10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

Abstract View


Evolution of the Complex Refractive Index of Secondary Organic Aerosols during Atmospheric Aging

QUANFU HE, Nir Bluvshtein, Lior Segev, Daphne Meidan, Michel Flores, Steven S. Brown, William Brune, Yinon Rudich, Weizmann Institute of Science

     Abstract Number: 854
     Working Group: Oxidation Flow Reactor: Development, Characterization, and Application to Aerosols

Abstract
Secondary organic aerosols (SOA) from biogenic and anthropogenic sources represent a large fraction of the tropospheric aerosol mass. The optical properties of SOA can vary as a function of wavelength, but only a few studies have reported measurements of the wavelength-dependent aerosol extinction cross section and complex refractive indices, particularly in the visible spectral range. Additionally, there is a large knowledge gap of how the optical properties of SOA in the visible spectral region evolve as a function of atmospheric aging. To address this issue, we applied a novel laboratory instrument to measure aerosol extinction as a function of wavelength from 400 to 650 nm, using a white light-broadband cavity enhanced spectroscopy (BBCES). The instrument was validated by measuring CO2 Rayleigh scattering cross section and extinction of reference materials. The new system was used to measure the changes in the optical properties of β−pinene and p−xylene SOA produced and aged in an oxidation flow reactor (OFR), simulating daytime aging by OH radical exposure under NOx-free conditions. It was found that these SOA are not absorbing in the visible range, and the real part of the refractive index (RI), n, for these two types of SOA showed slight spectral dependence in the visible range. With increased OH exposure, n first increased and then decreased, possibly due to an increase in aerosol density and chemical mean polarizability for SOA produced at low OH exposures, and a decrease in chemical mean polarizability for SOA produced at high OH exposures, respectively. A simple radiative forcing calculation suggests that atmospheric aging can introduce more than 40% uncertainty due to the changes in the RI for aged SOA.