Aerosol light absorption, black carbon, and elemental carbon at the Fresno Supersite, California

Abstract

Particle light absorption (b(ap)), black carbon (BC), and elemental carbon (EC) measurements at the Fresno Supersite during the summer of 2005 were compared to examine the equivalency of current techniques, evaluate filter-based b(ap) correction methods, and determine the EC mass absorption efficiency (sigma(ap)) and the spectral dependence of b(ap). The photoacoustic analyzer (PA) was used as a benchmark for in-situ b(ap). Most b(ap) measurement techniques were well correlated (r >= 0.95). Unadjusted Aethalometer (AE) and Particle Soot Absorption Photometer (PSAP) b(ap) were up to seven times higher than PA b(ap) at Similar wavelengths because of absorption enhancement by backscattering and multiple scattering. Applying published algorithms to Correct for these effects reduced the differences to 24 and 17% for the AE and PSAP, respectively, at 532 nm. The Multi-Angle Absorption Photometer (MAAP), which accounts for backscattering effects, overestimated b(ap) relative to the PA by 51%. BC concentrations determined by the AE, MAAP, and Sunset Laboratory semi-continuous carbon analyzer were also highly correlated (r >= 0.93) but differed by up to 57%. EC measured with the IMPROVE/STN thermal/optical protocols, and the French two-step thermal protocol agreed to within 29%. Absorption efficiencies determined from PA b(ap) and EC measured with different analytical protocols averaged 7.9 +/- 1.5. 5.4 +/- 1.1, and 2.8 +/- 0.6 m(2)/g at 532, 670, and 1047 nm, respectively. The Angstrom exponent (alpha) determined from adjusted AE and PA b(ap) ranged from 1.19 to 1.46. The largest values of alpha occurred during the afternoon hours when the organic fraction of total carbon was highest. Significant biases associated with filter-based measurements of b(ap), BC, and EC are method-specific. Correcting for these biases must take into account differences in aerosol concentration, composition, and sources. (C) 2009 Elsevier B.V. All rights reserved.