CMAQv5.1 VOC Emission Updates

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Brief Description

Summary of VOC Emission Updates

Updates to chemical mechanisms and Secondary Organic Aerosol Modules require additional emission species.

VOC Emission Species that are created with an emission processor such as SMOKE can be classified into 4 categories:

  • Mechanism Species: These lumped compounds or single compounds are integral to the chemical mechanism and play a role in the formation of other species
  • SOA Tracer Species: These species are "double counted" in the mechanism and in the SOA module. An SOA tracer species is used by the CMAQ model only in the SOA module and in transport
  • Toxic Tracer Species: These species are optional since they are not part of the chemical mechanism but are included for transport. A species may be both a toxic tracer and an SOA tracer.
  • QA species: a species output by the emission processor for Quality Assurance. CMAQ does not read these species. These species may represent additional mass from the TOG profile that is not identified or may be a count of inventory VOC to track the change in VOC when doing speciation.

For the Carbon Bond Mechanism, a list of VOC species names in CMAQ 5.0.2 and CMAQ 5.1 is provided as a reference:

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For CMAQ 5.1, SOAALK is a lumped SOA tracer species for all three mechanisms: CB05, SAPRC07, RACM2.

SOAALK is a precursor to alkane SOA and is a subset of ALK4 and ALK5 in saprc07 and part of PAR in cb05. These species include C6 and larger cyclic compounds, C8 and larger linear or branched. For SOA purposes, it is treated as a weighted sum relative to dodecane (see Pye et al. 2012). A Complete list of individual compounds in SOAALK is included here.

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Naphthalene is roughly half of the gas-phase PAHs in the inventory. In older versions of CMAQ, it was part of XYL and PAR (CB05) and ARO2 (SAPRC07). Since it is part of the gas-phase chemistry it has been removed from XYL and ARO2 to become an explicit mechanism species. This results in the creation of XYLMN for CB05 and ARO2MN for SAPRC07.

In the Carbon Bond 5 Mechanism, Naphthalene has 10 carbons and prior to CMAQ version 5.1, it was split as follows: 1 XYL (8 carbons) + 2 PAR (2 carbons).

Special Note on Naphthalene naming in CB05/SAPRC07T/RACM2

There can be some confusion regarding the species name for Naphthalene. We have elected the following convention for CB05/SAPRC07T/RACM2: NAPH refers to Naphthalene that is part of the VOC/TOG speciation NAPHTHALENE refers to the Naphthalene as estimated in the HAPS part of the NEI.

CB05 (Carbon Bond Mechanism) backward compatibility

If you have an emission files without XYLMN, SOAALK, and NAPH, you can estimate these as follows: These estimates are approximate and are based on the 2011 NEIv1. Use at your own discretion:

           NAPH (moles) = 0.002 * XYL (moles)
          XYLMN (moles) = 0.998 * XYL (moles)
        PAR(new)(moles) = PAR (moles) -0.00001*NAPH(moles)
         SOAALK (moles) = 0.108*PAR (moles)

SAPRC07 backward compatability

If you have an emission files without ARO2MN, SOAALK, and NAPH, you can estimate these as follows: These estimates are approximate. Use at your own discretion:

           NAPH (moles) = 0.04 * ARO2 (moles)
         ARO2MN (moles) = 0.96 * ARO2 (moles)
         SOAALK (moles) = 0.1 ALK4 + 0.7 ALK5 (moles)

In the event that you do not have ARO2MN and SOAALK, for an interim fix, use the following in you GC.nml:

instead of




These approaches may have a minor effect on gas-phase chemistry and lead to slightly different aerosol estimates.

Significance and Impact

The goal of these updates is to allow for better prediction of SOA from anthropogenic sources. See AERO6 update for significance and impact.

Affected Files

Emission inputs.


Pye, H. O. T. and G. A. Pouliot, Modeling the role of alkanes, polycyclic aromatic hydrocarbons, and their oligomers in secondary organic aerosol formation, Environ. Sci. Technol., 2012, 46 (11), 6041-6047. article


George Pouliot and Havala Pye, National Exposure Research Laboratory, U.S. EPA