Changes in CO 2 Emission Sources in Mexico City Metropolitan Area Deduced from Radiocarbon Concentrations in Tree Rings

We present radiocarbon ( 14 C) in tree rings from Mexico City and a reconstruction of fossil CO 2 concentrations for the last five decades, as part of a research program to understand the 14 C dynamics in this complex urban area. Background values were established by 14 C concentrations in tree rings from a nearby clean area. Agreement between background and NH-zone 2 values indicate Taxodium mucronatum is a good biomonitor for annual atmospheric 14 C variations. Values for the urban tree rings were significantly lower than background values, indicating a 14 C depletion from fossil CO 2 emissions. There is an increasing trend of fossil CO 2 between 1960 and 1990, in agreement with the population growth and the increasing demand for fossil fuels in Mexico City. Between 1990 and 2000, there is an apparent decrease in fossil CO 2 concentration, increasing again after 2000. The decrease in 2000, despite being of the same magnitude as the overall uncertainty, may reflect environmental policies that improved the energy efficiency and reduced CO 2 emissions in the area. The increase in fossil CO 2 concentration between 2000 and 2010 may be attributable to the significant growth of motor vehicle usage in Mexico City, which made transportation the main energy-demanding and -emitting sector.