Dissolved Gases and Air-Sea Exchange

1. Dissolved Gases and Air-Sea Exchange The oceans are full of gas! Photo by Will Drennan, NOAA

2. Spectrum of gases in the ocean • Proton-exchanging gases- CO2, H2S, NH3 , SO2 - Have ionic forms after losing or gaining a proton • Permanent Gases(unreactive) gases - N2, Ne, He (3He, 4He), Xe, 222Rn t1/2 = 3.5 d (from 226Ra), and Ar. • Reactive Gases (high chemical or biological reactivity) • O2 • H2 (3H, t1/2 = 12.7 y) • N2O (intermediate in the N-cycle; potent greenhouse gas) • CH4 • DMS (main sulfur input to atmosphere; anti-greenhouse gas) • COS (photochemical source, most abundant sulfur gas in atmosphere) • CO • Other trace gases • CH3I, CH3Br, CH3Cl, CH3Br; CHBr3 (bromoform), CHCl3 (chloroform) • Freons (unreactive, but catalytic for O3 destruction in the atmosphere- also strong greenhouse gases). • Ethane, propane, isoprene etc. also known as non-methane hydrocarbons (NMHC)

3. Why are gases of interest? • Highly mobile chemicals, move into and out of the ocean via the atmosphere and through different compartments within the ocean • Air-Sea exchange of gases is important for climate and atmospheric chemistry • Participate in many important biological reactions: • Photosynthesis/respiration • Nitrogen fixation/denitrification • Tracers of water mass movement 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

4. 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)

5. pA = 0.5 atm pB = 0.5 atm ptotal = (pA+pB) = 1 atm pA = 0.5 atm Simple Gas Laws Dalton’s law of partial pressures Each 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 http://id.mind.net/~zona/mstm/physics/mechanics/energy/heatAndTemperature/gasMoleculeMotion/gasMoleculeMotion.html

6. 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)

7. Ideal gas law pV = nRT Where p = pressure of the gas V = volume of the gas n = # of moles of the gas T= 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.

8. 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

9. 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.

10. 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 0.002-0.010 The range of concentrations is due mainly to variations in temperature, which affects the solubility of gases in equilibrium with the atmosphere.

11. Gas equilibrium Gas molecules Air Water 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.

12. A(g) Gas Solubility A(aq) 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. [A(aq)] = KH,A pA(g) Where KH,A is the Henry’s Law constant KH, Ais 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

13. 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)

14. 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). [Aaq] =  (PAmoist) Where [Aaq] is the concentration of gas in ml(@STP)/liter of water, PAmoist is the pressure of the gas A (the total pressure will be higher because of water vapor). The proportionality holds for any partial pressure of the gas A

15. 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

16. 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 Heavier gases have a larger temperature effect. More soluble CH4 Libes, Chap 6 Values are for 35 ppt SW

17. Gas solubility is a non-linear function of salinity, with greater solubility at LOW salinity From Pilson

18. 0 1 2 3 4 5 O2 minimum layer Sargasso Sea Depth distributions of oxygen ~ Atmos. Equilibrium values Conc.  Conc.  O2 minimum layer Depth km Eastern Tropical Pacific What about estuaries and nearshore waters? There is an O2-minimum layer in the thermocline nearly everywhere in the ocean!

19. Coastal hypoxia on Alabama shelf (Sep 8, 2010) FOCAL (M. Graham)

20. 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. Taken from NOAA report of dissolved O2 in Gulf –Sept, 2010 Month

21. Dissolved O2 concentrations in central Gulf of Mexico near Deepwater Horizon spill site O2 minimum (natural) Oil & gas layer Valentine et al 2010. Science Express.

22. CH4 concentration (log scale) O2 concentrations are depleted in the layer where natural gas (CH4) concentrations are high O2 concentration anomaly Negative anomalies = less O2 than expected Valentine et al 2010. Science Express.

23. 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) 100% O2 % Saturation Dauphin Island Mobile NEP http://www.mymobilebay.com/stationdata/chartdata/defaultb.asp?stationid=628&param=doper

24. Deviations from equilibrium Water mass mixing Temperature 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 Conservative mixing line Concentration -> Low end member Calculated saturation value Temperature -> Saturation anomaly

25. Deviations from equilibrium Bubble Exchange and Air Injection Gas 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!

26. Bubbles penetrate 5 m or more!

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

28. 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.

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

30. The flux equation can be approximated by: 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 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) 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! 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.

31. Turbulently mixed Turbulently mixed Thin film model of gas exchange at water-air boundary - a conceptual model 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 Gas concentration (or partial pressure) Air Csat Laminar layer “thin film”  z Concentration gradient dC/dz Water Boundary layers above and below interface Cw Bulk concentration (Cw) is uniform below laminar layer Depth (z) The thickness of the diffusive boundary layer will directly affect the flux! Concentration (or partial pressure)

32. 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.

33. 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. Wind Temperature (affects diffusion) [gas]

34. Wind speed affects the surface roughness, momentum transfer and ultimately the concentration gradient between the air and sea. The sea Surface is rarely smooth and “ideal”

35. Exchange coefficients are ~ exponential functions of wind speed Nightingale (2000) relation for exchange coefficient K600 = 0.222U2 + 0.333U 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

36. 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

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

38. Into the ocean Out of the ocean Spatial variation of CO2 flux from the ocean Out In In Out Out In In Emerson and Hedges

39. Climate forcing in the atmosphere Uncertainty - high Uncertainty - low Values relative to the year 1850 Wigley, 1999, The Science of Climate Change, Pew Center for Global Climate Change

40. CH4 CO2 N2O

41. 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.

42. 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. CH4 profile in Gulf of Mexico 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?

43. Coastal hypoxia on Alabama shelf (Sep 8, 2010) 25.9 9.7 7.3 29.7 3.5 10.7 3.2 4.1 7.9 24.2 3.5 3.5 27.3 15.3 Color contours indicate [O2]. Numbers are [CH4] in nM 28 38 FOCAL (M. Graham)

44. CH4 clathrate Exposed hydrate on seafloor 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). Hydrate crystals are stable only within certain temperature & pressure ranges. Typically > 500 m and < 10 oC. Methane hydrates look like ice! Images from http://www.netl.doe.gov/scng/hydrate/

46. Gas hydrates are stable at higher pressures and lower temperatures Hydrates stable Adding salt or N2 shifts boundary to left; adding CO2, H2S etc shifts boundary to right Trehu et al 2006

48. 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!

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

50. Location of major gas hydrates around the world