Impact of the new dry deposition in CMAQ v5.4 on coarse PM

After updating our modeling platform from CMAQ v5.3.1 to v5.4, we noticed that coarse mode sea salt concentrations were much lower with the new CMAQ version. So, we conducted some test runs, and the results are summarized in the slides linked here: Dropbox
This issue is similar to what’s reported in this thread: Low PM10 in CMAQv5.4
Unlike what’s reported there, however, we observed that turning on the AERO_M2USE options didn’t make much impacts.
Here are our findings:

  • Pleim et al. (https://doi.org/10.1029/2022MS003050) stated “[t]he most significant effect of the new model is to increase the mass dry deposition of the accumulation mode aerosols in CMAQ.” However, our test shows that differences between v5.3.1 and v5.4 are much larger in ASEACAT than in ANAJ (slide 3). The slide also shows that switching on the AERO_M2USE options made little difference on the modeled concentrations.
  • Slides 4-5 show process changes at the grid cell calculated by the IPR analysis. Both v5.3.1 and v5.4 show similar net process changes for ANAJ. For ASEACAT, however, a huge loss due to dry deposition (largely compensated by vertical diffusion) occurs during the first hour of the simulation with v5.4, which results in initial concentration drop shown in the bottom plot of slide 3. This seems unrealistic.
  • We also looked at other coarse PM components: ASOIL and ACORS (slides 7-9). ASOIL shows an initial concentration drop with v5.4 similar to ASEACAT. Its IPR result also shows a big dry deposition loss with v5.4 like ASEACAT. However, ACORS does not show such anomalies. This is strange; perhaps it has something to do with different size distributions among these coarse PM species?

My main concern is the huge difference in ASEACAT between v5.3.1 and v5.4 (this has a big impact on our sea salt model performance with the new modeling platform). Is this expected with the new dry deposition model?
I’d appreciate any insight/suggestion on this issue and will provide more details about our tests if needed.
Thanks!

There are many differences in aerosol dry deposition velocity between CMAQv5.3.1 and v5.4. The Pleim et al. 2022 paper shows these. To better understand the effects in your runs, you should compare the aerosol deposition velocities VMASSI, VMASSJ and VMASSC for Aitken, accumulation and coarse mode, respectively. Note that all aerosol species use these same deposition velocities.

Thanks @pleim.jon for your response!
Following your suggestion, I compared VMASSJ and VMASSC between v5.3.1 and v5.4 (see below).
v5.3.1 and v5.4 calculate comparable VMASSJ (with v5.3.1 showing somewhat higher peaks) whereas VMASSC is significantly higher with v5.4 throughout the simulation. This is consistent with what the concentration plots show on slide #3 of the PPT attached to the OP.
However, this seems to contradict what Pleim et al. (2022) says, that is, the main impact of the new model is on the accumulation mode. Perhaps, the assessment was based on comparison of deposition velocities for vegetated areas. The grid cell (55, 100) for my plots is located in a developed area (39% Developed Medium Intensity; 23% Developed Low Intensity; 20% Developed High Intensity; 13% Developed Open Space). Pleim et al. (2022) states that for accumulation mode, the effects of the new model are minor in developed areas compared to those in heavily forested areas (Figure 9d in the paper), but doesn’t mention what they look like for coarse mode.
I wonder if the large increases in VMASSC with v5.4 is consistent with the expected outcome of the new dry deposition model.
Hope you can shed some light on this. Thanks!