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. 2023 Jun 22;127(24):5209-5221.
doi: 10.1021/acs.jpca.3c01794. Epub 2023 Jun 7.

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

Lena Gerritz  1 , Meredith Schervish  1 , Pascale S J Lakey  1 , Tim Oeij  1 , Jinlai Wei  1 , Sergey A Nizkorodov  1 , Manabu Shiraiwa  1
Affiliations

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

Lena Gerritz et al. J Phys Chem A. .

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.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Structures of BQ and its photolysis products discussed in this study.
Figure 2
Figure 2
EPR spectra of (A) BQ solutions and (B) 1:10 mixture of BQ/LVG after 53 min of irradiation. The observed spectra were deconvoluted into BMPO-radical adducts: BMPO–OH (red), BMPO–R (blue), BMPO–OR (yellow), and BMPO–H (green).
Figure 3
Figure 3
Radical yield (the number of produced radicals normalized to the number of photons absorbed by BQ) over irradiation time for (A) BQ solutions and the (B) 1:10 BQ/LVG mixture trapped using ∼10 mM BMPO. Shaded regions represent one standard deviation of the measurements. Irradiation begins at time t = 0.
Figure 4
Figure 4
Radical yields (normalized to photons absorbed by BQ) for each solution: BQ only, BQ/LVG mixture with the ratio of 1:1, 1:10, and 1:100 after (A) 1 (beginning), (B) 29 (halfway), and (C) 53 min (end) of irradiation. Error bars represent one standard deviation of the measurements.
Figure 5
Figure 5
BMPO–OH formation over time for solutions of BQ only and 1:1 and 1:10 BQ/LVG mixtures with initial [BQ] = 2.5 mM and ∼10 mM BMPO. Shaded regions represent one standard deviation of the measurements. Irradiation begins at time t = 0.
Figure 6
Figure 6
LCMS chromatograms for the irradiated 1:10 BQ/LVG mixtures with 10 mM BMPO including the TIC and selected ion chromatograms for select BMPO adducts featured in Table 2.
Figure 7
Figure 7
Observed (markers) and modeled (red lines) time evolution of BMPO–OH concentrations (A) in the BQ solution and (B) in the 1:10 BQ/LVG mixture over 400 s of irradiation as well as the time evolution of BMPO–H concentrations and the model-predicted BMPO–OOH concentrations (C) in the BQ solution and (D) in the 1:10 BQ/LVG mixture over 3500 s of irradiation. The shaded regions represent the model uncertainties from the rate constant of R30. Irradiation begins at time t = 0.
Figure 8
Figure 8
Model-predicted time evolution of concentrations of (A) reactants (BQ, LVG, DO, and spin trap BMPO) and (B) ROS (•OH, H2O2, H•, and O2•–/HO2•) in the 1:10 BQ/LVG mixture with and without BMPO. Note that HO2• and O2–• are plotted together as they form the same BMPO–OOH adduct.
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