CMAQv5.3 ISAM - difference with reference data

Hi Dr. Napelenok et al.,

I enjoyed the 10/8/2019 CMAQv5.3 webinar, Dr. Napelenok mentioned “sum of target species concentrations in ISAM should be very close to the concentration of this species from basic CMAQ output”. I then tested CMAQv5.3 ISAM with benchmark


!!! The following are source definition text blocks in the format:
!!!   TAG NAME        |Three character text string
!!!   REGION(S)       |Keyword EVERYWHERE or variable names from the region file (multiple regions need to be comma delimited)
!!!   FILENAME(S)     |Emissions labels (multiple labels need to be comma delimited)

TAG NAME        |GR1


I then summed up O3_OTHER O3_ICON O3_BCON O3_GR1 and compared the data with O3 from original CMAQv5.3 output. scatter plot and domain average (total_O3_ISAM - O3_orig) look fine
bar difference

However, I saw some large differences in certain grid-cells
absolute difference (up to 16 ppb in some grids)

percent difference (up to 267%)

Are these large differences normal for CMAQv5.3 ISAM? BTW, I’ve checked my benchmark run with reference data without ISAM, and even in the two-weeks simulation the difference < 0.5%.

Thank you


Hi Joey,

The CMAQ benchmark case comes with ISAM output as well. Do your SA_ACONC files compare well with what is distributed with the release?


Hi Sergey,

The absolute differences are not significant

but if we look into percent differences, the numbers are huge in some areas, I believe these huge differences are due to small number in denominator

Let me know if you have any thoughts,



Hi Joey,

There is the expectation that the sum of the tags will diverge from the bulk concentration even for nonreactive species due to the non-linear nature of the transport processes in the model. Blowing around pieces of a thing should give you different results from blowing around the entire thing. The main concern would be if the error accumulates and increases in time. I don’t really see that in your case.

However, we are rewriting specifically the ozone apportionment algorithms and will release an updated version.