A comparison of atmospheric methods for determining the sources of atmospheric CO2 in urban ecosystems

Background/Question/Methods

The urban atmosphere has many sources of carbon dioxide (CO₂) including anthropogenic emissions from gasoline and natural gas combustion, in addition to respiration from vegetation and soil. There are several different methods of distinguishing between biological and anthropogenic CO₂ in the atmosphere and they have seldom been compared. In this study, we quantified the biogenic vs anthropogenic sources of CO₂ in Irvine, California within the Los Angeles basin. We compared stable isotopes (¹³C and ¹⁸O), radiocarbon (¹⁴C) and carbon monoxide (CO) as tracers for detecting various sources of CO₂.

Results/Conclusions

CO₂ concentrations in the study area commonly reached 450 ppm during stable nighttime conditions. The ¹⁴C composition of CO₂ in wintertime flask samples suggested that the sources of local (non-background) CO₂ were approximately 40% biogenic and 60% anthropogenic during early morning traffic rush hours. The CO tracer method similarly indicated a dominant gasoline CO₂ source during the high traffic sampling period. The stable isotope composition of CO₂ suggested that anthropogenic CO₂ was dominated by gasoline combustion, but contained a detectable amount of CO₂ from combustion of natural gas for electrical energy production. Finally, we also sampled urban air during the extensive California wildfires in October, 2007 and found that the radiocarbon keeling plot intercept was more enriched than typical values, suggesting a large source of CO₂ from combustion of modern biomass. While the radiocarbon method provides the most robust information for distinguishing between anthropogenic and biogenic sources, there can be good agreement with the CO tracer, which is easier to measure and can be monitored continuously. This initial study confirms the promising use of CO as tracer for fossil fuel CO₂ over large spatial scales when periodically validated with isotope methods.