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PreprintCC BYChemRxiv

From Bulk Hygroscopicity to Nanoscale Water Activity: Breakdown of Continuum Theories in Sub-4 nm Secondary Organic Aerosols

  • Jiaqi Wen1,2
  • Xiaohan Li3,4
  • Ian C. Bourg1,2,5
5 affiliations
  1. 1Department of Civil and Environmental Engineering
  2. 2Princeton University
  3. 3Department of Geophysical Sciences
  4. 4University of Chicago
  5. 5High Meadows Environmental Institute, Princeton Univerisity
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Abstract

Interactions between water and organic molecules in sub-10 nm clusters surrounded by air play a key role in the chemistry and microphysics of secondary organic aerosol (SOA). However, water-organic interactions in such small clusters are difficult to probe experimentally and remain poorly constrained. Here, we use molecular dynamics simulations to examine morphology, molecular interactions, and water evaporation energetics of nanoclusters formed by nine representative SOA compounds across sizes and water contents. Nanocluster water affinity depends on size and water content, with distinct controlling mechanisms emerging during RH-driven drying. At high and moderate water contents, clusters larger than 4 nm follow a Kelvin baseline based on their bulk-mixture surface tension. In contrast, sub-4 nm organic-shell clusters sometimes retain water much less strongly than expected based on the Kelvin relation as their phase-mixing state changes with cluster size. At low water content, loss of water-network connectivity fragments the water domain into isolated pockets, shifting control of water retention to specific water-organic and water–water interactions, ionic coordination, and steric accessibility, producing species-dependent water affinity that bulk hygroscopicity cannot predict. These results identify where continuum descriptions break down and inform the representation of water in SOA nanoclusters in atmospheric models.

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Cite

BibTeXRIS
BibTeX
@misc{wen2026from,
  title = {{From Bulk Hygroscopicity to Nanoscale Water Activity: Breakdown of Continuum Theories in Sub-4 nm Secondary Organic Aerosols}},
  author = {Jiaqi Wen and Xiaohan Li and Ian C. Bourg},
  howpublished = {ChemRxiv},
  year = {2026},
  month = oct,
  doi = {10.26434/chemrxiv.15010102/v1},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010102/v1},
  note = {CC BY},
}
RIS
TY  - UNPB
TI  - From Bulk Hygroscopicity to Nanoscale Water Activity: Breakdown of Continuum Theories in Sub-4 nm Secondary Organic Aerosols
AU  - Jiaqi Wen
AU  - Xiaohan Li
AU  - Ian C. Bourg
PY  - 2026
DA  - 2026/10/08
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15010102/v1
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010102/v1
AB  - Interactions between water and organic molecules in sub-10 nm clusters surrounded by air play a key role in the chemistry and microphysics of secondary organic aerosol (SOA). However, water-organic interactions in such small clusters are difficult to probe experimentally and remain poorly constrained. Here, we use molecular dynamics simulations to examine morphology, molecular interactions, and water evaporation energetics of nanoclusters formed by nine representative SOA compounds across sizes and water contents. Nanocluster water affinity depends on size and water content, with distinct controlling mechanisms emerging during RH-driven drying. At high and moderate water contents, clusters larger than 4 nm follow a Kelvin baseline based on their bulk-mixture surface tension. In contrast, sub-4 nm organic-shell clusters sometimes retain water much less strongly than expected based on the Kelvin relation as their phase-mixing state changes with cluster size. At low water content, loss of water-network connectivity fragments the water domain into isolated pockets, shifting control of water retention to specific water-organic and water–water interactions, ionic coordination, and steric accessibility, producing species-dependent water affinity that bulk hygroscopicity cannot predict. These results identify where continuum descriptions break down and inform the representation of water in SOA nanoclusters in atmospheric models.
ER  -