The manuscript entitled „New particle formation contribution to aerosol mass: regimes and implications for future urban environments” by Runlong Cai and coauthors presents an analytical approach to infer the mass contribution of nanoparticles formed by gas-to-particle conversion to aerosol loading under varying particulate pollution levels. To this end, three different regimes based on competing growth processes have been identified. In this context, condensation sink, mass accommodation coefficient, vapor source strength and number concentration of newly formed particles seem to be key parameters. The analysis suggests that under pristine background conditions like in boreal forest NPF does contribute substantially to aerosol mass, whereas in heavily polluted environments typical for East Asian megacities NPF does typically not play a pronounced role in aerosol mass concentration. However, due to the competition between condensation sink from background particles and emerging condensation sink related to the growing new mode this may eventually also allow particles from NPF to become a relevant contributor to aerosol mass. In view of decreasing primary emissions this effect may thus become important under future clean atmospheric conditions. Overall, my assessment of this manuscript is quite positive as it is well presented and the topic clearly fits the scope of the journal ACP. A few points mentioned below should, however, be considered before final publication.

One of the conclusions drawn, and also highlighted in the abstract, refers to the need of reductions in NPF precursors in order to achieve sustained reductions in aerosol mass loading. Since biogenic precursors will also contribute substantially to NPF it would be good to address pathways how such reductions in NPF precursors could be realized.

On page 7, lines 17-23, a somewhat more detailed discussion of the meaning of alpha_bg would be appreciable. This is partly due to terminology reasons but also because of some physically controversial behavior. On the one hand the authors use alpha_bg as a parameter in their calculations for mass accommodation coefficient, on the other hand they refer to effective uptake coefficients substantially below unity. What is the relation between accommodation and uptake coefficient? Furthermore, while I can follow the arguments for reduced vapor uptake due to unfavorable surface composition, the size effect is quite counterintuitive. Kinetic effects play a much more important role at smaller (~10 nm) particle sizes than at ~100nm where particles are in the transition regime and will also (at least partly) benefit from diffusion. What justifies the assumption of such low values of alpha_bg for 100 nm particles? Have some of the papers from the referenced literature (or other literature) seen such size dependent effects from experiments (decreasing mass accommodation coefficient with increasing size)?

Conclusions, page 8, line 17: To me the relation of high mass from new particles and CCN fraction is not clear. The impact of NPF on CCN is usually argued via number concentration. I guess a significant mass contribution under conditions representative for Hyytiälä is rather due to low CS/background. Some clarifying sentence here might help the reader in better understanding this point.

Editorial remarks/suggestions:

Abstract, page 1, line 14: “…NPF events in urban Beijing occur…”

Page 2, line 5 and page 6, line 2: I guess the word “competence” should be replaced by “competition”

Introduction, page 1, line 30: “…particle mode, after sufficient growth, and background…”

Introduction, page 1, line 31: The authors refer to “a prevailing hypothesis”. Is there a reference for this hypothesis?

Section 2.2, page 3, line 30: “covert”--> “convert”

Section 2.2, page 3, line 37: “… contributes negligibly due to…”

Section 3.1, page 4, line 21/22: “…reaching sizes (e.g., ~100 nm) that significantly contribute to aerosol mass after growth…”

Figure caption Fig. 1, page 5, line 10: model input parameters are presented. Are these values representative of a certain environment (e.g., Hyytiälä)? Or what were the reasons to choose these numbers?

Figure caption Fig. 2, page 7, line 1: model parameters are mostly very similar to values in Fig. 1 except for N_np0 which is higher by one order of magnitude. Is this correct or a typo?

Page 7, line 30: “Event” --> “Even”

Page 7, line 33: “which whether” does not make sense. Please reformulate.

Figure caption Fig. 3, page 8, line 5: “Statistical results of measured NPF events in relation to N and CS.“

Conclusions, page 8, line 15: would there be a reference to support the statement: “success in previous control of aerosol mass by reducing primary emissions”?

Conclusions, page 8, line 24: “… feedback is suppressed by the..”

The manuscript "New particle formation contribution to aerosol mass: regimes and implications for future urban environments" by Cai et al. proposes a conceptual regime framework for quantifying the contribution of new particle formation (NPF) to atmospheric aerosol mass, using a zero-dimensional aerosol dynamic model constrained by long-term observations from two different locales: urban Beijing and the Finnish boreal forest site at Hyytiälä. This work is important because it addresses a long-standing and genuinely open question of whether NPF events meaningfully contribute to aerosol mass or whether condensable vapors would simply partition onto background particles regardless. As such, it offers a useful framework that maps NPF mass contributions across regimes governed primarily by the background condensation sink. 

The authors' transparency about the simplifying assumptions underlying their model, including the constant vapor source hypothesis and the choice of mass accommodation coefficient, is appreciated and adds clarity to the scope of the work. However, in several key places the conclusions, particularly those with direct policy implications, are stated with a level of confidence that is difficult to fully justify given the acknowledged limitations of the modeling framework. Overall, this study presents a valuable conceptual contribution that merits publication, but requires major revisions to better align the strength of its conclusions with the evidence provided, as described in detail below.

1.    The abstract currently states that "control of NPF precursors represents an effective strategy for present and further particulate matter pollution." This is a direct policy recommendation derived from a model using a constant vapor source that the authors explicitly call "not supported by atmospheric measurements." The authors must qualify this statement, for example, framing it as a hypothesis warranting further investigation rather than a prescriptive conclusion. Similarly, the claim in the conclusions that "ongoing reductions in primary aerosol emissions, if not accompanied by equivalent reductions in NPF precursors, may disproportionately amplify NPF contributions to particle mass" should be clearly framed as model-dependent and conditional, not as a robust projection.

2.    The default value of alpha_bg = 0.1 is used throughout most of the main simulations, yet laboratory measurements cited in the paper itself span 0.01 to 1.0, and Liu et al. (2019), which is also cited, reports near-unity values for diverse aerosol types. Figure S11 shows clearly that the regime boundaries and NPF mass contribution ratios shift substantially across this range. The authors should either provide a stronger atmospheric justification for alpha_bg = 0.1 as a representative default, or present results more symmetrically across the full measured range rather than treating 0.1 as the primary scenario. As it stands, the default choice systematically favors larger NPF contributions relative to what higher alpha_bg values would produce.

3.    In Section 3.1, the authors report average mass concentrations of 39.7 ug/m^3 for the growing mode in Beijing, with values approaching ~100 ug/m^3 during severe haze, and use these figures to motivate NPF's significance to aerosol mass. However, just a few sentences later they acknowledge that "particles in the observed growing mode also originate from primary particle emissions and transport." These two statements are in direct conflict. The authors should more explicitly caveat the observational statistics in Figures S1 and S2, clarifying that these mass concentrations cannot be cleanly attributed to NPF alone. Presenting them as upper-bound estimates of NPF-relevant particle mass, rather than as direct NPF mass contributions, would be more scientifically defensible.

4.    The authors use the scatter in Figure S4, which spans roughly three orders of magnitude in growth rate, to conclude that "the rate of gas-to-particle conversion is not majorly limited by the competence of vapor between the two modes." This interpretation is then used to justify the constant vapor source assumption that underpins the entire model. However, the large scatter in Figure S4 is equally consistent with day-to-day variability in vapor sources, photochemical activity, or primary emission influences on particle sizing. The authors should acknowledge these alternative explanations and avoid using this ambiguous dataset as primary justification for a core modeling assumption.

5.    The thresholds of ~0.001 /s and ~0.01 /s separating Regimes I, II, and III are discussed throughout the paper as though they are generalizable atmospheric benchmarks. However, Figure S11 demonstrates clearly that these boundaries shift substantially with alpha_bg and Qv. The authors should more prominently and consistently qualify these thresholds as being specific to the parameter choices used in the main simulations, and caution against their direct application to other environments without recalibration.

6.    Currently, limitations are scattered throughout the text -- the unsupported vapor source assumption appears briefly in Section 3.1, the alpha_bg sensitivity is discussed in Section 3.2, and the mode separation problem appears in the Introduction. A dedicated limitations section would consolidate these acknowledgments and help ensure readers appropriately contextualize the conclusions. This section should explicitly address: the zero-dimensional model's inability to capture spatial heterogeneity and boundary layer dynamics; the constant vapor source assumption and its known departure from observations; the single-vapor representation; the fixed 48-hour simulation window with constant inputs; and the restriction of observational validation to just two sites.

7.    Given that the constant vapor source is explicitly acknowledged as a "conservative lower bound" and "not supported by atmospheric measurements," the authors should include at minimum a systematic sensitivity analysis testing how the regime boundaries and NPF mass contributions change under a range of vapor source scenarios, including temporally varying sources more representative of real urban diurnal cycles. This would help quantify the uncertainty introduced by this assumption and give readers a clearer sense of whether the conclusions are robust.

8.    The conclusions and Figure 4 extrapolate the regime framework broadly to "future urban environments" including East Asia and South Asia. However, the observational constraints come from only two sites -- urban Beijing and the Finnish boreal forest -- which represent very specific atmospheric regimes. The authors should either explicitly limit their geographic claims to environments similar to those studied, or provide some discussion of whether the regime boundaries and framework could be expected to hold under the substantially different emissions, chemistry, and meteorology of South Asian cities, European urban environments, or tropical regions.

Minor errors that need correction:

Abstract, line 14: "NPF events in urban Beijing occurs mostly" should be "occur mostly."

Abstract, line 16: "events at a Finnish forest site falls in regimes I and II" should be "fall in regimes I and II."

Abstract, line 17 (and repeated in the Conclusions): "an effective strategy for present and further particulate matter pollution": "further" should be "future."

Section 2.1, p.2: "Measurements in Beijing was conducted" should be "were conducted." The same error appears one sentence later: "Measurements at the Finnish boreal forest was conducted" again should be "were conducted."

Section 2.1, p.2: "provide constrains on typical conditions" should be "constraints."

Section 2.1, p.3: "characterized by a dramatical change in the mode diameter": "dramatical" should be "dramatic."

Section 1 (Introduction), p.4: "vapor uptake is kinetically influenced process" is missing an article; should be "is a kinetically influenced process."

Section 1, p.4: "vapor uptake by large background particles if often retarded": "if" should be "is."

Section 2.2, p.6: "we do not peruse a closed-formed analytical solution": "peruse" should be "pursue," and "closed-formed" should be "closed-form."

Section 3.2, p.7: "Event with the most favorable αbg (0.01)": "Event" should be "Even."

Section 3.2, p.7: "Under such condensations, new particles are rapidly removed":  "condensations" should be "conditions."

Sections 3.1 and 4 (pp.6 and 8, appearing twice): "not majorly limited by the competence of vapor between the two modes": "competence" should be "competition."

Section 4, p.8: "this feedback is suppressed the dominant CSbg" should be "suppressed by the dominant CSbg."

Section 4, p.9: "may not decrease with the same extend to that in CSbg0": "extend" should be "extent," and "to" should be "as" ("the same extent as that in CSbg0.")

Figure 2 caption (main text) and Figure S10 caption (supplementary): "Accomodation coefficient" should be "Accommodation"

Supplementary Figure S3 caption: "Measured evolution of atmospheric particles at on Sep. 8-10, 2019" should be "on Sep. 8-10, 2019."