| 07 Aug 2026
Comment to "Detecting supramolecular organic nanoparticles during heat wave by Zhang et al."
Abstract. Atmospheric new-particle formation is a major source of aerosol particles that influence air quality, cloud properties, and climate. Understanding the molecular mechanisms governing the initial steps of particle formation is therefore essential for accurately representing aerosol formation and its climatic effects in atmospheric models. Zhang et al. (2026c) recently proposed that hydrogen-bond–driven self-assembly of neutral carboxylic acids is a spontaneous and ubiquitous atmospheric new-particle formation mechanism. If correct, this would represent a fundamental shift in the current understanding of atmospheric nucleation.
In this comment, we show that the proposed nucleation mechanism is not adequately supported by the observations or theoretical analysis presented in Zhang et al. The reported particle-composition measurements lack sufficient validation to establish the proposed molecular composition of the smallest particles and do not directly constrain the molecular processes responsible for the earliest stages of particle formation. Furthermore, using the thermodynamic data reported by Zhang et al., we demonstrate that cluster evaporation overwhelmingly exceeds growth by molecular collisions, resulting in negligible particle-formation rates under the reported atmospheric conditions. These kinetic and thermodynamic analyses demonstrate that hydrogen-bond–driven clustering of neutral carboxylic acids cannot explain the reported observations and is unlikely to represent an atmospherically relevant new-particle formation mechanism.
Review of “Comment to Detecting supramolecular organic nanoparticles during heat wave by Zhang et al."
Myllys et al comments on the recent 2026 paper entitled “Detecting supramolecular organic nanoparticles during heat wave” by Zhang et al, published in Science. Zhang et al proposed that self-assembly (hydrogen bonding) of carboxylic acids is a spontaneous process leading to new-particle formation, even during extreme heat events. The submitted comment present evidence of misinterpretation of the experimental and theoretical results, essentially falsifying the conclusions of Zhang et al.
When I first read the study by Zhang et al, my immediate thought was that this paper completely ignores the immense knowledge obtained on organic enhanced cluster formation during the past two decades.It is by now well-established that it is most likely covalently bound dimers (accretion products) that drive organic nucleation at ombient conditions. In particular, Figure 5 in the paper by Zhang et al is highly misleading, as it only presents the standard free energies giving the impression that the clusters are stable. Considering the evaporation rates, or calculating the equilibrium cluster concentrations, would immediately demonstrate that the reported clusters are not sufficiently stable to explain the proposed mechanism. Hence, I am pleased to see the authors submit this critical comment, as I believe it exposes an important issue with the published paper. The comment is written by a strong team of highly qualified professionals. The critical analysis is technically sound, and the comment can essentially be published as is. I only have a few minor optional comments for the authors below.
Comments
Line 12: “These kinetic and thermodynamic analyses demonstrate that hydrogen-bond–driven clustering of neutral carboxylic acids cannot explain the reported observations and is unlikely to represent an atmospherically relevant new-particle formation mechanism.”
To be on the safe side, perhaps state that this refers to the “studied neutral carboxylic acids” in Zhang et al. We have recently shown that tricarboxylic acids might contribute to NPF, so there might be very special cases where neutral tricarboxylic acids can actually nucleate (if they exist).
Line 33: “These ∆G values appear unrealistically favorable, …”
I had the exact same thought when I first read the study. This is substantially more favourable than what have previously been found for carboxylic acid-carboxylic acid interactions. In addition, while addressed on page 7, it might also be worth pointing out already here that the free energy analysis is carried out at 298.15 K, while the paper states the mechanism to be relevant for up to 311.15 K (up to 38°C). This would further destabilize the studied carboxylic acid-carboxylic acid clusters.