Optical Properties of Absorbing Organic Aerosol Particles

Zarzana, Kyle J.

Graduate Thesis Or Dissertation

Optical Properties of Absorbing Organic Aerosol Particles Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/t722h896v

Abstract

Refractive indices can be used to quantify the amount of light scattered and absorbed by atmospheric aerosol particles. Accurate values for refractive indices are needed to determine the effect of aerosol particles on climate, but the refractive indices for many atmospheric species are poorly known, particularly for organic compounds. Determining refractive indices for organic aerosol is difficult because atmospheric aerosol contains numerous organic compounds, many of which are poorly characterized. While most organic aerosol is non-absorbing, a significant fraction is composed of a class of compounds termed brown carbon, which can absorb light at shorter wavelengths. Brown carbon is an important component of total radiative forcing, and can account for up to twenty percent of the total absorption at lower wavelengths. Even though brown carbon has been observed in many locations, the sources and properties are poorly known. One proposed source is aqueous phase reactions between carbonyls and species containing reduced nitrogen. Aqueous reactions are believed to be important sources of organic aerosol, and could also contribute to the formation of brown carbon. This thesis examines the optical properties of several brown carbon surrogate systems, as well as ways of accurately determining those properties. First, the optical properties of the products formed by reactions between carbonyls and amines were examined at a wavelength of 532 nm using only extinction data. These reactions form brown products with optical properties significantly different than non-absorbing organics. Second, a theoretical study was performed to determine the most effective way to accurately and precisely calculate the optical properties of both absorbing and non-absorbing aerosol. This study indicated the need for absorption data in addition to extinction data, and so a photoacoustic spectrometer was constructed to directly measure absorption. Finally, the optical properties of carbonyls reacted with ammonium were examined at a wavelength of 405 nm using the newly constructed instrumental setup, which is capable of simultaneous extinction and absorption measurements. These measurements represent some of the first refractive index values reported for these systems.

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