the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Sep 2025
Quiet New Particle Formation is a significant aerosol source in the Amazon boundary layer
Abstract. Aerosol particles formed by new particle formation (NPF) are essential for cloud condensation nuclei and can strongly influence cloud properties and climate. However, the mechanisms behind NPF in the Amazon boundary layer have remained elusive. Classical “banana” NPF events, common in other continental regions, are rarely observed in the Amazon, while most detected sub-50 nm particles have been linked to precipitation- and downdraft-related episodes, often called Amazonian banana events. Here, we analyse a decade of particle number size distributions (10–420 nm) from the Amazon Tall Tower Observatory (ATTO) during the wet season and demonstrate the presence of a distinct phenomenon called Quiet NPF. This process represents a subtle but persistent background particle formation, occurring on days without clear banana-type growth signatures. Using a statistical approach, we show that Quiet NPF links freshly formed 10 nm particles to growth into the Aitken mode. This mechanism is characterized by a growth rate of 2.3 ± 0.1 nm h⁻¹, about half that of Amazonian banana events, but occurs much more frequently. Quiet NPF accounts for ~45 % of 10–25 nm particle production during the wet season, revealing an overlooked but important source of nanoparticles that contributes to sustaining Amazonian aerosol populations.
Competing interests: Some authors are members of the editorial board of the journal Atmospheric Chemistry and Physics.
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
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4581', Anonymous Referee #1, 01 Nov 2025
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RC2: 'Comment on egusphere-2025-4581', Anonymous Referee #2, 10 Nov 2025
“Quiet New Particle Formation is a significant aerosol source in the Amazon boundary layer” by Meller et al. describes a phenomenon that the authors associate with “Quiet New Particle Formation (NPF).” Quiet NPF was first described by Kulmala and colleagues and refers to NPF and growth events that are almost undetected due to their low number concentrations but are nonetheless important as they are believed to occur on days that were previously assigned as non-NPF event days. This is an interesting study from an important field site. While I appreciate the brevity of this manuscript, I feel that the authors have left out some key details. Please see the following comments (specific questions are preceded by line number).
One major question I had upon reading this manuscript was whether the authors are assuming that quiet NPF is occurring every day that would normally be assigned as a non-event day. Figure 1 shows average size distributions, and in several places the analysis suggest that non-event days are significant and relevant to quiet NPF, but there is never anything said directly about this. Relating to this are comments 1-5:
- 64: This seems like a critical point to bring up the assumption that quiet NPF is NOT observed by viewing individual days, and that for the remainder of the manuscript the assumption is that this phenomenon is happening every day that a classical event is not observed. If this is not a correct assumption, then I am missing an important point of this paper and that, too, should be addressed.
- 88: If my point for line 64 is made clear, then it will be understandable why this potential temperature analysis is being performed on all non-event days. As it is currently written, it seems to suggest that all non-event days are relevant to quiet NPF. I would argue that this point needs to be clearer here.
- 102: It would be helpful to the reader to remind them that this growth rate analysis is being performed on the average normalized size distribution.
- 121: This line is the clearest indication thus far that the average properties of quiet NPF are being uniformly applied to all non-event days. But is it true that ALL non-event days have quiet NPF? What if this were not the case? It seems, to me, that average properties can be used for analysis of such things as growth rates, but individual days can be analyzed for whether or no quiet NPF could have occurred.
- 148: This statement requires the assumption that quiet NPF occurs on all days that are classified as non-events. This assumption again needs to be stated clearly and either be a caveat or justification needs to be provided.
Other questions are as follows:
6. 126: I am confused about what the dN/dt term is. The only description is that it “reflects the temporal distribution of the particle number,” however it appears to be plotted together with coagulation and growth. This needs to be clarified in the manuscript.
7. 130: Is the coagulation term really just one pixel in amplitude in Figure 3? Why is it so consistent across the entire day?
Citation: https://doi.org/10.5194/egusphere-2025-4581-RC2
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Dataset of Quiet New Particle Formation is a significant aerosol source in the Amazon boundary layer B. B. Meller et al. http://ftp.lfa.if.usp.br/ftp/public/Temp/Edmond_Meller2025_Paper/
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Please see comments in attached pdf file.