ASGAMAGE
A research
project to find out more about the way the sea and the atmosphere exchange
greenhouse gases
Here's a quick
guide to our web site:
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If you want
to know why ASGAMAGE might be important for your future, but you are not
familiar with climate research, look at: ASGAMAGE....
what's that,
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If you're a
bit more knowledgeable in climate research, try state
of the project
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If you want
to know which institutes participated, we have an item
project
organization
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If you're in
too much of a hurry to read those pages and reasonably well acquainted
with air-sea gas exchange work, here's a short summary.
Several
links to figures and texts will give you some more background information
in case you decide that you have somewhat more time after all.
Summary
Objectives
Field
experiments
Main
Conclusions
Acknowledgements
References
Objectives
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To find relationships
between the transport coefficients for the gas fluxes and any relevant
geophysical parameters.
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To test new
methods and new equipment for the measurement of air-sea fluxes of CO2,
DMS and other gases.
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To intercompare
different methods and systems to measure the transfer velocity of trace
gases over the sea.
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To find out
whether and, if at all, under what conditions there can be a significant
vertical gradients in the carbon dioxide concentration in the upper meters
of the water column.
[More
about the objectives]
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Field
experiments
Experiments
for ASGAMAGE, the MAGE project of the Air Sea GAs exchange programme,
took place in May and October of 1996. The experiments were performed at
and around Meetpost Noordwijk (MPN), a research
platform situated 9 km off the Dutch coast, and from the UK research vessel
"RRS Challenger", operating in the neighbourhood of the platform. At MPN
numerous
meteorological and oceanographic observations were performed. The activities
on the "Challenger" were primarily devoted to the differential tracer experiment
(tracer preparation, tracer release, sampling, analysis) and to meteorological
observations.
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Conclusions
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The ec
data indicate a diurnal trend in the transfer velocity, in the order
of a factor of two, which may be related to the sea water temperature.
Because the majority of the ec runs was made during the daytime,
when the fluxes were higher than at night, the resulting average was about
8% too high.
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Computer
model simulations of dt experiments suggest the existence of
near-surface (depth less than 1 m) concentration gradients of inert tracers,
which may cause a negative bias in dt results (10-30%).
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Similar simulations
of ec measurements of CO2 fluxes indicate that some
50% or more of the scatter in the data may be caused by the presence of
small patches of water with enhanced biological activity.
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The uncertainty
in the data is still too high to detect dependencies that are more subtle
than the one with the wind.
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Of the two
main contenders for the relationship between the transfer velocity and
the wind, the Liss-Merlivat (1986) and the Wanninkhof (1992) parameterizations,
the one due to Wanninkhof agrees best
with the experimental results.
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Differences
between the results from the various independent ec measurements
can largely be attributed to differences in the sensitivity of the instruments
to fluctuations in the water vapour concentration in the atmosphere.
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The relaxed
eddy accumulation measurements on DMS have provided sensible results, those
on CO2 were unsuccessful.
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CO2 concentration
gradients in the upper meters of the sea, seen during the '96 experiment
were very much smaller than those of the '93 experiment.
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(Technical)
The use of (semi) open-path CO2 detectors, based on the absorption of infrared
radiation, requires absolute cleanliness of the optical elements exposed
to the air stream.
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Acknowledgements
The European
institutes participating in ASGAMAGE were supported by the MAST-3
programme
of the European
Union
under contract MAS3-CT95-0044. The UK Natural
Environment Research Council provided ship time for the DT measurements
as part of its ACSOE
thematic program. ASGAMAGE was an integral component of the international
Marine
Aerosol and Gas Exchange (MAGE) initiative, a sub-activity of the International
Global Atmospheric Chemistry (IGAC) core project of the International
Geosphere-Biosphere Program (IGBP)
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References
Liss, P.S.
and L. Merlivat, 1986: Air-sea gas exchange rates: introduction and synthesis.
In: P. Buat Ménard (Ed.), The role of air-sea exchange in geochemical
cycling, D.Reidel Publishing Company, Dordrecht, 113-128.
Wanninkhof,
R., 1992: Relationship between wind speed and gas exchange over the ocean.
J. Geophys. Res.,
97, 7373-7382.
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