Gas Exchange Velocity (k)

The gas exchange velocity k, also called the transfer or piston velocity, is the rate constant for air-sea gas transfer, with units of velocity such as cm per hour. It links the flux F to the air-sea concentration difference through F = k (Cwater - alpha Cair), where alpha is solubility. Over the ocean k is parameterized from 10 m wind speed and scaled by the Schmidt number; the Wanninkhof relation gives k = 0.251 U10^2 (Sc/660)^-0.5 in cm per hour, normalized to CO2 in seawater at 20 C. Wind, bubbles, and surfactants control k, the largest single uncertainty in regional air-sea CO2 flux estimates.