Hygroscopic Properties of Atmospheric Aerosols

The hygroscopic properties of atmospheric aerosols govern their ability to take up and release water vapour, with profound implications for air quality, human health and climate processes. Water uptake alters particle size, phase state and optical properties, thereby influencing visibility, radiative forcing and cloud formation. Aerosols commonly consist of complex mixtures of inorganic salts, water-soluble organics and insoluble compounds. Interactions among these components give rise to non-ideal mixing, phase separation, glassy or semi-solid states and kinetic limitations that affect deliquescence, efflorescence and growth factors across relative humidity (RH) ranges. Such behaviour also modulates chemical reactivity—including pH-driven processes—and the cycling of reactive gases. Accurate representation of aerosol hygroscopicity in atmospheric models is essential for predicting cloud condensation nuclei (CCN) activation, regional precipitation patterns and the indirect effects of aerosols on climate. Advances in experimental and theoretical techniques continue to refine parameterisations of size-dependent phase transitions, mixing rule deviations and interfacial phenomena that underpin global aerosol dynamics.

Research from Nature Portfolio

Recent investigations have revealed that particle size exerts a decisive influence on phase transitions and hygroscopic growth in nanoscale aerosols. By applying a novel differential Köhler analysis to highly supersaturated droplets, researchers determined size-dependent solute activities and interfacial energies that resolve long-standing discrepancies between observations and models. The work shows that particles below roughly 20 nm remain liquid at room temperature, and that phase separation thresholds shift with decreasing diameter. These insights underscore the need to include particle size as an additional dimension in equilibrium phase diagrams and improve the fidelity of aerosol parameterisations in climate models.

Hygroscopic Properties of Atmospheric Aerosols publication trend

The graph below shows the total number of articles in hygroscopic properties of atmospheric aerosols across all publications each year (not limited to Nature Index journals).

Technical terms

Hygroscopicity: The capacity of aerosol particles to absorb or release water vapour, affecting their size, phase and optical characteristics.

Deliquescence relative humidity (DRH): The ambient RH at which a solid aerosol particle takes up sufficient water to dissolve and form an aqueous solution.

Efflorescence relative humidity (ERH): The ambient RH at which an aqueous aerosol droplet loses water and crystallises back into a solid phase.

Köhler theory: A theoretical framework combining solute thermodynamics and curvature effects to predict equilibrium water uptake and droplet activation onto CCN.

Cloud condensation nuclei (CCN): Atmospheric particles that facilitate the nucleation of cloud droplets by taking up water at supersaturated conditions.

References

1. Hygroscopic behavior and aerosol chemistry of atmospheric particles containing organic acids and inorganic salts. npj Climate and Atmospheric Science (2024).
2. Direct quantification of changes in pH within single levitated microdroplets and the kinetics of nitrate and chloride depletion. Chemical Science (2023).
3. Adsorbed Water Promotes Chemically Active Environments on the Surface of Sodium Chloride. The Journal of Physical Chemistry Letters (2023).
4. Size dependence of phase transitions in aerosol nanoparticles. Nature Communications (2015).
5. Amorphous and crystalline aerosol particles interacting with water vapor: conceptual framework and experimental evidence for restructuring, phase transitions and kinetic limitations. Atmospheric Chemistry and Physics (2009).