Deposition of greenhouse gases is a very important scientific and even political issue. The major questions for scientists are:
1. What are the uptake rates for CO2, CH4, and N2O by forest and soil and oceanic surfaces?
2. Is the ocean a sink or source of these gases?
3. How to reduce the uncertainty of the terrestrial and oceanic carbon sink?
There are no straightforward answers to these questions. One of the major studies on the subject were carried out within the EU CarboEurope project (http://www. bgc-jena.mpg.de). The project findings confirm that the biosphere is a sink of CO2. However, the magnitude and exact location of this sink are still fairly uncertain. Scaled air— water CO2 fluxes at European level show that the sink of atmospheric CO2 over continental shelves is highly significant and equivalent to the carbon sink in the terrestrial surfaces. Accurately quantifying the greenhouse gastransfer velocity between the air and the oceanic and/or terrestrial surfaces is problematic because it is influenced by a wide range of environmental variables.
Air-water gas exchange for gases is driven by molecular diffusion and turbulence. The process is defined by the length scales and velocity of turbulent eddies. Away from the air-water boundary, turbulent motions dominate gas transport. As the boundary is approached, these motions are progressively weakened due to the viscous properties of the water surface. This gives rise to the concept of 'diffusive sublayers' on either side of the interface, which provide resistance to gas transfer as it does over terrestrial surfaces, and in which gas concentration gradients develop. For gases that are very soluble or react with water, the atmospheric sublayer provides the greatest resistance to gas exchange, whereas for sparingly soluble gases transport through the aqueous sublayer is the rate-limiting step. Most gases of biogeochemical interest (e.g., CO2, O2, CH4, N2O, DMS, COS, CS2, CO) fall into the latter category and so for these the air-side boundary can be ignored for all practical purposes.
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