1. The Air-Sea Gas Transfer Velocity - Approaching it from Multiple Angles Mingxi Yang, T. Bell, P. Nightingale, J. Shutler (Additional contributions from B. Blomquist) Plymouth Marine Laboratory ESA/EGU/SOLAS Conference, Frascati, Oct 2014

3. Resistance on Airside/waterside analogous to two resistors in series
Gas Transfer Velocity (K) Controlled by Resistance on Airside/Waterside – Partitioning Depends on Solubility (α)
7 m/s and 20 °C, COARE model
Momentum/heat transfer airside controlled
Acetone subject to both airside & waterside control
Resistance on Airside/waterside analogous to two resistors in series
ra=1/ka
rw=1/kw
Highly soluble gases limited on airside
Sparingly soluble gases limited on waterside

4. Motivation
Reduce uncertainties in k (airside and waterside controlled gases), especially in high winds
Improve process level understanding in gas transfer
Approximate Uncertainty
3 < U < 10 m/s
U > 15 m/s ka
20%
50% kw
30%
80%
ktangential
kbubble
60%
Approach
Measure k of multiple gases with varying solubility in conjunction with observations of waves, bubbles, etc. Example: High Wind Gas Exchange Study (HiWinGS)

5. HiWinGS Cruise, Oct/Nov 2013

6. St Jude Storm 25~28 Oct, 2013
Telegraph
Guardian

8. List of Observations PML Contribution:
Directly quantify the air-sea transport of methanol & acetone
- Air concentration : PTR-MS w/ isotopic standard (Yang et al. ACP, 2013)
- Water concentration: PTR-MS w/ membrane inlet (Beale et al. ACA, 2011)
- Air-sea flux: Eddy Covariance w/ PTR-MS (Yang et al. ACP, 2013, 2014)

9. PML Eddy Covariance System
On Ship’s Foremast
(~20 m amsl)
Sonic anemometer (10 Hz)
Motion sensor (~15 Hz)
Gas inlet to PTRMS housed in lab van

10. Methanol & Acetone Concentrations and Flux
From High Resolution Proton-Transfer-Reaction
Mass Spectrometer (PTR-MS)
Eddy covariance flux
Wind velocity corrected for ship motion (Edson et al 1998)
Total transfer velocity from air perspective
- Yang et al PNAS 110, 50, 20034–20039, 2013
- Yang et al ACP 14, , 2014
Measuring at ~2.2 Hz
Soft chemical ionization
Isotopic standards added at inlet tip

11. Friction velocity consistent with COARE prediction
As is sensible heat

12. Momentum Transfer
Measurements better described by COARE model V3.5 than V3.0, especially in high winds Greater scatter in calmer conditions, when winds often came from the side of the ship (minor flow distortion)

13. Reduced sensible heat transfer during 25 Oct Storm - Sea spray/precipitation attenuates sensible heat flux?

14. Methanol & Acetone fluxes from air to sea
Close to bulk predictions (~18% relative RMS error)

15. Transfer Velocities of Methanol & Acetone (Ka = Flux/ΔC) in general agreement with COARE Model on the mean (some deviations in very high winds…)

16. Asymmetry between Airside and Waterside Transfer
Diffusion &
Micro turbulence
Ca,0
Cw,0 Ca Cw
Air
Water
Modified from Jaehne and Haussecker, 1998
Turbulence
~ 1 mm
~ 0.1 mm Zw
Airside transfer (ka) significantly limited by both turbulent (aerodynamic) resistance and molecular diffusive resistance
Waterside transfer (kw) mostly limited by molecular diffusion/micro turbulence

17. KHeat ~12% higher than KMeOH - Heat has higher diffusivity in air
KAcetone ~28% lower than KMeOH
Acetone has lower solubility in water and lower diffusivity in air
KHeat ~12% higher than KMeOH - Heat has higher diffusivity in air

18. Acetone Transfer Subject to Airside +Waterside Resistance - Estimation of kw by Difference
kw =1/(α(1/Ka – 1/ka))
At HiWinGs mean U10n of 12 m/s, we get:
kw = 9.1±4.3 cm/hr, normalized to Scw = 660
kw660 = 15.9±7.4 cm/hr
Ka : Total transfer velocity
ka : Airside transfer velocity kw : Waterside transfer velocity
α : Dimensionless solubility
Gas
Kw660 (cm/hr) at U10n=12 m/s
Reference
Acetone
15.9±7.4
This work
DMS
18~22
Yang et al. 2011
Dual Tracer 34
Nightingale et al 2000 37
Ho et al 2006
CO2 48
McGillis et al 2001
14C 39
Sweeney et al 2007
Indirectly estimated kw close to kDMS Tangential transfer
Additional bubble-mediated transfer for less soluble gases

19. Conclusions thus far from HiWinGS [Yang et al. accepted in JGR Oceans]
Turbulent transfer of momentum, sensible heat, methanol, and acetone largely follow expected trends up to U ~20 m/s
Reduction in heat/organics transfer in higher winds, possibly related to sea spray/precipitation?
Airside transfer velocity (ka) from methanol lower than that of sensible heat
Explained by difference in airside diffusivity
Waterside transfer velocity (kw) indirectly estimated from measurements of acetone transfer
Close to previous estimates of kDMS (tangential transfer)
Much lower than kw of less soluble gases

20. Outlook Expand the range of proxy tracers measured to better understand physical processes
Sparingly soluble
Carbon monoxide (Blomquist et al. AMT, 2012)
Terpenes?
Intermediate solubility
Acetaldehyde (Yang et al. ACP 2014)
Organohalogens?
Surface reactive
Ozone (e.g. Bariteau et al. 2011)
Sulfur dioxide (e.g. Faloona et al. 2010)
Heat
Modified Controlled Flux Technique (e.g. Nagel et al. 2014)
Sol.
Sc No. K ΔC
Flux
Modified from
Wanninkhof et al. 2009

21. Outlook (cont’) Remote sensing of other factors that control k (Coincident to in situ multi-gas k measurements)
1. Satellite altimeter backscattering more directly related to surface turbulence than wind speed
EC kDMS correlated to Ku band backscattering; better correlation with difference between Ku band and C band (Goddijn-Murphy et al. 2012, 2013)
How to increase overlap between altimetry data and in situ k observation?
Copernicus programme: Sentinel-3 mission
Aircraft? Geostationary satellite?
OSSPRE Cruise
U. Heidelberg, U. Washington
2. Mean squared wave slope
Scanning laser slope gauge (e.g. Bock and Hara, 1995; Frew et al 2004)
Reflective stereo slope gauge & medium angle slope gauge (e.g. Kiefhaber et al 2011)
Accounts for surfactant effect

22. Questions & Comments?

23. More Insights from HiWinGs in the Near Future
DMS
Comparison of kDMS with previous high wind measurements (e.g. SO GasEx 2008, Knorr 2011). Suppression in high winds?
CO2
Intercomparison of two closed-path sensors and comparison with previous measurements. Which wind speed dependence?
Multi-gas Comparisons
Difference between kDMS and kCO2 explained by bubbles?
Influence of wave state, bubble, and sea spray on waterside and airside transfer? …

24. Instrument Setup PML UH, NOAA PML, UCSD UH, NOAA Sonic anemometer
Sampling line
UH, NOAA

25. Airmass Back Trajectories (5-day HYSPLIT)

26. Sensible Heat Flux Mostly Consistent with COARE Model

27. Mean HiWinGs Cospectra Demonstrate Expected Behaviors of Atmospheric Turbulence
U’W’
Peak at 0.1~0.2 Hz due to wind-wave interaction or imperfect motion correction?
Attenuation of sensible heat transfer related to sea spray?
U’T’

28. High degree of correlation between the two suggests common sources (e
High degree of correlation between the two suggests common sources (e.g. terrestrial emission)
Lower
concentrations
In high
humidity
Higher atm. Acetone & methanol concentrations further south, esp. in southerly/westerly winds

29. NOAA-COARE Gas Transfer Model
Waterside
A & B are empirical constants
Airside
Woolf (97) model:
Gas solubility affects bubble-mediated transport (kb)
: Ostwald solubility
fwh: Whitecap fraction (~u3)

30. Motivation — Large Divergence in kw in High Winds
Obs rare in stormy seas
Existing obs suggest solubility dependence in kw
kw higher for CO2 than for dual tracer (not explained by COARE model)

31. Motivation (cont’) — Why Apparent Attenuation of kDMS in High Winds?
N. Atlantic
2011
S. Ocean, 2008
Yang et al., JGR, 2011 & Unpublished Data
Bell et al., ACP, 2013