ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-5-3033-2005 Processing of soot in an urban environment: case study from the Mexico City Metropolitan Area Johnson K. S. 1 Zuberi B. 1 4 Molina L. T. 1 Molina M. J. 1 Iedema M. J. 2 Cowin J. P. 3 Gaspar D. J. 2 Wang C. 2 Laskin A. 2 Department of Chemistry and of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, USA Chemical Science Division, Pacific Northwest National Laboratory, Richland, WA, USA now at: GEO2 Technologies, Woburn, MA, USA 14 11 2005 5 11 3033 3043 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/5/3033/2005/acp-5-3033-2005.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/5/3033/2005/acp-5-3033-2005.pdf

Chemical composition, size, and mixing state of atmospheric particles are critical in determining their effects on the environment. There is growing evidence that soot aerosols play a particularly important role in both climate and human health, but still relatively little is known of their physical and chemical nature. In addition, the atmospheric residence times and removal mechanisms for soot are neither well understood nor adequately represented in regional and global climate models. To investigate the effect of locality and residence time on properties of soot and mixing state in a polluted urban environment, particles of diameter 0.2–2.0 μm were collected in the Mexico City Metropolitan Area (MCMA) during the MCMA-2003 Field Campaign from various sites within the city. Individual particle analysis by different electron microscopy methods coupled with energy dispersed x-ray spectroscopy, and secondary ionization mass spectrometry show that freshly-emitted soot particles become rapidly processed in the MCMA. Whereas fresh particulate emissions from mixed-traffic are almost entirely carbonaceous, consisting of soot aggregates with liquid coatings suggestive of unburned lubricating oil and water, ambient soot particles which have been processed for less than a few hours are heavily internally mixed, primarily with ammonium sulfate. Single particle analysis suggests that this mixing occurs through several mechanisms that require further investigation. In light of previously published results, the internally-mixed nature of processed soot particles is expected to affect heterogeneous chemistry on the soot surface, including interaction with water during wet-removal.