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

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

Description

The SAPRC mechanism generation system, or MechGen, derives explicit mechanisms for the gas-phase reactions of many types of volatile organic compounds and their oxidation products when they react in the atmosphere in the presence of oxides of nitrogen and other pollutants. It then optionally uses the results to derive lumped mechanisms suitable for use in atmospheric models. This system was used to derive the SAPRC-22 chemical mechanism, which is the latest version of the SAPRC mechanisms. However, its primary use for most researchers in basic or atmospheric chemistry would be to derive fully explicit mechanisms that can be used for analyzing laboratory data or predicting atmospheric fates organic compounds when they react in the atmosphere.

MechGen is currently available online at http://mechgen.ees250027.projects.jetstream-cloud.org:8000 . This is subject to change, so check back at this page to see if it changed if it is not available at this address, or email me if it is down. Note that MechGen is no longer available at mechgen.cert.ucr.edu, which is now being used for updated information. Advanced users can also obtain their own copies of this program from the files below to run on their own computer, as discussed below.

The following provide more information about the system:

• A manuscript describing MechGen and its algorithms has been published is described by Carter et al, Geosci. Model Dev., 18, 8461–8483, 2025.
• The scientific basis for the chemical estimates and assignments used by MechGen is described by Carter et al., Atmos. Chem. Phys., 25, 199–242, 2025.
• The basic web users guide provides basic information on how to use the web system and is recommended for new users. The current version is dated September 13, 2024.
• The complete users manual describes the capabilities of the system and how to use them, and provides the most complete description of MechGen that is currentlyand currently available. The current version, describing MechGen-2.2, is dated May 16, 2026.
• The online system provides basic information on how anyone can create a free account log into the system, and once logged in there are links that provide additional information about how to use the system and also the chemical mechanism estimates and assignments.
  

The users manual also describes how interested advanced users can obtain their own copy of MechGen for their own use. This is necessary if users who need to carry out highly resources-intensive operations such as deriving mechanisms for large numbers of compounds, or compounds with more than 9 carbons or structural groups. It can be installed on both Windows or Unix or Linux based systems. The following files or links are available for this purpose:

• MechGen.zip contains the database and all the programs and files needed to run MechGen on a windows-based system. The current version of the MechGen database (v2.2) is dated May 16, 2026.
• Information how to install MechGen on Linux or Unix systems is available at the MechGen GitHub site. This can be used to obtain a version of the MechGen database that can be used for such systems..

Versions since 2025

• MechGen-1.0. 1/20/25. Version as documented by Carter et al, Geosci. Model Dev., 18, 8461–8483, 2025 , with chemistry as documented by Carter et al., Atmos. Chem. Phys., 25, 199–242, 2025.
• MechGen-1.1. 7/25/25. Incorporates software revisions, mainly related to SAPRC-22 mechanism generation. No changes in chemistry assignments or estimates, but minor software changes caused minor changes to complex generated mechanisms.
• MechGen-2.2. 5/16/26. Incorporates extensive software changes and some error corrections. Standard MechGen structure and SMILES strings are different for some compounds, but previous versions recognized as valid input. No changes in chemistry assignments or estimates, but software changes and corrections resulted in some changes to complex generated mechanisms. User manual updated.