The manuscript deals with the use of stable isotope ratio measurements for assessing sources and the formation of nitrogen-containing aerosols.

I found this manuscript interesting in a few aspects. First, the Authors used FTIR-ATR technique prior to isotopic measurements, which allowed to measure low amounts of nitrogen and oxygen in size segregated aerosol particles. Second, the Authors used low volume cascade impactor for day and night separate sampling and still were able to use individual plates for isotope ratio measurement. Third, the Authors demonstrate the use of a statistical isotope mixing model for aerosol source identification.

In my opinion, measurements were performed on the “edge” of isotope spectrometer capabilities, anyway is a nice attempt to look at the peculiarities of chemo-physical processes occurring in the cloud forest.

I would like to see more details on the measurement of the isotope ratio in the samples itself. This is actually a research that requires a lot of mastery because of the small amounts of analyte encountered. I would like the authors to provide more details in the supplementary material: what was the linearity of the spectrometer, what smallest samples did the authors measure with sufficient accuracy, or was the linearity tested with international standards of various sizes? All of these details will be useful to readers who apply similar analysis in the future.

Some specific comments:

Line 75 It’s not clear where samples were collected. It’s written that in Xitou experimental forest, but is not clear the location is up in the hill or in valley.

Line 105. There is no description how BC was measured with FTIR-ATR analysis. Does it is comparable with the measurements with other BC techniques, for example aethalometer?

Line 135. What stands for letter p in “p-NO₃^-=…”

Line 170. Fig. 29 (c) and 2(d). NH4+ is not in the Fig. 2(d).

Line 170. It’s not clear boundary level effect. Does it mean that the boundary level is always above the sampling station?

Line 190. I look at Fig. 3a and I see on average lower δ¹⁵N values in submicron range comparing to bigger particles. Authors say that the “trend o a higher NH4+ δ¹⁵N in submicron aerosol was also observed in Beijing”. I not understand how Authors compare different size bins.

Line 195. What is mean “daytime fractionation”?

Line 200. PM1-10 was higher  … similar to 0.32-1 μm aerosol. So no difference in all size bins, as almost the whole range fall in the 0.32 – 10 μm. This kind of differentiation seems artificial.   

Fig. 2. The legend must be revised. I suggest adding a legend to the (b) and (d) for clarification.

The authors report results from an aerosol sampling campaign in a rural cloud forest during December 2018. Different size fractions were sampled on filters taken during daytime and night-time and during some days fog events impacted the aerosol composition. The most important measured aerosol components were  ammonium, nitrate, sulphate, and black carbon. Ammonium and nitrate were also analysed for stable isotopes no nitrogen and oxygen.

The study nicely show local dynamics of aerosols and their partitioning into different size fractions. Differences in stable oxygen isotopes of nitrate during foggy conditions revealed a possible oxidation pathway involving peroxyl radicals.

My major concern is the performed source apportionment using the stable nitrogen isotopes and a mixing model (MixSIAR). Many aspects of the procedure are insufficiently described (e.g. what is posterior in this context, and how should probabilities interpreted). Table 4 seems to list the results of the source apportionment. I see mostly values around 20 with standard deviations around 15. A threshold of 20 is applied , but the choice of this value is not motivated. Overall, most values do not seem to be significantly different. I fail to see how any conclusions can be drawn from this model. Therefore, I suggest to remove this part.

Minor comments:

Language needs to be improved. Several issues… already in the first sentence of the abstract (aerosol components NOT compositions).

Figure 1: extend the figure to also indicate how daytime and night-time chemistry results in different stable isotope composition. A good description is given in the supplement. Maybe some of this can be incorporated in  Fig 1.

L132/133:  I am not sure if organic nitrogen can be neglected. There are several papers out reporting organic nitrates and other organic nitrogen compounds in aerosols. The authors should at least discuss how their results would change of there are significant fractions of other nitrogen compounds.

L214-216: This sentence has language issues. The argumentation does not seem to be logical.

L228-230: Was there any evidence for agricultural activity during that period? What was different compared to other periods?

L281-282: “The posterior probability of PM1 and PM1-10 nitrate sources has difference slightly:” This seems to be a mixture of poor English with lab/model-slang.