Microscopic droplets and particles make up aerosols in the air, clouds of haze, dust and soot that play vital roles in climate, absorbing or reflecting sunlight to help warm or cool the planet and serving as the seeds of precipitation. Now scientists find highly porous aerosol particles can form via freeze-drying in the atmosphere. These can be better at triggering ice and cloud formation than other aerosol particles and scatter light less efficiently, potentially playing a key role in the atmosphere. The findings are detailed in the Proceedings of the National Academy of Sciences.
When aerosol particles drift around clouds, chemical and physical properties such as composition and size can change, which in turn modifies how they influence weather and climate. What scientists know about the changes aerosols go through is generally limited to what goes on in water droplets in warm, low-altitude clouds. In contrast, within cold high-altitude clouds and highly swirling clouds in the tropics and mid-latitudes, aerosols can form ice, which can in turn sublimate — that is, change directly to vapor.
To learn more about the products of this atmospheric freeze-drying, researchers simulated ice cloud processes with experiments that generated aerosols, humidified the air so the particles could accumulate water, and freeze-dried them. The scientists then analyzed the size, density and shape of the resulting microscopic particles.
The investigators found aerosols that possessed organic material can undergo atmospheric freeze-drying and form particles with highly porous, corrugated surfaces that were both larger in diameter and lower in density than the initial aerosols. (Organic material is common in the kind of clouds the researchers studied where freeze-drying can take place.)
The researchers suggest than when the wet aerosol particles freeze, their ice and organic components separate, with ice forming pockets in an organic matrix. Freeze-drying then removes the ice, leaving behind pores. The remaining organic matrix of these porous aerosols is apparently glassy in nature — they are more akin to motionless liquids than solids, with a randomly ordered structure.
The porous nature of these particles give them more surface area to trigger ice and cloud formation than regular aerosols. Moreover, their structure means they should scatter light less efficiently. Satellite data suggest clouds with the combination of conditions for these highly porous aerosols to form are quite common in the tropics and midlatitudes — for instance, central West Africa.
The fact that highly porous aerosols are less dense than regular aerosols could mean they live in the atmosphere longer in dry and cold environments for prolonged effects on ice and cloud formation, the researchers speculated. Their reduced density could also change how they affect turbulence in clouds, further influencing cloud development and precipitation. Moreover, their increased surface area could lead to interesting chemical reactions happening on them — further research is needed to examine their size and structure and how they form and evolve under different atmospheric conditions.