Transfer velocities for a suite of trace gases of  emerging biogeochemical importance:  Liss and Slater (1974) revisited  ...
Motivation Lots of researchers need to calculate air-water exchanges from concentration difference measurements: Many not ...
Application of thin film model of interfacial mass exchange to the air-sea interface  Early estimates of k g  and k l  for...
For each compound the following data are required: Henry's law solubility (K H ) T-dependence of K H  (- Δ soln H/R )...
log(r g /r l )   for a suite of trace gases Log (r g /r l ) = 0 -> r g  = r l ->  50% contribution to total transfer...
K H  dependence of r g /r l For gases with solubility between 0.1 and 1000 mol/L/atm, both phases need to be considered in...
H2S CH3Cl C6H5CH3 CH3Br C2H5I CH3I HI CHCl3 CHI3 CH2CL2 DMS DES 2Butylnitrate Br2 2Propylnitrate CH2ICl BrCl DMDS 1Propyln...
r g /r l  compared with Liss and Slater 1974
Chemical enhancement of k l  (and k g ?): Hoover and Berkshire 1969 α  =  τ / {(τ-1) + (tanh(x)/x)} where x = z(k hyd .τ/D...
Gases other than CO 2  and SO 2 , reactions other than hydration Reversible reactions i) undersaturation ii) supersaturati...
Gases other than CO 2  and SO 2 , reactions other than hydration Irreversible reactions (e.g. photolysis) i) understaturat...
Effect of chemical enhancement / inhibition on K for gases of different solubilities
Rate constants to give  α  = 2 in both gas and liquid phases (90 gases plotted)
Rate constants required to give different  α for a gas of 'average' diffusivity
Selected reaction rates Compound Gas phase reaction Rate constant / s -1 Liquid phase reaction Rate constant / s -1 NH 3 U...
NH 3  pH = 8
NH 3  pH = 9
SO 2
CH 2 I 2
CH 4
Scopus citations since Jan 2008 COARE papers: Fairall et al 2003 Hare et al 2004
Total transfer velocity K H k g k l u 10 T S K H 0 - Δ soln H/R Sc g Sc l D g D l ν g ν l η g T η l T,S Sensitivity analys...
k l_660
Transfer velocities for a suite of trace gases of  emerging biogeochemical importance:  Liss and Slater (1974) revisited
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Transfer velocities for a suite of trace gases of emerging biogeochemical importance: Liss and Slater (1974) revisited

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Authors M. T. Johnson, P. S. Liss, T.G. Bell and C.Hughes and J. Woeltjen

Paper given at the 6th International Symposium on Gas Transfer at Water Surfaces, Kyoto, Japan, May 2010.

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Transfer velocities for a suite of trace gases of emerging biogeochemical importance: Liss and Slater (1974) revisited

  1. 1. Transfer velocities for a suite of trace gases of emerging biogeochemical importance: Liss and Slater (1974) revisited M. T. Johnson 1 , P. S. Liss 1 , T.G. Bell 1 and C.Hughes 1 and J. Woeltjen 1,2 1 Laboratory for Global Marine and Atmospheric Chemistry, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK 2 Now at: Helmholtz Centre for Environmental Research GmbH - UFZ, Permoser Strae 15, 04318 Leipzig, Germany. E-mail: martin.johnson@uea.ac.uk
  2. 2. Motivation Lots of researchers need to calculate air-water exchanges from concentration difference measurements: Many not experts in gas exchange Many for poorly studied gases (i,.e. Not GHGs, noble gases, O 2 or DMS) Concentration uncertainty is large so simple (wind driven parameterised) approach to transfer velocity is probably sensible Serious mistakes are often made in calculations e.g. for CH 3 OH using kl rather than kg leads to factor of 20 overestimation of flux! When is it appropriate to consider either k l =K l or k g =K g ? Notwithstanding the need to choose the 'best' transfer velocity parameterisations; solubility and diffusivity of the gas, and viscosity of the medium must be quantified for the gas of interest When is chemical enhancement potentially important?
  3. 3. Application of thin film model of interfacial mass exchange to the air-sea interface Early estimates of k g and k l for H 2 O and O 2 and some trace gases of interest: SO 2 , N 2 O, CO, CH 4 , CCl 4 , CCl 3 F, CH 3 I, DMS Showed that r g /r l was small (<10 -1 ) for all except SO 2 , where chemical enhancement in the liquid phase was shown to be important
  4. 4. For each compound the following data are required: Henry's law solubility (K H ) T-dependence of K H (- Δ soln H/R ) Molecular structure (in order to calculate liquid molar volume at boiling point, V b ) Wind speed, temperature, salinity Calculating temperature, wind-speed and salinity dependent transfer velocities Henry's law solubility and temp dependence mostly taken from Rolf Sanders compilation ( http://www.mpch-mainz.mpg.de/~sander/res/henry.html ), or primary literature where not compiled by Sander. Salinity dependence of K H determined from novel relationship derived from empirical data on gas solubilities in seawater Vb calculated using 'Schroeder' additive method Diffusivities of gases in air and water and viscosities of air and water calculated from best available paramterisations Transfer velocities: various parameterisations of k l and k g implemented. Nightingale et al 2000 (k l ) and Jeffrey et al 2010 (k g ) used here. Key assumptions: neutral bouyancy, all the assumptions made by the k l and k g parameterisations selected(!)
  5. 5. log(r g /r l ) for a suite of trace gases Log (r g /r l ) = 0 -> r g = r l -> 50% contribution to total transfer from both phases Log (r g /r l ) = 1 -> r g /r l = 10 -> 10% of total resistance due to liquid phase Log (r g /r l ) = -1 -> r g /r l = 0.1 -> 10% contribution to resistance from gas phase Log (r g /r l ) = 2 -> 1% contribution to transfer from liquid phase Log (r g /r l ) = -3 -> 0.1% contribution to transfer from gas phase
  6. 6. K H dependence of r g /r l For gases with solubility between 0.1 and 1000 mol/L/atm, both phases need to be considered in quantifying total transfer veloctiy
  7. 7. H2S CH3Cl C6H5CH3 CH3Br C2H5I CH3I HI CHCl3 CHI3 CH2CL2 DMS DES 2Butylnitrate Br2 2Propylnitrate CH2ICl BrCl DMDS 1Propylnitrate 1Butylnitrate HBr CH2Br2 SO2 Ethylnitrate CH2IBr CHBr3 Methylnitrate CH2I2 PPN I2 methylmethanoate PAN TEA methylethanoate TMA HCN propanal ethanal butanone HCl NHCl2 acetone OH DEA DMA nitromethane HNO2 MEA CH3CN NH3 2Nitrophenol HOBr NH2Cl MMA ICl MeOH EtOH IBr methylperoxide ethylperoxide IO HOI Phenol methanal HO2 K H dependence of r g /r l
  8. 8. r g /r l compared with Liss and Slater 1974
  9. 9. Chemical enhancement of k l (and k g ?): Hoover and Berkshire 1969 α = τ / {(τ-1) + (tanh(x)/x)} where x = z(k hyd .τ/D) 1/2 z = layer thickness (inversely related to wind speed) D = molecular diffusivity of gas in medium k hyd = rate of (hydration) reaction of gas in seawater τ = 1+ ([unreacted gas]/[reacted products]) Tanh(x)/x When k hyd slow, x is small, tanh(x)/x=1, α = 1 When k hyd v fast, x is large, tanh(x)/x=0, α max = τ / (τ-1) = e.g. 1+ [XH 2 O] /[X] Hoover and Berkshire assume stagnant film model, which probably underestimates potential chemical enhancement for reversible reactions Assumptions: 1. Stagnant film model applies 2. reaction can be represented by pseudo-first-order rate constant – i.e. rate is proportional to concentration of gas of interest and independent of all other factors
  10. 10. Gases other than CO 2 and SO 2 , reactions other than hydration Reversible reactions i) undersaturation ii) supersaturation
  11. 11. Gases other than CO 2 and SO 2 , reactions other than hydration Irreversible reactions (e.g. photolysis) i) understaturation 2) supersaturation For an irreversible reaction that produces the gas of interest in the surface layer, a flux out would be enhanced and a flux in would be inhibited... The physics is the same in the gas phase, so the Hoover and Berkshire equation will apply there too...
  12. 12. Effect of chemical enhancement / inhibition on K for gases of different solubilities
  13. 13. Rate constants to give α = 2 in both gas and liquid phases (90 gases plotted)
  14. 14. Rate constants required to give different α for a gas of 'average' diffusivity
  15. 15. Selected reaction rates Compound Gas phase reaction Rate constant / s -1 Liquid phase reaction Rate constant / s -1 NH 3 Uptake on acid sulfate aerosol 10 -5 protonation >10 9 CH 2 I 2 photolysis 10 -4 photolysis 10 -3 SO 2 - - hydration 10 6 CH 4 Oxidation by OH <10 -6 Biological turnover 10 -3 * CO 2 - - Hydration 0.04 Methanal (formaldehyde) ? ? Hydration 10 * estimated from bulk seawater bacterial methane turnover of 1 day -1 scaled up by factor of 100 for possible microlayer bacterial activity
  16. 16. NH 3 pH = 8
  17. 17. NH 3 pH = 9
  18. 18. SO 2
  19. 19. CH 2 I 2
  20. 20. CH 4
  21. 21. Scopus citations since Jan 2008 COARE papers: Fairall et al 2003 Hare et al 2004
  22. 22. Total transfer velocity K H k g k l u 10 T S K H 0 - Δ soln H/R Sc g Sc l D g D l ν g ν l η g T η l T,S Sensitivity analysis ρ g T ρ l T,S V b C D k g k l Estimated parameter /% uncertainty Highly soluble gas e.g. NH 3 Sparingly soluble gas. e.g CO 2 25 25 10 25 5 5 10 10 25 10 10 10 10 0.1 25 10 16 -0.04 4 0.05 -0.05 -1 10 1 -1 9 20 2 1 2 -0.2 2 4 4 -6 20 0.1 -0.1 1 Sparingly soluble gas. e.g CO 2 Highly soluble gas e.g. NH 3 Estimated parameter /% uncertainty D l D g 25 25 0.1 3 11 0.3 Table presents percentage change in total transfer velocity over range of parameter uncertainty
  23. 23. k l_660

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