Comparison of aqueous secondary organic aerosol (aqSOA) product distributions from guaiacol oxidation by non-phenolic and phenolic methoxybenzaldehydes as photosensitizers in the absence and presence of ammonium nitrate
<p>Aromatic carbonyls (e.g., methoxybenzaldehydes), an important class of photosensitizers, are abundant in the atmosphere. Photosensitization and nitrate-mediated photo-oxidation can occur simultaneously, yet studies about their interactions, particularly for aqueous secondary organic aerosol...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/23/2859/2023/acp-23-2859-2023.pdf |
| Summary: | <p>Aromatic carbonyls (e.g., methoxybenzaldehydes), an
important class of photosensitizers, are abundant in the atmosphere.
Photosensitization and nitrate-mediated photo-oxidation can occur
simultaneously, yet studies about their interactions, particularly for
aqueous secondary organic aerosol (aqSOA) formation, remain limited. This
study compared non-phenolic (3,4-dimethoxybenzaldehyde, DMB) and phenolic
(vanillin, VL) methoxybenzaldehydes as photosensitizers for aqSOA formation
via guaiacol (GUA) oxidation in the absence and presence of ammonium nitrate
(AN) under atmospherically relevant cloud and fog conditions. GUA oxidation
by triplet excited states of DMB (<span class="inline-formula"><sup>3</sup></span>DMB<span class="inline-formula"><sup>∗</sup></span>) (GUA <span class="inline-formula">+</span> DMB) was
<span class="inline-formula">∼</span> 4 times faster and exhibited greater light absorption than
oxidation by <span class="inline-formula"><sup>3</sup></span>VL<span class="inline-formula"><sup>∗</sup></span> (GUA <span class="inline-formula">+</span> VL). Both GUA <span class="inline-formula">+</span> DMB and GUA <span class="inline-formula">+</span> VL formed aqSOA
composed of oligomers, functionalized monomers, oxygenated ring-opening
species, and N-containing products in the presence of AN. The observation of N-heterocycles such as imidazoles indicates the participation of ammonium in
the reactions. The majority of generated aqSOA comprises potential brown carbon
(BrC) chromophores. Oligomerization and functionalization dominated in
GUA <span class="inline-formula">+</span> DMB and GUA <span class="inline-formula">+</span> VL, but functionalization appeared to be more important
in GUA <span class="inline-formula">+</span> VL due to contributions from VL itself. AN did not significantly
affect the oxidation kinetics, but it had distinct effects on the product
distributions, likely due to differences in the photosensitizing abilities
and structural features of DMB and VL. In particular, the more extensive
fragmentation in GUA <span class="inline-formula">+</span> DMB than in GUA <span class="inline-formula">+</span> VL likely generated more
N-containing products in GUA <span class="inline-formula">+</span> DMB <span class="inline-formula">+</span> AN. In GUA <span class="inline-formula">+</span> VL <span class="inline-formula">+</span> AN, the increased
oligomers may be due to VL-derived phenoxy radicals induced by <span class="inline-formula"><sup><span class="Radical">⚫</span></sup></span>OH or <span class="inline-formula"><sup><span class="Radical">⚫</span></sup></span>NO<span class="inline-formula"><sub>2</sub></span> from nitrate photolysis. Furthermore,
increased nitrated products observed in the presence of both DMB or VL and
AN than in AN alone imply that photosensitized reactions may promote
nitration. This work demonstrates how the structural features of
photosensitizers affect aqSOA formation via non-carbonyl phenol oxidation.
Potential interactions between photosensitization and AN photolysis were
also elucidated. These findings facilitate a better understanding of
photosensitized aqSOA formation and highlight the importance of AN
photolysis in these reactions.</p> |
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| ISSN: | 1680-7316 1680-7324 |