Mingxi Yang1
With intellectual contributions from:
P. Nightingale1,C. Fairall2, B. Blomquist3, P. Liss4, M. Johnson4
1
Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, UK
NOAA Earth System Research Laboratory, Physical Sciences Division, Boulder, CO, USA
3 Department of Oceanography, University of Hawaii, Honolulu, HI, USA
4 School of Environmental Sciences, University of East Anglia, Norwich, UK
2
SOLAS Conference, May 2012
Air-Sea Gas Flux — Bulk Formulation
~F
F  KC
∆C ~
F : Air-sea flux
K : Transfer velocity
∆C : Air-sea concentration difference
~ 1/K
Ohm’s Law
V = IR

Flux is often estimated
from ∆C assuming a
parameterization of K(U)
Climatological Mean ∆pCO2
Question:
How does transfer velocity vary with
wind speed?
Answer(s):
Short: Depends on the gas of interest
Long: Let’s have a look at the physical
processes near the air-sea interface

Classic ‘Two-Layer Model’ from Liss & Slater (1974)
— Molecular Diffusion Limits Gas Exchange
Ca
Ca,0
Air
Turbulent
Transfer
Molecular diffusion
Ca,0= Cw,0 / 
Molecular diffusion

Water
Cw
ra  1/ ka
rw  1/ k w

Ka  [
Kw  [
Cw,0
1
1 1

]  k a Large 
k a k w
1  1
 ]  kw
kw ka
Small 
Turbulent
Transfer
F  K a (Ca,0  Ca )
 K a (Cw /   Ca )
F  K w (Cw  Cw,0 )
 K w (Cw  Ca )
Gas

Sol ()
ra / total
Methanol
6067
0.96
Acetone
701
0.74
DMS
11.5
0.05
CO2
0.78
<0.01
Schmidt Number Dependence
Sc   / 
: Kinematic viscosity
: Molecular diffusivity

ka  Sca
kw  Scw
(α=0.78)
(α=11.5)
(α=413)
(α=701)
(α=6067)
Total Transfer
Velocity from Air
Perspective
1/ 2~2 / 3
1/ 2
Predicted K from
ka(Sca), kw(Scw), and 
Estimates from NOAA-COARE Gas
Transfer Model
(α=0.78)
(α=11.5)
(α=413)
(α=701)
(α=6067)
Total Transfer
Velocity from Water
Perspective
K a  F /(Ca,0  Ca )    F /(Cw  Cw,0 )    K w
Turbulence Drives Air-sea Transfer by:
- Disrupting diffusion layer & Causing Renewal
- Increasing turbulent diffusivity
Sources of turbulence to surface ocean include…
Calm conditions:
Buoyancy-driven
Instability
(sea warmer than air)
In moderate winds:
Shear stress ()
In rough seas:
Wave breaking
(from wind blowing tangentially
to sea surface)
(wave stress,
dissipation, whitecap)
U (z)
z
  u*2
A More Realistic Air-sea Interface
− Aerodynamic Resistance Important away from Interface
Resistance at U10=7 m/s
Air
NOAA-COARE, Fairall et al 2000
Ca,z
ra,t (za )   1 ln(za /a ) /u*a  U(za ) /u*a 2
Turbulence
~120 s/m at za = 10 m
δa
~ 1 mm
Diffusion 
&
renewal
Ca,0
δw
~ 100 m
&
Cw,0 Diffusion
renewal 
Turbulence
Zw
Cw,z
Modified from Jaehne and Haussecker, 1998

Water

ra,m  (ha /u*a )Sca1/ 2
~50 s/m for Sca = 1
rw,m  (hw /u*w )Scw1/ 2
~33000 s/m for Scw = 660
rw,t (zw )   1 ln(zw / w ) /u*w
~2600 s/m at zw = 5 m
Ka 
1
1

Ra ra,t  ra,m  (rw,t  rw,m ) / 
Additional Pathways for Gas Exchange
in High Winds
• Bubbles Injected
(complete dissolution)
• Equilibrated
(incomplete dissolution)
• Not equilibrated
(Incomplete dissolution)
Flux ~ fwh Ca
Stanley et al 2009
Flux ~ fwh ΔC α-1
Woolf 1997
Flux ~ fwh ΔC Scw-m α-n
Asher et al 1996; Keeling 1993
Sea spray potentially important for transfer
of surface enhanced/reactive gases
• HCl McInnes et al 1994
• CH3I Yokouchi et al 2001
• SO2 Sievering et al 1991
How well do we know kw?
Numerous Field/Lab Measurements of Different
Waterside Controlled Gases, though few in high winds
• Fair agreement in
low to moderate
winds
3He < CO2< DMS
• Large divergence
among different
gases/methods in
high winds
- Measurement
uncertainties &
differences
- Sea state, bubble
effects
How well do we know ka?
Based only on water transfer and theoretical studies
• Wind-wave tunnel
measurements higher
than model estimates
• ka sensitive to height
and atmospheric
stability
• Transfer of (Nonwater) airside
controlled gas only
measured in lab
- No field observation!
10000
Recent
Measurements
from
Circular
Wind-Wave
Tank
Ka (mean)
of the Air
Side
Controlled
Tracers
Demonstrate Dependence of Ka on , Sc, u*
methanol
acetone
MEK
acetaldehyde
ethyl acetate
Ka (cm/hr)
8000
6000
(a(α=4200)
= 4200)
(a(α=623)
= 623)
(a (α=524)
= 524)
(a
= 342)
(α=342)
(a(α=175)
= 175)
4000
2000
Still need field validation for
airside controlled gases
0
5
10
15
20
25
30
35
40
45
50
Ua* ( cm/s )
E. Mesarchaki1, J. Williams1, K. Richter2, C. Kräuter2, B. Jähne2
1Max
2Institute
Planck Institute for Chemistry in Mainz, Germany
for Environmental Physics, University of Heidelberg, Germany
55
60
Latitude
10
Ka from Oxygenated Volatile
-30
-20
Organic Compounds
(OVOCs: alcohols, ketones,
aldehydes..)
Produced naturally &
anthropogenically
•
Affect tropospheric chemistry
& climate via O3/OH cycling
•
Utilized by microbes in
surface ocean
30
-30
-40
Abundant & soluble
•
Methanol
Flux
-10Predicted
0
10
20
-10 in the Atlantic
(μmol m-2 day-1)
-20
Latitude
•
0
-50
Pre-dawn
-2
-1
F (umol m day )
Solar Noon
Figure from Beale, R.
Dissertation, 2011
Concurrent Flux Measurements of Methanol
and Acetone with Eddy Covariance
Vertical fluxes from the rooftop of
Plymouth Marine Laboratory
Methanol and acetone quantified
by a Proton-Transfer-Reaction
Mass Spectrometer (PTR-MS) 
H3O  CH3OH  H2O  CH 3OH  H 
H3O  CD3OH  H2O  CD3OH  H 
• Use continuous Isotopic standard
• Derive ambient [OVOC] from ratio
Initial Flux Results
Promising
Methanol
Acetone
F  W 'C'

• Similarity to heat spectrum confirms
robustness of OVOC flux
measurements
• Need uncontaminated flux and
seawater [OVOC] to estimate Ka
Why are lab measurements not enough?
• Different turbulence structure compared to nature
- Limited fetch and wall effects
- Fixed atmospheric stability
• Usually with fresh water
- Surface tension
- Chemical reactivity
- Microlayer enhancement
Need field validation for airside controlled gases
Near-surface Variability in
Microbial Loss of Methanol
SOLAS-DOGEE II Cruise
1000 ppt
500 ppt
N. Atlantic, U ~ 5 m/s
C: Catamaran
N: Near Surface Sampling Device
Dixon & Nightingale, BGD, 2012
300 ppt