Our research team from the University of Vienna’s Aerosol Physics and Environmental Physics Group has contributed to a study that identifies a previously overlooked, organic‑rich population of ultrafine particles in the lowest stratosphere. During NOAA’s SABRE mission (February 2023), NASA’s WB‑57 aircraft carried advanced instruments capable of detecting particles down to 3 nanometres—well below the thresholds of most satellites and balloon instruments. These measurements revealed an abundant mode of nanoparticles, mostly smaller than 0.11 micrometres, nearly 11 km above the surface. Despite their minute size, they account for up to 90% of the aerosol surface area available for atmospheric chemical reactions and condensation of trace gases, with implications for ozone chemistry and climate.
The Vienna team—Bernadett Weinzierl, Maximilian Dollner and Florian Kuderna — adapted and implemented an algorithm to classify each measurement as inside or outside the stratospheric polar vortex, separating two air masses with different origins and chemical histories. The newly recognised nanoparticles were found exclusively outside the vortex, in air recently influenced by the troposphere, and were virtually absent inside the long‑isolated vortex. This pattern, together with elevated nitrous oxide (N₂O)—a tracer of recent tropospheric influence—and particle mass spectrometry showing about 50% organic mass, confirms a tropospheric origin. The particles form from surface‑emitted organics in the upper troposphere and are transported into the stratosphere by tropical updrafts, convective storms and gradual uplift.
The observations also expose a blind spot in monitoring and modelling. A chemistry–climate model could not reproduce the observed bimodal size distribution that emerges as these nanoparticles mix with larger stratospheric sulphate particles. The model generated small particles via new particle formation in the lower stratosphere, inconsistent with in situ evidence, and its particles were often too large and largely sulphate‑only.
These findings bear directly on proposed stratospheric aerosol injection in the tropical or subtropical lower stratosphere—precisely where these organic nanoparticles dominate. As an extensive “condensation sink” and reaction platform, they would strongly shape the fate of injected material. While NOAA does not conduct SAI experiments, the study underscores the need for balloon instruments sensitive below 0.1 µm and improved representation of organic‑rich aerosols in models. The publicly available SABRE dataset will support these advances.
The full paper is available in Science (doi: www.science.org/doi/10.1126/science.adw8939), and APA has reported on the study here: science.apa.at/power-search/10690077941099698188