Large greenhouse
gas emissions occur via the release of carbon
dioxide (CO2) and methane (CH4) 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 betweenkand the physical
properties of the gas and water have led to the development of methods
to convertkbetween gases through Schmidt number
normalization. However, recent observations have found that such normalization
of apparentkestimates from field measurements can
yield different results for CH4and CO2. We
estimatedkfor CO2and CH4from measurements of concentration gradients and fluxes in four
contrasting lakes and found consistently higher (on an average 1.7
times) normalized apparentkvalues for CO2than CH4. From these results, we infer that several gas-specific
factors, including chemical and biological processes within the water
surface microlayer, can influence apparentkestimates.
We highlight the importance of accurately measuring relevant air–water
gas concentration gradients and considering gas-specific processes
when estimatingk.