The main aim of this study was to assess the natural and anthropogenic contributions of CO 2 in the urban atmosphere of Wrocław City (SW Poland) using combined quantitative (CO 2 concentrations) and qualitative analysis (δ 13 C of CO 2 ). Between 21 January 2011 and 22 December 2011, 17 sampling campaigns were performed at 3-week intervals and in total 255 samples were collected. The mean CO 2 concentration was 469±71ppm and the mean δ 13 C(CO 2 ) was −10.8±1‰.The measured δ 13 C(CO 2 ) values of major end-members for two winter heating seasons (−25.7‰ in January–March of 2011 and −27.6 ‰ in October–December of 2011) and for one vegetative season (−20.4‰ in April–September of 2011) suggest soil respiration as a main source of atmospheric CO 2 during the vegetative season, and a very significant impact of fossil fuel combustion during the winter heating seasons. There were significant increases of CO 2 concentrations at many sampling locations after the opening of a new motorway on 31 August 2011. The authors hypothesise that the new motorway contributes to the increase of CO 2 across the city.