How CMAQ-STM treat tracked species during deposition/advection/diffusion processes

Hi all,

Recently, I’m trying to understand the code of the CMAQ Sulfur Tracking Method and I am wondering if there is any material that can help me better understand the entire code flow and framework of this part.

Also, I have a question about how the model treats each tracked sulfate species during deposition, advection and diffusion processes because I couldn’t find any relevant STM code in these processes.

However, I saw in STM_MODULE.F: subroutine stm_wrap_ae that the proportion “mscor” is used to normalize tracked sulfate species to total modeled sulfate. Is this part considering the overall impact of these processes (deposition, advection, diffusion) on tracked sulfate species? If so, does it mean that the tracked sulfate species from different sources from upwind grid cells during transport are not considered?

If so, does it mean that it ignores the different sources from upwind grid cells and current grid cells of tracked sulfate species?

I appreciate any help from you.

The [User Guide] (CMAQ/DOCS/Users_Guide/ at main · USEPA/CMAQ · GitHub) may be helpful, though it likely does not have all the details you’re looking for.

Thank you for your response.
I have already read this document before, but it doesn’t provide more details as you mentioned. Could you (or anyone) please explain how the model treats sulfate tracers during the transport process?

Hi, Noor-

The Sulfur Tracking Method tells you what chemical and physical processes contribute to the sulfate concentration (ASO4I, ASO4J, and ASO4K) predicted in each cell for each timestep. The sulfate tags in STM are just particulate tracers, so they experience transport and deposition in CMAQ just like any other PM species (e.g., sulfate).

The option to normalize the sulfate tags is set in the CMAQ run script (STM_ADJSO4) and will readjust the sulfate tracers to the modeled sulfate concentration should the sum of the sulfate tags deviate slightly from the modeled sulfate concentration. The STM tags tell you about the modeled SO4 concentration in a particular cell, but that doesn’t mean that a particular process represented by a tag (e.g., secondary transformation in cloud droplets) necessarily occurred in that cell.

Hope that helps,