Chamber measurement of surface-atmosphere trace gas exchange: Numerical evaluation of dependence on soil, interfacial layer, and source/sink properties

We employed a three-dimensional finite difference gas diffusion model to si mulate the performance of chambers used to measure surface-atmosphere trace gas exchange. We found that systematic errors often result from convention al chamber design and deployment protocols, as well as key assumptions behi nd the estimation of trace gas exchange rates from observed concentration d ata. Specifically, our simulations showed that (1) when a chamber significa ntly alters atmospheric mixing processes operating near the soil surface, i t also nearly instantaneously enhances or suppresses the postdeployment gas exchange rate, (2) any change resulting in greater soil gas diffusivity, o r greater partitioning of the diffusing gas to solid or liquid soil fractio ns, increases the potential for chamber-induced measurement error, and (3) all such errors are independent of the magnitude, kinetics, and/or distribu tion of trace gas sources, but greater for trace gas sinks with the same in itial absolute flux. Finally, and most importantly, we found that our resul ts apply to steady state as well as non-steady-state chambers, because the slow rate of gas diffusion in soil inhibits recovery of the former from the ir initial non-steady-state condition. Over a range of representative condi tions, the error in steady state chamber estimates of the trace gas flux va ried from -30 to +32%, while estimates computed by linear regression from n on-steadystate chamber concentrations were 2 to 31% too small. Although suc h errors are relatively small in comparison to the temporal and spatial var iability characteristic of trace gas exchange, they bias the summary statis tics for each experiment as well as larger scale trace gas flux estimates b ased on them.