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Air-sea exchange of O2 and CO2: Processes controlling the transfer efficiency
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
Abstract [en]

World oceans cover more than 70% of the earth surface and constitutes a major sink of atmospheric CO2. Two of the most important gases in the marine carbon cycling are O2 and CO2 and hence accurate descriptions of the air-sea gas exchange of these gases are crucial. Still there is a lack of knowledge of the relative importance of processes controlling the efficiency of the air-sea gas transfer. This is especially true for Arctic and high latitude seas were studies on air-sea gas exchange are few. By studying processes causing water-side turbulence, using gases of different solubility and various measurement techniques, more knowledge on the governing processes can be obtained.

Here we present the very first air-sea fluxes of O2 using atmospheric eddy covariance measurements and investigate the dependence between the gas transfer velocity of O2 and turbulence generated by the mean wind. The instrument was found to suffer from the limited precision and time response, causing significant corrections on the O2 flux. After correcting for this, the O2 fluxes displays an anti-correlation with the air-sea fluxes of CO2 in agreement with the measured air-sea gradient of O2. The transfer velocities for O2 indicates a stronger wind dependence than other commonly used parameterizations of the transfer velocity for CO2 and O2, this especially for wind speeds > 5 m s-1 where the typical onset of wave breaking occur.

During two winter months eddy covariance measurements were taken over a high Arctic fjord. The data revealed a significant enhancement of the gas transfer velocity for CO2 from water-side convection, generated by cooling of surface waters. The dependence between water-side convection and gas transfer velocity were found for winds as high as 9 m s-1, but were strongest for wind speeds< 7  m s-1.  The data also showed on enhanced air-sea gas transfer of CO2 when conditions were unstable very close to neutral. This enhanced transfer were associated to increased contribution to the CO2 flux from downdraft of air with higher concentrations of CO2.  The combined effect of water-side convection and turbulence generated by wind results in a very effective transfer, thus the air-sea gas exchange at these latitudes may be significantly underestimated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , 42 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1471
Keyword [en]
air-sea flux, oxygen, transfer velocity, water-side convection, Arctic, UVCN
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
URN: urn:nbn:se:uu:diva-314166ISBN: 9789155498061 (print)OAI: oai:DiVA.org:uu-314166DiVA: diva2:1069501
Public defence
2017-03-17, Hambergsalen, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2017-02-24 Created: 2017-01-30 Last updated: 2017-02-24Bibliographically approved
List of papers
1. Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications
Open this publication in new window or tab >>Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications
2014 (English)In: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 31, no 11, 2498-2511 p.Article in journal (Refereed) Published
Abstract [en]

During the years 2010-13, atmospheric eddy covariance measurement of oxygen was performed at the marine site Ostergarnsholm in the Baltic Sea. The fast response optode Microx TX3 was used with two different types of tapered sensors. In spite of the increased lifetime, the optical isolated sensor is limited by the slower response time and is unsuitable for ground-based eddy covariance measurements. The sensor without optical isolation shows a -2/3 slope within the inertial subrange and attains sufficient response time and precision to be used in air-sea applications during continuous periods of 1-4 days. Spectral and cospectral analysis shows oxygen measured with the nonoptical isolated sensor to follow the same shape as for CO2 and water vapor when normalized. The sampling rate of the Microx TX3 is 2Hz; however, the sensor was found to have a limited response and resolution, yielding a flux loss in the frequency range f > 0.3Hz. This can be corrected for by applying cospectral similarity simultaneously using measurements of latent heat as the reference signal. On average the magnitude of the cospectral correction added 20% to the uncorrected oxygen flux during neutral atmospheric stratification.

National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-239999 (URN)10.1175/JTECH-D-13-00249.1 (DOI)000345008300009 ()
Available from: 2015-01-05 Created: 2015-01-05 Last updated: 2017-12-05Bibliographically approved
2. Using eddy covariance to estimate air-sea gas transfer velocity for oxygen
Open this publication in new window or tab >>Using eddy covariance to estimate air-sea gas transfer velocity for oxygen
2016 (English)In: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 159, 67-75 p.Article in journal (Refereed) Published
Abstract [en]

Air-sea gas transfer velocity for O2 is calculated using directly measured fluxes with the eddy covariance technique. It is a direct method and is frequently used to determine fluxes of heat, humidity, and CO2, but has not previously been used to estimate transfer velocities for O2, using atmospheric eddy covariance data. The measured O2 fluxes are upward directed, in agreement with the measured air-sea gradient of the O-2 concentration, and opposite to the direction of the simultaneously measured CO2 fluxes. The transfer velocities estimated from measurements are compared with prominent wind speed parameterizations of the transfer velocity for CO2 and O2, previously established from various measurement techniques. Our result indicates stronger wind speed dependence for the transfer velocity of O2 compared to CO2 starting at intermediate wind speeds. This stronger wind speed dependence appears to coincide with the onset of whitecap formation in the flux footprint and the strong curvature of a cubic wind -dependent function for the transfer velocity provides the best fit to the data. Additional data using the measured O2 flux and an indirect method (based on the Photosynthetic Quotient) to estimate oxygen concentration in water, support the stronger wind dependence for the transfer velocity of O2 O-2 to CO2.

National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-287887 (URN)10.1016/j.jmarsys.2016.02.008 (DOI)000375506200006 ()
Available from: 2016-04-26 Created: 2016-04-26 Last updated: 2017-11-30
3. Air-sea gas transfer in high Arctic fjords
Open this publication in new window or tab >>Air-sea gas transfer in high Arctic fjords
Show others...
2017 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 5, 2519-2526 p.Article in journal (Refereed) Published
Abstract [en]

In Arctic fjords and high-latitude seas, strong surface cooling dominates during a large part of the year, generating water-side convection (w*w) and enhanced turbulence in the water. These regions are key areas for the global carbon cycle; thus, a correct description of their air-sea gas exchange is crucial. CO2-data were measured via the eddy covariance technique in marine Arctic conditions and reveal that water-side convection has a major impact on the gas transfer velocity. This is observed even at wind speeds as high as 9 m s-1, where convective motions are generally thought to be suppressed by wind-driven turbulence. The enhanced air-sea transfer of CO2 caused by water-side convection nearly doubled the CO2uptake, after scaled to open sea conditions the contribution from  to the CO2 flux remained as high as 34%; this phenomenon is expected to be highly important for the total carbon uptake in marine Arctic areas.

Keyword
air-sea gas exchange, transfer velocity, surface cooling, water-side convection, CO2 flux, Arctic
National Category
Geosciences, Multidisciplinary
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-314161 (URN)10.1002/2016GL072373 (DOI)000398183700053 ()
Available from: 2017-01-29 Created: 2017-01-29 Last updated: 2017-05-04Bibliographically approved
4. Enhanced air-sea exchange of CO2 over a high Arctic fjord during unstable very close to neutral conditions
Open this publication in new window or tab >>Enhanced air-sea exchange of CO2 over a high Arctic fjord during unstable very close to neutral conditions
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Eddy covariance measurements over a high Arctic fjord reveals higher turbulent levels than normally found from classical surface layer theory. When conditions become unstable close to neutral i.e. -0.15<z/L< 0 the exchange coefficient for sensible heat, CH, is significantly enhanced compared to traditional parameterizations. Inspection of co-spectra of vertical wind (w) and temperature (T), wT show how a high frequency peak starts to develop around f≈1 Hz as z/L>-0.15, simultaneously quadrant analysis displays how the contribution from downdrafts to the vertical flux of temperature and CO2 increases. These findings are the signature of the evolving UVCN (Unstable Very Close to Neutral) regime, previously shown to enhance the vertical fluxes of temperature and humidity. In this study we show that the additional small scale turbulence related to these conditions also has the potential to enhance the vertical flux of CO2. Different to the vertical flux of temperature and humidity wq, the enhancement are not solely explained by the different properties of the air from aloft. We suggest that a part of the observed increase in CO2 flux and gas transfer velocity of CO2 when z/L> -0.1, also is generated by the increased levels of , causing higher levels of water-side turbulence. In winter the Arctic marine boundary layer is characterized by unstable stratification and during the nearly two months of measurements presented here as much as 36% of all data where associated to conditions with z/L in the range -0.15<z/L< 0.

Keyword
Transfer velocity, Arctic, UVCN, air-sea exchange
National Category
Natural Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-314162 (URN)
Available from: 2017-01-29 Created: 2017-01-29 Last updated: 2017-01-29

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