Optical Properties and Chemical Composition of Brown Carbon Aerosol Particles
Organic aerosol particles in the atmosphere range in color from white to brown to black and each type contributes differently to the atmospheric radiative energy budget. White aerosol particles scatter radiation causing a cooling effect, while black aerosol particles reduce this cooling by absorbing radiation and converting it into heat. The contribution of brown aerosol particles, also known as “brown carbon” (BrC), to this radiative forcing carries the most uncertainty due to its unique light-absorbing properties and chemical composition. Primary BrC is produced by biomass burning and other combustion processes, while secondary BrC is formed by a variety of atmospheric processes. To better quantify the ability of BrC to absorb near-UV and visible radiation, methods like UV-Vis and fluorescence spectroscopy are used to analyze its optical properties. In this dissertation, UV-Vis spectroscopy is used to determine the mass absorption coefficient of secondary BrC formed via the reaction of nitrogen-containing compounds, such as ammonia and amines, with several dicarbonyls, as well as through gas-phase photooxidation of a variety of aromatic precursors via the hydroxyl radical. Fluorescence spectroscopy is also used to characterize fluorescence yields and spectra of BrC. Additionally, different high-resolution mass spectrometry techniques have been used in order to characterize the different compounds that could be consider chromophores that contribute to this type of aerosol particles brown color. This color can be compromised by atmospheric aging, such as exposure to sunlight. By combining spectroscopy and mass spectrometry methods, the loss of BrC’s ability to absorb radiation upon UV exposure is used to determine its kinetics, as well as pin-point specific compounds that may be responsible for light absorption. This dissertation provides a comparison of the properties of BrC produced by different atmospherically-relevant systems and precursors, aiding in a better overall understanding of the influence of BrC on the climate.