Observations of smoke‐influenced aerosol during the Yosemite Aerosol Characterization Study: 2. Aerosol scattering and absorbing properties

Abstract

Direct observations and size‐distribution‐based estimates of light‐scattering coefficients (bsp) are presented for the Yosemite Aerosol Characterization Study (YACS), which took place during the summer of 2002, an active fire year. Uncertainty in the estimates of bsp were found to be sensitive to the choice of the aerosol refractive indices, which were retrieved by aligning optical particle counter measurements to differential mobility analyzer observations and, in addition, were calculated from composition measurements. Aerosol composition during the study was dominated by organic carbon, with highest levels observed during periods impacted by biomass burning smoke influenced hazes. As a result, estimates of the aerosol refractive index from composition measurements were highly sensitive to the assumed properties of organic carbon. Retrieved and calculated refractive indices were in agreement for reasonable assumptions for properties of organic carbon. Excellent agreement (within 6%) was found between measured bsp and size‐distribution derived bsp if the imaginary component of the refractive index, determined from composition measurements and assuming all species were internally mixed, was included in the Mie calculation. Under the internally mixed assumption, aerosols sampled during the study were modeled as weakly absorbing, with computed dry single scattering albedo ranging from 0.86 to 0.94. Calculated light absorption coefficients yielded a study‐averaged dry aerosol mass absorption efficiency of 0.37 ± 0.05 m2g−1.