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Processing of soot in an urban environment: case study from the Mexico City Metropolitan Area

by K.S. Johnson, B Zuberi, LT Molina, M.J. Molina, M.J. Iedema, J.P. Cowin, D.J. Gaspar, C Wang, A Laskin show all authors
Atmospheric Chemistry and Physics Discussions ()
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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 0.2–2.0μm were collected in the Mexico City MetropolitanArea (MCMA) during the MCMA-2003 Field Campaignfrom various sites within the city. Individual particle analysisby different electron microscopy methods coupled withenergy dispersed x-ray spectroscopy, and secondary ionizationmass spectrometry show that freshly-emitted soot particlesbecome rapidly processed in the MCMA. Whereas freshparticulate emissions from mixed-traffic are almost entirelycarbonaceous, consisting of soot aggregates with liquid coatingssuggestive of unburned lubricating oil and water, ambientsoot particles which have been processed for less than afew hours are heavily internally mixed, primarily with ammoniumsulfate. Single particle analysis suggests that thismixing occurs through several mechanisms that require furtherinvestigation. In light of previously published results,the internally-mixed nature of processed soot particles is expectedto affect heterogeneous chemistry on the soot surface,including interaction with water during wet-removal.

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