23 Aug 2022
23 Aug 2022
Status: this preprint is currently under review for the journal ACP.

Aqueous SOA formation from photosensitized guaiacol oxidation: Comparison between non-phenolic and phenolic methoxybenzaldehydes as photosensitizers in the absence and presence of ammonium nitrate

Beatrix Rosette Go Mabato1,2, Yong Jie Li3, Dan Dan Huang4, Yalin Wang3, and Chak Keung Chan1,2 Beatrix Rosette Go Mabato et al.
  • 1School of Energy and Environment, City University of Hong Kong, Hong Kong, China
  • 2City University of Hong Kong Shenzhen Research Institute, Shenzhen, China
  • 3Department of Civil and Environmental Engineering, and Centre for Regional Ocean, Faculty of Science and Technology, University of Macau, Macau, China
  • 4Shanghai Academy of Environmental Sciences, Shanghai 200233, China

Abstract. Aromatic carbonyls (e.g., methoxybenzaldehydes), an important class of photosensitizers, are abundant in the atmosphere. This study compared non-phenolic (3,4-dimethoxybenzaldehyde, DMB) and phenolic (vanillin, VL) methoxybenzaldehydes as photosensitizers for aqueous secondary organic aerosol (aqSOA) formation via guaiacol (GUA) oxidation under atmospherically relevant cloud and fog conditions. The effects of ammonium nitrate (AN) on these reactions were also explored. GUA oxidation by triplet excited states of DMB (3DMB*) (GUA+DMB) was ~4 times faster and exhibited greater light absorption than oxidation by 3VL* (GUA+VL). Both GUA+DMB and GUA+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 are potential brown carbon (BrC) chromophores. Oligomerization and functionalization dominated in GUA+DMB and GUA+VL, but functionalization appeared to be more important in GUA+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+DMB than in GUA+VL likely generated more N-containing products in GUA+DMB+AN. In GUA+VL+AN, the increased oligomers may be due to VL-derived phenoxy radicals induced by OH or NO2 from nitrate photolysis. Furthermore, increased nitrated products observed in the presence of both DMB or VL and AN than in AN alone implies 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 ammonium nitrate photolysis in these reactions.

Beatrix Rosette Go Mabato et al.

Status: open (until 04 Oct 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-595', Anonymous Referee #1, 15 Sep 2022 reply
  • RC2: 'Comment on acp-2022-595', Anonymous Referee #2, 16 Sep 2022 reply
  • RC3: 'Reviewer Comment on acp-2022-595', Anonymous Referee #3, 20 Sep 2022 reply
  • RC4: 'Comment on acp-2022-595', Anonymous Referee #4, 27 Sep 2022 reply

Beatrix Rosette Go Mabato et al.

Beatrix Rosette Go Mabato et al.


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Short summary
We compared non-phenolic and phenolic methoxybenzaldehydes as photosensitizers for aqueous secondary organic aerosol (aqSOA) formation under cloud/fog conditions. We showed that the structural features of photosensitizers affect aqSOA formation. We also elucidated potential interactions between photosensitization and ammonium nitrate photolysis. Our findings would be useful for evaluating the importance of photosensitized reactions on aqSOA formation, which could improve aqSOA-predictive models.