Yates et al. present an instrument to measure CO and N₂O aboard high altitude aircraft. They describe the modifications they made to a commercial analyzer in order for it to perform on an aircraft platform that samples up to 18 km in the atmosphere and in very cold and warm/humid conditions. They compare their CO measurements from one campaign on a NASA WB-57 platform to two other instruments that measured CO.  Correlation slopes between the other two instruments vs. the instrument described in depth in this manuscript had slopes that ranged from 1.10–1.15 and 0.94–1.10, respectively, depending on the flight that was compared.

Overall, this paper described modifications to a commercial instrument that could be helpful to the scientific community aboard high-altitude aircraft.  However, I think the authors should go into more details on both the modifications and for the other CO instruments that were compared to.  Therefore, I think major revisions are necessary.

First, more detail is needed for the inlet diaphragm pump assembly.  For example, the I was surprised that the calibration bottles were not inserted into the sample flow upstream of the diaphragm pump.  Were any tests done in the lab to show that this pump did not affect the CO or N₂O measurement?  What material were the diaphragm parts and seals made of?

Along those lines, I am not aware of people using Teflon FEP tubing to measure CO and N₂O. Are there any lab tests the authors can point to that show that this material does not affect CO and N₂O?

I also had some questions about the flow diagram in Figure 3.  What is the purpose of Port 1?  Is it the default flow path? 

line 68-73, how was cell pressure maintained?  Were the authors using the slow flow path that originally comes with an LGR/ABB instrument?  What was the flow rate in flight?

Figure 2, how are the “Solenoid” and “Ext front” temperatures below the ambient temperature?  Also, the text suggests this is in flight data, but the figure caption says the black trace is the environmental chamber pressure altitude?

line 173, I’m not sure why calibrations that differed by more than 4 standard deviations were removed?  Wouldn’t this reflect instrument performance and need to be retained? 

Figure 7, why the switch from NOAA standards to Matheson standards?  Could the authors discuss this switch?

Figure 9, could the authors use a different color for COMA?  It is hard to distinguish the dark blue from the light blue and/or black trace

line 229, could the authors add a short description of which groups operate COLD 2 and ACOS?  And how were they calibrated?  Did they get their standards from NOAA?

line 236, since the authors use two decimal places for the overall comparison, it is probably best to use two to describe the August 29 comparison

Typos/Grammar suggestions

line 115, NOAA ESRL no longer exists, and it is misspelled “ERSL”.  I would use “NOAA’s Global Monitoring Laboratory”.

line 121, the URL is missing “a.”.  Should be …gml.noaa.gov…

lines 93 and 129, don’t need to hyphenate “in-flight”

line 178, perhaps say “equally comprised of the residuals…”

line 217, remove comma after “N₂O”