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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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Reactivity of aminophenols in forming nitrogen-containing brown carbon from iron-catalyzed reactions
Abstract Nitrogen-containing organic carbon (NOC) in atmospheric particles is an important class of brown carbon (BrC). Redox active NOC like aminophenols received little attention in their ability to form BrC. Here we show that iron can catalyze dark oxidative oligomerization ofo- andp-aminophenols under simulated aerosol and cloud conditions (pH 1–7, and ionic strength 0.01–1 M). Homogeneous aqueous phase reactions were conducted using soluble Fe(III), where particle growth/agglomeration were monitored using dynamic light scattering. Mass yield experiments of insoluble soot-like dark brown to black particles were as high as 40%. Hygroscopicity growth factors (κ) of these insoluble products under sub- and super-saturated conditions ranged from 0.4–0.6, higher than that of levoglucosan, a prominent proxy for biomass burning organic aerosol (BBOA). Soluble products analyzed using chromatography and mass spectrometry revealed the formation of ring coupling products ofo- andp-aminophenols and their primary oxidation products. Heterogeneous reactions of aminophenol were also conducted using Arizona Test Dust (AZTD) under simulated aging conditions, and showed clear changes to optical properties, morphology, mixing state, and chemical composition. These results highlight the important role of iron redox chemistry in BrC formation under atmospherically relevant conditions.
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- Award ID(s):
- 1903744
- PAR ID:
- 10371955
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Communications Chemistry
- Volume:
- 5
- Issue:
- 1
- ISSN:
- 2399-3669
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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