Genetic Algorithm-determined Real Refractive Indicies of Various Secondary Organic Aerosols

Kim Hwajin University of California, Los Angeles
Brian Barkey University of California, Los Angeles
Suzanne Paulson University of California

Category: Aerosol Properties

Working Group: Aerosol Life Cycle

Image Thumbnail
GA-derived real refractive index of SOA particles generated from the ozonolysis of alpha-pinene in a solar reaction chamber as a function of SMPS-measured mean particle diameter. Each date represents the results from a separate experiment in which SOA particles are allowed to homogeneously nucleate and then grow over a period of 4 to 6 hours.

Real refractive indices (mr) of secondary organic aerosol (SOA) particles generated by oxidizing alpha-pinene, beta-pinene, and toluene in the UCLA solar reaction chamber are presented. The indices were derived from polar nephelometer measurements using 670 nm incident light polarized both parallel and perpendicular to the measurement scattering plane. Particles larger than about 200 nm gave sufficient signal to retrieve reliable refractive indices. A Genetic Algorithm (GA) optimization method is used to determine mr using the Mie-Lorenz scattering theory and particle size distributions measured with a scanning mobility particle sizer. As the measured size distribution often departs significantly from the expected lognormal distribution shape, a method to quantify and compensate for this distortion has been developed. The absolute error associated with the mr retrieval is ± 0.03, derived from an analysis of instrument electronic noise levels, errors arising from uncertainties in the calibration, and the efficacy of the GA method in determining the correct result from noisy data. The retrieved mr for the SOA studied indicate this property changes as particles grow and is different for each hydrocarbon precursor. For example, alpha- and beta-pinene SOA formed in the presence of NOx were similar, their mr increasing from 1.4–1.45 at 200 nm to 1.52 at 375–425 nm, and then declining gradually, reaching ~1.48 by 500 nm. Toluene SOA had a markedly different pattern and wider range of mr compared to the pinenes. Results for SOA from alpha- and beta-pinene reacted with ozone, as well as the dependence of retrieved mr on hydrocarbon precursor concentration will also be presented.

This poster will be displayed at ASR Science Team Meeting.

POSTER in PDF: for proper viewing, it should be viewed with Adobe Acrobat Reader. Download the latest version from the Adobe Reader website.