Probing the Role of Intermolecular Interactions in the Light Absorption of Biomass Burning Organic Aerosol

Colton Calvert, Holly Anthony, Ahmadraza Chaudhary, Micah Miles, ELIJAH SCHNITZLER, Oklahoma State University

     Abstract Number: 485
     Working Group: Aerosol Chemistry

Abstract
Light-absorbing components of biomass burning organic aerosol (BBOA), or brown carbon, influence air quality, visibility, and climate. Here, we explore the hypothesis that organic electron donor and acceptor species among the components of BBOA can participate in intermolecular interactions to form noncovalent complexes, which may contribute to the total light absorption of this material through charge-transfer. The candidate electron donors and acceptors considered here are methoxyphenols and quinones. Methoxyphenols are one of the most abundant classes of compounds emitted from biomass burning, i.e., from the thermal degradation of lignin. Quinones can form from the gas-phase oxidation of aromatic precursors; for example, naphthalene leads to naphthoquinone. The interactions between representative methoxyphenols (e.g., guaiacol, eugenol, vanillin, and others), both individually and in mixtures, and quinones (i.e., benzoquinone and naphthoquinone) were explored in bulk solutions, thin films, and levitated microdroplets. In bulk solutions, evidence for noncovalent complexes included enhanced light absorption that was greater than expected from the sum of the individual species as well as a reversible decrease in the absorption with increasing temperature, consistent with dissociation. In thin films, enhanced light absorption occurred upon uptake of benzoquinone from the gas phase to the condensed phase of pure and mixed methoxyphenols. The light absorption at the center of the thin films increased sigmoidally over the course of 24 h, characteristic of diffusion of benzoquinone through the thin films from the edge, where the benzoquinone exposure occurs. In levitated droplets, enhanced light absorption upon exposure to subliming benzoquinone was similarly observed, but it occurred much more rapidly, as the timescale required for diffusion within the microdroplets is orders of magnitude shorter than that required for diffusion in the thin films. Together, these results provide insights into the potential occurrence and role of organic non-covalent complexes in BBOA.