Confirming existing parameterizations for methane gas transfer velocity in lakes based on direct and high-frequent methods

Abstract

Freshwater systems are important sources of atmospheric methane (CH₄). However, estimated emissions are associated with high uncertainties due to limited knowledge about the temporal variability in emissions and their associated controls, such as air–water gas transfer velocity. Here, we determined the gas transfer velocity of CH₄ based on a novel measurement setup that combines simultaneous eddy covariance flux measurements with continuously monitored CH₄ water- and air-side concentrations. Measurements were conducted during a 10-d campaign in a freshwater lake in mid-Sweden. The gas transfer velocity fell within the range of existing wind-speed-based parameterizations derived for carbon dioxide in other lakes. For wind speeds below 4 m s⁻¹, the gas transfer velocity for CH₄ followed parameterizations predicting faster gas exchange, while for wind speeds above 5 m s⁻¹, it aligned with those predicting relatively lower gas exchange. This pattern can be explained by ebullition. Extending the wind speed range for such combined eddy covariance measurements with continuously monitored CH₄ water- and air-side concentrations would improve model reliability.