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Guaiacol, present in wood smoke, readily forms secondary organic aerosol (SOA), and, in the aqueous phase, brown carbon (BrC) species. Here, BrC is produced in an illuminated chamber containing guaiacol(g), HOOH(g) as an OH radical source, and either deliquesced salt particles or guaiacol SOA at 50% relative humidity. BrC production slows without an OH source (HOOH), likely due to low levels of radical generation by photosensitization, perhaps involving surface-adsorbed guaiacol and dissolved oxygen. With or without HOOH, BrC mass absorption coefficients at 365 nm generated by the guaiacol + OH reaction reach a maximum at ~6 h of atmospheric OH exposure, after which photobleaching becomes dominant. In the presence of soluble iron but no HOOH, more BrC is produced, likely due to insoluble polymer production observed in previous studies. However, with both soluble iron and HOOH (enabling Fenton chemistry), significantly less SOA and BrC are produced due to very high oxidation rates, and the average SOA carbon oxidation state reaches 2, indicating carboxylate products like oxalate. These results indicate that SOA and BrC formation by guaiacol photooxidation can take place over a wider range of atmospheric conditions than previously thought, and that the effects of iron(II) depend on HOOH. Multiphase guaiacol photooxidation likely makes a significant contribution to producing highly oxidized SOA material in smoke plumes.
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Kinetics and Oligomer Products of the Multiphase Reactions of Hydroxyacetone with Atmospheric Amines, Ammonium Sulfate, and Cloud Processing
Hydroxyacetone (HA) is an atmospheric oxidation product of isoprene and other organic precursors that can form brown carbon (BrC). Measured bulk aqueous-phase reaction rates of HA with ammonium sulfate, methylamine, and glycine suggest that these reactions cannot compete with aqueous-phase hydroxyl radical oxidation. In cloud chamber photooxidation experiments with either gaseous or particulate HA in the presence of the same N-containing species, BrC formation was minor, with similar mass absorption coefficients at 365 nm (<0.05 m2 g−1). However, rapid changes observed in aerosol volume and gas-phase species concentrations suggest that the lack of BrC was not due to slow reactivity. Filter-based UHPLC/(+)ESI-HR-QTOFMS analysis revealed that the SOA became heavily oligomerized, with average molecular masses of ∼400 amu in all cases. Oligomers contained, on average, 3.9 HA, 1.5 ammonia, and 1.6 other small aldehydes, including, in descending order of abundance, acetaldehyde, glycolaldehyde, glyoxal, and methylglyoxal. PTR-ToF-MS confirmed the production of these aldehydes. We identify C17H26O5, C10H22O9, C15H27NO7, C17H23NO5, and C18H32N2O9 as potential tracer ions for HA oligomers. We hypothesize that efficient oligomerization without substantial BrC production is due to negligible N-heterocycle (e.g., imidazoles/pyrazines) formation. While HA photooxidation is unlikely a significant atmospheric BrC source, it may contribute significantly to aqueous SOA formation.
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- Award ID(s):
- 2218491
- PAR ID:
- 10634089
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- ACS Earth and Space Chemistry
- Volume:
- 8
- Issue:
- 12
- ISSN:
- 2472-3452
- Page Range / eLocation ID:
- 2574 to 2586
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acsearthspacechem.4c00237
