Dissolved Gases

and

Air-Sea Exchange

The oceans are full of gas! Photo by Will Drennan,

NOAA

Proton-exchanging gasesProton-exchanging gases- CO- CO2, 2, HH22S, NHS, NH3 , 3 , SOSO2 2 - Have ionic forms after losing or - Have ionic forms after losing or

gaining a protongaining a proton

Permanent GasesPermanent Gases (unreactive) gases - N(unreactive) gases - N22, Ne, He (, Ne, He (33He, He, 44He), Xe, He), Xe, 222222Rn tRn t1/2 1/2 = 3.5 d = 3.5 d

(from (from 226226Ra), and Ar. Ra), and Ar.

Reactive GasesReactive Gases (high chemical or biological reactivity) (high chemical or biological reactivity)OO22

HH22 ( (33H, tH, t1/21/2 = 12.7 y) = 12.7 y)

NN22O (intermediate in the N-cycle; potent greenhouse gas)O (intermediate in the N-cycle; potent greenhouse gas)

CHCH44

DMS (main sulfur DMS (main sulfur inputinput to atmosphere; anti-greenhouse gas) to atmosphere; anti-greenhouse gas)COS (photochemical source, most COS (photochemical source, most abundantabundant sulfur gas in atmosphere) sulfur gas in atmosphere) COCO

Other trace gasesOther trace gasesCHCH33I, CHI, CH33Br, CHBr, CH33Cl, CHCl, CH33Br; CHBrBr; CHBr33 (bromoform), CHCl (bromoform), CHCl33 (chloroform) (chloroform)

Freons (unreactive, but catalytic for OFreons (unreactive, but catalytic for O33 destruction in the atmosphere- also strong destruction in the atmosphere- also strong

greenhouse gases). greenhouse gases). Ethane, propane, isoprene etc. also known as non-methane hydrocarbons (NMHC)Ethane, propane, isoprene etc. also known as non-methane hydrocarbons (NMHC)

Spectrum of gases in the oceanSpectrum of gases in the ocean

Why are gases of interest?Why are gases of interest?

Highly mobile chemicals, move into Highly mobile chemicals, move into and out of the ocean via the atmosphere and out of the ocean via the atmosphere and through different compartments and through different compartments within the oceanwithin the ocean

Air-Sea exchange of gases is important Air-Sea exchange of gases is important for climate and atmospheric chemistryfor climate and atmospheric chemistry

Participate in many important Participate in many important biological reactions: biological reactions:

Photosynthesis/respirationPhotosynthesis/respiration Nitrogen fixation/denitrificationNitrogen fixation/denitrification

Tracers of water mass Tracers of water mass movement and mixingmovement and mixing

If thermohaline circulation were to stop, the deep ocean waters would lose their connection to the atmosphere and they would go anoxic on time scale of ~1000 years

Gas composition of the atmosphere:

Like seawater, the bulk of the atmosphere is composed of just a few major chemicals.

N2, O2 and Ar total = 99.965%

From 1991 through 2005, the O2 content of the atmosphere has dropped by 0.00248% (Keeling data)

Simple Gas Laws

Dalton’s law of partial pressuresEach gas exerts a partial pressure independent of the other gases. (only true for dilute mixtures, but okay for us).

The total pressure is equal to the sum of the partial pressures.

Ptotal = p(A) + p(B) + p(C) ….For example: Air is mostly N2:O2:Ar in the ratio 78:21:1. Thus at 1 atm:

Ptotal = 1 atm = pN2 (0.78) + pO2 (0.21) + pAr (0.01)

If you take away oxygen and argon the total pressure would then be 0.78, equal to that of N2. For ideal gases, mole fraction = partial pressure

pA = 0.5 atm

pA = 0.5 atm

pB = 0.5 atm

ptotal = (pA+pB) = 1 atm

http://id.mind.net/~zona/mstm/physics/mechanics/energy/heatAndTemperature/gasMoleculeMotion/gasMoleculeMotion.html

Units of gas pressure

pascal - Standard SI unit for pressure (1 newton m-2).

bar = 100 kPa = 0.9869 atmospheres. 1 decibar ~ pressure of 1 meter column of seawater

atmosphere - 1 atm = 101,325 Pa or 101.325 kPa

mm or inches of Hg (still widely used)

Ideal gas law

pV = nRT

Wherep = pressure of the gasV = volume of the gasn = # of moles of the gasT= temperature in degrees Kelvin (absolute scale)R = Universal gas constant - has units consistent with other parameters: 8.314 liter kPa K-1 mol-1

0.08314 liter bar K-1 mol-1

0.08205 liter atm K-1 mol-1

From pV = nRT it is easy to determine that at 0 oC (273.13 oK) and 1 atm of pressure, 1 mole of gas has a volume of 22.4 liters.

For gases, the standard temperature and pressure (STP) is 0oC and 1 atm.

Units of gas concentration used in the aquatic sciences literature:

• ppm (based on partial pressure)• M (micromolar)• mol/kg of seawater• ml/liter sw (ml @Standard Temperature & Pressure (STP); must be converted from other conditions)

• mg/liter sw• mole fraction• mixing ratio’s

Leads to much confusion! It is best to use stoichiometric units - so use moles/liter or moles/kg sw

Fugacity - This term is analogous to that of activity for dissolved solids. A fugacity coefficient is similar to an activity coefficient.

For most work in surface seawater fugacity is very close (within a few percent) to partial pressure. At high pressures, gases do not behave ideally, and thus fugacities must be used.

Range of concentrations for important gases in seawater (Salinity = 35 o/oo)

mol/kg sw

N2 350-620 essentially inert chemically

O2 190-350 (can be zero in anoxic waters; can be higher than 350 mol/kg sw in algal blooms)

CO2 10-40 (compare with total DIC of ~2000 mol/kg sw)

CH4 0.002-0.010The range of concentrations is due mainly to variations in temperature, which affects the solubility of gases in equilibrium with the atmosphere.

As always, the equilibrium is dynamic - constant exchange between reservoirs.

At equilibrium the partial pressure is the same in both phases, but the concentration (mass/volume) is not necessarily the same between gas and liquid.

Gas equilibrium

Air

Water

Gas molecules

Gas Solubility

A(g)

A(aq)

[A(aq)] = KH,A pA(g)

Where KH,A is the Henry’s Law constant

The concentration of a gas in water, when the gas and the water phases are in equilibrium, depends directly on the partial pressure of the gas in the gas phase (pA) and a characteristic constant for that particular gas.

KH, A is a form of equilibrium constant for the dissolution

reaction:

A(g) <=> A(aq) Keq = [A(aq)]/[A(g)] This can be rearranged

to give: [A(aq)] = Keq [A(g)] Conc. of gas in the gas phase is in moles per unit volume (n/V).

From the ideal gas law, n/V = P/RT. Substituting P/RT for [A(g)] we get [A(aq)] = Keq P/RT = (Keq/RT)*P. The (Keq/RT) term = KH,A

Varieties of gas solubility coefficients(all are modifications of the Henry’s law constant using different units etc)

Ostwald coefficent- (concentration in the liquid phase)/(concentration in the gas phase)

(mol(g)/liter of liquid)/(mol(g) per liter of gas)

Bunsen coefficient (next slide)

Bunsen coefficient () - a form of Henry’s law constant that can be used to express the equilibrium concentration of a gas in (ml gas/liter of water).

The proportionality holds for any partial pressure of the gas A

[Aaq] = (PAmoist)

Where [Aaq] is the concentration of gas in ml(@STP)/liter of water, PA

moist is the pressure of the gas A (the total pressure will be higher because of water vapor).

Effects of temperature, pressure and salinity on solubility of gases (important!) As temperature goes up, gas solubility goes down

As pressure goes up, gas solubility goes up

As salinity goes up, gas solubility goes down

The effects are non-linear in all cases

Solubility of gases depends on molecular weight (heavy gases are more soluble). Solubility is a non-linear function of temperature, with greater solubility at LOW temperature

Libes, Chap 6

Heavier gases have a larger temperature effect.

CH4

Values are for 35 ppt SW

More

solu

ble

From Pilson

Gas solubility is a non-linear function of salinity, with greater solubility at LOW salinity

Depth distributions of oxygen

O2 minimum layer

Eastern Tropical Pacific

O2 minimum layer

Sargasso Sea

0

1

2

3

4

5

Dep

th

km

There is an O2-minimum layer in the thermocline nearly everywhere in the ocean!

What about estuaries and nearshore waters?

Conc. Conc. ~ Atmos.

Equilibrium values

Coastal hypoxia on Alabama shelf (Sep 8, 2010)

FOCAL (M. Graham)

Annual variations in dissolved O2 at 28.5 °N 88.5 °W (due south of Deepwater Horizon Blowout site). Climatological mean DO2 from the NODC 1° World Ocean Atlas 2009.

MonthTaken from NOAA report of dissolved O2 in Gulf –Sept, 2010

Dissolved O2 concentrations in central Gulf of Mexico near Deepwater Horizon spill site

Valentine et al 2010. Science Express.

Oil & gas layer

O2

minimum (natural)

Valentine et al 2010. Science Express.

CH4 concentration (log scale)

O2 concentration anomaly

O2 concentrations are depleted in the layer where natural gas (CH4) concentrations are high

Negative anomalies = less O2 than expected

Gas concentrations are often presented in % saturation. This is simply the deviation from the normal atmospheric equilibrium concentration (referred to as NAEC)

% Saturation = [Cin situ/Csat] x 100

where Cin situ is the concentration in situ and Csat is the predicted concentration in equilibrium with the atmosphere (@ 1 atm pressure)

Mobile NEP http://www.mymobilebay.com/stationdata/chartdata/defaultb.asp?stationid=628&param=doper

O2 % SaturationDauphin Island

100%

Deviations from equilibrium

Water mass mixingTemperature Mixing always results in supersaturation due to the non-linear (concave) relationship between solubility coefficients and temperature. If you mix two water masses that have different initial temperatures and gas concentrations, temperature will be conserved and the mass of gas will be conserved (i.e. fall on conservative mixing line), giving an intermediate value. However, the calculated equilibrium concentration at the new intermediate temperature will be lower than that observed, thereby giving an apparent supersaturation anomaly.

High end member

Low end member

Conservative mixing line

Calculated saturation value

Temperature ->

Con

cen

trati

on

->

Saturation anomaly

Deviations from equilibrium

Bubble Exchange and Air InjectionGas Bubble Exchange - As bubbles are forced deep into the water the pressure goes up and more gas will dissolve. The gases will dissolve according to their solubilities therefore more of a heavier gas will dissolve this way than a lighter gas. However, since heavy gases are likely to have high concentration in the water already (due to their greater solubilities) the % change due to bubble dissolution is small. Air Injection - If a bubble is completely dissolved, then even the insoluble gases are forced into solution and large saturation anomalies for those gases will be observed.

Many gases are supersatured in surface waters because of these processes!

Bubbles penetrate 5 m or more!

There is a slight supersaturation of O2 nearly everywhere in the surface ocean.

From Pilson.

Deviations from equilibrium concentrations result in net flux of gas into or out of the ocean.

Air-sea exchanges of gases are important globally.

Deviations from equilibrium arise from: • Biological production or consumption of gases• Photochemical production or destruction of gases• Physical processes such as water mass mixing, bubble dissolution etc.

Gas exchange across the Air-Sea interface boundaryFundamentally the flux of the gas across the boundary F(g) is governed by Fick’s first law:

z

CDF gg

)()(

Where

D(g) is the diffusion coefficient (units of m2/s; a function of the particular gas, temperature and salinity)

C/ z is the concentration gradient at the boundary

Thus, the greater the diffusion coefficient the greater the flux. Also, the greater the concentration gradient, the greater the flux.

z

CDF gg

)()(

The flux equation can be approximated by:

)()()( satCC

z

DF w

gg

Substituting this, and rearranging gives:

D(g)/z is called the transfer coefficient (K). It has units of (m2/s)/m = m/s. Thus: Flux = K (Cw- Csat)

C is the difference between the concentration of the gas at the very interface between air and water (assumed to be the equilibrium concentration with air, Csat), and the bulk water concentration (Cw) in the

mixed layer: C = Csat - Cw

If Cw > Csat flux is positive (to the atmosphere). If Cw < Csat then flux is negative and gas will have net transfer into the water phase.

K is also given the names: transfer velocity, exchange velocity, piston velocity, exchange coefficient (e(g)), exit coefficient - they all refer to the same thing!

Turbulently mixed

Laminar layer

“thin film” z

Concentration gradient dC/dz

Air

Turbulently mixed

Water

Concentration (or partial pressure)

Depth(z)

Gas concentration(or partial pressure)

Thin film model of gas exchangeat water-air boundary - a conceptualmodel

Assuming air-side boundary layer does not hinder gas diffusion to sea surface, The concentration of dissolved gas at the air-water interface is that predicted by the equilibrium solubility with the atmosphere

Bulk concentration (Cw) is uniform below laminar layer

Boundary layers above and below interface

The thickness of the diffusive boundary layer will directly affect the flux!

)()()(

satw

gg CC

z

DF

Cw

Csat

For some gases that are highly supersaturated in seawater (i.e. DMS) the concentration in equilibrium with the atmosphere is so low that it can be ignored. In such cases the concentration gradient approximates to the concentration in the bulk water.

C= Cw - Csat ≈ Cw

Thus, Flux = K Cw

For other gases, including CO2, CH4 and O2, the deviation from atmospheric equilibrium is small and the exact gradient should be calculated.

[gas]

Wind

The exchange coefficient (K) is not just a simple function of D(g) (which itself is a function of temperature).

It is also a complex function of wind speed, which can affect the thickness of the microlayer (z) and other transport properties at the sea surface.

Temperature (affects diffusion)

The sea Surface is rarely smooth and “ideal”

Wind speed affects the surface roughness, Wind speed affects the surface roughness, momentum transfer and ultimately the momentum transfer and ultimately the concentration gradient between the air and sea.concentration gradient between the air and sea.

Nightingale (2000) relation for exchange coefficient

K600 = 0.222U2 + 0.333U

Exchange coefficients are ~ exponential functions of wind speed

U is wind speed at 10 m above sea surface (in m/s) and the transfer velocity, Kc600, is in cm/h for a gas with a Schmidt number of 600

If K600 is needed in m d-1, multiply calculated value by 0.24

Schmidt Number: Sc = /D

Where is the kinematic viscosity (viscosity/density) of the seawater and D is the diffusion coefficient of the gas. Both and D have units of m2/s so the Sc is dimensionless.

Viscosity is the resistance to flow (internal friction). is essentially the diffusion coefficient for momentum of the liquid

The Nightingale equation holds for a Schmidt # (Sc) of 600 (the Sc value for CO2 @ 20 oC). To use this equation to calculate transfer velocities (Kc) for gases with other Schmidt #'s, use the relation:

Kc/Kc600 = (Sc)-0.5/(600)-0.5

So that Kc = Kc600 [(Sc)-0.5/(600)-0.5

Calculate the Sc of the gas of interest at the temperature and salinity under consideration. Plug this into the equation and calculate Kc

For a given wind speed, determine the Kc600 from the Nightingale relationship for Sc = 600. Plug this into equation.

Be sure to convert concentrations into appropriate units for use with the Kc!

Out of the ocean

Into the ocean

Spatial variation of CO2 flux from the ocean

Emerson and Hedges

Out

Out

Out

InInInIn

InIn

InIn

Climate forcing in the atmosphere

Wigley, 1999, The Science of Climate Change, Pew Center for Global Climate Change

Uncertainty - low Uncertainty - high

Values relative to the year 1850

CO2CH4 N2O

Methane (CH4) in seawater

Methane is a biogenic gas that is produced mainly by strict anaerobes from the Archaea domain. It is a very strong greenhouse gas.

CH4 is often high in rivers and coastal waters due to inputs from anoxic sediments.

CH4 profile in Gulf of Mexico

The Oceanic Methane Paradox - Open ocean surface seawater is 0.1 to 3x supersaturated (110-300% saturation) with CH4 throughout the world ocean and sub-surface maxima in the mixed layer and thermocline are usually even larger. The oceans are therefore a small net source of CH4 to the atmosphere. CH4 is produced only by strictly anaerobic microbes, so how can supersaturation be maintained? Anoxic microzones might be the reason, although proof is still lacking. Biological consumption of CH4 in the water column is extremely slow ( ~months-years), therefore air-sea exchange is the major loss of CH4.

Atmos. Equil. conc.

Karl and co-workers have found that methylphosphonate (an organic form of phosphorus) can be converted to CH4 by microbes in aerobic seawater. Is this the methane source?

Coastal hypoxia on Alabama shelf (Sep 8, 2010)

FOCAL (M. Graham)

4.1

3.5

38

3.2

27.3

28

3.5

15.3

3.57.924.2

7.310.7

9.7

29.7

25.9

Color contours indicate [O2]. Numbers are [CH4] in nM

Images from http://www.netl.doe.gov/scng/hydrate/

Dissolved CH4 also is high near methane hydrates, a crystalline solid in which water molecules hold a molecule of CH4 in a cage like structure (clathrate).

CH4 clathrate

Exposed hydrate on seafloorMethane hydrates look like ice!

Hydrate crystals are stable only within certain temperature & pressure ranges. Typically > 500 m and < 10 oC.

Gas hydrates are stable at higher pressures and lower temperatures

Trehu et al 2006

Hydrates stable

Adding salt or N2 shifts boundary to left; adding CO2, H2S etc shifts boundary to right

Core of methane hydrate recovered from the Johnson Sealink cruise in the Gulf of Mexico in July 2001. Photo courtesy Ian McDonald Texas A&M. http://www.netl.doe.gov/scng/hydrate/

Methane hydrates may contain more organic carbon than all the world's coal, oil, and non-hydrate natural gas combined!

http://marine.usgs.gov/fact-sheets/gas-hydrates/gas-hydrates-3.gif

Location of major gas hydrates around the world

Location of gas hydrates and seep chemosynthetic communities in the Gulf of Mexico.

Methane seeps in the deep sea support chemosynthetic communities.

Here a mussel community surrounds a brine pool on the bottom of the Gulf of Mexico. The brine is more dense than seawater so it sits in the depression. The brine is mostly devoid of O2, and CH4 diffuses out, supporting the mussels which grow along the “shore”.

The Brine Pool is a crater-like depression on the seafloor filled with very concentrated brines coming from the Luann Salt Layer. The brine contains a high concentration of methane gas that supports a surrounding dense mussel bed. (Image based on a mosaic created by Dr. Ian McDonald, Texas A&M University).. Image courtesy of Gulf of Mexico 2002, NOAA/OER.

Stop

bubble

microlayer microlayer

microlayer microlayer

Gas conc. high

Gas conc. low

The sea surface microlayer (film) may take on many forms, and change with time.

This will affect gradient, hence flux, of gases

Theoretical sea surface and microlayer

Dlugokencky et al. 1998, Nature, 393 (6684): 447-450.

CH4 concentration in the atmosphere is increasing worldwide! The rate of increase may be slowing – but then again…

Data of C. Keeling

Data from CSIRO

South Pole

Essentially all O2 in the atmosphere accumulated because organic matter (reduced carbon) and sulfides are sequestered in sediments (i.e. not yet oxidized)

Note: the equation above differs slightly from the Nightingale (2000) expression shown a few slides earlier because the slide at left, and the equation above, cover a narrower wind speed range than the final equation, which incorporated other data sets into the best fit equation. The equation to use is G600 = 0.222U2 + 0.333U which gives G600 in cm h-1. Multiply calculated G600 by 0.24 to get value in m d-1.

Nearly all of the exchange coefficient parameterizations are derived from empirical measurements of gas fluxes. Here the letters represent measurements made at different locations.

Wanninkhof

Nightingale

Liss & Merlivat

Sam

e a

s K

60

0

Gc(

600

) (c

m h

-1)

Wind speed @ 10 m height

Gas Transfer Coefficients are still highly uncertain and an area of intense research!

Points are experimental determinations of Gc.

Lines are fitted relationships.

From Ho et al, 2006

Nightingale 2000

Growth rate

Dlugokencky et al 2009. GRL

Growth of atmospheric methane concentrations generally slowed in the period 1985-2006. But growth resumed in 2007 & 2008

Other trace gases are produced biologically but are not shown

CH4 CO2 N2O

DMS CH4 & N2O CO2Bogs and peatlandsCoastal

Marshes

CO2

CH4

DMSCH4

N2O

Soils

CO2

CH4 & N2ODMS

Greenhouse GasesDMS

CH3-S-CH3

Anti-greenhouse gas

Bacteria and plants produce and consumegases that affect the climate system

LakesForest

CO2

CO2 DMS

Phytoplankton and bacterioplankton

DMS COS Bogs and peatlands

Coastal Marshes

COS

DMS

Soils

COSDMS

Lakes

Forest

COS

COS DMS

DMSP

COS

MeSH

DMS

Stratospheric sulfate layer

Sulfate aerosol and CCN

Direct backscatter

DMS

COS

Cloud reflection Cloud reflection (albedo)

MeSH

The natural biogenic sulfur cycle and the

atmosphere

Other gases such as CS2 and DMDS are also involved

Acid Precip.

Other trace gases are produced biologically but are not shown

CH4 CO2 N2O

DMS CH4 & N2O

CO2Bogs and peatlandsCoastal

Marshes

CO2

CH4

DMSCH4

N2O

Soils

CO2

CH4 & N2O

DMS

Greenhouse GasesDMS

CH3-S-CH3

Anti-greenhouse gas

Bacteria and plants produce and consumegases that affect the climate system

LakesForest

CO2

CO2

DMS

Phytoplankton and bacterioplankton

The EndHappy Mardi Gras

0

10

20

30

40

50

60

70

80

90

Nitrogen Oxygen Argon Trace gases(incl. CO2, CH4,

Ozone etc)

Gas

Fra

ctio

n o

f at

mo

sph

ere

(%

)

The atmosphere is dominated by only a few gases. Most of the “interesting” gases are present in only trace amounts, contributing <0.1% of the total mass of the atmosphere

78%

21%

1% <0.1%

Taken from Pilson, 1998

“The air-water boundary is not a simple case and the theoretical equations do not describe the empirical data well. Ledwell (1984) found that the exchange coefficient varied not as a direct function of the diffusion coefficient (D) but rather as D0.67 at a smooth surface and D0.5 for a rough surface. Furthermore the viscosity of water in the unstirred “laminar” layer just below the interface is important in affecting the transfer of momentum from wind to the water and from the bulk water towards the interface. Both viscosity and D vary with temperature and (very slightly) with salinity. In order to take account for these various effects we introduce the Schmidt number into the exchange coefficient:”

Sc = /D; where is the kinematic viscosity (viscosity/density) of the seawater and D is the diffusion coefficient of the gas. Both and D have units of m2/s so the Sc is dimensionless.

Schmidt numbers are used in all types Schmidt numbers are used in all types of mass transfer calculationsof mass transfer calculations

Nitrogen and argon are very unreactive gases but oxygen is very reactive. Why then is O2 so abundant in the atmosphere?Early Earth was anoxic. Evolution of

cyanobacteria, and eventually eukayotic algae/plants resulted in O2 production. But respiration consumes O2 formed in photosynthesis, so why did O2 build up ???

Rik Wanninkhof from NOAA developed a relationship, partly based on empirical data, to describe air sea exchange. The relationship took into account wind speed and other factors.

)(92.1 *

5.0

2

wi CCSc

uFlux

where u is the wind velocity at 10 m above the sea surface in m/s;Sc is the Schmidt # for the gas at the given temperature and C*

i-Cw is the concentration difference between the concentration at the sea surface in equilibrium with the atmosphere(C*

I) and the concentration in the bulk (well mixed) water below the surface (Cw). Note that the term (1.92u2/Sc0.5) is the exchange velocity coefficient and it has units of m/d - even though the wind speed is entered in m/s. The conversion is already in the factor of 1.92. It is necessary to express the gas concentration (C) in moles per m3 so that the units cancel properly.

Simply plug in wind speed in m/s, appropriate Sc (dimensionless), and the concentration difference (mol/m3) to get flux in mol m-2 d-1

Liss and Merlivat relationship for Air Sea exchange. Exchange velocity as a function of wind speed (10 m above sea surface). Transfer velocity is for a gas with a Sc of 600; Values must be scaled for other gases and temperatures. These equations hold for a Schmidt # (Sc) of 600. To use these equations to calculate transfer velocities for gases with other Schmidt #'s, use the relation: Kw/Kw600 = (Sc)-0.5/(600)-0.5 So that Kw = Kw600 [(Sc)-0.5/(600)-0.5]. (for u > 3.6 m/s). At winds speeds less than 3.6 m/s the exponent of the Sc is -0.667. Wind speed range

Transfer velocity Comments

< 3.6 m/s K = 0.17 u where u = wind speed in m/s at 10 m above sea surface

Calm conditions, no waves

3.6< u < 13 m/s K = 2.85u-9.65 Some waves but not enough to cause bubbles

>13 m/s K = 5.9u - 49.3 Breaking waves and many bubbles

Another widely used parameterization of transfer velocity as a function of wind is the empirical Liss and Merlivat relationship for air-sea exchange. Relationship defines 3 separate lines of wind dependency for Kw (same as eG)

0

20

40

60

80

100

120

0 5 10 15 20 25

Wind Speed (m/s)

Ex

ch

an

ge

co

eff

icie

nt

, e G

60

0

(cm

/h)

eG600 = 0.222U2 + 0.333U

Nightingale et al., 2000 – Relationship between wind speed, U (in m/s) at 10 m above sea surface and the transfer velocity, eG (in cm/h) for a gas with a Schmidt number of 600

Flux = eGw *[C*i-Cw]

If wind speed is entered in m/s, eg600 is predicted in cm/h. (don’t do conversions of those units).

If you want eg in m/d, multiply by 24/100. The eg in m/d can then be multiplied by concentration difference at the interface in mol/m3 to obtain a flux in mol/(m2*d)

0

20

40

60

80

100

120

0 5 10 15 20 25

Wind Speed @ 10 meters (m/s)

Tra

nsfe

r co

effic

ien

t , C 60

0

(cm

/h)

Gc600 = 0.222 U2 + 0.333 U

Nightingale et al., 2000 – Relationship between wind speed, U (in m/s) at 10 m above sea surface and the transfer velocity, Gc (in cm/h) for a gas with a Schmidt number of 600

First, calculate Gc for the given wind speed.

From Wanninkhof et al 2009

Gases in seawater - Outline

Why are gases of interest?

Gas composition of the atmosphere

Fundamentals of gas solubility in seawater

Deviations from equilibrium- physical, chemical & biological

Fluxes – air-sea exchange

Important biogenic gases other than CO2 and O2.

Blake Plateau gas hydrates

Gas solubilities as a function of salinity and temperature (Based on equations of Wiess, 1970)Calculations based on the following formula which assumes equilibrium with air:ln C* = A1 + A2(100/T) + A3 * ln(T/100) + A4(T/100) + So/oo * [B1 + B2(T/100) + B3(T/100)2]where C* is the equilibrium concentration of gas in ml gas @ STP per liter of solution at given S and T. Ax and Bx are coefficients listed below.Where S is salinity in ppt and T is temperature in degrees KelvinNote formula converts temperature from Celsius to KelvinEquations are valid over temperature range of 0-40 deg C and Salinities from 0 to 40.

Weiss' CoefficientsGas A1 A2 A3 A4 B1 B2 B3

O2 -173.43 249.6339 143.3483 -21.8492 -0.033096 0.014259 -0.0017

N2 -172.5 248.4262 143.0738 -21.712 -0.049781 0.025018 -0.0034861

Ar -173.51 245.451 141.8222 -21.802 -0.034474 0.014934 -0.0017729

O2 Equilibrium

Temperature solubility Solubility in Solubility in

deg C Salinity ln C ml O2/l µmol/liter mg/liter

22 32 1.6231 5.0688 226.0 7.23322 0 1.8093 6.1064 272.3 8.71420 18 1.7424 5.7111 254.7 8.14921 21 1.7058 5.5056 245.5 7.856

To calculate the concentration of O2, N2 or Ar in equilibrium with the atmosphere at sea level (1 atm pressure), enter the approriate temperature and salinity for each gas. The spreadsheet will calculate the solubility according to the equations of Weiss (1970).

For your information

Gas solubility is a function of temperature and salinity. The equations of Weiss are often used to calculate atmospheric equilibrium solubilities of O2, N2 and Ar for any temperature or salinity. (I’ll give you this spreadsheet).