DOGEE-SOLAS: The UK SOLAS Deep Ocean Gas Exchange Experiment

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DOGEE-SOLAS The UK SOLAS Deep Ocean Gas Exchange
Experiment

• Matt Salter

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Overview

• Why is gas exchange important?
• What determines gas exchange?
• What can we measure?
• What controls kw?
• The problem
• This project- the solution?
• What will we do?
• The tracer release
• Surfactants
• Further aims

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Why is gas exchange important?

• Gas exchange is an important term in many
  biogeochemical cycles but remains a major
  uncertainty
• Many important issues require accurate estimates
  of gas exchange rates
• CO2 uptake by oceans
• Marine source of major greenhouse gases such as
  nitrous oxide and methane
• Climate forcing involving DMS and iodocarbons

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What determines gas exchange?

• Gas exchange is determined by
• the concentration difference between the
  atmosphere and the surface ocean. This drives the
  flux of gases
• a kinetic transport term known as the gas
  transfer velocity, kw which is a function of the
  interfacial turbulence

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So what can we measure?

• Measuring the concentration driven gas flux is
  relatively easy
• However kw can only be estimated indirectly hence
  quantifying it is problematic
• Kw is a function of turbulence resulting from
  complex interactions between several forcings

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The controls

• Wave geometry
• Sea surface roughness
• Wave breaking
• Bubbles

• Rainfall

• Surfactants

• As many of the controls appear largely wind
  driven it was logical to derive simple empirical
  relationships for kw based upon wind speed

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• But...

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The problem

• Non-linear nature
• Strong divergence between predicted kws,
  especially at high wind speeds
• No agreement if any of the relationships are
  correct
• It would seem that using wind speed alone to
  predict kw is impossible

Figure 1. kw vs wind speed parameterisations
where k600 is a normalised value of kw
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• So...

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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

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The tracer release

• Involves a simultaneous release of two inert,
  non-toxic gaseous tracers SF6 and 3He
• The two gases are dissolved in a gas-tight tank
  of water through a headspace

SF6
3He
The ratio of SF6 concentration to 3He is measured
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The tracer release

• The tracers are then displaced by pumping
  seawater into the tank and are deployed at the
  release site

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The tracer release

• The tracers then become diluted due to horizontal
  and vertical mixing and loss through air-water
  exchange

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

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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)

• This will mimic the role of natural surfactants
  in modifying gas transfer
• We will compare the gas exchange data for the two
  patches- the first time this will have been
  attempted in the field

Tracer patch without surfactant
Tracer patch labeled with surfactant
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The effect of surfactants

• I would also like to quantify and characterise
  natural surfactants sampled from the microlayer
  as much as possible during the cruise
• I hope to achieve this using polarography, an
  electrochemical method

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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 C
• This is expressed by an adsorption isotherm, E vs
  C

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Summary and further aims

• We will measure kw indirectly during two cruises
  in the North Atlantic
• We 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

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Acknowledgments

• NERC
• PML for use of their facilities
• Professor Rob Goddard
• Dr Phil Nightingale, PML
• Dr Jo Dixon,PML
• Dr Alex Baker, UEA