Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 7 6 2007 10.5194/acp-7-1585-2007 http://www.atmos-chem-phys.net/7/1585/2007/ http://www.atmos-chem-phys.net/7/1585/2007/acp-7-1585-2007.html http://www.atmos-chem-phys.net/7/1585/2007/acp-7-1585-2007.pdf 1585 1598 2007-03-23 The T1-T2 study: evolution of aerosol properties downwind of Mexico City J. C. Doran christopher.doran@pnl.gov J. C. Barnard W. P. Arnott R. Cary R. Coulter J. D. Fast E. I. Kassianov L. Kleinman N. S. Laulainen T. Martin G. Paredes-Miranda M. S. Pekour W. J. Shaw D. F. Smith S. R. Springston X.-Y. Yu Pacific Northwest National Laboratory, Richland, WA, USA Desert Research Institute, Reno, NV, USA Sunset Laboratory, Inc., Tigard, OR, USA Argonne National Laboratory, Argonne, IL, USA Brookhaven National Laboratory, Upton, NY, USA As part of a major atmospheric chemistry and aerosol field program carried out in March 2006, a study was conducted in the area to the north and northeast of Mexico City to investigate the evolution of aerosols and their associated optical properties in the first few hours after their emission. The focus of the T1-T2 aerosol study was to investigate changes in the specific absorption α_ABS (absorption per unit mass, with unit of m² g^−1) of black carbon as it aged and became coated with compounds such as sulfate and organic carbon, evolving from an external to an internal mixture. Such evolution has been reported in previous studies. The T1 site was located just to the north of the Mexico City metropolitan area; the T2 site was situated approximately 35 km farther to the northeast. Nephelometers, particle soot absorption photometers, photoacoustic absorption spectrometers, and organic and elemental carbon analyzers were used to measure the optical properties of the aerosols and the carbon concentrations at each of the sites. Radar wind profilers and radiosonde systems helped to characterize the meteorology and to identify periods when transport from Mexico City over T1 and T2 occurred. Organic and elemental carbon concentrations at T1 showed diurnal cycles reflecting the nocturnal and early morning buildup from nearby sources, while concentrations at T2 appeared to be more affected by transport from Mexico City. Specific absorption during transport periods was lower than during other times, consistent with the likelihood of fresher emissions being found when the winds blew from Mexico City over T1 and T2. 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