Air–sea gas exchange in the coastal zone

Estuaries and coastal seas are important sources of climatically active atmospheric trace gases. There have been substantial advances towards understanding the air–sea gas exchange process during recent years; however, these are insufficient to allow adequate parameterisation of the fundamental controlling processes. The important environmental controls on air–sea gas exchange and progress towards their understanding and quantification are reviewed. Accurately defining the air–sea gas concentration difference, ΔC, is in theory comparatively straightforward, however, due to inherent spatial and temporal inhomogeneity obtaining the requisite spatial and temporal coverage for coastal waters can be a significant challenge. Accurately quantifying the gas transfer velocity, kw, is potentially more problematic because it is influenced by a wide range of environmental variables, most of which are strongly interlinked. Most relevant studies of kw have been in the laboratory or in the open ocean; for coastal waters observational and experimental data are comparatively very few and the additional controls on gas exchange in coastal waters further complicate analysis. Due to this complexity investigative approaches to date have tended to focus on processes individually. For some of these our understanding has improved in recent years, but while formal mathematical descriptions are being developed (e.g. for wave geometry and sea surface roughness), these are not yet definitive. For other processes, for example the role of the bacterioneuston, our understanding remains far more rudimentary. Consequently even for the open ocean a full and unique parameterisation of gas exchange remains some way off. In order to achieve this requires integrated and coherent approaches that can build on recent advances in understanding; recent new international initiatives are a significant step in this direction.