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

Abstract

Abstract. Our understanding of biosphere-atmosphere exchange has been considerably enhanced by eddy covariance measurements. However, there remain many trace gases, such as molecular hydrogen (H 2 ), 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 H 2 is poorly constrained by the limited available observations, and thus the ability to quantify and characterize the sources and sinks of H 2 by fluxgradient methods in various ecosystems is important. We developed an approach to make nonintrusive, automated measurements of ecosystem-scale H 2 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 CO 2 and H 2 O 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. H 2 fluxes calculated in a summer period agreed within their uncertainty and pointed to soil uptake as the main driver of H 2 exchange at Harvard Forest, with H 2 deposition velocities ranging from 0.04 to 0.10 cm s −1 .

Seasonal Evolution of Canopy Stomatal Conductance for a Prairie and Maize Field in the Midwestern United States from Continuous Carbonyl Sulfide Fluxes

“…To estimate the OCS flux ( ), we used the flux gradient approach as reviewed by Meredith et al (): where and are the above canopy gradients derived from the OCS analyzer and NEE is the net ecosystem exchange of CO derived from the EC system. From equation , we generated a 30‐min flux on the same time scale as the EC data and used the sign convention where flux towards the surface is positive.…”

Measurements and modeling of surface–atmosphere exchange of microorganisms in Mediterranean grassland

“…The flux-gradient method was used to estimate microbial fluxes from concentrations measured with the Burkard samplers. This methodology has been widely used to measure atmospheric fluxes of different scalars such as hydrogen (Meredith et al, 2014), nitrates and nitrogen compounds (Beine et al, 2003;Griffith and Galle, 2000;Taylor et al, 1999), mercury (Edwards et al, 2005;Fritsche et al, 2008;Lindberg et al, 1995), and particulate matter (Bonifacio et al, 2013;Kjelgaard et al, 2004;Park et al, 2011;Sow et al, 2009). The method follows the Monin-Obukhov similarity theory (Monin and Obukhov, 1954) and therefore assumes that in the atmospheric surface layer the flux of a certain scalar is a function of the gradient of the scalar measured at different heights, the heights themselves (z i ), and a transport velocity that is dependent on atmospheric turbulence and stability (a more detailed description of the methodology is provided in the Supplement).…”

N2O Emissions From Two Agroecosystems: High Spatial Variability and Long Pulses Observed Using Static Chambers and the Flux‐Gradient Technique

“…The flux‐gradient (FG) technique is a micrometeorological technique assuming that turbulent mixing in the atmosphere is analogous to Fick's law of molecular diffusion (Meredith et al, ). Fluxes are calculated by multiplying the molar density gradient of a trace gas by the eddy diffusivity, K : where F c is the flux of the N 2 O (nmol m −2 s −1 ), K is the eddy diffusivity (m 2 /s), and Δ C is the gradient of N 2 O molar density (nmol/m 3 ) over Δ z , the vertical gradient (m).…”