HR: 15:25h
AN: A23E-08 INVITED     [Abstracts]
TI: Continuous measurements of H2 and CO deposition onto soil: a laboratory soil chamber experiment
AU: * Ghosh, P
EM: pghosh@gps.caltech.edu
AF: Division of Geological Science, California Institute of Technology, Pasadena, CA 91125 United States
AU: Eiler, J
EM: eiler@gps.caltech.edu
AF: Division of Geological Science, California Institute of Technology, Pasadena, CA 91125 United States
AU: Smith, N V
EM: nicolev@gps.caltech.edu
AF: Division of Geological Science, California Institute of Technology, Pasadena, CA 91125 United States
AU: Thrift-Viveros, D L
EM: dljazz@caltech.edu
AF: Division of Geological Science, California Institute of Technology, Pasadena, CA 91125 United States
AB: Hydrogen uptake in soil is the largest single component of the global budget of atmospheric H2, and is the most important parameter for predicting changes in atmospheric concentration with future changing sources (anthropogenic and otherwise). The rate of hydrogen uptake rate by soil is highly uncertain [1]. As a component of the global budget, it is simply estimated as the difference among estimates for other recognized sources and sinks, assuming the atmosphere is presently in steady state. Previous field chamber experiments [2] show that H2 deposition velocity varies complexly with soil moisture level, and possibly with soil organic content and temperature. We present here results of controlled soil chamber experiments on 3 different soil blocks (each ~20 x ~20 x ~21 cm) with a controlled range of moisture contents. All three soils are arid to semi arid, fine grained, and have organic contents of 10-15%. A positive air pressure (slightly higher than atmospheric pressure) and constant temperature and relative humidity was maintained inside the 10.7 liter, leak-tight plexiglass chamber, and a stream of synthetic air with known H2 concentration was continuously bled into the chamber through a needle valve and mass flow meter. H2, CO and CO2 concentrations were continuously analyzed in the stream of gas exiting the chamber, using a TA 3000 automated Hg-HgO reduced gas analyzer and a LI-820 CO2 gas analyzer. Our experimental protocol involved waiting until concentrations of analyte gases in the exiting gas stream reached a steady state, and documenting how that steady state varied with various soil properties and the rate at which gases were delivered to the chamber. The rate constants for H2 and CO consumption in the chamber were measured at several soil moisture contents. The calculated deposition velocities of H2 and CO into the soil are positively correlated with steady-state concentrations, with slopes and curvatures that vary with soil type and moisture level. Increased moisture content is associated with increased deposition velocities at a given steady-state concentration: for every 5 % increase in soil moisture content, the ratio of deposition velocity to steady state concentration increases by 5.13ñ1.3 x 10-7cm2/s/ppb. Based on these observations, we conclude that uptake rate of H2 and CO in soil increases with increase in soil moisture content over the range characteristic of unsaturated soils, in contrast to the previous observation that increasing soil moisture level from 30% to 60% caused a large drop in hydrogen uptake rate (Yonemura et al., 1999)-a situation encountered during flood or heavy down pour. Our results indicate that the H2 consuming activity of soil is rapidly activated upon wetting and reaches a maximum at about 15% moisture level. This deduction supports the results obtained by several workers (Conrad and Seiler, 1980 ; Moxley and Smith, 1997) who showed that there is an optimum moisture level for microbiological hydrogen and CO uptake in soil. [1] Rahn T., Eiler, J.M., Kitchen, N., Fessenden, J.E. Geo. Res. Letters, (2002): 29(18): art no 1888. [2] Godde, M., Meuser, K., Conrad R. Hydrogen consumption and carbon monoxide production in soils with different properties, Bio Feril Soils (2000) 32:129-134.
DE: 0315 Biosphere/atmosphere interactions
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting