General comments:

This paper presents field data on the organic nitrogen composition of fog water and aerosol (interstitial and residual) obtained during the winter of 2022 FAIRARI campaign in the Po Valley of Italy. The measurements and analysis are novel and likely to be of great interest to the atmospheric chemistry community, and are mostly well presented in the manuscript. I have a few questions and points of confusion which I hope the authors can address.

Specific comments:

Line 74-75: You describe the aerosol pH decrease accompanying the fog water increase, and state that the aerosol pH decrease was “mainly contributed to ammonium” – do you mean, the pH decrease was attributed to a decrease in ammonium ion concentration? Clarify, and perhaps elaborate, since this is a bit counterintuitive (that water pH would increase which aerosol pH decreases).

Lines 139-144: In the discussion of inlet tubing – explain why you used mixed tubing. Is there any concern about particle losses at the junctions between tubing types? And maybe list diameters also in mm.

Figure 1: I spent a long time trying to understand this diagram, so I wonder if there’s a way you can help the reader get there more quickly. Maybe it would help to put red and purple lines next to eachother between the 3-way switch and the SP-AMS, so it’s clear that the SP-AMS switches between WAI and GCVI during fog/ non-fog? Or indicate with little arrows and labels “fog” “non-fog” which path the WAI follows in each case, at the red tee?

Around lines 162-164: does “dual” mode mean also BC or BC instead? The text reads like the 70 eV EI is substituted with 1064, but I think perhaps it is both simultaneous? Please clarify.

On line 185 you state that “During non-fog periods, both instruments measured behind the WAI.” This makes me expect some general statement comparing the two BC measurements, but I don’t see it.

Line 216: How did you quantify the concentrations of imidazole from the NMR spectrum? Here you describe, and in the SI figure, it is clear that you can identify the presence of the molecule from the existence of the peak, but how do you convert this to a mass concentration?

Around lines 241-247: You mention the moderate correlation between non-fog ambient aerosol ON and aerosol water content. Do you interpret this as indicating mainly water-soluble organic nitrogen compounds, or do you think both could be elevated during the colder episode and therefore correlated?

Figure 3: why are the data “spikier” during fog events? Maybe briefly explain in caption.

Figures 3 and 4 are nice clear overview figures of the differences in composition.

Around line 307: is the Pearson correlation coefficient you mention here just for the 1N compounds? Please clarify. Is r=0.89 the correlation coefficient for Figure S3? (all of the ON peaks) If so, mention this in the caption in the SI.

Lines 323-335: The imidazole results and comparison to NMR are very interesting indeed. On line 327 you mention they weren’t observed “in PM1” samples, but later you say the NMR derived concentrations correlate well with SP-AMS. Was the SP-AMS not measuring PM1?

Next paragraph:Do the imidazole peaks correlate at all with aerosol [NH4+]? You mention in lines 340-343 the correlation of CxHyN2+ with glyoxal being higher in the second event. Was the [NH4+] also higher?

At the end of the paragraph (lines 344-345) you mention a stronger linear correlation of CxHyN1+  with glyoxal. How do you interpret this?

Figure 6: I find the symbology on this figure confusing. Why are only the fog residuals shown as box & whisker plots and the others as bar charts behind? I think this would be much easier to read with all data as box & whiskers. The overlaying is not helpful, and you could colors to indicate which axis each corresponds to. Do you have ambient PM1 data also from the SP-AMS? This would provide another point of comparison.

Line 364: clarify that the lower temp and higher RH occurred during the first period. Do I correctly infer that this means you expect the nitrate to be more semivolatile than the OA mix?

Around line 375-377: Why would preferred partitioning of amines into the aqueous phase result in a strong correlation with glyoxal? If they have different sources, the gas-phase concentrations are not necessarily tracking.

Around line 381-382: Could the elevated CxHyN2+ during period 2 also be related to that period having dominant OA in the aerosol rather than NO3-? Does elevated NO3- provide another “sink” for NH4+ other than formation of imidazole? Again, I’d be interested to hear more about correlations with NH4+ as well.

SI:  In the caption to figure S1, I suggest to remind the reader briefly the nature of the correction applied.

Technical corrections:

Line 13 and elsewhere: when you use “1H-NMR” to refer to proton NMR, the 1 should be superscripted

Line 14: “medium” -> “media”

Line 27: suggest “understood” rather than “quantified”

Line 37: suggest “aerosol precursors” rather than “air pollutants”

Line 51: “atmospheric reactive nitrogen”

Line 88-89: suggest “The analysis of aerosol-fog interactions .. been restricted to offline chemical …” (make more general)

Line 98: “The findings from Kim … in the ambient aerosol, and this work extends this quantification to fog water and fog residuals.” (however doesn’t seem to fit here)

Line 124: remove “around”?

Line 134-135: suggest “Karlsson et al., 2021), therefore the sampled fog residual masses should be considered a lower limit of the real ambient values.”

Line 135: “Visibility was measurement by a Belfort Instrument sensor (Model 6400).”

Line 176: “calibration”

Line 181: does “within this range” mean “below m/z 106”? Or do you want to cite a m/z range here?

Line 190: on Ca and Mg the 2 should be part of the cation superscript

Line 340-341: suggest “(March 27-30), when CxHyN2+ ions were much more abundant in the ambient aerosol and fog residuals, a stronger linear correlation …”

Line 347: suggest “in the online fog residual measurements”

Line 366: suggest “comparison” > “contrast”

Line 379: suggest reordering: “The related fragments linked to imidazole have been confirmed by ¹H-NMR spectroscopy, and thus can now be used as tracer fragments."

The researchers performed online measurements of fog residuals and ambient aerosols, as well as offline analysis of fog water in the Po Valley, with a focus on the organic nitrogen components.   It was found that both the fog droplet residuals and fog water had enhancements of amino compounds and imidazoles. 

The manuscript is well written and informs an interesting topic area.  I only have a few comments:  

Do the authors have any comments on the primary sources of ON and relevant formation pathways of imidizoles in the Po Valley? Line 52 mentions vehicle exhaust, biomass burning and agriculture as possible sources, and at line 312 the authors seem to dismiss local traffic sources as a source due to differences from Saarikoski et al (2012). A sentence  or two in the conclusions will help showcase the relevance to other geographic regions.

Line 46: “Inorganic aerosol species also contribute to aqSOA. Nitrate (NO−3) is becoming more important as an aerosol component in many areas, due the reduction of other air pollutants (e.g. SO2), or where high levels of excess NH3 exist in the atmosphere (Bauer et al., 2007).” Is this sentence stating that high levels of nitrate can enhance aqSOA formation, or that inorganic nitrate is an important PM component formed in the aqueous phase.  If the latter, the first sentence should be revised as nitrate is not aqSOA. 

Line 191: Were these ions measured using the AMS or the offline IC that was briefly mentioned previously. If it was the AMS: how were K⁺, Ca⁺, Mg²⁺ and Na⁺ treated

Line 215: Does the NMR method differentiate between different substituted imidazoles?

Figure 6: I find this figure hard to interpret.  Is there another way to present these results such that it is clearer which bars correspond to which axis? I also find the simultaneous use of the box plot and bars distracting.