Gas Exchange at the Air–Water Interface

Abstract

Gas exchange at the air–water interface is controlled by a combination of chemical and physical factors. Chemically, exchange is driven by the concentration gradient across the air–water interface. The gradient can arise as a result of several processes, including changes in solubility and biochemical reactions which alter aqueous concentrations. Physical factors influencing the exchange of gases include turbulent and molecular diffusion. Several mechanisms can impart turbulence at the air–water interface, although wind stress is considered most often. Very near the air–water interface viscous forces reduce turbulence, and gas exchange is limited by molecular diffusion in the diffusive sub-layer. Several theoretical models attempt to explain the interaction between the turbulent and diffusive layers. In addition, empirical models are used to predict the gas exchange coefficient at the air–water interface. Several field methods for estimating gas exchange in lakes are discussed. Gas exchange in some other systems, such as streams, rivers, estuaries and wetlands, has been less well constrained, partially due to greater inherent variability. The aim of this article is to equip the reader with the knowledge to make rough estimates of gas exchange in lake settings, and to give an introduction to methods that result in more direct estimates of gas exchange in lakes and other environments.