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.