Characterizing The Evolution Of Fresh And Aged Water-Soluble Brown Carbon Chromophores In Wildfire Smoke.

Abstract

Wildfires are an important source of carbonaceous aerosol that impact air quality and the Earth’s radiative budget. Organic aerosol that is partially water soluble and absorbs in the ultraviolet to visible spectral range is referred to as “brown carbon” (BrC). There remains uncertainty regarding BrC’s optical properties and atmospheric evolution. This dissertation investigates BrC across laboratory, airborne, and ground-based measurements. Chapter two examines airborne measurements of western U.S. wildfire smoke aged 0-5 hrs during FIREX-AQ. Brown carbon absorption was measured online using a particle-into-liquid sampler (PILS) and aqueous samples were collected for offline SEC-UV analysis. Absorption was dominated by low molecular weight (MW) (< 500 Da) species and exhibited variable trends with plume age. Comparisons between the online PILS and offline SEC-UV absorption measurements revealed differences between absolute absorption and wavelength dependence. These results emphasized the importance of intercomparison between measurement techniques when investigating BrC absorption. Chapter three focuses on controlled combustion experiments from the FIREX FireLab study. Western U.S. fuels were pyrolyzed and BrC absorption, total aerosol absorption, and aerosol composition was investigated online and offline at ambient and thermal denuded conditions. Offline filters were characterized using SEC-UV, a volatility basis set framework, and the octanol water partitioning coefficient log(Kow) was estimated. At ambient conditions, absorption was dominated by low volatility compounds of low MW. After thermal denuding, a fraction of BrC persisted, where the absorption contribution by high MW species increased. The log(Kow) values indicated moderately hydrophobic (log(Kow) >3) species which provided insight into their atmospheric persistence. ii Chapter four investigates aged wildfire emissions during a smoke intrusion in Toronto. Size-resolved aerosols were collected for offline chromatographic analyses. Absorption was dominated by high MW species with moderate hydrophobicity, and elevated levels of alkylamines (NR3H+/NH4+ of 0.3-3.6) were observed. Alkylamines were not detected in fresh FIREX FireLab emissions, suggesting the ratios observed in Toronto reflect atmospheric aging and mixed urban biomass burning influences rather than direct wildfire emissions. This work demonstrates that BrC chemical and absorption properties evolve after emission, and highlights the importance of integrating chromatographic methods, laboratory experiments, and field measurements to constrain the evolution of BrC.