere–atmosphere exchange has been considerably
enhanced by eddy covariance measurements. However, there remain many trace
gases, such as molecular hydrogen (H2), that lack suitable analytical
methods to measure their fluxes by eddy covariance. In such cases,
flux-gradient methods can be used to calculate ecosystem-scale fluxes from
vertical concentration gradients. The budget of atmospheric H2 is poorly
constrained by the limited available observations, and thus the ability to
quantify and characterize the sources and sinks of H2 by flux-gradient
methods in various ecosystems is important. We developed an approach to make
nonintrusive, automated measurements of ecosystem-scale H2 fluxes both
above and below the forest canopy at the Harvard Forest in Petersham, Massachusetts, for
over a year. We used three flux-gradient methods to calculate the fluxes: two
similarity methods that do not rely on a micrometeorological determination of
the eddy diffusivity, K, based on (1) trace gases or (2) sensible heat, and
one flux-gradient method that (3) parameterizes K. We quantitatively
assessed the flux-gradient methods using CO2 and H2O by comparison
to their simultaneous independent flux measurements via eddy covariance and
soil chambers. All three flux-gradient methods performed well in certain
locations, seasons, and times of day, and the best methods were trace gas
similarity for above the canopy and K parameterization below it. Sensible heat
similarity required several independent measurements, and the results were
more variable, in part because those data were only available in the winter,
when heat fluxes and temperature gradients were small and difficult to
measure. Biases were often observed between flux-gradient methods and the
independent flux measurements, and there was at least a 26% difference
in nocturnal eddy-derived net ecosystem exchange (NEE) and chamber
measurements. H2 fluxes calculated in a summer period agreed within
their uncertainty and pointed to soil uptake as the main driver of H2
exchange at Harvard Forest, with H2 deposition velocities ranging from
0.04 to 0.10 cm s−1.

Atmospheric Measurement Techniques 2014/09/03 - 20:36 Czytaj