Sources and transport of Δ¹⁴C in CO₂ within the Mexico City Basin and vicinity

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.