A one-year time series of atmospheric CO<sub>2</sub> measurements from Bern, Switzerland, is presented. O<sub>2</sub>/N<sub>2</sub> and Ar/N<sub>2</sub> ratios as well as stable carbon and oxygen isotopes of CO<sub>2</sub> and δ<sup>29</sup>N<sub>2</sub>, δ<sup>34</sup>O<sub>2</sub> and δ<sup>36</sup>Ar were measured periodically during a one year period. Additionally, the <sup>222</sup>Rn activity was measured during three months in the winter 2004. Using the correlation from short-term fluctuations of CO<sub>2</sub> and <sup>222</sup>Rn we estimated a mean CO<sub>2</sub> flux density between February 2004 and April 2004 in the region of Bern of 95±39 tC km<sup>–2</sup>month<sup>–1</sup>. The continuous observations of carbon dioxide and associated tracers shed light on diurnal and seasonal patterns of the carbon cycle in an urban atmosphere. There is considerable variance in nighttime δ<sup>13</sup>C and δ<sup>18</sup>O of source CO<sub>2</sub> throughout the year, however, with generally lower values in winter compared to summertime. The O<sub>2</sub>:CO<sub>2</sub> oxidation ratio during the nighttime build-up of CO<sub>2</sub> varies between –0.96 and –1.69 mol O<sub>2</sub>/mol CO<sub>2</sub>. Furthermore, Ar/N<sub>2</sub> measurements showed that artifacts like thermal fractionation at the air intake are relevant for high precision measurements of atmospheric O<sub>2</sub>.