First, thanks for considering me as a reviewer for this nice work. I have read the manuscript, and I believe that it presents very good observations of frequent NPF in a high-altitude urban environment on the Tibetan Plateau. The dataset is interesting and potentially important for understanding aerosol formation in unique mountain urban regions. However, several central conclusions depend on subjective event classification (that’s a well-known general problem) and some limitation on NPF mechanistic understanding. I like the study and I would like to see it published in ACP but it would need some revisions.

Here my comments:

Introduction:

The introduction is overall well structured; However, it might be that the literature review is somehow narrow. I do understand that the focus of the paper is the Tibetan Plateau however there are several other studies with a focus on high altitude measurements and location very close to the one done in this study. This introduction focuses strongly on recent Tibetan Plateau background studies while giving limited attention to the broader high-altitude and free-tropospheric NPF literature. I can imagine that a comparison done with measurements presented in Himalaya such as Bianchi et al. (2021, Nature Geoscience) but also many other studies could be good for this work. Also, this comparison can been done when discussing the possible precursors. In this way the authors can also benefit by distinguish their urban Lhasa case from other mountain NPF types.

Classification event:

It is well known that the actual classification method is subjective and probably quite qualitative. However, I have the feeling that in this case can be improved. Also, because many central results depend on the classification, like NPF frequency, J/GR statistics, and CCN formation. For example, in figure 2, I would even think that type II and even undefined can be considered as a class I event. Mostly, it seems that, the separation between Class II and Undefined events is not really clear. Something useful could be a day-by-day classification table, including event start/end times, growth-rate time windows, and so on. An additional approach could be to use a ranking method that classified that days in function of particles that have been produced.

Monsoon NPF mechanism and NPF mechanism in general:

The study says that monsoon NPF is mainly associated with SO₂ availability and therefore H₂SO₄ formation. This is can also be reasonable, but since the authors couldn’t measure all the parameters such as sulfuric acid, or bases such as ammonia or amines, or organic molecules and their oxidation products. SO2 is clearly quite high and therefore can be a reason for the NPF. One possibility could be to calculate Sulphuric Acid proxy from SO2 and see how it compare to the formation rate. Because of all of that I would say that the interpretation that monsoon NPF is sulfur-precursor-driven should be softened unless additional analysis such as at least Sulphuric Acid proxy Vs Formation rate. Also, the paper discusses possible VOC-related oxidation in the pre-monsoon season but cannot constrain it. We understand that these measurements are very complicated, especially in such location, however, considering recent literature on HOMs/OOMs in mountain and TP environments, the author should be more critical in highlighting these limitations. They have a beautiful data set also without going too much in details on the NPF mechanism.

Urban primary emissions vs NPF:

I think it would be good if the authors could try to better separate regional NPF from local primary ultrafine particle emissions. This could include analysis of wind direction, BC/NO₂/SO₂ spikes, rush-hour timing, and whether events occur simultaneously across broader size ranges. Considering the unique location this would add strength and very interesting new information to the study.

Other minor comments:

Line 25 – Define TU. I guess you mean TP.

Figure 1 – “Tibetan Plateau” is not visible. Also, define TU in the caption.

Figure 2 – Fix the Panel a Y-axes description. Caption is missing explanation.

Figure 3 – fixe axes labels here as well. Too many overlaps.

Caption in general could contain more information related to the figures.

The manuscript describes a dataset of particle number size distribution, precursor vapors and meteorology measurements at an urban high-altitude site. While the dataset is limited to two campaign-wise measurement periods of one month each, the dataset and its analyses provide valuable information on atmospheric new particle formation from a location not studied in this aspect before. Therefore, the manuscript is well in the scope of Atmospheric Chemistry and Physics. However, as pointed out by Referee #2 there are certain aspects in the present version of the study that would need to revised before the manuscript can be accepted for publication. I suggest the authors take into consideration the issues raised by Referee #2. My additional comments are following:

1. I would recommend removing the distinction between NPF event Class I and Class II, and suggest these to be called NPF events. As seen from the supplement figures S1 and S2, there seem to be some days (Oct 26, March 25 and March 26) classified as Class I days, which by definition of Class I should show uninterrupted growth of particles but there are clear gaps in the measured PNSD data. On the other hand, some days (at least Oct 10 and Oct 19) seem to have quite continuous growth of particles, but these days have been classified as Class II days.
2. Particle formation rates J3 and J7 seem to have almost identical magnitude and diurnal variation (Fig 6) for Class I and Class II events. This would also support having only one NPF event class (combination of the Class I and Class II days).
3. Have the authors compared the calculated CCN concentrations (calculated assuming const. hygroscopicity and supersaturation) to the measured accumulation mode number concetrations N_100-700, which is in many studies used as proxy for CCN? Based on Figures 5d and 10a, the diurnal variation of both these seems similar but the calculated CCN concentrations seem to be higher.
4. Please define all symbols used in Equations 6 and 7