the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Sep 2025
Investigating small ion number size distributions: insight into cluster formation and growth
Abstract. Small ions, consisting mostly of charged molecular clusters with mobility diameters below 2 nm, exist continuously in the atmosphere. Here, we studied small ion number size distributions measured with Neutral cluster and Air Ion Spectrometer measurements in Hyytiälä, Finland and Beijing, China. We found that in Hyytiälä, there is a strong positive relationship between the concentration and diameter of small ions of both polarities and highly oxidized organic molecule (HOM) and sulfuric acid concentrations, and that the relationship with the former is especially strong. The relationship between the negative sulfuric acid cluster ions and the small ion number size distribution in Hyytiälä was found to be more complex, but overall positive. In contrast to Hyytiälä, we found that in Beijing the small ion number size distribution does not have a clear relationship with sulfuric acid or oxidized organic molecule (OOM) concentration. However, in both locations, the impact of growth on the small ion number size distribution during periods of intense cluster formation and new particle formation is clearly seen.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Aerosol Research. Authors have no other competing interests to declare.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
(4175 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 05 Nov 2025)
- RC1: 'Comment on ar-2025-28', Anonymous Referee #1, 14 Oct 2025 reply
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 168 | 12 | 9 | 189 | 11 | 14 |
- HTML: 168
- PDF: 12
- XML: 9
- Total: 189
- BibTeX: 11
- EndNote: 14
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The authors present data from two sites with long-term observations of small ion number size distributions of both polarities, focusing on very small clusters of <1 nm to 2 nm in diameter. Complementary observations of sulfuric acid and organic molecular clusters are used to examine their impacts on the dynamics of cluster growth toward new particles. The two sites, Hyytiälä (remote forest) and Beijing (polluted urban), contrast in their environments and these contrasts are reflected in different dependencies of cluster dynamics on the measured precursors. These differences are illustrated with case study examples.
The data sets are interesting and unique, particularly in that they span multiple seasons and years. The introductory discussions of charged particles in the atmosphere and the charged general dynamic equation are well written and helpful in providing background for non-specialists. The observations are presented clearly, but the discussion and interpretation seemed less in-depth than suggested by the title “Insight into cluster formation and growth”. Relationships are pointed to, but it’s not entirely clear whether and how they shed new light into what is known already about cluster formation and growth, or how they might be applied to predict processes in other locations.
With the presentation and discussion of equation (2), I expected the data would be interpreted in that context, considering the different terms’ influence on the evolution of the cluster sizes and particularly how the two sites support different magnitudes of some of the rates. I was therefore surprised that the analysis did not consider coagulation scavenging (CoagS). The discussion on lines 95-109 seems to indicate that this rate plays a key role, competing with the growth rate (GR) in shaping the small ion size distribution. Can the authors explain and justify why this term was not included in the analyses (Lines 108-109)? In particular, I anticipated that Beijing, as an urban region with higher particulate and precursor concentrations, would have lower overall ion concentrations due to competition for the ions by the coagulation sink. Indeed, Figure 2 shows that Beijing had lower number concentrations - but a rationale for this observation, supported by the data, is not provided here. On lines 342-344, the authors note that “Due to the high concentrations of both low volatility vapors and large particles, the dynamics of small ions in a megacity such as Beijing are different than in a rural site such as Hyytiälä.” This, and other observations that note that the sites are “different”, seems reasonable; but as an expected result, that isn’t further explored with the dataset, it doesn’t rise to the level of “insight”.
On line 421, the authors note: “In Beijing, CoagS is crucial in determining whether the growing clusters will survive to larger sizes or not.” The subsequent section goes on to use new particle formation (NPF) ranking as a means of sorting for conditions supporting growth of cluster into larger sizes. If CoagS could not be determined explicitly and used as a sorting variable, but NPF ranking is a suitable proxy, this argument could be motivated and explained in the introduction. That would help explain more explicitly the reasons for the use of NPF ranking and would provide a clearer link to equation 2.
I suggest the authors also address two other points that might be helpful to make explicit. First, what do the ion polarities depend on? Are some molecules preferentially charged with one polarity over another and does composition play a role? The role of polarity in cluster dynamics and growth is not addressed extensively, so it wasn’t clear why data were separated into positive and negative polarities for analysis and what should be gleaned from that, versus presenting the distributions of all charged ions together and how these totals trend with, e.g., sulfuric acid concentrations. Would different diurnal and seasonal trends in dominant polarity be expected, why, and what impacts on cluster dynamics are expected?
Second, the role of transport and meteorology is not included explicitly in the discussion. The assumption seems to be that the observations are very localized (e.g., line 457) and can be interpreted as such; if this is the case, it would be helpful to explicitly state this and provide some justification. For example, is this approach justified because the lifetimes of these ions are so short? Some of the observations do seem to suggest that impacts from other influences are seen in the data (e.g., line 491 mentions discontinuous growth in a Hyytiälä case study; Lines 539-540 suggest changes in ambient aerosol and meteorology affected the Beijing case study). If the behaviors are highly localized, what is their broader relevance?
Overall, the data sets are very interesting and report characteristics of an understudied class of atmospheric particles. I suggest the manuscript and its utility for the community would be enhanced if the authors provide some additional interpretation (perhaps shortening the discussion that just points out trends, or move some to the Appendix), addressing the comments above.
Minor comments:
Line 328: “compared to results for Hyytiälä, the differences in the [Beijing] size distributions are small for either polarity.” This refers to Figure 6a, compared with 5a or 3a or 3. This confused me, because in Figure 6a the shift in the peak diameter between positive and negative polarities looks larger than those in the other two figures.
Line 81: should be “the changes IN”
Line 120: should be “from which data WERE”
Line 131: should be “WERE also used”
Line 171: should refer to TABLE 1 (not Table 2)
Line 197: should refer to TABLE 2 (not Table 3)
Line 290: note to author left in “add percentiles here”
Line 582: remains to be IDENTIFIED