# Question about CRACMM2 species properties **URL:** \ **Category:** CMAQ\ **Created:** [February 1, 2026, 4:06pm UTC](https://forum.cmascenter.org/t/question-about-cracmm2-species-properties/6144 "2026-02-01T16:06:10Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![afernandez](https://avatars.discourse-cdn.com/v4/letter/a/c0e974/32.png) [@afernandez](https://forum.cmascenter.org/u/afernandez)\ **Post date:** [February 1, 2026, 4:06pm UTC](https://forum.cmascenter.org/t/question-about-cracmm2-species-properties/6144/1 "2026-02-01T16:06:10Z")
Hello, Q1 - I have some emission rates for propane, alkanes (with 4 or more carbons) and propene. Looking at the CRACMMv2.0 Emissions Mapping (3/17/2025), the first 2 should be evaluated based on whether they’re slow, intermediate or fast reacting. A search is telling me that both propane and butane/pentane are fast reacting. Should I just lump them as HC5 or would it need further refinement? What about propene, should it just go as OLT? Q2 - Do the species OLT, HC10, HC5 & HC3 have equivalent molecular weights? Thanks ---
**Author:** ![Havala.Pye](https://avatars.discourse-cdn.com/v4/letter/h/67e7ee/32.png) [@Havala.Pye](https://forum.cmascenter.org/u/Havala.Pye)\ **Post date:** [February 3, 2026, 5:09pm UTC](https://forum.cmascenter.org/t/question-about-cracmm2-species-properties/6144/2 "2026-02-03T17:09:28Z")
Hello, The definition of slow vs fast reacting is based on the OH rate constant for the species. You can manually map species using this [PDF](https://github.com/USEPA/CRACMM/blob/main/emissions/cracmm2/cracmm2_mapping_flowchart.pdf). You can also map using [python code](https://github.com/USEPA/CRACMM/blob/main/utilities/cracmm2_mapper.py) ([jupyter notebook tutorial calling the function here](https://usepa.github.io/CRACMM/utilities/smiles2cracmm.html)). A large set of mapped compounds from SPECIATE is available as [a csv file](https://github.com/USEPA/CRACMM/blob/main/emissions/SPECIATEInputs/SPECIATEv5.2x_fromCRACMM.csv). The molecular weights are stored in the [GC.nml in CMAQ](https://github.com/USEPA/CMAQ/blob/main/CCTM/src/MECHS/cracmm2/GC_cracmm2.nml). We also tabulate them for use outside of CMAQ in [markdown (for web-based viewing) and csv (for expanded information)](https://github.com/USEPA/CRACMM/tree/main/metadata/cracmm2). Each piece of data in these files is propagated via code from a single source, so they all have the same info (just displayed in different formats with more or less detail). More information is available at [GitHub - USEPA/CRACMM: The Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM) is a description of atmospheric NOx-VOC-ozone-aerosol chemistry and associated metadata.](https://github.com/USEPA/CRACMM/tree/main) which is also displayed as a github pages site ([Overview — CRACMM 2.0 documentation](https://usepa.github.io/CRACMM/)). If you are mapping species where you don’t know properties (such as OH rate constant), the documentation provides guidance on how to use structure-activity relationships to obtain them. ---
**Author:** ![afernandez](https://avatars.discourse-cdn.com/v4/letter/a/c0e974/32.png) [@afernandez](https://forum.cmascenter.org/u/afernandez)\ **Post date:** [February 4, 2026, 2:44am UTC](https://forum.cmascenter.org/t/question-about-cracmm2-species-properties/6144/3 "2026-02-04T02:44:33Z")
Hi. Thanks. That is the PDF that I was using. The issue is that, as you mention in the second paragraph, I wasn’t sure how to evaluate the OH rate constant for butane or pentane to mention 2 alkanes. I need to double-check whether the SPECIATE csv file has this info for relatively simple species or how to use the documentation that you mention (but will have to wait until tomorrow for several unexpected reasons). P.S. The molecular weights are listed at the CG namelist file -don’t know what I was thinking. ---
**Author:** ![Havala.Pye](https://avatars.discourse-cdn.com/v4/letter/h/67e7ee/32.png) [@Havala.Pye](https://forum.cmascenter.org/u/Havala.Pye)\ **Post date:** [February 4, 2026, 1:15pm UTC](https://forum.cmascenter.org/t/question-about-cracmm2-species-properties/6144/4 "2026-02-04T13:15:30Z")
Also check out the github pages Emissions page. It provides resources for how to get kOH for any species through EPA’s Chemicals Dashboard. ---
**Author:** ![system](https://canada1.discourse-cdn.com/flex027/uploads/cmas/original/1X/9a72c78e529465c3f1053af404e3033b0875428c.png) [@system](https://forum.cmascenter.org/u/system)\ **Post date:** [February 17, 2026, 2:39pm UTC](https://forum.cmascenter.org/t/question-about-cracmm2-species-properties/6144/5 "2026-02-17T14:39:43Z")
Closed on February 17, 2026, 2:39pm UTC This topic was automatically closed 7 days after the last reply. New replies are no longer allowed.