Nir Bluvshtein

An Approach for Faster Retrieval of Aerosols’ Complex Refractive Index Using Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy (CRD-S) is widely adapted for studying light extinction by aerosol in laboratory and field studies. The complex refractive index (RI) of an aerosol can be retrieved by finding the theoretical Mie theory curve that best fits the measured extinction efficiency for as many aerosol diameters as possible. In this study, we introduce a new retrieval approach for the complex RI of aerosols using extinction measurements at only two carefully selected size parameters. We show that for three model substances measured (ammonium sulfate, Suwannee river fulvic acid, and nigrosin) and for 22 other reanalyzed datasets, the “2-points” measurement approach enables the retrieval of complex refractive indices with comparable uncertainty to the traditional measurement and retrieval procedure. The advantages and disadvantages of the new approach are discussed. Copyright 2012 American Association for Aerosol Research

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

The wavelength-dependence of the complex refractive indices (RI) in the visible spectral range of secondary organic aerosols (SOA) are rarely studied, and the evolution of the RI with atmospheric aging is largely unknown. In this study, we applied a novel white light-broadband cavity enhanced spectroscopy to measure the changes in the RI (400-650 nm) of β-pinene and p-xylene SOA produced and aged in an oxidation flow reactor, simulating daytime aging under NO x-free conditions. It was found that these SOA are not absorbing in the visible range, and that the real part of the RI, n, shows a 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.