The SAPRC-18 Atmospheric Chemical Mechanism

William P. L. Carter
Research Chemist Emeritus
College of Engineering, Center for Environmental Research and Technology
University of California, Riverside, CA, 92521, USA
Phone: (951) 788-8425

Mechanism last updated November 8, 2020
Page last updated June 14, 2023


The SAPRC-18 mechanism represents the results of the first complete update of all aspects of the SAPRC series of mechanisms since the development of SAPRC-07 . (SAPRC-11 is an incremental update of SAPRC-07, and SAPRC-16 was never finalized.) The objective was to provide airshed modelers with an up-to-date mechanism with more of the chemical detail needed for more reliable toxics and SOA modeling, but not too large that it is impractical for use in 3D airshed models. An additional objective is to use the SAPRC mechanism generation system to assure a direct and documented relationship between the mechanism and the underlying kinetic and mechanistic data, theories, and estimates. This is consistent with the systematic mechanism development approach outlined by Kaduwela et al (2015), involving first developing detailed mechanisms based on available data, mechanism generation tools, and structure-reactivity and other estimation methods, evaluating the detailed mechanisms using chamber and ambient data, then utilizing various methods to reduce or condense the detailed mechanism for specific airshed model applications. In this work, the objective was to derive a mechanism that contains as much chemical detail as might be needed for model applications that benefit from chemical detail, but not more than necessary for this purpose. Such applications include modeling formation and destruction of toxic organic process, explicitly representing individual compounds that are important in emissions or of particular environmental concern, and predicting the formation of secondary organic aerosol (SOA) precursors, and serving as a basis for deriving more reduced mechanisms.

This mechanism is its final form, and is included among the mechanisms distributed with the SAPRC modeling software and files. However, this has now been superceded by SAPRC-22, so anyone considering using SAPRC-18 should use SAPRC-22 instead, which has a version with similar level of detail as SAPRC-18 and a more condensed version that gives essentially the same results. But if you are using SAPRC-18, the files should be dated May 13, 2020 or later, and the kinetic parameters for the OH+NO2 in the .RXN files should be corrected (see the current SAPRC18.RXN file). Please contact me if you have questions about using SAPRC-18.

Files Available

S18doc.pdfAvailable documentation of the mechanism. Describes its general features and the mechanism generation system it uses, how they differ from previous versions, lists the model species, reactions, and rate parameters used., gives a summary of the results of the evaluation and adjustments using chamber data, and compares results of box model simulations of simplified ambient scenarios with simulations using the earlier version of SAPRC. This version of this document is dated May 29, 2020, but describes the version of the mechanism dated November 8 (after a minor error was corrected)..
Saprc18.xlsThis is an electronic supplement to S18doc.pdf, containing the larger tables that are not in the main document. It also includes inputs and outputs of a test calclation to verify correct implementation of this mechanism. This version is dated June 6, 2020. Note: The May 29 file had an out-of-date reaction listing on Table A-4. Please replace with the current version of this file was downloaded before June 6.
SAPRC18.RXNReactions in the mechanism for atmospheric simulations in SAPRC format. Created November 8, 2020. (The .RXN file version dated May 13, 2020 had an error in the kinetic parameters for the OH+HNO3 reaction. This has only a minor effect on simulations but was corrected on November 8, 2020, and uploaded on June 14, 2023.) Any versions downloaded before June 14, 2023 need to be replaced.
SAPRC18.MECReactions in the mechanism for atmospheric simulations in CMAQ mech.def format. Updated March 19, 2021. The version created May 13, 2020 was created improperly and had severe errors, so any versions dowloaded before March 19, 2021, need to be replaced. (Note that the May 13 .MEC file does have the OH + HNO3 error.)
S18-KPP.zipFiles for implementing the mechanism for test simulations using the KPP solver (gives photolysis rates calculated for Jeffries STD640 light model, Z=0, full sun). Note that these are also available in spreadsheet format in Saprc18.xls. Created May 13, 2020 and corrected June 13, 2021. Any versions dowloaded before June 14, 2021, need to be replaced. (Note that the May 13 KPP files do not have the OH + HNO3 error.)
PHF.zipAbsorption cross section and quantum yields for photolysis reactions. Zip file created October 7, 2019. Newest file is dated October 14, 2016, but is still applicable to this version.
WALLS.RXNReactions in mechanism to implement the chamber wall model in SAPRC format (not needed for atmospheric simulations).
Speciation websiteAccess to database and files giving assignmnets mapping categories used in emissions profile database to model species in various mechanisms. This now includes assignments for processing emissions for this version of SAPRC-16. File date is September, 2014.
MechGen web interfaceAccess to the SAPRC-18 version of the SAPRC mechanism generation system. The online version allows you to generate reactions for individual compounds and view the associated documentation for the data or estimation methods used. It also gives documentation of the various estimation methods and assignments used.
MechGen.pdfIncomplete documentation of the SAPRC Mechanism generation system, giving an overview, operations, and use of the system. Note that the represenation of the chemistry in  the system is documented separately. Although incomplete, the documentation covers most of the operations of the system that would be needed by most users. Documentation of installation and maintainng the system is still largely incomplete. This version is dated May 25, 2019.
SAPRC software and files websiteAccess to files for downloading SAPRC software and data files to conduct simulations and evaluate chamber experiments using various mechanisms, including SAPRC-18.
(no link yet)
Documentation of the chemical basis of the SAPRC mechanism generation system is still in preparation, and will be made available here when it is sufficiently complete for initial distribution and review. However, users can obtain infromation and documentation about estimates and assignments of various reactions from within the system by the following methods:
  • Derivations of the mechanisms of individual reactions: Generate the reactions in a step-by-step manner will output documentation text on the estimation methods or assignments used..
  • Information on estimation methods for selected types of reactions: Select "estimathon methods" from the main menu after logging in, then select the type of reaction of interest.
  • Assigned rate constants and product distributions in reactions of a compound:  Select "estimathon methods" from the main menu after logging in, then select either the type of reaction or a link to produce a list of compounds for which assignments have been made. Selecting the compound will give all assignments used when deriving the mechanism for that compound.

Log of Updates
June 14, 2023Error in SAPRC18 RXN files corrected.
March 19, 2021The SAPRC18.MEC file created May 13 was found to have errors and was replaced by a corrected version.
June 6, 2020
May 29, 2020
May 13, 2020
Updated the mechanism to version S18E, which has re-adjusted mechanisms for phenols and bicyclic aromatics, and re-adjusted photolysis rates of aromatic photoreactive products. This version gives less biased fits to the data for most aromatics. The mechanism files were uploaded on May 13 but the documentation was updated on May 29. Saprc18.xls corrected June 6.
May 8, 2020An updated version of the mechanism, designatee S18C, is made available here with its associated documentation. The mechanism generation (MechGen) system at has also been updated, and is consistent with this version as of this date. Although updates to MechGen are continuing, the version used to derive this S18C version of the mechanism has been archived and is available upon request.
Jan 22, 2020The MechGen system available at updated to a new version. This version has more documentation of estimation methods and updates estimates of Criegee biradical and some carbon-centered radical reactions. This will result in changes to the mechanism and documentation that have not yet been implemented. See the note above. The previous version of MechGen, which is consistent with the 2019 version of the mechanism, is still available, but at
Nov 20, 2019The Current version of SAPRC-18, derived from MechGen on 5/11/19, with input files generated on 10/7/19 and its documentation are made available here. Site created Nov 18; minor updates Nov 20, December 9.
2016 - 2019Updates made to the memechanism generation system and mechanism. Mechanism name changed to SAPRC-18. Documentation updated.
Oct 29, 2016The version pf SAPRC-16 used by Venecek et al (2018) and its associtied documentation was made available. This incorporates modifications relative to the September 15 version. These filese are now available at the SAPRC-16 archive web site.
Sept.15, 2016The preliminary version of SAPRC-16 was uploaded.

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