Optical, physical, and chemical properties of tar balls observed during the Yosemite Aerosol Characterization Study

The Yosemite Aerosol Characterization Study of summer 2002 (YACS) occurred during an active fire season in the western United States and provided an opportunity to investigate many unresolved issues related to the radiative effects of biomass burning aerosols. Single particle analysis was performed on field‐collected aerosol samples using an array of electron microscopy techniques. Amorphous carbon spheres, or “tar balls,” were present in samples collected during episodes of high particle light scattering coefficients that occurred during the peak of a smoke/haze event. The highest concentrations of light‐absorbing carbon from a dual‐wavelength aethalometer (λ = 370 and 880 nm) occurred during periods when the particles were predominantly tar balls, indicating they do absorb light in the UV and near‐IR range of the solar spectrum. Closure experiments of mass concentrations and light scattering coefficients during periods dominated by tar balls did not require any distinct assumptions of organic carbon molecular weight correction factors, density, or refractive index compared to periods dominated by other types of organic carbon aerosols. Measurements of the hygroscopic behavior of tar balls using an environmental SEM indicate that tar balls do not exhibit deliquescence but do uptake some water at high (∼83%) relative humidity. The ability of tar balls to efficiently scatter and absorb light and to absorb water has important implications for their role in regional haze and climate forcing.