Stable carbon isotope constraints on mixing and mass balance of CO2 in an urban atmosphere: Dallas metropolitan area, Texas, USA

The concentrations and δ13C values of atmospheric CO2 were measured in not, vert, similar150 air samples collected at 8 sites in the Dallas metropolitan area over the period August 1998 to December 1999. Measured concentrations (C) of atmospheric CO2 ranged from 369 to 475 ppm, while the δ13C values ranged from –12.0 to –8.1‰. These values contrast with a “global” concentration at the time of this study of approximately 367 ppm and a corresponding δ13C value of about –8.0‰. δ13C was linearly correlated with 1/C for samples collected at heights of not, vert, similar2 m at 3 sites adjacent to streets with significant automobile traffic. Extrapolation of this two-component mixing line to 1/C=0 yielded a δ13C value of about –27‰ for the CO2 input—i.e., the same as that of gasoline. A simple box model, incorporating photosynthesis, respiration, and anthropogenic addition of CO2, indicates that differences between downwind and upwind concentration-weighted δ13C values (Δ[C*δ13C]) of atmospheric CO2 may be linearly correlated with downwind and upwind differences in concentration (Cd*−Cu*), where C* is reported as mol/m3. The model predicts that measurable effects of photosynthetic withdrawal of atmospheric CO2 are manifested by data arrays with slopes more positive than about –16. This effect of photosynthesis is evident in a linear array of “warm weather”, Dallas atmospheric CO2 data (slope of –12.7‰). Collectively, the data for all 8 sites exhibited considerable scatter about binary mixing lines that depict the addition of CO2 from combustion of natural gas and gasoline. However, when model slopes (m) were calculated for binary mixing between a “background” atmospheric CO2 and each individual sample, it was found that, in general, m increases with decreasing temperature. The effects of photosynthesis and respiration complicate this relationship, but the overall pattern suggests that, as temperature decreases, the proportion of anthropogenic CO2 derived from combustion of natural gas increases. This increase appears to reflect increased use of natural gas for home heating, etc., in cooler weather. Therefore, seasonally changing patterns of fossil fuel use are detectable in the atmospheric CO2 of this urban environment.