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.