Breakthrough Study Introduces Predictive Model for Aerosol Phase Behavior

Researchers have unveiled a new generic framework that clarifies how mixtures of ammonium sulfate, ammonium nitrate, and organic matter behave under changing humidity conditions. The work, published online on 23 June 2026 in the Journal of Aerosol Science, addresses long-standing gaps in understanding the hygroscopic cycle of atmospheric aerosols.

The study focuses on internally mixed particles containing ammonium sulfate (AS), ammonium nitrate (AN), and organic matter (OM). These components dominate many polluted air masses and influence everything from cloud formation to air quality forecasts. Lead contributors include Seong Hyun Kim, Taehyeon Kim, Atta Ullah, Hye-Jung Shin, Gookyoung Heo, and Ho-Jin Lim.

Why Hygroscopic Properties Matter for Atmospheric Science

Hygroscopicity describes a particle's ability to absorb water vapor from the surrounding air. When relative humidity rises, many aerosols undergo deliquescence, transitioning from solid to liquid. The reverse process, efflorescence, occurs as humidity falls. These phase changes alter particle size, optical properties, and chemical reactivity.

Accurate representation of these transitions improves predictions of aerosol effects on climate and human health. Inorganic salts such as AS and AN have well-characterized behavior, yet real-world particles contain substantial organic fractions that complicate the picture.

Study Design and Experimental Approach

The team generated ternary mixtures with varying mass fractions of AS, AN, and a surrogate OM blend. The organic surrogate combined succinic acid, palmitic acid, and naphthalene to represent hydrophilic, hydrophobic, and aromatic classes common in the atmosphere.

Particles were produced via atomization, dried, and deposited on hydrophobic substrates. An optical microscope coupled with a flow cell allowed real-time observation of morphological changes across controlled relative humidity ramps. This setup captured both complete and partial phase transitions.

Key Observations on Deliquescence and Efflorescence

Single-component particles showed expected behavior: ammonium sulfate deliquesced near 80 percent relative humidity, while ammonium nitrate transitioned at lower values. Organic-rich mixtures displayed greater complexity.

As organic content increased, complete deliquescence relative humidity (DRH) shifted upward. Initial water uptake, marked by mutual deliquescence relative humidity (MDRH), was inhibited. Efflorescence relative humidity (ERH) and mutual efflorescence relative humidity (MERH) also varied significantly, with hydrophilic organics tending to raise DRH while lowering ERH.

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Development of Quantitative Parameterization Equations

Multiple linear regression applied to the experimental dataset yielded predictive equations for DRH, ERH, MDRH, and MERH. These equations express transition points as functions of the dry-mass fractions of AS, AN, and OM.

Validation against independent literature data produced correlation coefficients exceeding 0.96, demonstrating strong predictive skill across a wide compositional range. Separate parameterizations address general OM mixtures and hydrophilic-dominant cases.

Implications for Climate and Air Quality Modeling

Existing thermodynamic models such as E-AIM and AIOMFAC require extensive compound-specific parameters that remain unavailable for many organics. The new empirical framework offers a practical, composition-based alternative that complements these tools.

Improved phase-state predictions will refine estimates of aerosol optical depth, cloud condensation nuclei activity, and heterogeneous reaction rates. Regional air quality models may also benefit from more accurate representation of particle growth and deposition.

Broader Context in Aerosol Research

Atmospheric aerosols influence radiative forcing, precipitation patterns, and visibility. Organic matter can constitute more than half the submicron aerosol mass in many environments. The mixing state between inorganics and organics governs whether particles remain solid, liquid, or exhibit liquid-liquid phase separation.

This study builds on prior binary-system investigations while extending coverage to ternary mixtures that better approximate ambient conditions. The resulting dataset fills a documented gap in comprehensive DRH and ERH measurements for AS–AN–OM systems.

Future Research Directions and Model Integration

The authors note that additional work could test the framework with ambient samples and more diverse organic surrogates. Integration into global climate models could proceed through simplified lookup tables or direct implementation of the regression equations.

Collaborations between laboratory experimentalists and model developers will be essential to realize the full potential of these parameterizations. Ongoing monitoring networks may incorporate composition-resolved hygroscopic measurements to validate predictions.

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Access the Original Publication

The full study appears in the Journal of Aerosol Science as article 106841. Readers can access the abstract and related content at https://www.sciencedirect.com/science/article/abs/pii/S002185022600100X. The DOI is 10.1016/j.jaerosci.2026.106841.

Relevance to Academic and Research Communities

Environmental science departments and atmospheric chemistry groups will find immediate value in the new parameterization. Graduate students and postdoctoral researchers can use the equations for sensitivity studies or as benchmarks for more complex thermodynamic simulations.

University libraries subscribing to Elsevier journals provide direct access. Faculty may incorporate the findings into courses on aerosol physics or climate modeling.