Higher Apparent Gas Transfer Velocities for CO2 Compared to CH4 in Small Lakes

Highergas transfer velocities for CO2 than CH4 inlakes challenge previous results and commonly made assumptionsand highlight the importance of gas-specific transport in aquaticgreenhouse gas exchange. Large greenhousegas emissions occur via the release of carbondioxide (CO2) and methane (CH4) from the surfacelayer of lakes. Such emissions are modeled from the air-watergas concentration gradient and the gas transfer velocity (k). The links between k and the physicalproperties of the gas and water have led to the development of methodsto convert k between gases through Schmidt numbernormalization. However, recent observations have found that such normalizationof apparent k estimates from field measurements canyield different results for CH4 and CO2. Weestimated k for CO2 and CH4 from measurements of concentration gradients and fluxes in fourcontrasting lakes and found consistently higher (on an average 1.7times) normalized apparent k values for CO2 than CH4. From these results, we infer that several gas-specificfactors, including chemical and biological processes within the watersurface microlayer, can influence apparent k estimates.We highlight the importance of accurately measuring relevant air-watergas concentration gradients and considering gas-specific processeswhen estimating k.