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

The variability in time and space of the atmospheric molar fraction of CO 2 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 CO 2 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 CO 2 using the difference in partial pressure between air and sea, uncertainties in the atmospheric molar fraction of CO 2 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.