Inferring biogenic and anthropogenic carbon dioxide sources across an urban to rural gradient

We continuously monitored CO₂ concentrations at three locations along an urban-to-rural gradient in the Salt Lake Valley, Utah from 2004 to 2006. The results showed a range of CO₂ concentrations from daily averages exceeding 500 p.p.m. at the city center to much lower concentrations in a non-urbanized, rural region of the valley. The highest values were measured in the wintertime and under stable atmospheric conditions. At all three sites, we utilized weekly measurements of the C and O isotope composition of CO₂ for a 1-year period to evaluate the CO₂ sources underlying spatial and temporal variability in CO₂ concentrations. The results of an inverse analysis of CO₂ sources and the O isotope composition of ecosystem respiration (delta(18) O (R)) showed large contributions (>50%) of natural gas combustion to atmospheric CO₂ in the wintertime, particularly at the city center, and large contributions (>60%) of biogenic respiration to atmospheric CO₂ during the growing season, particularly at the rural site. delta(18) O (R) was most enriched at the rural site and more isotopically depleted at the urban sites due to the effects of irrigation on ecosystem water pools at the urban sites. The results also suggested differences in the role of leaf versus soil respiration between the two urban sites, with seasonal variation in the contribution of leaf respiration at a residential site and relatively constant contributions of leaf respiration at the city center. These results illustrate that spatial and temporal patterns of urban CO₂ concentrations and isotopic composition can be used to infer patterns of energy use by urban residents as well as plant and soil processes in urban areas.