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Abstract

Seeking for baseline conditions has biased the carbon dioxide (CO₂) monitoring networks towards remote marine stations, missing part of the variability that is due to regional anthropogenic as well as land biota activity. We present here a five-year record of atmospheric CO₂ mixing ratios and oxygen/nitrogen ratios (O₂/N₂) from the coastal stations Lutjewad (LUT), the Netherlands and Mace Head (MHD), Ireland, derived from flask samples. Oxygen mixing ratios, concurrently measured with CO₂, help determine regional CO₂ fluxes by separating land fluxes from sea fluxes. Mace Head is the closest marine baseline station to Lutjewad, located at the same latitude, and therefore is taken as a reference. During the period of this study, we observed an average increase of CO₂ in the atmosphere of (1.7±0.2) μmol*(mol dry air)−1 per year, and a change of the O₂ fraction of (−20±1) per meg per year. The observed acceleration of the oxygen decrease during the study period seems to confirm the existence of a net oxygen sink other than the combustion processes alone. The difference between the CO₂ summer minimum and the winter maximum is 14.4 μmol*(mol dry air)−1 and 16.1 μmol*(mol dry air)−1 at Mace Head and Lutjewad, respectively, while the opposite variation in the O₂ signal equals 113 per meg and 153 per meg, respectively. We also studied the APO (atmospheric potential oxygen) tracer at both stations. By this analysis, evidence has been found that we need to be careful when using APO close to anthropogenic CO₂ sources. It could be biased by combustion-derived CO₂, and models have to take into account daily and seasonal variations in the anthropogenic CO₂ production in order to be able to simulate APO over the continents.