Correlating gas exchange across the air-water interface to water-side velocity statistics

Abstract:

Wetlands emit a number of trace gases that affect climate, biogeochemistry, and human health. Diffusive flux across the air-water interface of wetlands can be approximated with the thin-film method, which relates gas flux to the product of the gas transfer velocity and concentration gradient. In this laboratory study, we correlated the gas transfer velocity to water-side velocity statistics. Using a model wetland, we conducted a series of experiments where dissolved oxygen was chemically removed from the water column and the resulting oxygen flux was observed under three wind-generated stirring regimes. Water velocity was measured using an automated, submersible camera that tracked neutrally buoyant tracer particles in the water column. The relationship between gas transfer velocity and the standard deviation of vertical velocity was fit to a linear regression with a positive slope and non-zero intercept. This positive slope suggests that fluctuations in vertical velocity enhanced gas exchange, possibly through bringing parcels of oxygen from the interface to the bulk of the water column and thinning the water-side viscous boundary layer. This correlation could be used to estimate the gas transfer velocity in wetlands, and therefore flux, when the standard deviation of vertical velocity is known and between 1·10^-4 m-s^-1 and 6·10^-4 m-s^-1.