This manuscript investigates atmospheric new particle formation (NPF) taking place in the central Tibetan Plateau. To my knowledge, there are no prior publications of NPF in this location, so the obtained results can be considered worth publishing. The paper itself if well organized and the conducted analysis appears to be scientifically sound. There are, however, two major issues that require further consideration.

First, the measurements periods are rather short, about 4 weeks for the pre-monsoon season and less than 2 weeks for the monsoon season. As a result, it remains unclear how representative the obtained results are for this location during these two seasons.

Second, many of the conclusions made in the paper rely on SO2 and VOC concentrations. Unfortunately, there are very limited measurements on these 2 trace gases (only VOCs during the monsoon season), instead their concentrations were estimated from large-scale model simulations. The simulated concentrations may have large uncertainties, which are not quantified by any means in the paper.

These two issues, representativeness of the measurements and uncertainties related to SO2 and VOC concentrations, need to acknowledged much better when discussing the results and when making conclusions about NPF mechanisms etc.

Other important scientific comments

The description on how CCN concentrations were calculated (section 2.4) is incomplete. Apparently, the authors used equations 4 and 5 to determine the critical diameters corresponding to different supersaturations, and from these critical diameters one then gets the number of CCN using measured particle number size distributions. However, this calculation cannot be done without knowing the hygroscopicity parameter cappa. Did the authors simply assumed a fixed value for cappa or did they estimate it from some chemical data?

The discussion on the role of condensation sink (CS) in favoring/disfavoring NPF is not logical. The authors first say that their result differs from those found in earlier studies (lines 243-244), but then mention a few studies which actually agree with their findings (lines 245-247). Please reformulate this part of the text, as it causes confusion in its present form.

The discussion on the role of VOCs (lines 266-277) could be improved as well. First, considering the typical variability of VOC concentrations in ambient measurements, I would think a 20% higher VOC concentration is slightly rather than noticeably higher (line 268). It is also confusing that for the pre-monsuun season the VOC concentration difference is given as % while for the monsoon season it is given as an absolute value (ppb).

Wind direction is a very local quantity, and does not necessary tell correctly air pollutant sources or transport pathways. I wonder whether the authors have information on air mass trajectories which would provide more direct support for their statements on lines 283-295.

The enhancements of CCN concentrations due to NPF is reported in 3 different ways in section 3.4: 1) using an enhancement factor EF, 2) using percentage increases, and 3) stating that something is N times higher than... This is confusing. I highly recommend the authors to unify this discussion.

Minor comments:

line 49: I suppose that the authors mean quantities like the particle formation and growth rate as referring to parameters of NPF. I do not feel that parameter is a good wording here, rather suggesting something as characteristics of NPF.

lines 127-128: Classification of a NPF event seems untypical. Has the performance of this classification method tested and has it been used in other studies besides Fang et al. (2020)? The word obviously does not fit into this context.

lines 225-233: The authors list a number of things that potentially affect the occurrence of NPF. The list misses one highly relevant quantity: the intensity of solar radiation. This quantity should be mentioned here.

Unlike the Aitken mode, the nucleation mode is usually written using a lower-case letter (lines 326, 359, 383)

In several places (lines 225, 254, 258, 261, 284, 296, 299, 340), the use of tense is somewhat wrong, or at least uncommon. Please reconsider which tense to use in these places.

New particle formation (NPF) at high altitudes is crucial to understand sources of aerosol and CCN in the free troposphere. In this study, the authors conducted intensive measurements at Nam Co station (4379 m a.s.l) in the central TP to understand the new particle formation during pre-monsoon and monsoon seasons. They identified the frequency of NPF during monsoon seasons was significantly higher than during pre-monsoon seasons. This study did provide valuable observation data. But the explanation that higher VOCs triggered the frequent NPF during monsoon season is unconvincing. Therefore, the manuscript is not recommended to be published on ACP unless the authors can address the following major concerns.

1. Sulfuric acid (SA), Ultra/Extremely Low Volatility Organic Compounds (U/ELVOCs), and bases, e.g., NH3 or DMA, are known as the essential precursor of NPF. Their concentrations determine whether NPF can occur, as well as the intensity. However, in this study, all these key precursors were not measured. Even the precursors of these “direct precursors”, e.g. SO2 and VOCs who can form low-volatile oxidation products, were not well measured either. First, the simulated concentration of SO2 used in this study without any verification by observation data is not convincing. Since SO2 is a very reactive species, one needs to use the simulated value very carefully. Second, although 99 types of VOCs were measured during pre-monsoon using a GC-MS/FID, they are key precursors of ozone formation and are not suitable as indicator precursors for ELVOCs. The author, at least, needs to provide the concentration of monoterpenes, which are well-known sources of ELVOCs. In addition, the simulation of VOCs during monsoon is needed to be verified. In summary, the authors need to provide more solid evidence to support their main conclusion that higher VOCs triggered the frequent NPF during monsoon season.
2. The observation period is a bit too short, especially, with only 10 days during monsoon. One cannot be sure that the high NPF frequency observed during this 10-day observation can be representative of the entire monsoon period.
3. The authors grouped their data into “NPF-pre days”, “NPF-monsoon days” and “non-event days” in Fig. 3- Fig. 7 and related discussions. I may suggest separating “non-event days” into pre-monsoon and monsoon non-event days.