PM25 composition and estimation of PM25 water (CMAQv5.3.1)

Dear all,
I use CMAQv5.3.1 and I have used the combine postprocessing script. Among the species I can find in the outputs, there are PM25_TOT and some PM25 components (such PM25_OC, PM25_NO3 etc.). For information, I use the cb6r3_ae7 scheme, so the script reads the file SpecDef_cb6r3_ae7_aq.txt.
First of all, I have seen that the sum of the values of each PM25 component listed in this file does not correspond to the values given by PM25_TOT. I guess some species are doubled counted, such as OM and OC. But is there a clear split I can use to have all the components corresponding to the total PM25?
Then, I have also guessed that the particle bound water is not included in this PM25_TOT, so it corresponds to the dry PM25. Is it right? If so, how can I can I calculate it? Should I add a line like: AH2OI[1]*PM25AT[3]+AH2OJ[1]*PM25AC[3]+AH2OK[1]*PM25CO[3] to get PM25_water? I tried it but it gives large values. Thus, I think by doing this, it overestimates the mass of water.
Thank you for help.
Best regards, Matthieu

Particulate water, specifically AORGH2OJ and AH2OJ (and I and K modes if relevant), equilibrates with ambient conditions. Including or excluding those species in PM25 totals depends on what you are comparing to. An in situ measurement in equilibrium with atmospheric conditions will contain AORGH2O and AH2O in the total PM2.5 mass. However, a laboratory filter-based measurement of PM2.5 is often equilibrated at some reference humidity and part of AORGH2O and AH2O will evaporate. The AORGH2O+AH2O is not strongly bound to the particle. I evaluated particulate water for SOAS in Fig 11e here: You can conclude from that figure that both AORGH2O and AH2O are needed to reproduce observed (ambient) liquid water content (blue line alone is AH2O and underestimates the total). Overestimates in OA (red+purple, panel a) will propagate to overestimates in AORGH2O. The hygroscopicity of organics (driving AORGH2O, kappa value, d) is actually underestimated and not responsible for liquid water overestimates. So, if your organic and inorganic aerosol concentrations are reasonable, the AORGH2O+AH2O total should represent a realistic amount of particulate water under ambient conditions.

Thank you.
Thus, my understanding, it is:

  1. by default, the PM25_tot retrieved with the combine postprocessing script corresponds to the dry PM2.5. Is it correct?
  2. There is no clear indication to get PM25 concentration for conditions at RH=50% and T=20C, as required according to the Reference Method for sample equilibration (e.g. ACP - To what extent can aerosol water explain the discrepancy between model calculated and gravimetric PM10 and PM2.5?), since the particulate water in CMAQ is given for ambient conditions.

Hi Matthieu,

There was an effort made to account for PBW (along with other measurement artifacts) based on the SANDWICH technique published by Neil Frank ( From this article, PBW can be estimated as a function of the measured SO4 and NH4 (accounting for NH4 loss), such that:

PBW = 0.24*(SO4 + NH4 - NH4_loss)

The species definition file distributed with the CMAQ code contains a section titled “FRM PM Equivalent Calculation” which was developed to account for measurement artifacts from the CSN, most specifically NH4 loss from the filter and PBW, and is based on the information contained in the Young paper. At the time this work was specifically focused on the CSN network measurements. I’m not 100% sure that the information provided in the article is still fully applicable to recent CSN FRM measurements, and methods and techniques can change over time.

We do not routinely use the artifact corrected (i.e. PM_FRM and PM25_FRM) values calculated using the species definition file in our comparisons between modeled and observed values, as the difference between our standard PM2.5 calculation and the PM25_FRM calculation are on average relatively small. However, if you wish to attempt a correction for measurement artifacts, the Frank paper might be a good starting point.

And I’m happy to discuss further if you’d like.


Thanks a lot!
I guess you meant Neil H. Frank and not Neil Young :wink:

Ha! Sure did. I even looked at the paper to double check. Lot of good it did.

I corrected my previous response to correct Young to Frank. Thanks!


To follow up on this initial question, the specific aggregated components as defined in the SpecDef file and calculated from the underlying CMAQ aero7 species that would sum up to PM25_TOT are

PM25_TOT = PM25_UNSPEC1 + PM25_CL + PM25_EC + PM25_NA + PM25_NH4 + PM25_NO3 + PM25_OC + PM25_SOIL + PM25_SO4

as can be seen by rearranging this equation from the SpecDef file:

PM25_UNSPEC1 ,ug m-3 ,PM25_TOT[0]-(PM25_CL[0]+PM25_EC[0]+PM25_NA[0]+PM25_NH4[0] \ +PM25_NO3[0]+PM25_OC[0]+PM25_SOIL[0]+PM25_SO4[0])

You are correct that the SpecDef file also defines aggregates that are not part of this equation, such as OM, and that including all of the defined aggregates in PM25_TOT would have resulted in double counting of some underlying aero7 species. The motivation for including additional aggregates into the SpecDef file is to support different analyses with a single ‘combine’ output file, and the definitions that are included in the default SpecDef file should cover many routine analyses. However, there may be cases in which different or additional aggregates are desired, and the SpecDef interface to combine offers the opportunity to construct such different aggregates from the underlying aero7 species.

Thank you Christian, that confirms my understanding of your speciation and the SpecDef_cb6r3_ae7_aq.txt file.

May I ask other questions in this discussion, since it is related to the PM2.5 speciation?

I also try to split my organic aerosol as primary and secondary; and also as biogenic and anthropogenic.
I base this split on the information given in your SpecDef_Dep_cb6r3_ae7_aq.txt, AE_cb6r3_ae7_aq.nml, and the Schmedding‘s paper (ACP - Predicting secondary organic aerosol phase state and viscosity and its effect on multiphase chemistry in a regional-scale air quality model).

I am confused because:

  1. I found inconsistencies in the POA and SOA definitions given in your SpecDef_Dep_cb6r3_ae7_aq.txt and AE_cb6r3_ae7_aq.nml files.

In ASOMJ, defined in SpecDef_Dep_cb6r3_ae7_aq.txt, there are ALVOO1J[1] + ALVOO2J[1] + ASVOO1J[1] + ASVOO2J[1], but these species are described as POA (and not SOA) in AE_cb6r3_ae7_aq.nml.
The Schmedding‘s paper or even the, do not help to know if there are primary or secondary OA.

  1. APOCI (or J), APNCOM, AIVPO1, AMT1 ,AMTHYD: I am not sure about the category of these species. Are they anthro or biogenic?
    I have not found a source of information which can confirm the category given in your SpecDef_Dep_cb6r3_ae7_aq.txt.

@Havala.Pye and @Ben_Murphy are the experts on this topic, but here are my quick thoughts:

  1. The entry in the “AE2AQ SURR” column of the AE_cb6r3_ae7_aq.nml file controls how different aerosols are treated in the aqueous chemistry component of CMAQ, so they are just that - surrogates. While I don’t know the details, I would suspect that low-volatility aerosols of both primary and secondary origin share similar physical properties so that both ALVPO* and ALVOO* can be treated with the same approach for aqueous chemistry purposes and that therefore they are being mapped to the same surrogate used in the aqueous chemistry code. The name of that surrogate (e.g. POA_ACCUM) should not be interpreted as indication of the primary vs. secondary nature of the aerosol - that information is solely contained in the actual species name.

  2. The following links contain additional information on the definition of the CMAQ organic aerosol species:

cb6r3_ae7_aq species table

aero7 release notes

aerosol science documentation

list of CMAQ aerosol references


  1. The LVOO series represent oxygenated species derived from evaporated POA. These are mostly anthropogenic (except for wind-blown dust, sea spray, and wildfires depending on your definition of anthropogenic), and they have reacted at least once in the atmosphere, so I would recommend designating them as SOA.

  2. APOC and APNCOM are POA, and almost always anthropogenic, unless you have chosen to route some wind-blown dust or sea spray aerosol mass into these species (this is not default). There should not be much mass there if you are running the ae7 mechanism. AMT1 and AMTHYD are biogenic SOA species.

Best wishes,

Thank you. It is very helpful!