This is a very well-written and timely study on factors affecting the sensitivity of CI-MS measurements. I warmly recommend its publication, and have only two very small suggestions for further improvement.

1)As illustrated by reaction R6, the neutral benzene concentration (or neutral to ionised benzene ratio) can affect the detection sensitivity. The same feature applies also to nitrate ionisation (where sensitivities for some compounds can be very different in NO3-  - dominated charging regimes vs (HNO3)NO3- dominated ones, indeed some compounds can only be detected in the former). I understand this issue is very specific to the particular combination of charger ion an analyse, and thus somewhat beyond the scope of the present study on general factors affecting CI-MS sensitivity, but maybe you could still briefly discuss it, and how it might be affected by the parameters already studied here?

2)The sentence on line 134 beginning with “Through quantitative...” seems to be missing some words, at least I don’t understand the syntax. Please rephrase. 

This is a nice manuscript that is leveraging access to many CIMS instruments to probe the drivers of sensitivity. While many studies previous to this one have characterized the sensitivity drivers considered here, the contribution of this work is to show that among multiple instruments we can reproducibly determine sensitivity by holding the controlling factors constant across instruments. While not a surprising result, this is the first work to methodically show that result in quantitative detail. Additionally the authors provide a rather new perspective, that regardless of the ion used the collisional limit should remain constant as it is controlled by geometry, mixing and reaction timescale. I am in support of publication of this work with minimal corrections. 

Comments: 

- I feel like this work would benefit from a better discussion on the external sampling factors that would influence net sensitivity. There is minimal discussion of complicating factors like the impact of RH changes when sampling, sampling transmission impact that would go into a net sensitivity function, and potential changes in sampling pressures. The manuscript presents the most stable/idealized version of the sensitivity experiments which may give the reader false confidence that these system do not require frequent calibration checks to ensure stability and correct for transient effects, like RH changes. These types of effects will not impact all ion chemistries equally and individual practices such as material use, operating pressures, and other customizations which are common place among the scientific user community will significantly impact the sensitivity. This feeds into the overall tone of the manuscript where one of the final statements is "... reduce calibration requirements" (page 26, line 724). I do not actually believe any of the work in this manuscript reduces the need for calibration it just provides more detail on the drivers of sensitivity and a better understanding of why comparison of the different customized CIMS instruments over the years have proven challenging. The sensitivity function drivers from real world sampling challenges can not be ignored and will require calibration checks. 

- Page 3, line 68: The statement "As the number of chemical present in the atmosphere continues to increase" warrants a citation. While I do believe that our awareness of the number of chemicals is increasing due to rapid advancements in measurement technology, I am not sure it is a true statement that the actual number of chemicals is increasing. 

- Page 5, line 134: The sentence that begins "Through quantitative comparison..." needs editing. 

- Page 11, line 329: In the phrase "The sensitivity increases approximately quadratically due to changes in collision frequency.." quadratically in what parameter space? It is unclear what is meant here. 

- Page 14, line 406, "A change in sample gas temperature change..." Delete the second instance of change.

- Page 14, line 409. With regards to operating at the lowest feasible temperature, this is specifically true for sensitivity but there may be detrimental impacts from surface absorption and time responses. I think a qualifying statement should be made here. 

-Page 15, line 436. The discussion on pressure impacts. It is worth noting that there can also be negative impacts from secondary ion chemistry as has been shown by several CIMS studies. Those secondary reactions can complicate interpretation of mass spectra. 

- Page 18, line 513. Your state "we determined that levoglucosan systematically reacts ~20% below collisional limit" but then at several locations through the remainder of the manuscript repeatedly state that "Levo. is known to ionize with iodide anions near the collision limit" (Figure 4 caption), (page 22, line 629), (and elsewhere). 

-Page 22, line 607. You are talking about the N2O5 measurement and state that transmission efficiency changes "... adds to the major challenge of generating a quantitative in situ source of N2O5." However, generating a quantitative calibration source of N2O5 has nothing to do with the instrument transmission. Determining the collisional limit sensitivity of a molecule that has multiple reaction pathways is made difficult when considering the transmission, but having a quantitative N2O5 calibration source and calibrating your peak responses is unrelated to that.