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Molecular Composition, Optical Properties, and Chemical Aging of Primary and Secondary Organic Aerosol


Aerosols are composed of solid or liquid particles and the gases they are suspended in. Primary aerosols are directly emitted and secondary organic aerosols (SOA) are formed in the atmosphere from the oxidation of volatile organic compounds (VOCs). The molecular composition of aerosol particles determines their optical properties, and affects the radiation balance of the Earth. Organic aerosol particles largely scatter solar radiation, however, there is a subset of organic particles called brown carbon (BrC) that absorb near-ultraviolet and visible radiation from the sun. Also, adverse effects on human health from exposure to aerosols are related to their molecular composition. The molecular composition of aerosol particles can also change during atmospheric transport, for example, due to the effects of sunlight and water vapor; and this makes the optical properties and human health effects of aerosols even harder to predict.

Chapters 2 and 3 examine the composition of smoke from biomass-burning cookstoves in rural India. This was done for two fuel types (wood and dung), as well as two stoves that are common to the area, the chulha and angithi. Chapter 2 quantifies the emission factors (EFs) of VOCs and PM2.5 from these emission sources, and estimates ozone and SOA production from these emissions. The major finding was that dung fuels and angithi cookstoves significantly increase the EFs for most VOCs. In Chapter 3, the molecular composition of smoke particles are characterized using high resolution mass spectrometry techniques. We observed that particles produced from dung-burning cookstoves were more complex (in terms of numbers of components), and contained a higher fraction of nitrogen-containing molecules (by mass abundance).

Chapter 4 characterizes BrC chromophores found in aerosol particles produced by simulated wildfires during the FIREX campaign. Then, we estimate lifetimes of individual BrC chromophores as well as bulk BrC absorbance. We find that BrC absorption has lifetimes of 10 to 41 days due to photochemical aging.

Chapter 5 investigates the evaporative browning of various SOA extracts, acidified with sulfuric acid to pH=2. The optical properties were quantified for SOAs of various precursors, both anthropogenic and biogenic, and oxidants (OH, O3, NOx). The molecular composition of BrC chromophores were determined, all of which were organosulfates.

Chapter 6 explores the effect of water vapor and aerosol liquid water on the molecular composition of SOA, as it is formed and chemically aged. We have evidence that for α-pinene/OH SOA formed in the presence of ammonium sulfate seed, water likely participates in hydrolysis reactions that result in less viscous particles.

These results provide key inputs to air quality models to more accurately predict regional climate; including EFs for Indian cookstoves and lifetimes of BrC in biomass burning organic aerosol. Other observations regarding secondary BrC formation in CCN mimics and hydrolysis reactions occurring in model SOA further our understanding of the evolution and chemical processing of atmospheric aerosols.

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