Total carbon samples were collected during the MILAGRO field project in March of 2006 at the Instituto Mexicano de Petroleo (IMP) site also known as T-0 in Mexico City, D.F., and at the Universidad Technologica de Tecamac in the state of Mexico known as T-1. These sites are about 35 km apart. Samples were collected on quartz fiber filters for carbon isotopic analyses (13C/12C and 14C). Biomass burning as well as trash burning in both Mexico City and the surrounding areas can be evaluated using the stable and radioactive isotopes of carbon. Since fossil fuels are million of years old, all of their original 14C has decayed and they have zero 14C content. Biogenic materials such as grasses, woody plants, trees, and carbon paper products in trash burning, as well as charcoal or wood burning for fuel are enriched in 14C. So a measure of the 14C content can give a solid indication of the relative amounts of biomass vs fossil derived carbon. Stable carbon isotopes are also useful in that the various grasses and plants that have either C-4 or C-3 biochemical pathways can be distinguished. The C-3 plants or Calvin-Benson photosynthetic cycle have a more selective chemistry and fractionate the heavier carbon by about 12 parts per thousand as compared to the less selective C-4 or Hatch-Slack pathway. Data from the T-0 and T-1 site are presented that clearly indicate a significant biomass fraction in the total aerosol carbon at both sites. The T-1 site shows indications of higher biomass fractions consistent with the regional fires observed during the MILAGRO study. The stable carbon isotopes show indications of C-4 grass contributions consistent with these observations. We are currently completing thermal separations on the samples to determine the organic carbon and elemental carbon isotopic contributions to evaluate the sources of secondary organic and black carbon to fine aerosol loadings during MILAGRO.

Acknowledgements: This work was conducted as part of the Department of Energy's Atmospheric Science Program as part of the Megacity Aerosol Experiment – Mexico City during MILAGRO. This research was supported by the Office of Science (BER), U.S. Department of Energy Grant No. DE-FG02-07ER64328. We also wish to thank Mexican Scientists and students for their assistance from the Instituto Mexicano de Petroleo (IMP) and CENICA.