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: martin.johnson@uea.ac.uk

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?

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

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(!)

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

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

H2SCH3ClC6H5CH3CH3BrC2H5ICH3IHICHCl3CHI3CH2CL2DMSDES2ButylnitrateBr22PropylnitrateCH2IClBrClDMDS1Propylnitrate1ButylnitrateHBrCH2Br2

SO2EthylnitrateCH2IBrCHBr3MethylnitrateCH2I2PPNI2methylmethanoatePANTEAmethylethanoateTMAHCNpropanalethanalbutanoneHClNHCl2acetone

OHDEADMAnitromethaneHNO2MEACH3CNNH32NitrophenolHOBrNH2ClMMAIClMeOHEtOHIBrmethylperoxideethylperoxideIOHOIPhenolmethanalHO2

KH dependence of r

g/r

l

rg/r

l compared with Liss and Slater 1974

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

Gases other than CO2 and SO

2,

reactions other than hydration

Reversible reactions

i) undersaturationii) supersaturation

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...

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

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

NH3 pH = 8

NH3 pH = 9

SO2

CH2I2

CH4

Scopus citations since Jan 2008

COARE papers:Fairall et al 2003Hare et al 2004

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

kl_660