I am conducting source apportionment studies using both the Zero-out method and ISAM in CMAQ, with identical emission and meteorological parameters. I’ve noticed a significant discrepancy between the results from these two methods that I hope you could help clarify.
With the Zero-out method, the contribution of a specific emission source to O3 shows both positive and negative values across different regions, which is understandable due to the varying regional sensitivities and the titration effect of NOx emissions on O3. However, when using ISAM, the contributions are consistently positive across all regions.
This substantial difference between the two methods raises some questions:
Why do these two methods yield such different results?
Does ISAM not account for the titration effect of NOx on O3?
Welcome to the forum and CMAQ and/or CMAQ-ISAM modeling. The answers to your simple questions are complex. You will get pretty far answering them yourself if you review the documentation accompanying the release of the model and the various publications that are out supporting it. Even a simple google search will be helpful.
Here’s my understanding of ISAM, I don’t know if there’s anything wrong with it：
In ISAM, the chemical equations continuously track how “chemical components released from that source (tag)” generate the final O3. If NO from one source reacts with O3 tagged from another source, ISAM typically lets “the O3 tagged from that other source” “bear the responsibility” of being consumed, rather than allowing the “original source” to show negative contributions.
In other words, ISAM tends to answer: “Which ‘raw materials’ (NOx, VOCs, etc.) did this portion of O3 transform from?” rather than “How much O3 produced by others did this source consume through its NOx?”