Influence of coastal upwelling on the air-sea gas exchange of CO2 in a Baltic Sea Basin

Norman, M; Parampil, SR; Rutgersson, A; Sahlee, E
2013 | Tellus Ser. B-Chem. Phys. Meteorol. | 65

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 pCO(2) 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 pCO(2). 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%.

Impact of improved air-sea gas transfer velocity on fluxes and water chemistry in a Baltic Sea model

Norman, M; Rutgersson, A; Sahlee, E
2013 | J. Mar. Syst. | 111 (175-188)

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 O-2 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 02 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. 2012 Elsevier B.V. All rights reserved.

Methods for Estimating Air-Sea Fluxes of CO2 Using High-Frequency Measurements

Norman, M; Rutgersson, A; Sorensen, LL; Sahlee, E
2012 | Boundary-Layer Meteorology | 144 (3) (379-400)

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).

Atmospheric CO2 variation over the Baltic Sea and the impact on air-sea exchange

Rutgersson, A; Norman, M; Astrom, G
2009 | Boreal Environ. Res. | 14 (1) (238-249)

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.

The annual cycle of carbon dioxide and parameters influencing the air-sea carbon exchange in the Baltic Proper

Rutgersson, A; Norman, M; Schneider, B; Pettersson, H; Sahlee, E
2008 | J. Mar. Syst. | 74 (1-2) (381-394)

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. Buts 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 pCO(2)(w) 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. (C) 2008 Elsevier B.V. All rights reserved.

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