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Air-Sea Fluxes of CO2: Analysis Methods and Impact on Carbon Budget
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (Awep)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Carbon dioxide (CO2) is an important greenhouse gas, and the atmospheric concentration of CO2 has increased by more than 100 ppm since prior to the industrial revolution.  The global oceans are considered an important sink of atmospheric CO2, since approximately one third of the anthropogenic emissions are absorbed by the oceans. To be able to model the global carbon cycle and the future climate, it is important to have knowledge of the processes controlling the air-sea exchange of CO2. In this thesis, measurements as well as a model is used in order to increase the knowledge of the exchange processes.

The air-sea flux of CO2 is estimated from high frequency measurements using three methods; one empirical method, and two methods with a solid theoretical foundation. The methods are modified to be applicable for various atmospheric stratifications, and the agreement between methods is good in average.

A new parameterization of the transfer velocity (the rate of transfer across the air-sea interface), is implemented in a Baltic Sea model. The new parameterization includes also the mechanism of water-side convection. The impact of including the new parameterization is relatively small due to feedback processes in the model. The new parameterization is however more representative for flux calculations using in-situ measurement or remote sensing products. When removing the feedback to the model, the monthly average flux increases by up to 20% in some months, compared to when water-side convection is not included.

The Baltic Sea carbon budget was estimated using the Baltic Sea model, and the Baltic Sea was found to be a net sink of CO2. This is consistent with some previous studies, while contradictory to others. The dissimilarity between studies indicates the difficulty in estimating the carbon budget mainly due to variations of the CO2 uptake/release in time and space. Local variations not captured by the model, such as coastal upwelling, give uncertainties to the model. Coastal upwelling can alter the uptake/release of CO2 in a region by up to 250%. If upwelling would be included in the model, the Baltic Sea might be considered a smaller sink of CO2.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , 47 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1020
Keyword [en]
air-sea exchange, carbon dioxide, Baltic Sea, eddy-covariance method, inertial dissipation method, cospectral-peak method, Baltic Sea measurements, CO2 fluxes, Galathea 3 expedition, Baltic Sea modeling, water-side convection, coastal upwelling, carbon budget
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
URN: urn:nbn:se:uu:diva-194960ISBN: 978-91-554-8599-3 (print)OAI: oai:DiVA.org:uu-194960DiVA: diva2:606708
Public defence
2013-04-05, Hambergsalen, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2013-03-14 Created: 2013-02-20 Last updated: 2013-04-02Bibliographically approved
List of papers
1. The annual cycle of carbon dioxide and parameters influencing the air-sea carbon exchange in the Baltic Proper
Open this publication in new window or tab >>The annual cycle of carbon dioxide and parameters influencing the air-sea carbon exchange in the Baltic Proper
Show others...
2008 (English)In: Journal of Marine Systems, ISSN 0924-7963, Vol. 74, no 1-2, 381-394 p.Article in journal (Refereed) Published
Abstract [en]

A land-based field station, two moored buoys and data from the Finnpartner ship were used to investigate the variability of the air-sea CO2-flux and parameters controlling the flux during one year in the Baltic Sea region.The agreement between the sea surface partial pressure of CO2 measured near the tower and from the ship in the central parts of the Baltic Proper was relatively good during most of the period.But, during periods with intense biological activity or strong upwelling there were significant differences.The flux of CO2 was measured with the eddy-correlation method.The transfer velocity was calculated from the flux measurements and the instrumental uncertainty in calculations of the hourly values of transfer velocity was of the order of 20%.The calculated value of the transfer velocity increased with increasing the wind speed.The relation showed, however, great scatter and no clear wind-dependent relation could be determined.It was shown that for the measured flux and for transfer velocities estimated from measurements it is important to know the variability of pCO2w in the footprint area.This is of particular importance when investigating the processes influencing the flux.When calculating the air-sea flux of CO2 the greatest uncertainty is in the determination of the transfer velocity, but it was shown that also the partial pressure of CO2 in the surface water is crucial to determine with good accuracy.

Keyword
Carbon dioxide, air-sea interaction, air–sea exchange, transfer velocity parameterization, CO2 exchange, eddy-correlation measurements
National Category
Natural Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-85618 (URN)10.1016/j.jmarsys.2008.02.005 (DOI)000261256100023 ()
Available from: 2008-10-23 Created: 2008-10-23 Last updated: 2013-04-30Bibliographically approved
2. Atmospheric CO2 variation over the Baltic Sea and the impact on air–sea exchange
Open this publication in new window or tab >>Atmospheric CO2 variation over the Baltic Sea and the impact on air–sea exchange
2009 (English)In: Boreal environment research, ISSN 1239-6095, Vol. 14, no 1, 238-249 p.Article in journal (Refereed) Published
Abstract [en]

The variability in time and space of the atmospheric molar fraction of CO2 over the Baltic Sea was investigated using data from seven stations from the World Data Center for Greenhouse Gases. The variation on a monthly timescale of CO2 was divided into a global trend, a regional anthropogenic contribution and a natural seasonal cycle. For the Baltic Sea stations the anthropogenic and terrestrial contributions were largest at the coastal sites in the southern Baltic Sea (an offset of 9 ppm), decreasing towards the north over the Baltic Sea (to about 2 ppm). When calculating the air–sea flux of CO2 using the difference in partial pressure between air and sea, uncertainties in the atmospheric molar fraction of CO2 were shown to be of secondary importance as compared with uncertainties in other parameters (< 10%). Realistic uncertainties in the sea surface partial pressure, wind speed or transfer velocity resulted in significantly larger uncertainties in a calculated air–sea flux.

Keyword
Air-sea exchange, carbon dioxide, Baltic Sea, NORTHERN FINLAND, SEASONAL CYCLE, BOREAL SITE, WIND-SPEED, VARIABILITY, PLATFORM, FLUXES, SERIES
National Category
Natural Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-98566 (URN)000264429500023 ()
Available from: 2009-02-26 Created: 2009-02-26 Last updated: 2013-04-30Bibliographically approved
3. Methods for Estimating Air-Sea Fluxes of CO2 Using High-Frequency Measurements
Open this publication in new window or tab >>Methods for Estimating Air-Sea Fluxes of CO2 Using High-Frequency Measurements
2012 (English)In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 144, no 3, 379-400 p.Article in journal (Refereed) Published
Abstract [en]

The most direct method for flux estimation uses eddy covariance, which is also the most commonly used method for land-based measurements of surface fluxes. Moving platforms are frequently used to make measurements over the sea, in which case motion can disturb the measurements. An alternative method for flux estimation should be considered if the effects of platform motion cannot be properly corrected for. Three methods for estimating CO2 fluxes are studied here: the eddy-covariance, the inertial-dissipation, and the cospectral-peak methods. High-frequency measurements made at the land-based Ostergarnsholm marine station in the Baltic Sea and measurements made from a ship during the Galathea 3 expedition are used. The Kolmogorov constant for CO2, used in the inertial-dissipation method, is estimated to be 0.68 and is determined using direct flux measurements made at the Ostergarnsholm site. The cospectral-peak method, originally developed for neutral stratification, is modified to be applicable in all stratifications. With these modifications, the CO2 fluxes estimated using the three methods agree well. Using data from the Ostergarnsholm site, the mean absolute error between the eddy-covariance and inertial-dissipation methods is 0.25 mu mol m(-2) s(-1). The corresponding mean absolute error between the eddy-covariance and cospectral-peak methods is 0.26 mu mol m(-2) s(-1), while between the inertial-dissipation and cospectral-peak methods it is 0.14 mu mol m(-2) s(-1).

Keyword
Baltic Sea measurements, CO2 fluxes, Cospectral-peak technique, Eddy covariance, Galathea 3 expedition, Inertial-dissipation technique
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-174821 (URN)10.1007/s10546-012-9730-9 (DOI)000308316700004 ()
Available from: 2012-05-28 Created: 2012-05-28 Last updated: 2017-12-07Bibliographically approved
4. Impact of improved air–sea gas transfer velocity on fluxes and water chemistry in a Baltic Sea model
Open this publication in new window or tab >>Impact of improved air–sea gas transfer velocity on fluxes and water chemistry in a Baltic Sea model
2013 (English)In: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 111, 175-188 p.Article in journal (Refereed) Published
Abstract [en]

The air–sea exchange of gases is largely controlled by the efficiency of the transfer across the interface (parameterized by the transfer velocity). A biogeochemical model of the Baltic Sea is used to study the impact of an improved formulation of the transfer velocity on the air–sea fluxes and water chemistry. Two parameterizations using the concept of resistance are applied in the model for calculating carbon dioxide and oxygen air–sea fluxes. One parameterization includes the water-side convection, which has demonstrated to increase the transfer velocity during unstable atmospheric stratification and at great mixing depths. Including the water-side convection changes the seasonal cycle of CO2 and O2 fluxes, although the changes are relatively small due to feedback processes in the model. When not taking the feedback processes into account, the impact of water-side convection on the fluxes is significantly greater, with a maximum difference in the order of 20%. The vertical water profiles are also slightly modified when including water-side convection, the accumulated effect being greatest in the deeper part of the basin. Furthermore, CO2 uptake and O2 emissions decrease by 6.5% and 4.5%, respectively, when water-side convection is included in the model. Compared to the great difference between previous studies, the differences between the model runs in the present study are small, indicating that the choice of formulation for the transfer velocity in a model is not crucial although it is more physically correct.

National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-186146 (URN)10.1016/j.jmarsys.2012.10.013 (DOI)000314375500015 ()
Available from: 2012-11-28 Created: 2012-11-28 Last updated: 2017-12-07Bibliographically approved
5. Influence of Coastal Upwelling on the Air-Sea Gas Exchange of CO2 in a Baltic Sea Basin
Open this publication in new window or tab >>Influence of Coastal Upwelling on the Air-Sea Gas Exchange of CO2 in a Baltic Sea Basin
2013 (English)In: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 65, no 21831, 1-16 p.Article in journal (Refereed) Published
Abstract [en]

During coastal upwelling cold water from the ocean interior with high CO2 concentration is brought up to the surface, allowing this water to interact with the atmosphere. This sets the stage for events with potentially altered sea–air CO2 fluxes. Four upwelling events off the east coast of Gotland in the Baltic Sea were analyzed to assess the impact of upwelling on the air–sea exchange of CO2. For each event, the observed pCO2 were found to be a function of sea-surface temperature (SST) in the upwelling area, which allowed satellite observations of SST to form a proxy for surface water pCO2. A bulk formula was then used to estimate the air–sea CO2 flux during the upwelling events. The results show that the CO2 fluxes in the study area are highly influenced by the upwelling. Comparing with idealized cases without upwelling yields relatively large differences, ranging between 19 and 250% in reduced uptake/increased emission of CO2. Upwelling may also influence the CO2 fluxes on larger scales. A rough estimate indicates that it may also be of significant importance for the average annual CO2 flux from the Baltic Sea. Including upwelling possibly decreases the Baltic Sea annual average uptake by up to 25%.

Keyword
coastal upwelling, carbon dioxide, air–sea exchange, Baltic Sea measurements, remote sensing
National Category
Earth and Related Environmental Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-194940 (URN)10.3402/tellusb.v65i0.21831 (DOI)000328681800001 ()
Available from: 2013-02-20 Created: 2013-02-19 Last updated: 2017-12-06Bibliographically approved

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