Black Carbon Aerosol over the Los Angeles Basin during CalNex

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 m(expn -3), with an average of 0.172 micro g m(expn -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.
Published in the Journal of Geophysical Research, v117 article ID D00V13, 20 Apr 2012. Prepared in cooperation with the Division of Engineering and Applied Science, California Institute of Technology, Pasadena, the Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, the Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, the Department of Chemical and Environmental Engineering, University of Arizona, Tucson, and the Department of Atmospheric Sciences, University of Arizona, Tucson.