1NASA Langley Research Center, Hampton, Virginia, USA
2University of California, Irvine, California, USA
3National Institutes of Aerospace, Hampton, Virginia, USA
4NASA Ames Research Center, Moffett Field, California, USA
Abstract. Radiocarbon samples taken over Mexico City and the surrounding region during the MILAGRO field campaign in March 2006 exhibited an unexpected distribution: (1) relatively few samples (23%) were below the North American free tropospheric background value (57±2‰) despite the fossil fuel emissions from one of the world's most highly polluted environments; and (2) frequent enrichment well above the background value was observed. Correlate source tracer species and air transport characteristics were examined to elucidate influences on the radiocarbon distribution. Our analysis suggests that a combination of radiocarbon sources biased the "regional radiocarbon background" above the North American value thereby decreasing the apparent fossil fuel signature. Likely sources include the release of 14C-enhanced carbon from bomb 14C sequestered in plant carbon pools via the ubiquitous biomass burning in the region as well as the direct release of radiocarbon as CO2 from other "hot" sources. Plausible perturbations from local point "hot" sources include the burning of hazardous waste in cement kilns; medical waste incineration; and emissions from the Laguna Verde Nuclear Power Plant. These observations provide insight into the use of Δ14CO2 to constrain fossil fuel emissions in the megacity environment, indicating that underestimation of the fossil fuel contribution to the CO2 flux is likely wherever biomass burning coexists with urban emissions and is unaccounted for as a source of the elevated CO2 observed above local background. Our findings increase the complexity required to quantify fossil fuel-derived CO2 in source-rich environments characteristic of megacities, and have implications for the use of Δ14CO2 observations in evaluating bottom-up emission inventories and their reliability as a tool for validating national emission claims of CO2 within the framework of the Kyoto Protocol.