This manuscript proposes and calibrates a method to measure size‑resolved condensation sink (CS) using an ELPI+ (diffusion charger + cascade impactor) by deriving stage‑specific conversion factors from measured electric current to CS. The method is then applied across multiple urban environments (Finland, Germany, Czechia, India), and the authors introduce CS0.1 (CS attributable to particles <100 nm) as a metric potentially relevant for cloud activation and health (lung deposition) considerations. The paper addresses a relevant and timely gap, moving beyond total CS to size-resolved CS and demonstrating its practical measurement and interpretive value across diverse settings. However, methodological assumptions (notably effective density, condensing vapour choice, and accommodation coefficient), uncertainty quantification, and validation depth need to be strengthened before publication in ACP.

Major comments

The CS derivation relies on assumptions about effective density (ρ_eff) for converting between mobility and aerodynamic diameters, condensing vapour properties (sulfuric acid), mass accommodation coefficient (assumed = 1), and ambient T/P conditions; all of these feed into Equations (2-7) and the stage‑specific conversion factors (Table 1). While sensitivity to ρ_eff is qualitatively discussed and conversion factors are tabulated, the manuscript lacks a consolidated quantitative uncertainty budget (per stage and total), including contributions from (i) ρ_eff choice and its size‑dependence, (ii) accommodation coefficient, (iii) vapour selection (H₂SO₄ vs. semi‑volatile organics), (iv) temperature/pressure deviations from the assumed NTP, and (v) ELPI+ charging model/flow tolerances. Please provide a formal error propagation and a sensitivity analysis. This is crucial for interpreting inter‑site differences and the CS0.1 metric.

Minor comments

* The manuscript argues that size‑resolved CS can improve climate/air‑quality modelling. Can you specify why this would improve climate modelling and how it would improve climate direct or indirect effects?

* Figures do not include error bars or shaded envelopes, could you somehow estimate errors of the results?

L56-58 – NPF events have also been observed in polluted environments (e.g. Kulmala et al., 2017) or particle loadings (e.g. Casquero-Vera et al., 2023), so the CS is expected to decrease NPF occurrence but depending on the CS efficiency (e.g. Tuovinen et al., 2021). I think those ideas are also important to include, since it shows the current lack of knowledge on the effect of CS in NPF events occurrence.

L59-69 – Following the major comment, here the text suggests that diffusion‑charger–based instruments (ELPI+) better capture surface area without requiring shape/density assumptions, unlike DMPS. However, ELPI sizing still involves effective‑density assumptions and sticking efficiency. Could the authors clarify whether ELPI‑based CS measurements (total and size‑resolved) avoid or still rely on particle density/shape assumptions, and how these uncertainties compare to DMPS? It is not clear this point when later the authors use sulfuric acid as the reference vapor.

References

Casquero-Vera, J. A., Pérez-Ramírez, D., Lyamani, H., Rejano, F., Casans, A., Titos, G., Olmo, F. J., Dada, L., Hakala, S., Hussein, T., Lehtipalo, K., Paasonen, P., Hyvärinen, A., Pérez, N., Querol, X., Rodríguez, S., Kalivitis, N., González, Y., Alghamdi, M. A., Kerminen, V.-M., Alastuey, A., Petäjä, T., and Alados-Arboledas, L.: Impact of desert dust on new particle formation events and the cloud condensation nuclei budget in dust-influenced areas, Atmos. Chem. Phys., 23, 15795–15814, https://doi.org/10.5194/acp-23-15795-2023, 2023.

Kulmala, M., Kerminen, V. M., Petäjä, T., Ding, A. J., and Wang, L.: Atmospheric gas-to-particle conversion: Why NPF events are observed in megacities?, Faraday Discuss., 200, 271–288, https://doi.org/10.1039/c6fd00257a, 2017. 

Tuovinen, S., Kontkanen, J., Cai, R., and Kulmala, M.: Condensation sink of atmospheric vapors: the effect of vapor properties and the resulting uncertainties, Environ. Sci.-Atmos., 1, 543–557, https://doi.org/10.1039/D1EA00032B, 2021.