Noncovalent Interactions in Brown Carbon Surrogates Lead to Enhanced Absorption of Visible Light

COLTON CALVERT, Holly Anthony, Nathan Huskins, Micah Miles, Elijah Schnitzler, Oklahoma State University

     Abstract Number: 409
     Working Group: Aerosol Chemistry

Abstract
Biomass burning produces brown carbon, which absorbs visible light. However, there is an ongoing debate if the non-covalent interactions in brown carbon aerosol give rise to charge-transfer complexes, leading to additional absorbance. Guaiacol and its derivatives are known to be the most abundant products of biomass burning from gymnosperms, while benzoquinone and naphthoquinone are also well-known components of brown carbon. Furthermore, the electron donating groups on guaiacol and electron withdrawing groups on the quinones lead to an increased likelihood of the formation of a charge-transfer complex. Using UV-vis spectroscopy, we demonstrated complexation when the components are mixed in n-heptane, which serves as a proxy of the non-polar organic phase of biomass burning organic aerosol. Enhanced light absorption occurred instantaneously upon mixing and did not change with time. However, absorbance did change with temperature, and the initial absorbance was recovered simply by returning to the initial temperature. Furthermore, gas phase quinones are shown to partition into liquid guaiacol and guaiacol-like compounds to form charge-transfer complexes, and photoacoustic extinctiometer measurements are used to explore complexation in the aerosol phase. From these measurements, the physical and optical properties of the complexes are calculated, providing insight into the potential contributions of charge-transfer complexes to the total absorbance of brown carbon.