| 06 Oct 2025
Unveiling the organic contribution to the initial particle growth in 3–10 nm size range
Abstract. Organic compounds play an important role in driving atmospheric particle formation and initial growth along with sulfuric acid. However, the detailed chemical composition of newly formed particles remains limited due to analytical challenges. In this study, we conducted the laboratory experiments using a flow tube reactor to investigate the roles of sulfuric acid (SA) and oxygenated organic molecules (OOMs, from α-pinene oxidation) in nanoparticle growth. For the first time, the size-resolved hygroscopicity parameter (κ) and organic mass fraction (forg) of 3–10 nm particles were measured using a custom-built scanning flow condensation particle counter (SFCPC). The hygroscopicity of SA decreased 49 % as particle size increased (from 0.413 ± 0.011 at 3 nm to 0.209 ± 0.004 at 10 nm) and declined by up to 18 % with increasing RH, primarily due to hydration effects. In contrast, the κ values of OOMs increased with RH by as much as 57 %, attributable to changes in oxidation product. Size-resolved forg revealed that larger particles contained a greater proportion of organics, indicating that SA dominates the growth of small particles, whereas OOMs contribute more significantly to growth at larger sizes. Moreover, elevated humidity enhanced the organic contribution to particle growth by up to 81 %. This enhancement was more pronounced for 5–10 nm particles due to the incorporation of increased yields of more volatile oxidation products and Kelvin effect. These valuable information on hygroscopicity and chemical composition of 3–10 nm particles during new particle formation and subsequent growth could further the understanding of related atmospheric mechanisms.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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This manuscript investigates the contribution of sulfuric acid and organic vapours (essentially alpha-pinene oxidation products) on the growth of sub-10 nm size particles as a function of particle size and relative humidity based on laboratory experiments. The investigation appears to be robust and novel enough to be published in a scientific publication. There are a few issues, however, that should be addressed before final acceptance of this work.
Scientific issues
Line 17-18: Hydration effects and changes in oxidation products are given as scientific facts for explaining the observations in the abstract. However, when reading the paper, it appears that these explanations, while likely, are speculations rather than facts. I recommend that the authors pay more attention what is interpreted as a scientific fact, a probable explanation, or speculation when discussing the observations.
Lines 59-66: This paragraph lists what will be done/investigated in the paper. The authors should also give either concrete scientific goals of the study or, alternatively, research questions aimed to be answered here.
Lines 249-250: This statement (This behaviour is attributed to …) is given without a proper justification. The authors should add some reasoning(s), or at least speculations.
Technical issues:
Lines 126-129 and 194-196: Although the procedure of determining the OOM mass fraction is relatively straightforward, the paper would benefit from having an example plot on how this works in practice for the data applied here.
Line 169: Differ from --> different from
The given values in percent on lines 198 and 218 are too accurate. I would recommend using an accuracy of 1% or, in maximum, the accuracy of 0.1%.