Gateway to the SAPRC Mechanism Generation System

as user with password
(User name must be given. Password not needed if you accessed this system as this user before)

SAPRC Mechanism Generation System
for the Atmospheric Reactions of Volatile Organic Compounds in the Presence of NOx

Prepared for the California Air Resources Board
Contract No. 11-761

William P. L. Carter
Research Chemist Emeritus
College of Engineering, Center for Environmental Research and Technology
University of California, Riverside, CA, USA

Mailing Address: CE-CERT, University of California, Riverside, CA 92521
Phone: (951) 788-8425.  FAX: (951) 781-5790


The SAPRC mechanism generation system, or MechGen, derives explicit mechanisms for the gas-phase reactions of many types of emitted organic compounds and their oxidation products when they react in the atmosphere in the presence of oxides of nitrogen and other pollutants. It then uses the results to derive lumped mechanisms suitable for use in atmospheric models. This system was used to derive the SAPRC-18 chemical mechanism, which is the latest version of the SAPRC mechanisms. The preliminary documentation of the of the system, which is in the process of being updated, is available here. The documentation of the scientific basis of the estimation methods and chemical assignments is still in preparation, but documentation information is provided within this web system. This documentation will be available at the SAPRC-18 web site when it becomes available.

How to access this system

Access to this system is free, but you need to provide a username and password to access it. No personal or contact information is requested, though I would appreciate it if you would email me at if you find the system useful in your work, and especially if you use it in any published work. The user name is used to direct you to a "web reactor" that is created when you create your account and you can configure for your own use, and where you can return to your work from a previous system. The password is required distinguish you from other users who may chose the user name, and does not need to be particularly secure. If you forget your password you will not be able to log in with this username, and will have to create another account. Inactive accounts are periodically deleted, and accounts are also subject to deletion when the system is periodically updated. However, it is unlikely that you would lose much if your account has to be re-created, unless you have an extensive set of user assignments or large user mechanisms, which can be downloaded and re-entered if needed.

To create a page you can bookmark or save to give you a direct link to your web reactor, do a "reload" on the main menu, and bookmark or save the URL that is returned. This will allow you to bypass the login screen as long as your user has not been deleted. If it is no longer valid, you will just be redirected to the login screen.

How to use this system to generate reactions and mechanisms

Details on how to use the system are given in the available documentation, but knowledge of these details should not be needed for you to check out reactions predicted for compounds you select or specify. To use this system once you've logged on, either use the form to create a new reactant by giving its structure, or look for a reactant of interest in the list of available model species. Click the "help on create" link for information on how to specify a reactant by its structure. After you create a structure or chose one from the list of model species and you find it in the list of "currently selected reactants", clicking on it will give you a page where you can chose which reactions to generate. Clicking on web-linked radical will automatically generate its reactions, and clicking on any web-linked product will generate the menu for its reactions. Each page has a "reload main menu" link that you can use to get to the main menu if needed.

This system has recently provides a capability for you to derive modified versions of the SAPRC-18 mechanism, optionally incorporating your assignments of rate constants and branching ratios that may be different from MechGen and SAPRC-18 defaults. You can also modify which species are represented explicitly, to create more chemically detailed versions of SAPRC-18. The modified mechanisms can be downloaded and used as input to the SAPRC box modeling simulation software to conduct model simulations. A document describing these new features is available here.

Please notify me at if you encounter problems, errors, or unreasonable mechanism results at this site. If you get a traceback, please copy and send it to me, along with enough information so I can duplicate it. I will then attempt to correct the errors, or at least have the system give proper error messages if it cannot proceed.

Page last modified February 25, 2021