the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Sep 2026
Unappreciated role of sulfate radicals in the aqueous aging process of methoxyphenols derived from biomass burning
Abstract. Methoxyphenols are widely acknowledged tracers of biomass burning emissions, undergo complex chemical transformations in atmospheric aqueous environments that significantly modulate aerosol properties. The role of sulfate radicals (SO4•–), the highly potent electrophiles prevalent in cloud and fog waters, remains poorly constrained. Herein, we investigated the aqueous-phase kinetics and aqueous secondary organic aerosol (aqSOA) formation of three representative methoxyphenols containing different substituents, namely vanillic acid (VAA), vanillin (VAL), and coniferyl aldehyde (CFA), upon oxidation by SO4•–. The determined second-order rate constants for VAA, VAL, and CFA with SO4•– were in the range of (2.40-3.37) × 109 M-1 s-1, revealing that side-chain substituents critically govern reactivity through π-electron density modulation. These reactions efficiently generated aqSOA with maximum mass yields of 63.12%-70.49%, characterized by high oxidation degree comparable to that of atmospheric low-volatility oxygenated organic aerosols (LV-OOA). Notably, the oxidation process driven the generation of humic-like substances (HULIS) with pronounced light-absorbing capabilities in the near-ultraviolet and visible regions, thereby contributing to atmospheric brown carbon. Furthermore, the oxidation products exhibited significantly elevated oxidative potentials compared to their precursors, posing enhanced health risk. Our findings identify SO4•–-initiated aqueous chemistry as a critical yet previously overlooked pathway that transforms biomass burning emissions into toxic and light-absorbing secondary aerosols. Therefore, it is necessary to incorporate SO4•–-related processes into atmospheric models to accurately predict air quality and climate forcing.
- Preprint
(1208 KB) - Metadata XML
-
Supplement
(1227 KB) - BibTeX
- EndNote
Status: open (until 10 Nov 2026)
- RC1: 'Comment on egusphere-2026-5353', Anonymous Referee #2, 13 Oct 2026 reply
-
RC2: 'Comment on egusphere-2026-5353', Anonymous Referee #1, 14 Oct 2026
reply
Publisher’s note: this comment was edited on16 October 2026. The following text is not identical to the original comment, but the adjustments were minor without effect on the scientific meaning.
This work provides updated kinetic data for aqueous-phase reactions of three representative substituted methoxyphenols with SO4 •– , and further elucidates aqSOA mass yields, optical properties, and oxidation potentials. It was found that SO4 •– -initiated aqueous chemistry is a critical yet previously overlooked pathway that transforms biomass burning emissions into toxic and light-absorbing secondary aerosols. The kinetic data have important implications for the fate of methoxyphenols in atmospheric aqueous environments. Overall, the manuscript provides a novel and valuable investigation of SO4 •–initial reactions in SOA formation. However, there are still some minor points that need to be illustrated. Therefore, the publication of this manuscript in Atmospheric Chemistry and Physics is supported if the authors can properly address the following comments.
1. What is the main sink of sulfate radicals in the ambient atmosphere?
2. Lines 81-82, provide more information about the atmospheric scenario where sulfate radicals and methoxyphenols coexist.
3. Please elaborate on how ionic strength may affect the reaction kinetics, while not discussed in Supplement information.
4. More information about the calculation of aqSOA mass yields should be supplemented in the Experimental Section.
5. Fig. S5 shows the changes in solution color before and after the reaction, yet the corresponding reaction time points are not provided. This omission hinders readers from fully understanding the detailed experimental progression. The similar omission also occurs in Fig. S6.
6. Field observations in China have shown that the mass fraction of nitrate in PM2.5 increases rather than decreases. Nitrates are highly hygroscopic, which elevates the water content of atmospheric aerosols. Therefore, the significance of aqSOA formation merits attention. However, this relevant discussion is absent from the Atmospheric Implication section.
7. The sizes of Figures 1, 2 and 3 should be adjusted so that they can be presented in landscape orientation.
8. The manuscript would benefit from copy-editing for grammar and syntax, because some grammatical errors are found in this manuscript.
9. Line 26: “63.12%-70.49%” may be changed to “63.12-70.49%”.
10. Lines 42-43 and 406: “3.6 × 10-13 to 2.3 × 10-15 mol L-1” may be changed to “2.3 × 10-15 to 3.6 × 10-13 mol L-1”.
11. Line 45: The “.” before SO4•– should be a “,”.
12. Line 421: The “.” before the mass should be a “,”.Citation: https://doi.org/10.5194/egusphere-2026-5353-RC2
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 227 | 54 | 24 | 305 | 52 | 35 | 35 |
- HTML: 227
- PDF: 54
- XML: 24
- Total: 305
- Supplement: 52
- BibTeX: 35
- EndNote: 35
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The manuscript by Chen et al. systematically investigates the aqueous-phase oxidation of three representative biomass-burning methoxyphenols by sulfate radicals (SO4•–). The authors determine second-order rate constants using a relative-rate method and report substantial aqueous secondary organic aerosol (aqSOA) mass yields. The aqSOA is highly oxidized, with O/C ratios and average carbon oxidation states comparable to atmospheric low-volatility oxygenated organic aerosol (LV-OOA). The reaction products also show increased light absorption in the near-UV and visible regions, and the authors identify humic-like substances (HULIS) as important products. DTT assays indicate that the oxidation products have higher oxidative potential than the parent methoxyphenols. The authors conclude that SO4•–-initiated aqueous chemistry is an overlooked pathway for transforming biomass-burning emissions into light-absorbing, potentially toxic secondary aerosols, and recommend incorporating SO4•–-related processes into atmospheric models. Overall, the manuscript is generally well organized. I recommend a minor revision to improve the clarity of several methodological descriptions and the presentation of the results.
Line 28: “Notably, the oxidation process driven the generation”, grammar error: should be “drove the generation”.
Line 42-43: “spans 3.6×10-13 to 2.3×10-15”, range written high-to-low; should be low-to-high, same issue for line 406.
Line 105: “the possible impacts of temperature and ion strength”, “ion strength” should be “ionic strength”.
Line 113-116: The k value for ferulic acid is attributed to Herrmann et al. (2010), while Table 2 attributes it to “(Zhu et al., 2006)”, please check.
Line 139: 5,5’-dithiobis (2-nitrobenzoic acid) (DNTB), “DNTB” should be DTNB.
Line 278: “and its classes” should be “their classes”.
Line 292: “The substitutes are either electron-withdrawing groups”, where “substitutes” should be “substituents”.