air-sea gas exchange - school of ocean and earth science ... · methods to determine gas transfer...
TRANSCRIPT
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Air-Sea Gas ExchangeOCN 623 – Chemical Oceanography
17 March 2015
Readings:Libes, Chapter 6 – pp. 158 -168
Wanninkhof et al. (2009) Advances in quantifying air-sea gas exchange and environmental forcing, Annual Reviews of Marine Science
© 2015 David Ho and Frank Sansone
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Overview
• Introduction
• Theory/models of gas exchange
• Mechanisms/lab studies of gas exchange
• Field measurements
• Parameterizations
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Why do we care about air-water gas exchange?
Globally, to understand cycling of biogeochemically important trace gases (e.g., CO2, DMS, CH4, N2O, CH3Br)
Regionally and locally To understand indicators of water quality
(e.g., dissolved O2) To predict evasion rates of volatile
pollutants (e.g., VOCs, PAHs, PCBs)
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Factors Influencing Air-water Transfer of Mass, Momentum, Heat
From SOLAS Science Plan and Implementation Strategy
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Gas transfer velocity(cm s-1)
“resistance”
Concentration gradient(mol cm-3)
“driving force”Flux
(mol cm-2 s-1)
k: Gas transfer velocity, piston velocity, gas exchange coefficient
Cw: Concentration in water near the surfaceCa: Concentration in air near the surfaceα: Ostwald solubility coefficient (temp-compensated
Bunsen coeff)
Basic flux equation
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Basic conceptual model
Atmosphere
Ocean
Ca
Cw
zFilm
Laminar Stagnant Boundary Layer (transport by molecular diffusion)
Turbulent bulk liquid
Turbulent atmosphere
F = k(Cw-αCa)
Cws
Cas
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He ≈ 0.01O2 ≈ 0.03CO2 ≈ 0.7
DMS ≈ 10CH3Br ≈ 10
PCB's ≈ 100-1000 H2O ≈ ∞
Air/water side resistanceMagnitude of typical Ostwald solubility coefficients
Water-side resistance
Air-side resistance
Air- and water-side resistance
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Gas transfer velocity k of insoluble gases (α < 10), function of:• Environmental forcing of water boundary layer• Thermodynamic property of gas and liquid
k = (ν/D)n f(q,L)
ν = kinematic viscosity of water D = Diffusion coefficient of gas in water (ν/D) = Schmidt number (Sc) n = Schmidt number exponent f(q,L) = function of turbulent velocity and length scale
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Overview
• Introduction
• Theory/models of gas exchange
• Mechanisms/lab studies of gas exchange
• Field measurements
• Parameterizations
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Stagnant Film Model (Whitman, 1923; Lewis and Whitman, 1924)• Film thickness is independent of gas• k only changes as a function of diffusion coefficient• Commonly used in "historical work"• Generally viewed as incorrect. Can introduce significant error in
comparing k of different gases (up to a factor of 2).• Implies that k is proportional to Sc-1
Atmosphere
Ocean
Ca
Cw
zFilm
Laminar Stagnant Boundary Layer (transport by molecular diffusion)
Turbulent bulk liquid
Turbulent atmosphere
F=k(Cw-αCa)
k=D/z
Cws
Cas
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Is the Stagnant Film Model realistic?
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Solid wall model (Deacon, 1977)
• k proportional to Sc-2/3
• Relationship holds for exchange over smooth surfaceswithout waves
• Based on analogy between transfer of momentum andmass
• k is related to Schmidt number (Sc) and friction velocity in air (u*a):
Friction velocity (aka shear velocity) is the square-root of the kinematic stress. Close to the surface, friction velocity is not a function of height. A general rule is that the shear velocity is about 1/10 of the mean flow velocity. ρa/ρw is the density ratio of air and water.
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Film replacement model (Higbie, 1935; Danckwerts, 1970)• k proportional to Sc-1/2
Cws
Cw
Cws
Ca
Eddy impingement model (Ledwell, 1984; Coantic, 1986; McCready, 1984; Kittagoroddski, 1984; Asher, 1989) • k proportional to Sc-1/2 if all eddies reach surface
Gas exchange related to replacement time
Gas exchange related to turbulent length scales
Stagnant fluid close to the interface is replaced periodically with material from the bulk
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Overview
• Introduction
• Theory/models of gas exchange
• Mechanisms/lab studies of gas exchange
• Field measurements
• Parameterizations
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Wade McGillis
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Wind Wave Facility
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Wind-wave tank results:3 regimes
• Smooth surface-k =f(u*) Sc -2/3
• Wavy surfacek = f(u*) Sc -1/2
• Breaking wavesk = f(bubble, u*) Sc -x
All bets are off
Issues:• Scaling• Tank (wall artifacts)
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Example: tank artifact due to fetch
Gas Exchange and Environmental Forcing: Tanks
Ocean estimate
100-m long tank
30-m long tank
10-m long tank
Wind-wave and other laboratory tanks are ideal systems to study processes, but absolute magnitudes generally cannot be applied to oceans
Wanninkhof et al., 1991, Donelan and Drennan
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Summary of laboratory studies
Variable affecting k:
• Wind
• Waves
• Bubbles/breaking waves
• Surfactants
• Temperature & humidity gradients
• Rain
• Tunnel shape?
• Tunnel dimensions??
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Good for studying mechanisms
Difficult to extrapolate results from wind/wave tanks to coastal seas and open oceans due to: •Limited fetch•Lack of gusting•Boundary conditions•Use of ‘clean’ freshwater
Summary of laboratory studies
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Surfactants
Two types• Insoluble - an extra barrier (i.e., monolayer or additional liquid
phase)• Impedes molecular diffusion across the surface• Effect only at very low wind speeds; easily dispersed by
wind and waves
• Soluble - change surface tension • Modifies sea surface hydrodynamics • Dampens capillary and gravity waves• Reduces microscale wave-breaking and subsurface
turbulence
Dependence of k on wind speed is a function of surfactants
Ubiquitous in nature
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Biological control on gas transfer?
Laboratory versus field measurements
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Microscale Wave Breaking
Skin-layer disruption by microbreaking eventsproduces thermal signatures that can bedetected and quantified using IR imagery.
Incipient breaking of small scale waves that do not entrain air.
Hypothesis: Microscale wave breaking isthe underlying physical process thatcontrols gas transfer at low to moderatewind speeds.
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Gas Transfer by Bubbles
• Mechanisms by which breaking waves can enhance gas exchange
– Bubble-mediated• Air in bubble exchanges directly with the bulk seawater • Bypasses the air-sea interface• Bubble approaches equilibrium more quickly for soluble
gases• Bubble dissolves or rises
– Surface disruption– Turbulent Patches
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Mechanism 1: Bubble-Mediated Transfer
Woolf, 97
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Mechanism 2: Surface Disruption
Gas transfer at the sea surface is usually limited by low diffusion across the ‘surface skin’. This can also lead to a “cool skin” measurable by a thermal camera (left). When a bubble plume surfaces (right), the skin is disrupted and - by implication - gas transfer enhanced.
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Mechanism 3: Patches of Turbulence
•Time series of dispersion (size of a dyed patch, border labeled by curve) and turbulent intensity (size of crosses).
•Wave breaking introduces a patch of higher near-surface turbulence that gradually decays in intensity but expands in size.
•Gas transfer through the sea surface depends on near-surface turbulence intensity. Therefore, gas transfer is enhanced by the turbulence generated by breaking waves.
Rapp and Melville, 1990
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Overview
• Introduction• Theory/models of gas exchange• Mechanisms/lab studies of gas exchange• Field measurements• Parameterizations
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1.Balance of (known) decay and invasion/evasion rates
• 14C - Both natural and bomb produced
• 222Rn - Radon deficiency method
2.Based on water column inventories:
• 3He/SF6 deliberate tracers
3.Measuring fluxes in the atmosphere (micrometeorological techniques)
• Eddy covariance• Eddy accumulation• Atmospheric gradient
Methods to determine gas transfer velocities in the ocean
Natural 14C
Bomb 14C
222Rn
3He/SF6
Micromet. Meas.
Opportunistic tracers
Deliberate tracers
Direct flux measurements
Scales of techniques to quantify air-sea gas exchange
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Slide from Radioisotope Tracers lecture:
Soluble, conservative
Particle-active
Soluble, from sed.
Inert gas
Rel. insoluble
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Opportunistic tracers of air-sea gas exchange
Natural 14CO2/12CO2 invasion
14CO2/12CO2 out gassing
14N + n 14C
Decay: 14C 14N + e-
Pre-industrial assumption:14CO2 in = 14CO2 out + Decay
0.061 mol m-2 y-1 µatm-1
= 21.4 cm h-1
222Rn 218Po + 4He
226Raaq 222Rngas + 4He
Evasion of Radon
0.062 mol m-2 y-1 µatm-1
= 21.9 cm h-1
[Rn]
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Tracer Infusion and Injection
3He and SF6 dissolved in 4000 L of seawater in gas-tight tank through a headspace
SF6
3He
The tracers are injected at the release site by pumping 3He and SF6-infused seawater from the ship
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• The tracers become diluted due to horizontal and vertical mixing and loss through air-water gas exchange.
• Gas transfer velocities can be calculated from the change in ratio of the two tracers over time.
SF6 and 3He mix horizontally & vertically
3He diffuses more rapidly than SF6 across the air-water interface
3He/SF6 dual tracer technique
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3He/SF6 Deliberate Tracers
Strong dependence on wind
Similarities between coastal ocean and global ocean
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Direct Flux Measurements
• Eddy covariance• Eddy accumulation• Atmospheric gradient
Work well over landVery difficult over seas/oceans
Why? Small fluxes (requirement for high precision/accuracy)Ship MovementFlow distortion
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Eddy Covariance
• Covariance of vertical wind (w) and CO2 or DMS (c)
• Downward air flows have higher CO2 levels than upward air flow (due to CO2 uptake by ocean)
• Requires very fast measurements of gas conc and vertical wind velocity
• Timescale ~10-20 minutes
• Footprint ~1 km2
Gas Flux
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DMS Eddy Covariance Cruises
Knorr_07
PHASE-1
Knorr_06
TAO
VOCALS
BIO
SO GASEX
DOGEE
DMS Eddy Covariancehourly average
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Overview
• Introduction• Theory/models of gas exchange• Mechanisms/lab studies of gas exchange• Field measurements• Parameterizations
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Motivation
• Why relate gas exchange to wind speeds?– Wind generates near surface turbulence and bubbles (i.e.,
the main drivers for gas exchange in the ocean)– Wind speed is widely measured
• Relationships between gas transfer velocity (k) and 10-m wind speed (u10) used:
– in global biogeochemical models– in combination with ocean pCO2 climatologies to
determine ocean CO2 uptake
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Climatology of global ocean pCO2
Basic flux equation
=
http://www.ldeo.columbia.edu/res/pi/CO2/
Calculating Ocean CO2 Uptake Using pCO2 Disequilibrium
?Relationship between wind speed and gas transfer velocity
×
Global wind field
×
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Frequently used wind speed/gas exchange parameterizations
Wanninkhof & McGillis [1999]547
Wanninkhof [1992]2433
Wanninkhof et al. [2009]186
Ho et al. [2006]187
Nightingale et al. [2000]691
Liss & Merlivat [1986]1516
Total Citationsas of Apr 13, 2014from Google Scholar
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Why are relationships different?(Why do observations scatter?):
Experimental uncertainty
Variability in forcing
Other parameters influence air-sea gas transfer
Gas Exchange vs. Wind Speed
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27% difference !