SAPRC Atmospheric Chemical Mechanisms and VOC Reactivity Scales

W.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

Last updated July 7, 2024

The SAPRC atmospheric chemical mechanisms are used in airshed models to model the gas-phase, ground-level atmospheric photochemical reactions of organic compoundsd in the presence of oxides of nitrogen, for the purpose of predicting formation of secondary pollutants such as ozone and toxic organic products. Versions of this mechanism have also been used to derive ozone reactivity scales that quantify the relative amounts of ozone formed when different volatile organic compounds (VOCs) are emitted under various conditions. This page gives links to versions of the mechanism and its reactivity scales. 

Quick Links

The SAPRC-22 Mechanism is  now available.

The latert version of the SAPRC mechanisms is SAPRC-22, which was updated on Juner, 2024 to incorporate improvements in in mechanism generation and lumping procedures. It should now be finalized unless correctable problems are found. Like previous SAPRC mechanisms, it uses the SAPRC mechanism generation system (MechGen) to derive explicit mechanisms for atmospheric reactions of organic compounds, which are then processed using various reduction methods to produce a lumped mechanism for airshed modeling. Its development is consistent with the systematic mechanism development approach outlined by Kaduwela et al (2015). SAPRC-22 incorporates the latest updates MechGen, which are being documented in a journal article to be submitted for peer review. SAPRC-22 is similar to SAPRC-18 in chemical detail, but uses a more efficient method to reprsent peroxy radical reactions that requires use of fewer model species and reactions. Since SAPRC-22 is more up-to-date and has somewhat greater chemical detail than SAPRC-07 or 11 but has similar computer demands, it is an appropriate update for models that currently use SAPRC-07 or 11.

The SAPRC-18 mechanism is superceded by SAPRC-22.

The SAPRC-18 mechanism is first complete update of all aspects of the SAPRC series of mechanisms since the development of SAPRC-07. The objective was to provide an up-to-date mechanism with chemical detail needed for model applications that need it, but not more than necessary or appropriate given current knowledge. It uses the SAPRC mechanism generation system (MechGen) to derive explicit mechanisms for atmospheric reactions of organic compounds, and its development is also consistent with the systematic mechanism development approach mentioned above. Although the mechanism has been evaluated against available environmental chamber data and is considered finalized, it has not been implemented in 3D models, in part because it is much larger than other available mechanisms. SAPRC-22 was developed to address the size issue, and should be used rather than SAPRC-18 if size is an issue. The full version of SAPRC-22 is comparble to SAPRC-18 in size.

The SAPRC-16 mechanism is no longer supported.

The SAPRC-16 mechanism is a preliminary version of SAPRC-18 that was used in the study of  Venecek et al, titled "Analysis of SAPRC16 chemical mechanism for ambient simulations," Atmos Environ, 192, 136-150, 2018. It is similiar to SAPRC-18 but is based on an earlier version of the SAPRC mechanism generation system. Because it was used in a publication, the SAPRC-16 web site containing its preliminary documentation and files is being retained for archival purposes.

The SAPRC-11 mechanism is an incremental update to SAPRC-07

The SAPRC-11 mechanism is an update to SAPRC-07 that has a new version of aromatic chemistry that performs better in simulations of environmental chamber data. The representation of non-aromatic chemistry and the degree chemical detail has not been signficiantly modified. This mechanism is documented by Carter and Heo "Development of Revised SAPRC Aromatics Mechanisms," Atmos. Environ. 77, 404-414, 2013. It has been implemented in various airshed models and used to update the SAPRC-07 VOC reactivity scale, though the reactivity changes are not signficant for most compounds, and the SAPRC-07 scale (see below) is still the most widely used. The SAPRC-11 mechansim has also been used to develop a preliminary, chemicaly-based model for secondary organic aerosol (SOA) formation.

The CSAPRC-07 mechanism is a condensed version of SAPRC-07 

The CSAPRC-07 mechanism is a condensed version of SAPRC-07 that is described by Carter, "Development of a Condensed SAPRC-07 Chemical Mechanism," Atmospheric Environment, 44, 5336-5345, 2010. It was developed by examining effects of various reduction approaches on predictions of ozone and other measures of reactivity, and adopting those that have the least effects on ozone predictions. Its development is consistent with the systematic mechanism reduction approach outlined by Kaduwela et al (2015). It gives essentally the same ozone predictions desspite its much smaller size. The level of condensation is similar to that used in the latest Carbon Bond mechanisms. Files for this version of the mechanism are available on request.

The SAPRC-07 mechanism is the most widely used version of SAPRC

The SAPRC-07 mechanism is a complete update to SAPRC-99 and is described by Carter, "Development of the SAPRC-07 Chemical Mechanism," Atmospheric Environment, 44, 5324-5335, 2010. It is widely used in airshed modeling studies and was used to develop the SAPRC-07 reactivity scale that is used or has been considered for use in California's reactivity-based VOC regulations. It is the first version of the SAPRC mechanisms that utilized a mechanism generation system to help derive mechanisms for reactions of some organics. Links to information about this mechanism and its reactivity scales are given below.


The SAPRC-99 mechanism is a complete update to the SAPRC-90 and incorporates mechanisms for a wide variety of VOCs. It has been superceded by SAPRC-07 or SAPRC-11 for airshed model applications, and its reactivity scale is superceded by the SAPRC-07 reactivity scale.


The SAPRC-90 is the first published version of the SAPRC mechanisms and is described in the journal article “A Detailed Mechanism for the Gas-Phase Atmospheric Reactions of Organic Compounds,” Atmos. Environ., 24A, 481-518, 1990. It is the first version of the mechanism that provides for reactions of individual organic compounds to be added to the condensed mechanism for atmospheric mixtures for deriving a reactivity scale, and is the basis for the first version of the MIR and other VOC reactivity scales described in the original paper “Development of Ozone Reactivity Scales for Volatile Organic Compounds,” J. Air & Waste Manage. Assoc., 44, 881-899, 1994. This mechanism, and its reactivity scale, have been superceded by the later versions described above.

Update History

Date last updated
September 12, 2023Added a link to the MIR and other reactivity scales calculated using SAPRC-11. These are the latest set of reactivity values because they were not updated for later versions of the mechaimsm.
September 9, 2023
May 15, 2023
Website updated to note that SAPRC-22 has been updated.
April 7, 2023Website updated to note that SAPRC-22 to provide a link to the SAPRC-22 page, where files for this new mechanism are available.
March 12, 2023Website updated to mention the development of SAPRC-22 as an update to SAPRC-18
November 20, 2019Website completely updated and the SAPRC-18 mechanism is added and information on SAPRC-11 is updated
September 14, 2013  This page
February 22, 2013Revised version of SAPRC-11 (Gas phase only)
May 21, 2012Preliminary SAPRC-11 Aromatics and Aromatic SOA mechanisms
May 21, 2012 Mechanism Documentation
January 25, 2010 SAPRC-07 mechanism
January 25, 2010 Condensed versions of SAPRC-07
January 25, 2010"Toxics" versions of SAPRC-07
September 14, 2013 VOC reactivity scales (SAPRC-07) (May 11, 2011 for most compounds)
January 25, 2010 Mechanism implementation files for CMAQ
June 21, 2012
Mechanism implementation files for SAPRC software
December 15, 2011
Emissions assignments (see Speciation Database page)

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