transfer velocities for a suite of trace gases of emerging biogeochemical importance: liss and...
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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.TRANSCRIPT
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Transfer velocities for a suite of trace gases of
emerging biogeochemical importance:
Liss and Slater (1974) revisited
M. T. Johnson1, P. S. Liss1, T.G. Bell1 and C.Hughes1 and J. Woeltjen1,2
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: [email protected]
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Motivation
Lots of researchers need to calculate air-water exchanges from concentration difference measurements:
Many not experts in gas exchangeMany 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 calculationse.g. for CH
3OH using kl rather than kg leads to factor of 20
overestimation of flux!
When is it appropriate to consider either kl=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?
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Application of thin film model of interfacial mass exchange to the air-sea interface
Early estimates of kg and k
l for H
2O and O
2 and some trace gases of interest:
SO2, N
2O, CO, CH
4, CCl
4, CCl
3F, CH
3I, DMS
Showed that rg/r
l was small (<10-1) for all except SO
2, where chemical enhancement in the
liquid phase was shown to be important
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For each compound the following data are required:
Henry's law solubility (KH)
T-dependence of KH (-Δ
solnH/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 (kl) and Jeffrey et al 2010 (k
g) used here.
Key assumptions: neutral bouyancy, all the assumptions made by the kl and k
g
parameterisations selected(!)
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log(rg/r
l)
for a suite of trace gases
Log (rg/r
l) = 0 → r
g = r
l
→ 50% contribution to
total transfer from both phases
Log (rg/r
l) = 1 → r
g/r
l = 10
→ 10% of total resistance due to liquid phase
Log (rg/r
l) = -1 → r
g/r
l = 0.1
→ 10% contribution to resistance from gas phase
Log (rg/r
l) = 2
→ 1% contribution to transfer from liquid phase
Log (rg/r
l) = -3
→ 0.1% contribution to transfer from gas phase
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KH 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
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H2SCH3ClC6H5CH3CH3BrC2H5ICH3IHICHCl3CHI3CH2CL2DMSDES2ButylnitrateBr22PropylnitrateCH2IClBrClDMDS1Propylnitrate1ButylnitrateHBrCH2Br2
SO2EthylnitrateCH2IBrCHBr3MethylnitrateCH2I2PPNI2methylmethanoatePANTEAmethylethanoateTMAHCNpropanalethanalbutanoneHClNHCl2acetone
OHDEADMAnitromethaneHNO2MEACH3CNNH32NitrophenolHOBrNH2ClMMAIClMeOHEtOHIBrmethylperoxideethylperoxideIOHOIPhenolmethanalHO2
KH dependence of r
g/r
l
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rg/r
l compared with Liss and Slater 1974
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Chemical enhancement of kl (and k
g?):
Hoover and Berkshire 1969
α = τ / {(τ-1) + (tanh(x)/x)}
where x = z(khyd
.τ/D)1/2
z = layer thickness (inversely related to wind speed)D = molecular diffusivity of gas in mediumk
hyd = rate of (hydration) reaction of gas in seawater
τ = 1+ ([unreacted gas]/[reacted products])
Tanh(x)/xWhen k
hyd slow, x is small, tanh(x)/x=1, α = 1
When khyd
v fast, x is large, tanh(x)/x=0, α
max = τ / (τ-1) = e.g. 1+ [XH
2O]/[X]
Hoover and Berkshire assume stagnant film model,
which probably underestimates potential chemical enhancement
for reversible reactions
Assumptions: 1. Stagnant film model applies2. reaction can be represented by pseudo-first-order
rate constant – i.e. rate is proportional to concentrationof gas of interest and independent of all other factors
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Gases other than CO2 and SO
2,
reactions other than hydration
Reversible reactions
i) undersaturationii) supersaturation
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Gases other than CO2 and SO
2,
reactions other than hydration
Irreversible reactions(e.g. photolysis)
i) understaturation2) 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...
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Effect of chemical enhancement / inhibition on K for gases of different solubilities
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Rate constants to give α = 2 in both gas and liquid phases (90 gases plotted)
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Rate constants required to give different α for a gas of 'average' diffusivity
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Selected reaction rates
Compound Gas phase reaction
Rate constant/ s-1
Liquid phase reaction
Rate constant/ s-1
NH3
Uptake on acid sulfate aerosol
10-5 protonation >109
CH2I2
photolysis 10-4 photolysis 10-3
SO2
- - hydration 106
CH4
Oxidation by OH
<10-6 Biological turnover
10-3 *
CO2
- - 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
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NH3 pH = 8
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NH3 pH = 9
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SO2
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CH2I2
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CH4
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Scopus citations since Jan 2008
COARE papers:Fairall et al 2003Hare et al 2004
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Total transfer velocity
KH
kgk
l
u10TSK
H0 -Δ
solnH/R
Scg
Scl
Dg
Dl
νg
νl
ηgTη
lT,S
Sensitivity analysis
ρgTρ
lT,S V
bC
Dk
gk
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
Dl D
g
25 25
0.1 3
11 0.3
Table presents percentage change in total transfer velocity over range of parameter uncertainty
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kl_660