| 03 Mar 2026
Thin Organic Films Unexpectedly Enhance Alcohol Uptake on Soot Analogs: Critical Implications for Aerosol Aging
Abstract. Organic coatings strongly influence how gases are taken up by soot particles, yet the underlying kinetics are poorly understood. Environmental molecular beam experiments combined with time-of-flight mass spectrometry and molecular dynamics simulations were used to examine interactions between butanol clusters and graphite surfaces with thin and thick organic coatings over 180–300 K. Bare graphite shows two desorption pathways: a fast, temperature-insensitive channel and a slower channel peaking near 210–220 K. Thin organic coatings suppress the slow pathway entirely, consistent with rapid formation of a condensed alcohol layer that stabilizes surface-bound molecules. In contrast, thick organic layers enhance slow desorption and shift complete release to lower temperatures, indicating reduced molecular stability on corrugated organic surfaces. Analysis reveals similar activation energies and rate parameters for delayed desorption on graphite and thick coatings, pointing to a shared cluster-mediated mechanism. Translating these kinetics into an effective uptake framework shows that gas-particle exchange shifts between kinetic retention and desorption-limited regimes depending on coating structure and temperature. Simulations further demonstrate how surface morphology and coating thickness control cluster adsorption, reflection, and stability. Together, these findings show that thin organic films on aged soot can strongly enhance retention of semi-volatile organics, while thicker organic layers promote delayed release, with important implications for aerosol aging, secondary organic aerosol formation, and climate effects.
This is a well-written excellent example of how "chemical physics" experiments may be used to gain insight into atmospheric chemistry. The combination of molecular beam scattering results and molecular dynamics simulations is powerful and appropriate. The experiments and simulations are well described and I have no issue with the authors' interpretation of their results.
The question of interest - how do organic coatings on growing SOA particles influence accretion of gas phase organic compounds - is of real interest and importance. One school of thought is that equilibrium partitioning models capture all of the important processes; this (and similar) experiment suggests that the molecular interactions governing the gas-surface collision play an important role before equilibrium is reached. That is to say - one must acknowledge that the timescale for establishing equilibrium is necessarily longer than the collision time scales, so the possibility of non-equilibrium processes must be considered for a true picture of SOA formation and growth to emerge.
That all said, I do think that some more thought is required to highlight the real atmospheric significance of these findings. First, the very narrow and highly directed energy distribution in a molecular beam is far from representative of the thermal distributions important in the troposphere and lower stratosphere. Does this difference give rise to different dynamics at the surface as measured using the scattered beam than those of importance in the atmosphere? I think the authors should comment on this.
Second, the region of interest for SOA formation and growth is at a considerably higher temperature than the surface temperature range explored here. Presumably, this higher temperature will give rise to greater surface mobility, which may well impact the results at realistic temperatures. Similarly, the presence of water vapour in the troposphere is key to many heterogeneous chemical processes, but is neglected here (by necessity, in the experiments). I would like to see some consideration given to how these features of the real atmosphere might impact the results, and also the overall interpretation, presented here.