Uncertainty analysis of output from Sulfur Tracking Method (STM) in CMAQ v5.3.3

Hello everyone!

I am a CMAQ (v5.3.3) user and I have questions about STM results. These days I am using the STM module to quantify the relative importance of primary emission and chemical processes to the final sulfate concentration. My research domain covers east Asia region and modelling period is Jan, Apr, Jul and Oct of 2019.

My question is how to evaluate my STM outputs and make sure that my results are reliable enough? Or, are there any references elucidating the uncertainty of this module. I have searched on the CMAS forum and the CMAQ github webpage but the information there didn’t solve my problem about uncertainty analysis. The references listed in CMAQ manual are all about sulfate conversion mechanisms. I have this question because the STM results are an important part in my reseach.

Any responce will be appreciated!


As you know, the sulfur tracking model (STM) tracks the relative contribution of the various processes leading to the modeled sulfur burden. These process breakdowns in the STM diagnostics in themselves do not have much uncertainty in representing the modeled aggregate S concentrations and modeled burden (i.e., their sum totals the modeled ambient S concentrations and modeled atmospheric burden). They, however, inherit any uncertainty in the input data (as in emissions) or in the underlying kinetics of the represented gas and aqueous pathways in the model. The starting point for evaluating the STM outputs should be the bulk concentrations of modeled SO2 and SO4 relative to observations. Discrepancies between modeled and observed concentrations of these species could be influenced both by uncertainties in the emissions used as well as the lack of representing possible (and uncertain) chemical pathways (e.g., Redirecting; https://doi.org/10.1073/pnas.1616540113). Depending on the objectives of the study one could perform additional sensitivity simulations to assess how the uncertainties in emissions or missing pathways influence the sulfur budget terms output by the STM relative to the bulk concentrations. Adding additional chemical pathways (gas, aqueous or heterogeneous) would require modifications both the base CMAQ code as well as to the STM. A couple of examples of use of STM to assess impacts of different gas phase chemical mechanisms that may be of interest: Diagnostic Analysis of the Three-Dimensional Sulfur Distributions over the Eastern United States Using the CMAQ Model and Measurements from the ICARTT Field Experiment | SpringerLink and ACP - Sensitivities of sulfate aerosol formation and oxidation pathways on the chemical mechanism employed in simulations


Thank you for your detailed response which well solved my question. I will refer to the references mentioned above and evaluate the modeled bulk sulfate and SO2 concentration to check the uncertainties.