Photoenhanced Radical Formation in Aqueous Mixtures of Levoglucosan and Benzoquinone: Implications to Photochemical Aging of Biomass-Burning Organic Aerosols.

Abstract

The photochemical aging of biomass-burning organic aerosols (BBOAs) by exposure to sunlight changes the chemical composition over its atmospheric lifetime, affecting the toxicological and climate-relevant properties of BBOA particles. This study used electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping agent, 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO), high-resolution mass spectrometry, and kinetic modeling to study the photosensitized formation of reactive oxygen species (ROS) and free radicals in mixtures of benzoquinone and levoglucosan, known BBOA tracer molecules. EPR analysis of irradiated benzoquinone solutions showed dominant formation of hydroxyl radicals (•OH), which are known products of reaction of triplet-state benzoquinone with water, also yielding semiquinone radicals. In addition, hydrogen radicals (H•) were also observed, which were not detected in previous studies. They were most likely generated by photochemical decomposition of semiquinone radicals. The irradiation of mixtures of benzoquinone and levoglucosan led to substantial formation of carbon- and oxygen-centered organic radicals, which became prominent in mixtures with a higher fraction of levoglucosan. High-resolution mass spectrometry permitted direct observation of BMPO-radical adducts and demonstrated the formation of •OH, semiquinone radicals, and organic radicals derived from oxidation of benzoquinone and levoglucosan. Mass spectrometry also detected superoxide radical adducts (BMPO-OOH) that did not appear in the EPR spectra. Kinetic modeling of the processes in the irradiated mixtures successfully reproduced the time evolution of the observed formation of the BMPO adducts of •OH and H• observed with EPR. The model was then applied to describe photochemical processes that would occur in mixtures of benzoquinone and levoglucosan in the absence of BMPO, predicting the generation of HO2• due to the reaction of H• with dissolved oxygen. These results imply that photoirradiation of aerosols containing photosensitizers induces ROS formation and secondary radical chemistry to drive photochemical aging of BBOA in the atmosphere.