In Penn et al, the authors perform an inversion using GEOS-Chem and methane from GOSAT and AIRS to optimize both methane emissions and OH concentrations. Including both GOSAT and AIRS allows the authors to determine the effect of including methane from both shortwave (GOSAT) and thermal (AIRS) infrared instruments. They find that including AIRS in the inversion provides little additional information beyond that found from using GOSAT alone. While the method cannot resolve information on OH at mid-latitudes due to correlation of errors, it was able to provide information on interannual variability and the interhemispheric ratio. This was an interesting paper that both demonstrates the need for continued methane observations by satellites as well as their utility in constraining OH at the global and hemispheric scales. It is suitable for publication in ACP once the very minor comments listed below are addressed.

Minor Comments:

Line 49: Should be “resulting in a lifetime of ~1 second”. Currently missing “of”.

Line 56: Also differences in UV flux in the troposphere. See, for example, Nicely et al, 2020.

Line 159: You use a uniform correction of 19 ppb but figure 1 shows a clear latitudinal bias in AIRS with respect to the GEOS-Chem simulation you are using as “truth”. Why not use a latitudinally dependent correction? In the end, I imagine this doesn’t really affect your results since including AIRS doesn’t seem to have much benefit.

Line 176: Where do the 50% and 20% numbers come from? Are they just “best guesses”, in which case, how sensitive are your results to these numbers?

Line 211: Similar question here as for line 176. What’s your justification for using an error of 50%?

Line 385: You refer to “Boreal North America” as “East Canada” in Figure 6. Be consistent with labeling.

Line 387: Probably shouldn’t start two consecutive sentences with “remarkably”.

Conclusions: One of the main conclusions from this work seems to be that including information from AIRS doesn’t really seem to improve the inversions and that information from the SWIR retrieval (GOSAT, in this case) is sufficient. It could be helpful to have a discussion on whether this is just a result of limitations of the AIRS instrument, or if you think other TIR instruments could prove more useful. What would need to be done to make the TIR retrievals more useful for these types of studies, either improvements to the retrievals or to the instruments themselves? Similarly, when discussing the inability of this methodology to resolve information on OH across more latitude bands, what could be done to improve this, if anything? Using a different or larger set of independent simulations? Or is this simply a limitation of this methodology and it is unlikely to ever yield spatially resolved information on OH distributions?

References:

Nicely, J. M., Duncan, B. N., Hanisco, T. F., Wolfe, G. M., Salawitch, R. J., Deushi, M., et al. (2020). A machine learning examination of hydroxyl radical differences among model simulations for CCMI-1. Atmospheric Chemistry and Physics, 20(3), 1341-1361.

My apologies for the delay in my review.  See attached pdf for comments.