In this study, an analysis of upwelling and biological activities impact on the seawater pCO2 variability was done to improve the knowledge about the pCO2 variability in seawater in the Baltic Sea. During upwelling activity, CO2 rich waters are upwelled to the surface. This influences air-sea CO2 flux and thus the net uptake/emission of CO2 by the sea. pCO2 and SST measurements from a SAMI sensor, located at the Östergarnsholm site in the Baltic Sea, and SST satellite data, was used to identify periods affected by upwelling activity. A strong variability due to upwelling activity was observed on the pCO2 concentration. The frequency of upwelling activity at the Östergarnsholm site was estimated to be around 20 % of the time from May-November. The relationship between pCO2 and SST during upwelling activity was found to depend of the month. In November, this relationship can be used to characterise the effect of upwelling, but for the other months there are no differences between the none- upwelling periods and the upwelling periods. Another type of period, characterized by a diurnal variability of pCO2 mainly driven by the biological effect, was also observed. / I denna studie undersöktes hur uppvällning och biologisk aktivitet påverkar koncentrationen av pCO2 i havet. Detta gjordes för att få ökad förståelse för korttidsvariationen av pCO2 i Östersjön. Under uppvällning pressas CO2-rikt vatten upp till ytan. Detta påverkar flödet av CO2 mellan hav och atmosfär och då även det totala upptaget/utsläppet av CO2 från havet. I studien användes pCO2 och SST mätningar från en SAMI-sensor, placerad vid Östergarnsholm i Östersjön, och en del satellitdata för att identifiera perioder påverkade av uppvällning. Det observerades att uppvällnings inverkan på koncentrationen av pCO2 varierar kraftigt. Frekvensen av uppvällning vid Östergarnsholm uppskattades vara ungefär 20 % av tiden under maj-november. Relationen mellan SST och pCO2 under perioder med uppvällning observerades variera från maj-november. I november kan denna relation användas till att uppskatta effekten av uppvällning men under de andra månaderna är relationen under uppvällning för lika relationen under icke- uppvällning. Även en annan slags perioder observerades; under dessa perioder observerades en dygnscykel av pCO2, driven av biologisk aktivitet.

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.