Photoacoustic Measurements of Amplification of the Absorption Cross Section for Coated Soot Aerosols

A quantitative understanding of the absorption and scattering properties of mixed soot and aerosol particles is necessary for evaluating the Earth's energy balance. Uncertainty in the net radiative forcing of atmospheric aerosols is relatively large and may be limited by oversimplified models that fail to predict these properties for bare and externally mixed soot particles. In this laboratory study of flame-generated soot, we combine photoacoustic spectroscopy, particle counting techniques, and differential mobility analysis to obtain high-precision measurements of the size-dependent absorption cross section of uncoated and coated soot particles. We investigate how the coating of soot by nonabsorbing films of dibutyl phthalate (chosen as a surrogate for sulfuric acid) affects the particles’ morphology and optical properties. Absorption measurements were made with photoacoustic spectroscopy using a laser at λ = 405 nm. We report measurements and model calculations of the absolute cross section, mass absorption coefficient, and amplification of the absorption cross section. The results are interpreted and modeled in terms of a core–shell geometry and Lorenz–Mie theory of scattering and absorption. We discuss evidence of soot particle and collapse as a result of the coating process and we demonstrate the ability to resolve changes in the coating thickness as small as 2 nm.

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