This note reports calibration experiments conducted at NOAA Chemical Sciences Laboratory for two I- chemical ionization mass spectrometer instruments towards two distinct isomers with molecular formula C2H3ON, namely methyl isocyanate (CH3NCO) and hydroxyacetonitrile (HOCH2CN).  The results indicate that these instruments are highly sensitive to HOCH2CN while insensitive to CH3NCO.

It is not at all surprising that I- forms moderately strong complexes with HOCH2CN as compared with CH3NCO.  The hydrogen in the polarized (acidic) H-O bond is undoubtedly largely responsible for this difference.  Similarly, the polar H-NCO bond is likely the dominant reason for I- forming clusters with HNCO.  These functionalities promote stronger binding to reagent ions through hydrogen bonding (J. Phys. Chem. A 2018, 122, 269−279).

For the most part this paper is clear and well-written and the subject matter is pertinent for the AMT journal and I recommend that it be published after the authors have addressed the following points to the editor’s satisfaction:

0). The authors should modify the strong, confrontational language used in abstract, introduction and conclusion where they apply results from this work to reports of previous measurements. Such statements may be hard to prove, and serve no good end.  Statements including modifiers such as ‘likely’, ‘may have’, and ‘it may be’ should suffice to convey the authors' point.

1).  Are there other stable isomers having formula C2H3NCO besides the two discussed in this paper?  If so, authors should discuss the likelihood that these could contribute to ambient CIMS signals.  Are there other ‘nearby’ isobars (ie, different atomic composition), considering the resolution of spectrometers in question, which could contribute to signal with nominal mass of C2H3NCO clusters?

2). The IR absorption bands and cross-sections used to quantify CH3NCO should be included. How stable was this compound in AL cylinders?  How do Nr calibration of CH3NCO mixtures in AL bottles agree with FTIR determinations?

3). Are there other possible ‘N’ compounds produced in the syringe pump std method for HOCH2CN?  How does the mixing ratio calculated from pumping rate and gas flow compare with Nr determination?

Figure 1: y-axis of Panel B is Nr signal or something else?  If Nr, you should keep this label, and state in text explicitly the assumption that 100% (or whatever the assumption is) Nr signal is comprised of HAN.

Figure 3:  How do the authors interpret and deal with the ToF 30C and 35C and Quad 20C curves that seem to be out of family with the other curves, and the mechanism. Are these curves reproducible?

LN272-281:  This PP should be reformulated.  Suggest that if the authors wish to put forward the idea that HAN is observed in the atmosphere it would be more appropriate, straightforward, and convincing,  if they present their own data, rather than simply re-assigning previously published by other groups.  [reviewer notes that the instrumentation calibrated within this work has been deployed I number of previous field campaigns from aircraft and ground-based platforms, with plenty of biomass burning influence].  In addition, the authors should discuss the more general importance of HOCH2CN to the nitrile budget.  What fraction of nitriles does HOCH2CN comprise?  Is there reason (and if so what are the reasons) to study its chemistry in more detail?