Abstract:

The propagation of radiation through Earth’s atmosphere is directly affected by the absorption and scattering of incoming solar radiation by atmospheric aerosols of both primary and secondary origin, prompting research on the relation between the complex molecular composition of aerosol particles and their optical properties. While all organic aerosols (OA) scatter incoming solar radiation, only a subset of organic aerosols known as “brown carbon” (BrC) appreciably absorb visible radiation. BrC is largely emitted from combustion processes such as through the burning of biomass and human-engineered (or urban) materials. Fires occurring at the wildlandurban interface (WUI) are increasing in both frequency and magnitude, suggesting that the composition and properties of wildfire related OA are dictated not only by burning biomass but also by burning urban materials. Optical properties of BrC in biomass burning organic aerosol (BBOA) and urban OA are highly variable and are coupled to their fuel source and combustion conditions. Moreover, the molecular composition of BBOA and OA can evolve through atmospheric transport, such as through photochemical aging and condensed phase chemistry, to impact its optical properties. The role of various atmospheric aging processes on both the optical and chemical properties of BBOA and urban OA is still poorly understood. This dissertation defense will focus on two main topics. First, the interaction of BBOA with atmospheric iron (Fe(III)), a common component of aerosol liquid water from dissolved mineral dust, will be examined. Evidence will be presented that Fe(III) can catalyze oligomerization reactions with a broad range of phenolic compounds, abundantly present in biomass burning smoke, to produce strongly light absorbing and water insoluble particles. This work showcases that that biomass burning smoke can become more light absorbing as it mixes with mineral dust particles during atmospheric transport, and may, therefore, have a stronger effect on climate after aging. The second half of the defense will focus on the chemical composition, optical properties, and effect of sunlight on particulate matter (PM) emitted during urban fires. High resolution mass spectrometry was used to characterize compounds acting as BrC chromophores in urban smoke and determine their fate after photolytic aging. Results demonstrate that fires at the wildland-urban interface efficiently produce chemically complex BrC PM, which evolves into species with higher light absorptive properties after UV aging. The most important result of this PhD work is the observation that smoke can become darker in color (i.e., more light-absorbing) as a result of two very different mechanisms – transition metal catalyzed oligomerization occurring in the dark and photolysisinduced aging occurring in sunlight.