Colloquium: Friday, June 2, 2006, 2:00PM in room 6085A.

Aerosol Optical Properties: Dependence on Particle Size-Resolved Chemical Composition

Abstract:

Aerosol affects the radiative properties of the atmosphere by absorbing and scattering solar radiation. These interactions play an important role in determining the global climate. Optical properties of aerosol, such as absorption and scattering coefficients, depend on particle size and chemical composition. Our ability to predict the radiative effects of aerosol depends on the quality and resolution of size and composition measurements, as well as our understanding of the physics governing the relevant processes.

 Volatile components of the aerosol composition can now be measured with good size and time resolution by the Aerodyne aerosol mass spectrometer (AMS), thereby permitting similarly resolved dependent properties, such as refractive index and particle density, to be derived.  This study attempts to use these newly available data to investigate the variability of calculated aerosol scattering extinction and to compare it to measured values. Achieving this goal required the development of a data analysis algorithm that not only computes size-resolved refractive indices, but also carries out the scattering extinction calculations, after first constructing a comprehensive size distribution for particles ranging in size from 20 nm to 20 μm, from aerosol size distributions measured by three different instruments (SMPS, PCASP, and APS).  The complexity of the data analysis required that most steps be automated.  Consequently, the data analysis software tool “EMLACE tropo analysis” (ETA) was developed.

The feasibility of the data analysis methods and the quantitative performance of the tool were assessed by analyzing, first, data from a series of laboratory experiments using well-characterized aerosol, and, secondly, a comprehensive set of ambient aerosol measurements made during phase I of the Elk Mountain/Laramie Aerosol Characterization Experiment (EMLACE).  This colloquium will present a discussion of the algorithm and the data analysis methods on which it is based, together with case study results from their application to EMLACE measurements.