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DOGEE-SOLAS: The UK SOLAS Deep Ocean Gas Exchange
ExperimentMatt Salter
Overview
Why is gas exchange important?What determines gas exchange?What can we measure?What controls kw?The problemThis project- the solution?What will we do?
The tracer releaseSurfactants
Further aims
Why is gas exchange important?
Gas exchange is an important term in many biogeochemical cycles but remains a major uncertaintyMany important issues require accurate estimates of gas exchange rates
CO2 uptake by oceans
Marine source of major greenhouse gases such as nitrous oxide and methaneClimate forcing involving DMS and iodocarbons
What determines gas exchange?
Gas exchange is determined by: 1. the concentration difference between the
atmosphere and the surface ocean. This drives the flux of gases
2. a kinetic transport term known as the gas transfer velocity, kw which is a function of the interfacial turbulence
ΔC
So what can we measure?
Measuring the concentration driven gas flux is relatively easyHowever kw can only be estimated indirectly hence quantifying it is problematic Kw is a function of turbulence resulting from complex interactions between several forcings…
The controls
Wave geometrySea surface roughnessWave breakingBubbles
RainfallSurfactants
As many of the controls appear largely wind driven it was logical to derive simple empirical relationships for kw based upon wind speed…
But...
The problem
Non-linear nature Strong divergence between predicted kw’s, especially at high wind speedsNo agreement if any of the relationships are correctIt would seem that using wind speed alone to predict kw is impossible
U10 Wind speed ( m s-1)K
600 (
cm h
-1)
WM99
W92N2000
LM86
Figure 1. kw vs wind speed parameterisations where k600 is a normalised value of kw
So...
We will measure kw
We will utilise a dual tracer method to obtain indirect estimates of kw at sea during two cruises in the North Atlantic in December 06’ and May 07’
The tracer release
Involves a simultaneous release of two inert, non-toxic gaseous tracers SF6 and 3HeThe two gases are dissolved in a gas-tight tank of water through a headspace
SF63He
The ratio of SF6 concentration to 3He is measured
The tracer release
The tracers are then displaced by pumping seawater into the tank and are deployed at the release site
Seawater
The tracer release
The tracers then become diluted due to horizontal and vertical mixing and loss through air-water exchange
SF6 and 3He mix horizontally and vertically
However 3He diffuses more rapidly than SF6 across the air-water interface
Thus values for kw can be calculated from the change in ratio of the two tracers over time
The effect of surfactants
On the second cruise we plan to release two tracer patches in close proximity, one of which will be labeled with a surfactant (a surrogate for natural sea surface surfactants)
Tracer patch without surfactant
Tracer patch labeled with surfactant
This will mimic the role of natural surfactants in modifying gas transferWe will compare the gas exchange data for the two patches- the first time this will have been attempted in the field
The effect of surfactantsI would also like to quantify and characterise natural surfactants sampled from the microlayer as much as possible during the cruiseI hope to achieve this using polarography, an electrochemical method
A.C. Polarography
Surfactants reduce the capacitance C formed by the Hg surface and the electrolytic double layer A relationship exists between the surfactant concentration c in the electrolyte and the density of the deposited material, and therefore also between c and CThis is expressed by an adsorption isotherm, E vs C
Summary and further aims
We will measure kw indirectly during two cruises in the North AtlanticWe will investigate the role of surfactants on kw in the field
During the same cruises, other groups will be investigating the role of other controlling variables on kw
In the end, we hope to combine all this data in order to gain a clearer picture of the controls on kw
Acknowledgments
NERC
PML for use of their facilitiesProfessor Rob GoddardDr Phil Nightingale, PMLDr Jo Dixon,PMLDr Alex Baker, UEA