AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Measuring Aerosol Scattering and Absorption - Limitations of the Extinction-Minus-Scattering Method
SUJEETA SINGH, Damon Smith, Marc Fiddler, Solomon Bililign, North Carolina A&T State University
Abstract Number: 303 Working Group: Instrumentation and Methods
Abstract Aerosols from biomass burning represent a significant source of particles in the atmosphere. An accurate measurement of the optical properties of these aerosols is critical for measuring their effect on climate, since they represent a large uncertainty. Optically dark aerosols, such as soot, affect the climate system by both cooling (due to scattering) and by warming (due to absorbing), dependent on the aerosol’s size, composition, and shape. The aim of this work is to quantitatively determine the absorption and scattering cross-section, single scattering albedo, and angstrom coefficient of soot. Particle extinction (absorption + scattering) was measured with a cavity ring-down spectrometer (CRDS), scattering was measured with an integrating nephelometer, and a condensation particle counter (CPC) measured particle number density. In this work, we perform an assessment of the contributions to systematic and random errors for CRDS extinction measurements. Cross section measurements are limited by the accuracy of the CPC and, to a lesser extent, measurement variability and cell geometry. Statistical fluctuations of aerosol particles, recapturing of forward-scattered light, laser mode noise, and laser bandwidth effects are negligible. For nephelometer measurements, scattering cross section errors resulting from extrapolation between measured values limits the accuracy of this method. A comparison between CRDS, nephelometery, and Mie Theory predictions for non-absorbing polystyrene latex spheres shows mean measured cross section values at 584 nm to have an excellent agreement, though all were systematically larger than predicted values. Backscatter values showed poor agreement, and all nephelometer cross sections were limited by fitting errors (those from extrapolating between measured wavelengths). The resulting effects of propagated errors on the accuracy of absorption cross section and single scattering albedo will be discussed along with preliminary measurements on absorbing polystyrene and reaerosolized soot.