Higher Apparent Gas Transfer Velocities for CO 2 Compared to CH 4 in Small Lakes.

Large greenhouse gas emissions occur via the release of carbon dioxide (CO 2 ) and methane (CH 4 ) from the surface layer of lakes. Such emissions are modeled from the air-water gas concentration gradient and the gas transfer velocity ( k ). The links between k and the physical properties of the gas and water have led to the development of methods to convert k between gases through Schmidt number normalization. However, recent observations have found that such normalization of apparent k estimates from field measurements can yield different results for CH 4 and CO 2 . We estimated k for CO 2 and CH 4 from measurements of concentration gradients and fluxes in four contrasting lakes and found consistently higher (on an average 1.7 times) normalized apparent k values for CO 2 than CH 4 . From these results, we infer that several gas-specific factors, including chemical and biological processes within the water surface microlayer, can influence apparent k estimates. We highlight the importance of accurately measuring relevant air-water gas concentration gradients and considering gas-specific processes when estimating k .