# How to use Combine tool to process VOC units into ppbV and μg/m3?

Hello everyone.
I am doing CMAQ post-processing and found that the Combine tool can only process the units of VOC as ppbC, how can I process it as ppbV and μg/m3, can you provide the exact formula?

To obtain a sum of emitted VOC species in ppbV instead of ppbC, you could edit the VOC equation shown in the SpecDef files to remove the multiplication of each species with its carbon number and just compute the straight sum, but that’s probably not a meaningful quantity.

To convert a species from ppm to ug/m3, you need the ratio of the molecular weight of that species to the molecular weight of air (28.97 g/mole) as well as the air density from the MCIP file. The SpecDef file shows several examples of how to do that:

HNO3_UGM3 ,ug m-3 ,1000.0*(HNO3[1]*2.1756*DENS[2]) where 2.1756 is the ratio of 63.03 to 28.97
NH3_UGM3 ,ug m-3 ,1000.0*(NH3[1]*0.5880*DENS[2]) where 0.5880 is the ratio of 17.03 to 28.97
SO2_UGM3 ,ug m-3 ,1000.0*(SO2[1]*2.2118*DENS[2]) where 2.2118 is the ratio of 64.07 to 28.97

How do I calculate the molecular weight of VOC? Do I need to know the chemical formula of each VOC species to do the calculation?

Because each of the species used in this VOC summation has its own molecular weight (which you can view in the second column of the GC_{mech}.nml file), there is no unique molecular weight for VOC as it would depend not only on the molecular weights of the individual species but also their relative abundance which varies in space and time.

This is the reason that ppbC is often used when working with a sum of individual VOC species/compounds. Each of these species/compounds has one or more carbon atoms, and the molecular weight of those carbon atoms (12) is of course the same across species/compounds. So multiplying each mixing ratio with that carbon number when creating a “VOC” sum in ppbC yields a quantity that focuses on the aspect that all of these species/compounds have in common. This quantity can also easily be converted to ugC/m3 by using the molecular weight of carbon.

All that said, whether and how summing across mixing ratios of different VOC species/compounds is useful or desirable really depends on the objectives of a particular analysis. When comparing to measurements, for some specific species/compounds it’s also important to consider if the chemical mechanism used in CMAQ follows a lumped-structure approach to species aggregation like CB6 or a lumped-molecule approach like for example SAPRC, but this goes well beyond my area of expertise. You might also want to read this recent discussion on different ways to think about VOC.

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I have found the molecular weight of each species based on your suggestion.
Currently my total VOC calculation formula is shown below. I would like to know the full name of each VOC species in the formula in English. Where should I read to find out? I am using the CB05 chemical mechanism.

!VOC ,ppbC ,1000.0* (PAR[1] +2.0ETHA[1] +3.0PRPA[1] +MEOH[1]
! +2.0ETH[1] +2.0ETOH[1] +2.0OLE[1] +3.0ACET[1]
! +7.0TOL[1] +8.0XYLMN[1] +6.0BENZENE[1]
! +FORM[1] +3.0
GLY[1] +4.0KET[1] +2.0ETHY[1]
! +2.0ALD2[1] + 2.0ETHA[1] + 4.0IOLE[1] + 2.0ALDX[1]
! +5.0ISOP[1] + 10.0TERP[1]+ 10.0*NAPH[1])

Starting in CMAQv5.3, markdown files with species tables for all supported mechanisms can be found in https://github.com/USEPA/CMAQ/tree/main/CCTM/src/MECHS/mechanism_information, for example cb6r3_ae7_aq

Since CB05 is no longer supported in CMAQv5.3, you need to look at the documentation of older releases. In CMAQv5.2.1, corresponding tables for several mechanism were included in Appendix A of the User Manual, e.g. for cb05e51_ae6_aq

Full documentation of the original CB05 mechanism and the subsequent updates to toluene chemistry can be found in these publications:

Yarwood, G., Rao, S., Yocke, M. and Whitten, G.Z. (2005) Updates to the Carbon Bond Chemical Mechanism: CB05. Final Report Prepared for the United States Environmental Protection Agency

Gary Z. Whitten, Gookyoung Heo, Yosuke Kimura, Elena McDonald-Buller, David T. Allen, William P.L. Carter, Greg Yarwood, A new condensed toluene mechanism for Carbon Bond: CB05-TU, Atmospheric Environment, Volume 44, Issue 40, 2010, Redirecting

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