Sources and transport of Δ<sup>14</sup>C in CO<sub>2</sub> within the Mexico City Basin and vicinity

Abstract

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 14 C-enhanced carbon from bomb 14 C sequestered in plant carbon pools via the ubiquitous biomass burning in the region as well as the direct release of radiocarbon as CO 2 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 14 CO 2 to constrain fossil fuel emissions in the megacity environment, indicating that underestimation of the fossil fuel contribution to the CO 2 flux is likely wherever biomass burning coexists with urban emissions and is unaccounted for as a source of the elevated CO 2 observed above local background. Our findings increase Correspondence to: S. A. Vay (stephanie.a.vay@nasa.gov) the complexity required to quantify fossil fuel-derived CO 2 in source-rich environments characteristic of megacities, and have implications for the use of 14 CO 2 observations in evaluating bottom-up emission inventories and their reliability as a tool for validating national emission claims of CO 2 within the framework of the Kyoto Protocol.

Measurements of volatile organic compounds at a suburban ground site (T1) in Mexico City during the MILAGRO 2006 campaign: measurement comparison, emission ratios, and source attribution

“…difference is probably due to differences in automobile emission systems and fleet ages between the two countries. CO to CO 2 ratios of about 45 ppbv ppmv −1 were observed during the MILAGRO campaign (Vay et al, 2009); a value similar to the ratio observed in plumes originating in China and twice as large as the 10-20 ppbv ppmv −1 observed in industrialized nations like Japan and the United States (de Gouw et al, 2004;Takegawa et al, 2004). It is worth noting, therefore, that VOC emission ratios per kg fuel burned are probably closer to a factor of 6 ± 2 larger in Mexico City than those for US cities.…”

Impact of Mexico City emissions on regional air quality from MOZART-4 simulations

“…Karl et al (2009) estimate 0-10% of the observed aromatic compound concentrations are from biomass burning. Another potential source of information about the relative contribution of anthropogenic and biomass burning emissions is from 14 C isotopic ratio measurements, however interpretation of the results is quite complex due to the contamination of wood by nuclear bomb radiocarbon and other enriched sources (Vay et al, 2009;Aiken et al, 2010). The fraction of fire emissions in the inventory used here (see Table 1) varies greatly among species, depending both on the fire emissions factors and on the magnitude of anthropogenic and natural emissions in this region.…”

Aerosol Absorption and Scattering Measurements: Field Measuremnets and Laboratory Characterizations.

“…The FZKENDURO Ultialight aircraft was sta tioned at Hermanos Serdan Airport near Huejotzingo, Puebla . The scientific payloads of the three laiger aiicraft are summarized in Table 5 for the C130, Ta ble 6 for the Gl, and Table 2a of Singh et al (2009) for the DC8. The 131 carried mostly radiometric instruments used to measuie aerosols, water vapor, clouds, Eaith surface properties, and radiation fields (see Sect.…”

“…Measurements of the spatially varying wind speed, wind direction, temperature, pressure, and humidity over central Mexico were collected by the G-1 , C-130 (de Carlo et al, 2008), DC-8 (Singh et al, 2009), Twin Otter , and the FZK-ENDURO ultralight aircraft . The J-31 ) measured temperature, pressure, and humidity.…”

“…The model explains the observed modern carbon as the sum of similar contributions from regional biogenic SOA, biomass burning POA and SOA, and POA and SOA from urban sources of modern carbon . Sources of "hot" aerosol carbon (enriched in 14 C) in the Mexico City area and/or stored in forests could produce a bias in this method and need to be further investigated (Vay et al, 2009). …”