Ozone formation BCON CMAQ

Hi people,
Does anyone know to explain why the CMAQ model has a high dependence on the BCON for the formation of ozone? I’ve performed some simulations with the ISAM that pointed out that about 70-80% of ozone came from the BCON. I also removed all sources from my domain and run again the test, which the concentrations haven’t significantly changed. If I don’t consider ozone in my BCON profile, the ozone concentrations will be largely affected. I am trying to find an explanation for this behaviour, which I would say is because of the CMAQ model.
Thank you in advance.

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Depending on the size of your modeling domain, meteorological conditions, and emissions within your domain, ozone specified at the lateral boundaries can indeed be expected to be a (very) significant contributor to your simulated ozone concentrations. While ozone near the surface might have a lifetime of a few days, free tropospheric ozone has a lifetime on the order of weeks, so transport from the boundaries can be quite important for smaller domains and even continental-scale domains.

The exact contributions will depend on the domain, the meteorological conditions (transport from the boundaries vs. circulation within the domain), the abundance of emissions within the domain, and the time periods considered (specific high ozone episodes vs. average conditions), so it’s hard to generalize what the number should be in a specific situation.

The following publications may be useful for additional context: