Im running CMAQ-ISAM 5.4 in order to understand sources contribution to NO2 concentrations in my domain. My domain comprehends a big urban area, but with very small NOx emissions outside of it.
I find large relative contributions from BCO to NO2 concentrations (around 40% depending on time and site) which brings to the following question:
As NOx emissions are small outside my domain I tend to believe that the BCO contribution that I am seeing is not only due of primary concentrations but also secondary ones. Is this correct? I believe this has changed with ISAM versions, but I believe this is true for CMAQ 5.4.
The BCO contribution to NO2 captures any NO2 directly transported from the boundaries as well as any NO2 formed from any other nitrogen-containing species (e.g. PAN) traced back to the boundaries, in the same fashion as the BCO contribution to O3 captures any O3 directly transported from the boundaries as well as any O3 formed from O3 precursor species traced back to the boundaries.
Even though (anthropogenic) NOx emissions may be small “outside” your urban area of interest (though this probably depends on how far upwind you look), depending on where your boundary conditions came from, they may contain non-negligible impacts of natural NO sources (soil, lightning).