ESA study finds ultrafine aerosols

The story is about stratospheric aerosols — tiny particles floating high above the weather layer that help shape climate and ozone chemistry. For years, scientists mostly treated the lower stratosphere as a place where sulfate particles dominate and the really tiny stuff does not matter much. But measurements published in *Science* on April 24, 2026 upended that picture. A team led by NOAA, CIRES, and university researchers found a widespread population of ultrafine, mostly organic particles in the lower stratosphere, and those particles seem to control more surface area for chemistry than the larger particles models usually focus on. ### What did they actually find? They flew high-altitude instruments on NASA’s WB-57 aircraft during NOAA’s SABRE campaign in February 2023 and sampled air up to about 19 km. In that air, particles smaller than 150 nanometers were not a side show — they dominated the aerosol surface area in the lower stratosphere and produced a bimodal size distribution that a chemistry-climate model failed to reproduce. Is surface area the key thing? Because chemistry happens on particle surfaces. A small amount of mass can still create a lot of reactive area if it is split into huge numbers of tiny particles. That is the catch here — these aerosols are tiny, so older instruments and simplified models could miss them, but they still offer a lot of real estate for reactions that affect ozone and other trace gases. ### Where are these particles coming from? Turns out they do not look like classic sulfate-only stratospheric aerosols. Mass spectrometry showed the very small particles are rich in organics, with about 50% of their mass tied to organics from surface sources. The picture the researchers sketch is that organic-rich nanoparticles form in the upper troposphere from emissions below, then get lofted into the lower stratosphere, where they mix and coagulate with the background aerosol layer. ### Why was this missed before? Mostly because the particles are hard to see. They are roughly 100 times smaller than a dust particle, and many observing systems were tuned to the larger aerosol sizes long assumed to matter most. If your instruments undersample the smallest end, and your models assume sulfate dominates, you can end up with a clean-looking stratosphere that is not actually there. ### Does this change climate modeling? Probably, yes — though not in the simple “warming goes up” or “cooling goes down” way people might want. The immediate point is that chemistry-climate models may be getting aerosol surface area density and heterogeneous reaction rates wrong in parts of the lower stratosphere. That can feed into ozone calculations, radiative forcing estimates, and the way scientists simulate particle growth and removal. ### Why are geoengineering people paying attention? Because many solar aerosol injection ideas target the same tropical and subtropical lower stratosphere where these particles appear to dominate. If the background aerosol environment already contains lots of tiny organic-rich particles, then injected material would not evolve in a purely sulfate world. That changes the condensation sink, mixing behavior, and reaction environment engineers have been assuming. ### So what is the real takeaway? Basically, the lower stratosphere just got more complicated. Not chaotic — just less sulfate-only and less settled than the standard picture suggested. The new result does not overturn everything scientists know about aerosols, but it does reopen an important part of the atmosphere that many models had simplified too aggressively.