the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Oct 2025
Oxidation-driven acceleration of NPF-to-CCN conversion under polluted atmosphere: Evidence from mountain-top observations in Yangtze River Delta
Abstract. To what extent the new particle formation (NPF) contributed to the CCN remained unclear, especially at the boundary layer top (BLT) in polluted atmosphere. In this study, measurements at a mountain top background site in southeastern China during spring 2024 quantified NPF growth dynamics under different air masses, exploring the nucleation mechanism and its potential contribution to CCN. Eight NPF events were observed, and three of them occurred in the polluted conditions (NPF-P) which associated with regional transportation while the rest five events appeared in the clean conditions (NPF-C). The average formation rate (J2.5: 2.4 vs. 0.7 cm⁻³ s⁻¹) and growth rate (GR: 6.8 vs. 5.5 nm h⁻¹) were significantly higher in NPF-P compared to NPF- C, accompany by higher concentrations of sulfuric acid and ammonia, suggesting the important role of ammonia that enhancing sulfuric acid nucleation. In addition, much higher CCN enhancement factor was observed in NPF-P (EFCCN: 1.6 vs. 0.7 in NPF-C) due to the regional transported of anthropogenic pollutants from the urban cluster regions and their secondary transformation under enhanced atmospheric oxidation capacity. Furthermore, the duration of NPF-to-CCN conversion was quantified using “Time Window (τ)”, revealing polluted condition accelerated NPF-to-CCN conversion by 17.0 % (τ = 16.4 h vs. 19.8 h), with nitrate playing an important role in maintaining rapid GR and compressing τ, thereby enabling clouds to form more readily during NPF days. These findings reveal that polluted air masses enhance both the efficiency and speed of CCN production at the BLT through elevated atmospheric oxidation capacity.
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Status: open (until 03 Dec 2025)
- RC1: 'Comment on egusphere-2025-4901', Anonymous Referee #1, 06 Nov 2025 reply
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This manuscript investigates CCN production associated with atmospheric new particle formation using a few observed cases at a mountain-top measurement station. The scientific approach appears to be robust, and the paper has some novel features, such as introduction of the "time window" concept. Interpretation of the results requires, however, considerable revisions here and there. The language of the paper requires also improvements. My detailed comments in this regard are given below.
Scientific issues
Lines 29-32 in Abstract: While changes in CCN concentrations influence many cloud properties via changes in cloud droplet concentrations, the ability of a cloud to form is not really dependent on CCN concentrations (unless CCN are missing altogether, which is extremely rarely the case). I recommend mentioning changes in cloud properties in this context, not talking about cloud formation.
Lines 218-219: Talking about hygroscopic growth in this context is misleading. Being a CCN at a given supersaturation (SS) is an aerosol property, dictated by aerosol size and chemical composition. Becoming a CCN means increasing either particle “dry” size (by condensation growth) or its hygrospicity to a sufficient degree. This should not be mixed with particle hygroscopic growth which happens when CCN respond to ambient RH or are activating into cloud droplets.
Lines 311-313: The logic of this statement does not work. The growth rate of a newly formed particle is determined solely by the condensation flux of low-volatile (and partly semi-volatile) compounds into it, being influenced mainly by the gas-phase concentration of such compounds. To a first approximation, the hygroscopicity of the condensing compounds do not matter in this context, as growth rates are usually determined for “dry” particles or for particles at low RH. It is true that the hygroscopicity of the condensing material influences the size of a growing particles at elevated RH, so that particles having more hygroscopic material have a higher “wet” size, which enhances the condensation flux and thereby growth rate of particles to some extent. But this a secondary effect compared with gas-phase concentrations.
Lines 326-327: Besides these theoretical calculations and laboratory experiments, there is also evidence form field measurements on the involvement of ammonia/amines. I recommend citing also field evidence in this context.
Line 359: Was there some specific reason for why only a single SS (0.2%) was used in the calculations. Having two or even more values of SS in consideration may have given additional information on how newly formed particle reach CCN sizes. This is especially so when noting that with typical particle growth rates, it takes quite a while until newly formed particle grow into sizes relevant for CCN at 0.2% (at higher values of SS, which are certainly possible, newly formed particles become CCN much quicker).
Lines 362-364: The logic of this statement does not work either. It is true that for 2 particles of similar size, the one with a higher organic fraction requires a higher SS to activate into a cloud droplet (as organics tend to have much lower hygroscopicity than the main inorganic compounds). However, a higher organic fraction is not expected to suppress condensation of other material from the gas phase (not even water unless these organics reduce e.g. the accommodation coefficients of condensing water). Please modify.
Lines 390-393: It is unclear to me how the degree of oxidation of organics can be estimated from VRF in mixtures of inorganic and organic compounds?
Lines 505-507: There is something strange in this sentence. I am able to understand what is meant to be said.
Technical issues
The paper has several typos, especially in section 3, which should be corrected in the revised version. Just a few examples of these: (line 231: event is, line 247: particles, line 248: the in-cloud, line 282: which is close to, line 296: GR values are higher by, lines 402-403: non-volatile)
The percentages given in the paper appear overly accurate (one digit, e.g lines 289, 292 and 294, but also elsewhere). Maybe 1% accuracy would be more relevant here.
A similar accuracy issue concerns critical diameters in lines 360-362 (4 digits too many).
All the figures should be understandable based on available figure legends and figure captions, and the figures should be consistent with what is said in the text. This is not the case for many of the figures:
In Figure 1, it is not mentioned which one (a or b) corresponds to clean or polluted conditions (I suppose a is clean and b is polluted based on the distributions).
In Figure 2g, it is not explained which bars represent J and which ones GR (this info can now only be gotten by reading the main text).
The text says (lines 345-346) that Figs. 3b and 3c give J as a function of SA and ammonia product, but this is not true for Fig. 3b.
The text says (lines 373-374) that Figs. 4b reveals something about the coupling between critical diameter and temporal particle size evolution. This is practically impossible to see from Figure 4b alone, but it requires additional information from other figures and text.
In Figure 5b, the 3 red cases seem to refer to polluted ones, while the blue cases correspond to clean ones. This is not explained in figure caption, which is confusing especially as red and blue mean totally different things in Figure 5a.