Jeffrey S. Gaffney, Environmental Research Division, Argonne National Laboratory
Mexico City Revisited: Natural Radioactivity, Ammonia, and Black Carbon Measurements in April 2003

Mexico City is one of the world's largest megacities (>10 million population).  Field research during April of 2003 was undertaken for the Department of Energy's Atmospheric Science Program in collaboration with the Mexico City Metropolitan Area (MCMA) field study organized and led by Drs. Mario and Luisa Molina of MIT.  Presented here are some of the results from that study, including results on natural radioactivity measurements in the submicron aerosol fractions, ammonia, and black carbon.  Ammonia measurements were taken in Mexico City using an open path tunable diode laser system in the near-IR by our group.  A telescope and retro-reflector system with a 250 m open path is used to obtain the concentration measurements in the low ppb range.  Ammonia were found to be high in Mexico City and, working with Aerodyne Research, Inc., it was confirmed that a major source of the ammonia was due to the use of three-way catalytic converters on new vehicles.  Black carbon (BC) measurements were made by using an aethalometer that measures optical absorption at seven wavelengths.  BC measured before, during, and after Holy Week clearly indicate that the levels of black carbon are much lower during low diesel traffic periods, consistent with diesel engines as a major source of BC in Mexico City.  Although the air quality in Mexico City is being improved, BC data indicate that this has not occurred for soot.  Natural radioactivity measurements were also taken, including 14C measurements that indicate a significant amount of carbonaceous aerosols are from biogenic sources.  Biomass fires from the Yucatan area, local sources of biogenic aerosols from fruit drying industry and urban refuse/wood burning, all are suspected contributors.  The results will be discussed and compared to measurements taken of BC in Chicago, at The University of Chicago Urban Atmospheric Observatory and with regard to further plans for Mexico City in February 2006.