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).
Thank you for your answer. But I have a question for you based on your answer. If natural sources such as lightning and NOX emission from soil are not included in my CCTM script, is there a natural contribution of NO2 in the simulation result?
Best
pbwang
If your simulations do not include NO emissions from natural sources within your modeling domain, you would not be able to configure ISAM to attribute NO2 to such natural NO sources. Depending on how NO emissions from natural sources were handled in the simulation used to generate boundary conditions for your modeling domain, your simulated NO2 might still include a contribution from natural sources. In ISAM any such natural source contributions from outside your domain would then be included in the BCO tag, along with contributions from all other sources outside your domain that contributed to the boundary conditions.