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Black Carbon Aerosol over the Los Angeles Basin during CalNex

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Journal article

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Refractory black carbon rBC mass and number concentrations were quantified by a Single Particle Soot Photometer SP2 in the CalNex 2010 field study on board the Center for Interdisciplinary Remotely-Piloted Aircraft Studies CIRPAS Twin Otter in the Los Angeles LA Basin in May, 2010. The mass concentrations of rBC in the LA Basin ranged from 0.002 0.530 micro g mexpn -3, with an average of 0.172 micro g mexpn -3. Lower concentrations were measured in the Basin outflow regions and above the inversion layer. The SP2 afforded a quantification of the mixing state of rBC aerosols through modeling the scattering cross-section with a core-and-shell Mie model to determine coating thickness. The rBC particles above the inversion layer were more thickly coated by a light-scattering substance than those below, indicating a more aged aerosol in the free troposphere. Near the surface, as the LA plume is advected from west to east with the sea breeze, a coating of scattering material grows on rBC particles, coincident with a clear growth of ammonium nitrate within the LA Basin and the persistence of water-soluble organic compounds as the plume travels through the outflow regions. Detailed analysis of the rBC mixing state reveals two modes of coated rBC particles a mode with smaller rBC core diameters 90 nm but thick 200 nm coating diameters and a mode with larger rBC cores 145 nm with a thin 75 nm coating. The weekend effect in the LA Basin results in more thickly coated rBC particles, coinciding with more secondary formation of aerosol.

Subject Categories:

  • Physical Chemistry
  • Optics
  • Air Pollution and Control

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