Optical and Physicochemical Properties of Atmospherically Processed Brown Carbon using Novel First-Principle Instrumentation
Date of Award
Doctor of Philosophy (PhD)
Atmospheric processing of brown carbon (BrC) – a class of spherical, internally-mixed, light-absorbing organic aerosol – emitted from smoldering biomass combustion is an understudied phenomenon with implications for climate science, air quality models, and satellite retrieval algorithms. BrC aerosols have received significant attention as a strong contributor to atmospheric light absorption in the shorter visible wavelengths and a driver of UV photochemistry. Their complex refractive indices (m=n+ik), size distributions, and carbon oxidation states dictate their optical properties, atmospheric residence times, and chemical interactions, respectively. There is currently a gap in our understanding of these fundamental particle properties and their evolution with atmospheric processing. Long-range transport and oxidation by O3, OH, and other atmospheric oxidants, as well as exposure to UV light present significant challenges when parameterizing these complex processes.This dissertation is broadly divided into three parts. The first part is a series of laboratory studies and the development of novel mathematical tools to provide a foundational understanding of chemical, physical, and optical properties of BrC aerosol and their evolution upon simulated atmospheric aging. The properties of primary BrC were studied as functions of moisture content, fuel source depth, geographic origin, and fuel packing density. No clear functionality in moisture content, source depth, or geographic origin were observed, however, the fuel packing density was found to have a significant impact on the resulting BrC optical properties. Additionally, the morphology and internal structure of BrC was studied using a centrifugal particle mass analyzer, and the long-standing assumption that BrC is spherical and homogeneous was rigorously verified. This result justifies the application of a new Mie Theory inversion algorithm to obtain the aerosol complex refractive index from laboratory measurements, which serves as an important input parameter in climate models and atmospheric remote sensing. The second part identifies the need for compact, robust, high-sensitivity aerosol instrumentation suitable for laboratory or field studies, and communicates the design, construction, and revision of a new multiwavelength integrated photoacoustic-nephelometer (MIPN). This new instrument is a field-portable instrument that directly measures the aerosol absorption and scattering coefficients at four wavelengths. The final part of this dissertation brings closure to the insights gained in laboratory studies by applying the MIPN to a series of real-world wildfires during FIREX-AQ, a large multiagency field campaign that took place in 2019. Daytime (OH-driven) and nighttime (NO3-driven) oxidation was performed on biomass burning aerosol using a Potential Aerosol Mass reactor aboard the Aerodyne Mobile Laboratory as it sampled wildfire events in Arizona and Oregon. The knowledge gained during these studies will help inform more accurate climate models and satellite remote sensing algorithms to better attribute the effects of atmospherically-processed BrC to global radiative transfer and climate change.
Rajan K. Chakrabarty Brent Williams
Richard Axelbaum, Pratim Biswas, Andrew Lambe, Jay Turner,