Measurement of Optical Properties of Soot Using Cavity Ring-Down Spectroscopy and Integrating Nephelometry

Since black carbon and brown carbon are among the greatest contributors to radiative forcing (black carbon being second only to carbon dioxide), this work focuses on the laboratory measurement of their optical properties using cavity ring-down spectroscopy (CRDS) and integrating nephelometry. Water soluble soot is collected using an impinger and cascade impactor by burning different fuel types to mimic ambient aerosols dominant in regions where biomass burning is the main source of aerosols. Using an optical parametric oscillator (OPO) as a light source, we are able to measure extinction and scattering over a wide range of wavelengths. A correction factor is calculated using a method by Anderson and Ogren to reconcile scattering from the nephelometer to extinction from the CRDS. The extinction-minus-scattering method is then used to determine absorption. Purely scattering polystyrene latex (PSL) spheres of known sizes (100--700 nm) are used in the lab to calibrate the system for this study. Measurements of optical properties of soot collected from different fuel sources at different stages of burning are reported.