the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 May 2026
Comprehensive Treatment of C4-C6 Alkanes and Their Oxidation Products in CAM-chem: The MOZART-T3 Mechanism
Abstract. Most global atmospheric chemistry models represent ≥C4 alkanes using lumped surrogates, limiting both detailed simulation of their oxygenated products and evaluation against comprehensive observational datasets. We present MOZART-T3, which replaces the lumped BIGALK representation with a more explicit treatment of individual C4−C6 alkane species and resolves propane peroxy radical isomers, enabling more mechanistically consistent alkane chemistry in the Community Atmosphere Model with chemistry (CAM-chem). Global simulations demonstrate that T3 maintains similar total alkane burdens compared to previous mechanisms, while substantially altering oxygenated product budgets and distributions. Relative to MOZART-T1, T3 significantly reduces the global burden of methyl ethyl ketone (MEK) primarily through incorporation of more comprehensive n-butane oxidation chemistry, with additional contributions from increased photolysis rates and updated emission speciation. T3 introduces six additional C5−C6 ketone species that contribute ∼40% to global ketone sources but only ∼2% to the total burden due to their short lifetimes. The acetaldehyde burden decreases by 8−14% through compound-specific yields that replace the fixed yield in previous mechanisms. The choice of anthropogenic emission inventory drives larger variations in alkane burdens (∼24%) than does mechanism complexity (∼4%), but mechanism choice dominates for oxidation products. T3 enables evaluation of previously unrepresented species including individual alkanes, propanal, and peroxypropionyl nitrate, with generally improved simulation of oxygenated compounds, although the evaluation results vary temporally and spatially. While lumped approaches sufficiently represent global-scale major pollutant concentrations, T3’s detailed treatment enables more comprehensive evaluation and is expected to be more important for urban air quality applications using higher-resolution regional simulations.
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Status: open (until 10 Jul 2026)
- RC1: 'Comment on egusphere-2026-1868', Anonymous Referee #2, 09 Jun 2026 reply
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- 1
This paper describes an enlargement of the organic mechanism within the MOZART suite of models. It updates some aspects of the propane chemistry and then develops the butane, pentane and larger chemistry. In general this paper is a good description of the updates made. I think there should be some clarrifications and additions before the paper is published but these should be minor.
Larger issues:
The authors could do a better job of justifying the additional of a more complex chemistry scheme. There are still outstanding significant science questions where a more complicated chemistry could help to answer these questions. The authors could try and do more to justify the need for additional chemistry.
Presumably some of the potential impacts could be to global oxidation. It would be useful in the abstract and text to discuss how much the O3 budget, OH concentration etc are changed by these updates. Even if they are very small it would be useful to discuss this.
There are no isomer of propane. There are 2 of butane. There are 3 of pentane (n-pentane, isopentane (methyl-butane), neopentane (di-methyl-propane)) but only 2 of the pentane isomers are discussed. Why is this? Is there some justification? It might be obvious but it would be good to state this obvious.
There are a lot of simulations discussed here. It would be useful to keep some consistent description of there. Table 1 constains some of these detail but I think it would be useful to have a section which give some more description of these even if it repeat information that has already been given. Can it be repeated what the difference is between T1.3_T3 vs T2.3_T3. This is quite confusing. It would be useful to repeat some information to clarify.
Minor comments.
Line 34. I don’t think speculation about the potential use of the scheme in urban air quality is worth commenting upon unless there is some evidence in the paper to support it.
Line 45. Compared to the millions of potential species, its not that “most” of the species are unmeasured, the overwhelming number are not measured.
Line 57. It would be useful to say what the authors mean by “better” model performance.
Figures 1 and 2. I might move this to after the description of the chemistry rather than having them first.
Line 99. It would be useful to indicate that molecule names in capital without subscripts (C3H6) refers specifically to the model tracer, whereas if they are indicated by subscripts (C3H6) this indicates the molecule in general. Same with other model species.
Line 115. There are other potential initiation reactions of these molecule. They react with halogens, and IUPAC contains rate constants for NO3+n-C4H10. Perhaps some words about why other oxidants have been ignored.
Line 120. When talking about the primary, secondary and tertiary channel it might be useful to describe what is meant exactly. Is this the first, second and third generation products, or it is peroxy radicals which are formed from attacking the different carbon atoms in the carbon chain?
Line 130. By assuming that ALKNIT is C5 I think this conserve nitrogen but leads to a lack of conservation of Carbon. This isn’t a problem but it should be stated that this could lead to an imbalance in the carbon budget.
Line 215. I found this quite confusing. The original publication is said to be Emmons et al., 2020 so how can there be a prior version to this? This be section should be rewritten to increase its clarity. Perhaps Table 1 could be extended to provide more information and detail about what is different between all of these versions?
Line 270. I’m confused about how the species have been mapped here. Is there no mapping to the butane isomers? Could this be given as a table? With more of an explanation about what is being emitted by the biosphere?
Line 333. Why does the propane concentration change at all? Presumably this is due to changes in the OH between the simulations rather than differences in the propane oxidation rate?
Line 343. It should explain that the BIGALK for T3 is the sum of a set of species. Its not a real specie.s
Line 367. I’m not sure what the (0.31-3.06 Tg yr-1( means. Is that it increases by 0.31 from 3.06 or is this a range of changes?
Line 386. “decrease significant in T2” Compared to what simulation? T1?
Line 423. It would be useful to show whether NOy changes from these different simulations. Overall does NOy change?
ATOM figures. Its clear that the model significantly underestimates the alkane concentrations under all simulations. It would be useful to make some comments about what this means. This seems to suggest that the emissions are likely significantly underestimated. It would be useful to explicitly say this