AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Evaluation of Combined Electrical Mobility and Optical Sizing Techniques for Deriving Aerosol Refractive Index
Stephen Zimmerman, RICHARD MOORE, Bruce Anderson, Andreas Beyersdorf, Chelsea Corr, Michael Shook, Kenneth Thornhill, Edward Winstead, Luke Ziemba, NASA Langley Research Center
Abstract Number: 310 Working Group: Instrumentation and Methods
Abstract The next generation of aerosol satellite instruments will include multi-spectral polarimetric measurements to retrieve aerosol size distributions and complex refractive index (RI) at several wavelengths (Hasekamp et al., 2011; NRC, 2007), which is the “only means of constraining aerosol chemical composition from space” for passive sensors (Mischenko et al. 2007). Over past decades a number of methods have been developed to derive refractive index from combined optical and electrical mobility sizing instruments. For example, the so-called “Alignment Method” of Hand and Kreidenweis, 2002, combines joint differential mobility analyzer and optical particle counter size distributions to estimate the aerosol real refractive index. Moteki et al, 2010, used the SP2-derived scatting cross section, OPC-derived size distribution, and APM-derived mass-mobility form factor to constrain the complex refractive index of black carbon aerosols. Lack et al., 2012, present an iterative method for finding the refractive index of brown carbon coating on black carbon (BC) using SP2 and photoacoustic absorption spectrometric techniques. Finally, Mie theory can be used with DMA-derived size distributions and absorption or extinction measurements to theoretically constrain the complex refractive index of aerosols.
In this work, we evaluate the sensitivity of two commercially available, high-resolution optical particle counters for determining the size-resolved real refractive index of laboratory-generated aerosols when used with a DMA via a modified form of the “Alignment Method”. We focus on characterizing the Ultra-High Sensitivity Aerosol Size Spectrometer (UHSAS) from Droplet Measurement Technologies and the Laser Aerosol Spectrometer (LAS) from TSI, Inc. using model aerosol species whose real refractive indices range from 1.3 to 1.6. Absorbing BC and BC-surrogates are also explored. While the scattering geometries of both instruments are the same, the principal difference between the instruments relates to the laser wavelengths. The sensitivity of each instrument to changes in compositionally-driven aerosol refractive index will be discussed.