Phase Behavior and Properties of Atmospheric Organic Aerosols

Atmospheric organic aerosols encompass a complex mixture of emitted and formed particles that range from freshly condensed vapours to highly processed secondary material. Their phase behaviour—encompassing liquid, semi-solid and glassy states—determines key processes such as water uptake, gas-particle partitioning, heterogeneous chemistry and optical properties. Environmental factors including temperature, relative humidity and chemical composition dictate transitions between phases, often through non-ideal mixing and kinetic limitations. Liquid-liquid phase separation can produce coexisting organic-rich and aqueous phases, altering diffusion rates and reaction pathways. Conversely, viscosity and glass transition impose barriers to mass transport, prolonging particle lifetimes and influencing long-range transport of pollutants. Advances in experimental techniques, from microfluidic viscometry to single-particle levitation, alongside thermodynamic modelling frameworks, have begun to unveil the intricate interplay between composition and phase state. This understanding is critical for accurate climate models, as phase state modulates aerosol-cloud interactions, radiative forcing and the formation of cloud condensation nuclei. Moreover, insights into volatility and activity coefficients guide predictions of particle evolution in urban and remote regions, informing air quality management and health impact assessments.

Research from Nature Portfolio

Recent studies have characterised the viscosity of atmospherically relevant organic particles across a broad range of environmental conditions, demonstrating transitions from liquid to semi-solid or glassy states as a function of composition and humidity. These findings revealed that many secondary organic aerosols exist in highly viscous semi-solid phases under typical tropospheric conditions, with implications for diffusion-limited uptake of water and oxidants. In parallel, global modelling efforts have mapped the phase state of secondary organic aerosols in both the boundary layer and free troposphere, showing predominantly liquid phases in humid regions, semi-solids in temperate zones and glassy solids at higher altitudes and lower relative humidities. Such global distributions underscore the importance of phase state for ice nucleation, long-range transport and pollutant ageing processes.

Phase Behavior and Properties of Atmospheric Organic Aerosols publication trend

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

Technical terms

Secondary organic aerosol (SOA): Particulate matter formed by atmospheric oxidation of volatile organic compounds.

Liquid–liquid phase separation (LLPS): The spontaneous formation of coexisting organic-rich and aqueous phases within a single aerosol particle due to immiscibility.

Viscosity: A measure of a fluid’s resistance to flow, determining diffusion rates of gases and solutes within particles.

Glass transition temperature (Tg): The temperature below which an amorphous material transitions to a rigid glassy state with extremely high viscosity.

Activity coefficient: A factor that quantifies deviations from ideal solution behaviour, influencing the volatility and partitioning of species.

References

1. Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity. Environmental Science and Technology (2023).
2. Estimation of the Volatility and Apparent Activity Coefficient of Levoglucosan in Wood-Burning Organic Aerosols. Environmental Science & Technology Letters (2024).
3. Probing the evaporation dynamics of semi-volatile organic compounds to reveal the thermodynamics of liquid–liquid phase separated aerosol. Chemical Science (2024).
4. The viscosity of atmospherically relevant organic particles. Nature Communications (2018).
5. Global distribution of particle phase state in atmospheric secondary organic aerosols. Nature Communications (2017).