Using Extinction-Minus-Scattering Technique to Measure Single Scattering Albedo of Fresh Biomass Aerosols from Pine, Red Oak, and Cedar

An accurate measurement of optical properties of aerosols is critical for quantifying the effect of aerosols on climate. There has been a substantial improvement in the measurements of aerosol physical and chemical properties and the direct and indirect radiative effects of aerosols both in laboratory and in the field measurements. However, uncertainties still persist and measurement results vary significantly. There is still a need for studying the properties of aerosols under controlled laboratory conditions to develop a mechanistic and quantitative understanding of aerosol formation, chemistry, and dynamics.;The factors that affect measurement accuracy and the resulting uncertainties of the extinction-minus-scattering method are measured using a combination of cavity ring-down spectroscopy (CRDS) and integrating nephelometry at a wider range of optical wavelengths. Purely scattering polystyrene latex (PSL) spheres with diameters from 107--303 nm and absorbing polystyrene spheres (APSL) with 390 nm diameter were used to determine the consistency and agreement, within experimental uncertainties, of CRDS and nephelometer values with theoretical calculations derived from Mie theory for non-absorbing spheres. Overall uncertainties for extinction cross-section were largely 10%--11% and dominated by condensation particle counter (CPC) measurement error. Systematic uncertainties due to particle losses, RD cell geometry (RL), CPC counting efficiency, ring-down regression fitting, blank drift, optical tweezing, and recapturing of forward scattered light are also investigated. The random error observed in this work for absorbing spheres is comparable to previous reported measurements. Our analysis showed that this method should result in an error in a wavelength dependent nephelometer correction factor (C(lambda)soot) of < 2%, which translates to a mean single scattering albedo (SSA) error of < 2.9%. Our analysis shows that this method would allow for SSA accuracies of < 0.03 which are required for successful modeling efforts.;The technique and the error analysis developed are applied to measure the optical properties (extinction, scattering, absorption and angstrom coefficient coefficients, as well as SSA) of fresh soot (size 300 and 400 nm) produced from burning of pine, red oak and cedar. We have demonstrated a system that allows measurement of optical properties at a wide range of wavelengths than previously attempted (500--680 nm) to determine "featured" absorption cross sections as a function of wavelength. This is important since the accurate measurement of aerosol optical properties over the entire solar spectrum is a technological challenge that must be addressed to quantify the impact of aerosols on climate.;The work demonstrated a new soot collection method for laboratory studies by impinging soot in distilled water. This may have a potential for use as soot sample collection for laboratory studies. The Size distribution measured with a scanning mobility particle sizer (SMPS) at the soot generation system and after re-suspension showed no change in size distribution over the course of weeks even though we cannot rule out possibility of compaction and chemical change.;SSA values measured were nearly flat ranging from 0.45 to 0.6. The result also demonstrates that SSA of fresh soot is nearly independent of wavelength of light in the 500-680 wavelength range with a slight increase at longer wavelength. The values are within the range of measured values both in the laboratory and in field studies for fresh soot.