BCON and OTH tag in CMAQ-ISAM v5.4

Hi all,

I am running CMAQ-ISAM v5.4 with a model setting that includes five domains. Domain 5 uses bcon from domain 4. I have noticed that the PM2.5 concentration contour plot in the BCON tag shows a power plant plume pattern at 10 layers, especially for the variables PM25_NO3- and PM25_OM from CCTM_SA_ACONC.nc. Is this correct? I had assumed that BCON contributions should solely come from the boundary and should not exhibit a power plant plume pattern within the middle of domain 5. Is it possible that CMAQ ISAM is overcontributing apportionment of NO3- and OM resulting from the chemical reaction between BCON and other emissions to the BCON tag?

My second question pertains to the OTH tag (contributions from all non-tagged emission streams and other processes within the model). Are the processes of VBS classified under the OTH tag?

Could anyone provide me with some advice? Thank you!

Some species that the boundary conditions tag (BCON) tracks are reactive. When these encounter other sources, particularly those with highly concentrated emissions such as from a power plant, they will react. A fraction of the products of these reactions will be attributed to BCON, because BCON provided the reactants that created these products. This can be controlled by the user to some extent through runscript options for ISAM. There was a very recent webinar that explained this in more detail. Perhaps, someone can link it here for you.

I am not sure what you mean by your second question. ISAM tracks emissions streams, ICON, BCON, and potential vorticity at the top if you have that turned on. If you are tracking an emissions stream(s) that to provides precursors into any CMAQ module, these these will show up in your tagged species. If you don’t specify some emissions stream, it will be lumped into the OTH tag.


The meaning of the second question is as follows: The isam_control.txt file I used has already tagged all 11 emission files in the input file. According to the useguide website, the “OTH tag” indicates the contribution from all non-tagged emission streams and other processes in the model (source: CMAQ/CMAQ_UG_ch11_ISAM.md at main · USEPA/CMAQ · GitHub).

Since I have already tagged all the emissions I used, I have noticed that the PM2.5 concentration contour plot in the OTH tag shows a power plant plume pattern at 10 layers, particularly for the PM25_OM variable, which makes up almost 9X% of PM25_TOT from the CCTM_SA_ACONC.nc file. The power plant emissions include POC and PNCOM, which will allocate to Semivolatile POA (VSVP, VIVP, ALVP, and ASVP) according to CMAQ_Control_DESID_cb6r3_ae7_aq.nml. We suspect that these PM25_OM concentrations may be generated by the CMAQ-VBS module. Therefore, I would like to know if the CMAQ-VBS module is classified under the OTH tag.

ISAM will track all that you tagged through the entire model including the aerosol module, which contains the VBS module. Everything not selected will be included in the “OTH”. This may include inline calculated emissions streams such as biogenics, lightning, dust, etc. There is also some minor contribution to “OTH” from second-order interactions between sources. See the bottom of the following document:


I appreciate your answer, it helped me a lot, thanks!

Always glad to help Bruce Lee!