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

[摘要] Chemical composition, size, and mixing state of atmospheric particles arecritical in determining their effects on the environment. There is growingevidence that soot aerosols play a particularly important role in bothclimate and human health, but still relatively little is known of theirphysical and chemical nature. In addition, the atmospheric residence timesand removal mechanisms for soot are neither well understood nor adequatelyrepresented in regional and global climate models. To investigate the effectof locality and residence time on properties of soot and mixing state in apolluted urban environment, particles of diameter 0.2–2.0 μm werecollected in the Mexico City Metropolitan Area (MCMA) during the MCMA-2003Field Campaign from various sites within the city. Individual particleanalysis by different electron microscopy methods coupled with energydispersed x-ray spectroscopy, and secondary ionization mass spectrometryshow that freshly-emitted soot particles become rapidly processed in theMCMA. Whereas fresh particulate emissions from mixed-traffic are almostentirely carbonaceous, consisting of soot aggregates with liquid coatingssuggestive of unburned lubricating oil and water, ambient soot particleswhich have been processed for less than a few hours are heavily internallymixed, primarily with ammonium sulfate. Single particle analysis suggeststhat this mixing occurs through several mechanisms that require furtherinvestigation. In light of previously published results, theinternally-mixed nature of processed soot particles is expected to affectheterogeneous chemistry on the soot surface, including interaction withwater during wet-removal.

[学科分类] 大气科学