The SAPRC-22 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 September 9, 2023
Page last updated November 14
, 2023


This page provides files and documentation for the SAPRC-22 mechanism, which is the latest version of the the SAPRC series of gas-phase atmospheric  mechanisms for airshed models. SAPRC-22 is an updated version of SAPRC-18, which in turn is an updated version of the widely used SAPRC-07 mechanism. As with previous SAPRC mechanisms, the representation of reactions of almost all of the organics were derived using the SAPRC mechanism generation system (MechGen), which has been updated and documented prior to its use to derive SAPRC-22, though the documenation is still under review. Therefore, SAPRC-22 incorporates some potentially significant updates to the organic chemistry, which the underlying chemistry incorporated in MechGen now being documented in a separate paper that is being submitted for publication. The standard version of SAPRC-22 is a suitablereplacement to SAPRC-07, with more updated chemistry, more tracking of toxic species, and comparable size and computer demands, being significantly smaller in size than SAPRC-18 but representing similar chemical detail.

Please contact me if you are interested in possibly using the mechanism and have downloaded the documentation and files, so I can notify you if corrections are needed or if there are new versions of the documentation and associated files. However, this should be the final version unless needs for corrections are found.

Please note: Significantly new versions of the mechanism, the documentation report, and supplementary data have been uploaded on Septermber 9, 2023. The update was necessary in order to incorporate improved estimates of auto-oxidation reactions of peroxy reactions. Please upload new versions of these files if you have versions that were uploaded before then. See the file dates on the table below. Note also that only the "standard" version of SAPRC-22 is now recommended for use in airhsed models, so"SAPRC22F" files are no longer included in this distribution. However, files containing the other versions discussed in the documentation report are also avaiilable.

Files Available

Available documentation of the mechanism. Describes the versions of SAPRC-22 that were developed and how they  differ from each other and previous versions, discusses how the mechanisms for the organics were derived from the mechanism generation system, documents the base mechanism for inorganics and common organics, discusses box model test calculations comparing versions of the mechanism, and provides information on how to implement the mechanism. This version of this document is dated September 9, 2023, and supercedes the previously posted draft dated July 13, 2023.
S22supp.xlsThis is an electronic supplement to S22doc.pdf, containing the larger tables and complete mechanism listings 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 eptember 9, 2023 and supercedes the version dated dated July 12, 2023..
SAPRC22.RXNReactions in standard SAPRC-22 for atmospheric simulations in SAPRC format. Created September 8, 2023.
SAPRC22.MECReactions in standard SAPRC-22 for atmospheric simulations in CMAQ mech.def format.
Files for implementing standard SAPRC-22 for test simulations using the KPP solver (gives photolysis rates calculated for Jeffries STD640 light model, Z=0, full sun). Note that the procedures used to create KPP files have not been tested to see if this was done properly and I'm unsure how to define photolysis reactions for KPP. Please contact me if you use these files and let me know how well they worked. Created  July 12, 2023..
PHF.zipAbsorption cross section and quantum yields for photolysis reactions. Zip file created July 6, 2019, which is when the file C2CHOabs.phf replaced C2CHO.phf when the mechanism was updated in July, 2023.. Newest file other than that is dated October 14, 2016, but is still applicable to this version.
Spreadsheets that can be used to create files implementing either the standard, full, or reduced version of SAPRC-22 that are discussed in the documentation report. Each contains full listings of the model species and rate constant parameters (other than absorption cross sections and quantum yields that are in and macros used to produce the distributed .RXN, .MEC, or .KPP files. Uploaded  September 9, 2023.
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 SAPRC-22 (they are the same for both versions). Updaed March 9, 2023.
MechGen web interfaceAccess to the SAPRC-22 version of the SAPRC mechanism generation system (MechGen). 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.pdfDraft documentation of the MechGen mechanism generation system. Note that the representation of the chemistry in  the system is documented separately, as indicated below. The current draft MechGen documentation covers most of the operations of the system that would be needed by most users. Documentation of installation and maintaining the system as a stand-alone system and advanced capabilities is in preparation. This version is dated November 14, 2023
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-22.
(no link yet)
Documentation of the chemical basis of the SAPRC mechanism and the MechGen mechanism generation system is given in a manuscript that has been submitted to ACP for publication. This manuscript is currently under review, and is available for public comment until November 15, 2023. Please contact me if you have comments or questions.

Users can also 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
Nov. 4, 2023
Nov 14
Available documentation for MechGen and the status of the manuscript describing the chemical estimates and assignments it uses were updated. Made available on Nov. 4 and updated Nov 14.
Sept. 9Mechanism was updated to incorporate an updated estimation method for autooxidation reactions of peroxy radicals that were implemented in MechGen since the last version was developed. These updates resulted in increased predicted importance of autooxidation reactions and resulted in a significant increase in size for "full" version of the mechanism, making itimpractical for use in most modeling applications. However, an improved method for lumping peroxy radical reactions was incorporated in the standard version, making use of the full version not necessary for most applications, so only the standard version is now recommended for use.
July 13Added new model species to better represent formation of non-volatile products from catechols.
July 7Corrected errors in treatment of aldehyde photolysis to make them more consistent with current recommendations. Also, corrected error in MechGen that resulted in underpredictions of formations of catechols from phenols.
July 1Corrected an error in MechGen that caused fast isomerization reactions of acyl peroxy radicals to be ignored. The version uploaded on July 1 should be the final version unless more needs for corrections are found.
April 7Updated version of the report and mechanism, that incorporates results of external reviews and some error fixes. These are superceded by later versions.
Mar. 8, 2023New version of the mechanism and documentation report were updated. These are superceded by later versions.
Dec. 30, 2022 This page was created and the first versions of the mechanism and documentation files were uploaded. These are superceded by later versions.

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