| 21 Aug 2026
Comparison of the GECKO-A and SAPRC MechGen Atmospheric Chemical Mechanism Generators
Abstract. Atmospheric chemical mechanisms describe the photolysis and oxidation reactions of volatile organic compounds (VOCs), including bimolecular and unimolecular reactions of product radicals. Fully explicit representation of a single VOC can require thousands to millions of chemical reactions and species. Consequently, explicit chemical mechanism generation has proceeded through the use of automated tools that rely on observational data and structure activity relationships (SARs) to estimate rate coefficients and branching ratios. Here we compare, for the first time, two automated mechanism generators, GECKO-A (Generator of Explicit Chemical Kinetics of Organics in the Atmosphere and SAPRC MechGen (SAPRC Mechanism Generator). We specifically compare mechanisms for ten representative VOCs and the resultant properties of generated product mixtures as relevant for atmospheric chemistry. The observed differences in product mixtures can be explained by systematic differences between the mechanism generators after the initial reaction steps. GECKO-A has a more detailed representation of photolysis, including Norrish type-2 photolysis, which results in the formation of reactive alkenes from aldehydes. MechGen includes more alkoxy radical isomerization processes and autoxidation via H-shift reactions, the latter of which are not currently included in GECKO-A. Also MechGen assumes fast cyclization of alkyl radicals to form lactones or carbonates, which is not assumed by GECKO-A, and for which available theoretical predictions are inconsistent. These differences highlight needs for additional experimental data, particularly for uncertain products and yields, as well as for near continuous updating of these mechanism generation tools as sufficient data and new SARs become available.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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The authors present a modelling study comparing the chemical mechanisms describing the atmospheric oxidation of a series of compounds representative of typical species in the atmosphere generated from two automated mechanism generators, GECKO-A and SAPRC MechGen. The manuscript is well written and provides useful detail regarding the differences between results of simulations. The manuscript will be of interest to the atmospheric science community and I have only minor comments that should be addressed prior to publication.
Although the manuscript is rather long, it would benefit from the inclusion of a brief discussion outlining previous work on mechanism validation, particularly involving comparison to experiment and/or theory. Examples of previous work in the literature using either of the mechanism generators and any previous mechanism intercomparison studies would also be beneficial.
In addition, although the authors have identified key assumptions in the mechanism generators that lead to differences in simulations for some systems, it would be helpful, if possible, to identify which mechanism might be more reliable in each instance.
Recommendations are made for additional experiments to improve mechanism development and to help resolve such differences, it would be helpful to provide some specific detail of the type of experiments (temperature-dependent kinetics, product studies, or site-specific kinetics for example) or systems of interest (classes of compound for example) that would be most beneficial for the development of automated mechanism generators.
The summary is a little focussed towards future developments in GECKO-A, can the authors comment on any needs or plans for development for MechGen?
Other minor comments:
Line 101 onwards: Are the cut-offs in MechGen fixed or user supplied?
Line 138: How does aggregation of properties in post-processing as opposed to consideration of heterogeneous processes at run-time potentially impact results? Are any physical losses included in the simulations?
Line 160: What are the differences between the inorganic chemistry in the two mechanisms?
Line 160-165: Please clarify how the constraint on RO2 and RCO3 potentially impacts results. What are the eight RO2 types?
Line 171: Please quantify ‘approximately the same’.
Line 181: Is OH reactivity defined here as the rate of OH consumption (-d[OH]/dt) or the rate coefficient describing the rate?
Line 213: Section numbers need checking.
Line 234: How much difference does assessment of mPOHR compared to summing the reactivity from species in the mechanism make? How significant are the differences in the amount of “lost carbon” between the two mechanisms?
Line 242 onwards (and elsewhere): Are there any chamber measurements that can be compared to the simulations, particularly for simulations where the two mechanisms differ?
Line 362: Are the units correct for 4-6 × 10-12?
Line 427: Is there any value in comparisons between mechanisms containing more than one parent VOC? Is this an area of potential interest for future work?
Line 450: Are values taken for specific time intervals in the simulations?
Line 464: What are the values based on in each mechanism? Are there experimental values available for comparison?
Line 473 (and elsewhere): The image quality for some of the figures should be improved before final publication.
Line 559: Are experimental data available for comparison or validation of either mechanism?
Line 573-574: Over what time period are the yields time-integrated?