Intermolecular Interactions Enhance the Light Absorption of a Methoxyphenol Constituent of Biomass Burning Emissions

Brown carbon (BrC) components of biomass burning organic aerosol (BBOA) absorb sunlight at visible wavelengths. However, it is not clear whether the total light absorption of this BrC is simply the sum of the contributions of the individual components or whether the components can bind noncovalently to give additional absorption through charge transfer. Here, intermolecular interactions between guaiacol and quinones (1,4-benzoquinone and 1,4-naphthoquinone) were identified in proxies of the nonpolar, water-insoluble phase of BBOA, using UV-vis spectroscopy. Guaiacol and its derivatives are some of the most abundant emissions of smoldering coniferous species. Enhanced light absorption occurred instantaneously upon mixing colorless guaiacol with either quinone in n-heptane and did not increase with time, in contrast to the absorbance changes that would be expected for a covalent product. This enhancement decreased by about 25% as the temperature increased from 303 to 323 K, consistent with exothermic association to give complexes, yielding enthalpies of complexation of -13.3 ± 0.6 and -12.3 ± 0.4 kJ mol^-1 for guaiacol with benzoquinone and naphthoquinone, respectively. Enhancement was also observed upon gas-liquid partitioning of benzoquinone into thin films of guaiacol, for example, with a thickness of 20 μm. This multiphase processing, mimicking partitioning of quinones into liquid BBOA, produced absorption comparable to moderately absorbing BrC from other sources, suggestive of the atmospheric relevance of these interactions.