Methane flux across the air-water interface: air velocity effects

Methane flux from the air-water interface of freshwater-covered wetland ponds is shown to increase approximately as the square of the air velocity over the water surface. CH₄ fluxes were measured using a partitioned chamber placed over the water surface where the air velocity along the surface can be controlled. Correlating measurements of CH₄ fluxes and surface water dissolved CH₄ concentrations to air velocities provide a means to evaluate the liquid phase exchange coefficient (k₁) for a two-layer model of the gas-liquid interface. The liquid phase exchange coefficient for CH₄ is calculated as k₁ = 1.7 cm^-1 for air velocities of zero and as k_t = 1.1 + 1.2 v^1.96 cm h^-1 for air velocities from 1.4 to 3.5 m s^-1 and water temperatures of 2O°C. Surface water dissolved CH₄ concentrations of the wetland ponds used in this study varied from 10 to 780μg l^-1, resulting in CH₄ flux measurements from 0.01 to 1.22 g m^-2 day^-1.