Please see my attached review comments.

This manuscript describes a new coupled model system based on the MPAS meteorological model and CMAQ air quality model. The coupled system with a variable mesh grid has numerous advantages over other model systems such as a consistent transport scheme for pollutants and meteorological variables and the advantages of finer resolution without the need to nest domains, which can introduce interpolation errors.  The manuscript is very well written. I recommend publication with only minor changes.

Line 40. I feel like there should be a reference to MPAS somewhere in this paragraph.

Line 75. Can you please check this sentence. The sentence states enhancements for physics options and then states “namely the Pleim-Xiu land-surface model” which is a surface model and not physics model.

Line 125.  Can you provide a bit more detail. Are the soluble and insoluble species in the PM phase? What are the other components needed in MPAS-CMAQ system to model the online aerosol direct effect?

Line 185.  One of the advantages of the MPAS-CMAQ system is the variable mesh which enables finer scale without the need for nesting grids. This removes the need for interpolation of lateral boundary conditions. However, for the tests performed here, the emissions were interpolated from rectangular grids to the MPAS grid which introduces errors and may counter the gain from not needing nesting. In future work, it would be beneficial to do the emissions processing directly onto the MPAS grid rather than interpolate from other grids.  

Line 218.  I feel there should be more information on how plume rise is calculated in MPAS-CMAQ since this is a key process where meteorology impacts pollutant dispersion. Is plume rise handled in MPAS or CMAQ? If in CMAQ, what parameters are transferred to CMAQ to simulate the vertical mixing. Does the water vapor in the point source feedback to the meteorology and provide more latent heat?

Line 246.  Does the MPAS-CMAQ system include ozone data assimilation in the stratosphere. This may improve the ozone low bias in winter/spring, particularly in free troposphere.

I would recommend larger size for some of the figures (e.g. Figure 7,8), maybe not side by side.

Grammar Corrections

Line 29. Change “a” to “an”.

Line 50. Change “entities” to “objectives”

Line 230.  Remove “has”.

Line 251.  Correct MERA to MERRA

Line 282. Change CAMQ to CMAQ