Articles | Volume 26, issue 21
https://doi.org/10.5194/acp-26-15853-2026
© Author(s) 2026. This work is distributed under the Creative Commons Attribution 4.0 License.
Drivers of biogenic secondary organic aerosol from the past to the future
Download
- Final revised paper (published on 11 Nov 2026)
- Supplement to the final revised paper
- Preprint (discussion started on 26 Mar 2026)
- Supplement to the preprint
Interactive discussion
Status: closed
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
| : Report abuse
- RC1: 'Referee Comment on egusphere-2026-1570', Anonymous Referee #1, 23 May 2026
- RC2: 'Comment on egusphere-2026-1570', Anonymous Referee #2, 26 May 2026
- RC3: 'Comment on egusphere-2026-1570', Anonymous Referee #3, 27 May 2026
- AC1: 'Comment on egusphere-2026-1570', Yang Shi, 15 Aug 2026
Peer review completion
AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload
AR by Yang Shi on behalf of the Authors (15 Aug 2026)
Author's response
Author's tracked changes
Manuscript
ED: Referee Nomination & Report Request started (02 Sep 2026) by Stefania Gilardoni
RR by Anonymous Referee #2 (18 Sep 2026)
ED: Publish as is (28 Sep 2026) by Stefania Gilardoni
AR by Yang Shi on behalf of the Authors (06 Oct 2026)
Post-review adjustments
AA – Author's adjustment | EA – Editor approval
AA by Yang Shi on behalf of the Authors (27 Oct 2026)
Author's adjustment
Manuscript
EA: Adjustments approved (30 Oct 2026) by Stefania Gilardoni
This manuscript describes a series of modeling experiments performed in CESM2 to test the importance of newly elucidated pathways in biogenic volatile organic compound (VOC) chemistry to the formation of secondary organic aerosol (SOA), and the resulting changes in that SOA under past and future climate scenarios. Specifically, the authors implement a number of new peroxy radical (RO2) reaction pathways in the model's chemical mechanism, some of which do not form SOA but compete with SOA-forming pathways (for isoprene) and some of which have SOA-forming capabilities of their own (isomerization and RO2 + RO2 chemistry, especially from monoterpenes). Using parameters from the literature for these new pathways, they authors find that monoterpene-RO2 isomerization is a hugely important pathway, accounting for nearly half of monoterpene-derived SOA. They also find that the effect on SOA of changes in biogenic VOC emissions from preindustrial to present to future conditions overwhelm the effects of changing chemistry (via temperature-dependent reaction rates and changing bimolecular RO2 reaction partners) and thermodynamics (gas-particle partitioning), and will likely cause increased in biogenic SOA in a warmer future climate.
The results of this study are certainly important and useful, and worthy of publication following some clarification as described below. I'm also very impressed by the clarity of the writing. The edits made to the model and mechanism are extensive, but I felt they were clearly enough laid out and walked through that I was never lost. The step-by step discussion of modeling results is excellent; in particular the paragraph on separating chemistry from emissions (L 551-565) does an exceptional job of explaining the experimental setup and highlighting the important results in a concise and clear manner. I think the finding that the change from PD to F is completely due to emissions -- and in fact that chemistry & thermodynamics would lead to a decrease in SOA with BVOC emissions held constant -- is an important one that should be highlighted in the abstract!
My main concern is that a lot of the mechanistically-specific conclusions (that monoterpene isomerization is hugely important to SOA formation; that RO2 + RO2 chemistry is not) seem based on the particular parameters the authors chose out of many highly uncertain ones, and this uncertainty is not satisfactorily expressed in the places where quantitative conclusions about the contributions of these pathways are noted. While some of these uncertainties are discussed in the methods section, they are then glossed over in places like the abstract and conclusions where headline numbers about the importance of isomerization, e.g., are reported. In particular, I *don't* think it's worth highlighting quantitatively in the abstract the very precise fraction of monoterpene SOA formed by isomerization (44-46%), which makes it look like the uncertainty range on this is tiny when in fact the parameterization that went into it is highly uncertain.
I write more detail below on the specific elements of uncertainty (or at least the specific parameters that seem highly uncertain) that come to mind, but overall I think the magnitudes of these compounding uncertainties in reaction rates, yields, and simplifying parameterizations (e.g. lumping of all terpenes and treating them like a-pinene) merit more discussion -- for example, how big a range could the isomerization fraction of monoterpene SOA actually span given the uncertainty on parameters like its rate, its SOA yield, the isomerize-able fraction (especially given the lumping of all terpenes into a-pinene), the low volatility and photorecalcitrance of its products? Not that you need to do more simulations testing all this -- just some back-of-the-envelope type analysis to temper the conclusions would be useful, I think.
- L174-175 --> but ISOP makes SOA mostly through heterogeneous reactivity (IEPOX uptake being a reactive process), not volatility, so isn't that problematic? Not suggesting that you fully revise the isoprene SOA scheme here, but has there been any previous assessment you can point to here of the errors introduced by treating IEPOX SOA as a volatility-driven process, and/or can you speculate on how that would influence your findings about PI vs PD vs F for isoprene SOA?
- L177-179 --> lumping of MTERP RO2s seems problematic given their very different structures; they'll likely have very different reaction pathways and balances between fragmentation vs functionalization. It's fine if the existing mechanism is tuned to get this right given the distribution of monoterpenes, but will that distribution change under preindustrial or future scenarios, and what influence would that have?
- L182-183 isn't that work by Pye et al just for a-pinene? Do these rates and yields hold across MTERPs, and if not, how much uncertainty is introduced from your treatment here?
- L188-195 the isom pathway seems extremely poorly constrained and heavily oversimplified; is there any constraint on the high SOAG yields and subsequent SOA production from this pathway? Does that match any ambient or chamber observations? And is there any information at all informing the implementation of an equivalent rate and product yield for BCARY? I'm glad you acknowledge the uncertainties here, but later in the results section (e.g. L 313-321) it sounds like it's taken as fact, since you're describing the huge impact of isomerization-derived SOA there without any mention of the high uncertainty.
- Table 2 --> it appears that these added RO2 + RO2 reactions in Part 2 consume gas-phase RO2s that weren't part of the VBS scheme, which is contrary to how this mechanism's previous VBS chemistry seems to work, in that none of the reactions in Table 1 consume the gas-phase compounds implicated in the reactions (i.e. the reactions of precursors with OH still have the precursor and OH on the right side of the equation, so they effectively don't consume it, and the reactions of VBS compounds with HO2 & NO don't consume those either). Does the newly-added consumption of gas-phase RO2 in your scheme influence gas-phase chemical outcomes (I'm especially thinking of oxidant recycling) in places where this chemistry happens abundantly? I wouldn't be surprised, for example, if this consumption of RO2 in, say, the Amazon leads to somewhat lower HOx levels there...
- Also in Table 2 - Is it known that the reactions of MTERP/BCARY RO2's with CH3O2 and CH3CO3 make no SOA? (And ISOPRO2 + CH3O2/CH3CO3, for that matter, though that seems less likely anyway). And if that's known known experimentally, why is that assumed? Presumably these RO2-RO2 reactions could lead to a small fraction of accretion products (at least for the RO2 + CH3O2 reactions) -- the equivalent accretion products do show up as an appreciable chunk of the total in some locations in Mayhew et al (https://acp.copernicus.org/articles/25/17027/2025/, Fig 12, bottom, "sesquiterpenes + small VOCs" & "monoterpenes + small VOCs"). Even for the branching of these RO2 + RO2 reactions to non-accretion alkoxy radicals, some moderate-volatility products could be produced (especially for BCARY), for which volatilities could at least be estimated. Could you at least provide an estimate of what fraction of the mono-/sesquiterpenes go down these pathways to give the reader a sense of the uncertainty here?
- Speaking of the branching to non-accretion products -- am I correct in interpreting the 0.04 SOAG0 yield from all the BCARYO2VBS + RO2 reactions as an attempt to represent just the accretion product, meaning that there's an assumed SOA yield of zero from non-accretion pathways? Again, this seems unlikely for the non-accretion pathways from sesquiterpenes and even monoterpenes, which leave you with more-oxygenated C10 and C15 compounds than you started with and could easily form compounds with low-enough volatility to contribute to SOA. Overall it seems this representation is likely to underestimate SOA from RO2 + RO2 pathways if it only accounts for accretion products, and with a fixed yield of 4%, despite mounting evidence that yields increase with molecular size (Berndt et al., 2018, DOI 10.1002/anie.201710989; Franzon, 2023, DOI 10.1021/acs.jpca.3c01890; Frandsen et al., 2025, DOI 10.1021/acsearthspacechem.4c00355). The justification for this fixed 4% number (L 222-224) needs to be stronger.
- I assume "SOAG0" is a single compound with a fixed effective molar mass; if that's the case, surely the 0.04 molar yield of SOAG0 represents a fixed mass yield of this SOA compound from each of the RO2 + RO2 reactions, but that's not the same as saying that the reactions have the same dimer yields; a BCARYO2 + BCARYO2 dimer would have 30 carbons compared to a MTERPO2 + ISOPRO2 dimer's 15. So doesn't this method of fixing the yield at 0.04 SOAG0 skew the product formation, such that lighter precursors effectively have higher molar SOA yields? This is compounded by the fact that the C30 BCARYO2 + BCARYO2 dimer should be less volatile than the C15 MTERPO2 + ISOPRO2 dimer, and so should have a higher propensity to form SOA. You acknowledge this on L226-229, but it seems like it would be reasonable to estimate the volatilities of these dimers and correctly apportion them into volatility bins rather than just putting them all in the same extremely-low-volatility bin together.
- The overall sense here is that many compounding factors are making the SOA yield parameterizations from this RO2+RO2 chemistry hugely uncertain and likely a low estimate, and as with the isomerization chemistry discussed above, I think that merits more mention of this uncertainty later on when large claims are made (e.g. L 575-576 in the conclusions that "The RO2 + RO2 reactions contribute moderately to the MTERP RO2 fate but make a minor contribution to the MTERP SOA formation" -- I think it's worth stating right in that sentence that this is perhaps only because the estimated SOA yields from RO2 + RO2 are quite low.
- L 224-225 it's unclear what's meant by "these two"
- Table 3 - are the ISOP-RO2s not allowed to cross react with other ISOP-RO2s? Presumably this pathway could be important in some high-isoprene-emission locations, and would lead to a small yield of accretion products with sufficiently low volatility to contribute to SOA?
- L 261-262 does the implementation of this photorecalcitrant fraction in just the lowest-volatility bin and not spread out across the bins -- which seems based in convenience rather than any physical or chemical characteristics of that volatility bin -- potentially bias your results toward overemphasizing the importance of the lowest volatility bin, which just happens to be where you've put a lot of the new isomerization and RO2+RO2 SOA? How large could this effect be?
- L 284 "reduced future anthropogenic emissions" of what? A priori it seems directly counter to the high CO2 stated in the previous sentence. Reduced emissions of NOx? How is that compatible with increased CO2 that likely comes from fossil fuel combustion?
- L 403-406 - the introduction of new ISOPRO2 reactions that compete with the SOA-forming pathways and decrease overall isoprene-derived SOA brings up an important point: were the original SOA yields from ISOPRO2's HO2 and NO pathways based exclusively on chamber experimental results, or were they at all tuned in order to match ambient observations of isoprene-derived SOA? Because if it's the latter, then adding new pathways that compete with those old ones means you should artificially bump up the SOA yields to compensate; otherwise, you're now biased low on isoprene SOA formation relative to the previous tuning. If it's the former, keeping those old SOA yields should be okay -- but you're really only getting the decrease because you've assumed zero SOA formation from the newly added isomerization and RO2 + RO2 pathways for isoprene, which is likely not correct (e.g. Berndt et al 2025 for SOA formation from isomerization, Mayhew et al 2025 for the contribution of isoprene RO2 + isoprene RO2 accretion products to SOA).