Review of “Enhancing characterization of organic nitrogen components in aerosols and droplets using high-resolution aerosol mass spectrometry” by Ge et al.

The authors presented careful characterization of aerosol mass spectrometric (AMS) measurements of 75 nitrogen-containing organic compounds using high-resolution AMS (HR-AMS). Fragmentation patterns of these atmospherically relevant compounds are discussed and tracer ions are proposed. In addition, based on calibration of 18 nitrogen-containing organic compounds (mixed with ammonium sulfate), an average relative ionization efficiency (RIE) of 1.52 was found to be not much different from the RIE of organic aerosol (OA) in general (1.4); a new calibration factor of 0.79, however, is recommended for N/C estimation for ambient datasets, instead of the widely used 0.96. After the methodology was established, it was used to evaluate the nitrogen-containing organic compounds in two PM1 datasets (Fresno and NYC) and one fog water dataset (Fresno), with both concentrations of nitrogen-containing organic compounds and N/C ratios quantified. Finally, limitations of the methodology are also discussed. This is a well designed and rigorously conducted experiment that gives very useful methodological essence in using the HR-AMS to quantify nitrogen-containing organic compounds in atmospheric aerosols and waters. The manuscript is also very well written with clear demonstration of results and conclusions. I therefore recommend Minor Revision with a few comments as follows.

Specific:

1. Please clarify why organic nitrates (and nitriles, as the authors stated in P8/L240) are not used in this study.
2. Please clarify how the compound class was categorized if a compound contains more than one functional group in the list, for instance compounds #31, #32, and #60 in Table S1.
3. P8/L250: it would be good to put a row of fractional signals of averages/standard deviations for M^+ in Table 1 to echo the discussion here, such that readers have a broad idea on what compound classes have higher molecular ion contribution without refereeing to the SI. Although it does not align well with the mass contribution of nitrogen-containing ions to total organic nitrogen mass, it would be good to have such information easily referred to. Better yet, it would be good to make another section in the table to show the fraction signals of main nitrogen-containing ion (and M^+) to the overall mass spectra.
4. P11/L325: the bimodal size distributions of ON in marine aerosols here might be linked to the difference between Fresno PM1 and fog water results discussed right above it. It would be good to have one sentence to make the linkage.

Technical:

1. P6/L193: I do not see it necessary to use “respectively” here.
2. P8/L226: “Additional” to “Additionally”
3. P8/L249: “- a simple amide” to “(a simple amide)”
4. P9/L274: Change this “Furthermore” to “In addition”?
5. P9/L279: “70 ev” to “70-eV”, also in P11/L335.

This paper employs high-resolution aerosol mass spectrometry (HR-AMS) to provide an exhaustive characterization of organic nitrogen compounds within the atmosphere. The study encompasses an analysis of 75 distinct types of organic nitrogen standards. The authors not only explore the mass spectral features and N/C correction factors but also propose identifiable ion series, which prove invaluable in facilitating the speciation analysis of organic nitrogen. Furthermore, the authors extend the utility of this method to investigate three environmental samples, analyzing the contents and compositions of organic nitrogen across various regions and phases. This is an important paper for the AMS community. However, there are some minor issues that need to be addressed before publication:

1. I would suggest to make a table summarizing all the previous AMS measurements of the ON standards. In this way, the novelty of this paper would be obvious.

2. The introduction does not address the limitations of HR-AMS technology. These limitations include an inability to detect substances with low volatility or ionization, differentiate between isotopes, and provide structural insights. To enhance the paper's quality, it is suggested to explain these constraints in the method or discussion section and assess their impact on the obtained results.

3. The paper does not consider the possible mixing effects when analyzing the fingerprint ion series, namely, different types of ON may coexist in the same sample, resulting in signal superposition or interference. It is suggested to discuss this situation in the discussion section and provide methods to distinguish or resolve it.

4. Mixtures of nitrogen-containing organic compounds and sulfate were combined in a 1:1 mass ratio. Why using this mass ratio? Will mass ratio affect RIE of different compounds?

5. How did the authors measure fog water with the AMS? I cannot find any experimental details. Did the water be atomized and then dried? Please provide these important details so that other can replicate these experiments.

6. Overall, the experimental section is too brief and lack of many important experimental details. For example, the aerosolization of ON standards is not well described. Again, it is very important to provide all these details so that others can replicate these experiments.

The authors used an expanded set of ambient-relevant nitrogen-containing organic compounds standards to improve the characterization and quantification methods of submicron organic nitrogen species by the Aerodyne high-resolution aerosol mass spectrometer (HR-AMS). This study provided the improved N/C conversion factor and RIE for quantifying organic nitrogen species (non-organonitrates). The authors also did a detailed examination of the mass spectral feature of standards and ambient samples and proposed tracer ions for identifying different types of organic nitrogen species. This work enhanced our understanding of the performance of the HR-AMS and provided insight into the sources and processes of nitrogen-containing organic compounds in the atmosphere. The manuscript is very well written and easy to follow. I suggest publication after addressing the following comments:

1. The study excluded organonitrates, which are also nitrogen-containing organic species, in the analysis. Can the authors comment on that? I suggest the authors clarify the reasons why organonitrates are not included in this method development and perhaps use an alternative term instead of “organic nitrogen (ON)” throughout the manuscript to avoid confusion.
2. What were the rough concentrations for the standard solutions? How were fog water samples collected? How were samples stored and prepared before nebulization?
3. Can the authors explain the importance of RIE? One entire section in the Results and Discussion talks about RIE, and yet it is not clear what this value is and why it is important.

Other comments:

1. Line 19, a calibration factor of 0.79 is for what?
2. Table 1, why are some numbers in bold? Please also clarify what does “± xx” represent
3. Figure 1, please explain how is the “Avg. Err” calculated.
4. Figure 2, do amino acids show CxHyO+ ions in the MS? Why only present the one or two ion families for each species?