AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Photocatalyzed Autoxidation of Polycyclic Aromatic Hydrocarbons in the Production of Humic-like Substances
JOHN HAYNES, Keith Miller, Brian Majestic, University of Denver
Abstract Number: 656 Working Group: Aerosol Chemistry
Abstract This study evaluates the possibility of humic-like substance (HULIS) production by oxidation of aqueous suspensions of polycyclic aromatic hydrocarbons (PAH) with exposure to sunlight. HULIS material is presented as a collective of compounds, each containing significant aromatic and carboxylic acid character. They are found to be a major component of brown carbon and are implicated with radiative forcing in the atmosphere. PAH and HULIS are commonly observed in particles sourced from pyrogenic processes, e.g. biomass burning and vehicle exhaust. The extended atmospheric lifetimes of PAH lend themselves to longer exposure times to sunlight and oxidants, potentially producing other branched and oxidized PAH (oxPAH) products such as anthraquinone and naphthol, which then may act as intermediates for larger structures, e.g. HULIS. Particle surfaces containing PAH may use ultraviolet radiation as a catalyst in an electron transfer interaction with elemental oxygen or water molecules to produce radicals, such as superoxide or hydroxyl radicals. These radicals are highly reactive oxidation agents and may initiate several oxidizing pathways, resulting in the production of HULIS via oxPAH.
Reaction conditions use varying light exposure, sample composition, and duration. Analytical techniques, including HPLC, SPE, GCMS, FTIR, UV-vis, and pH, are used to elucidate the number and character of these products. Initial PAH data include many new HPLC peaks during and following light exposure periods, which indicate the production of several species of oxPAH. This array of products is emblematic of HULIS materials. The growth of peaks at specific retention times throughout the reaction demonstrates the evolution of explicit reaction pathways toward predictable products. Evaluating the formation of products from dissolved and suspended PAH solutions with photo-reactions conducted within purified water, formate buffer, and suspended soil allows for the determination of specific reaction mechanisms.