A Significant Mode of Small, Tropospheric Particles in the Lower Stratosphere

The stratospheric aerosol layer plays an essential role in stratospheric chemical and radiative processes. Emissions of SO₂ from small-to-midsized volcanic eruptions are typically introduced into the lower stratosphere, where a background aerosol already exists. The radiative and chemical consequences of these eruptive emissions depend in part upon the characteristics of these background particles, whose number originates from the troposphere. In-situ observations of sub-0.1 µm diameter particles in the upper troposphere/lower stratosphere are rare, but are of particular importance in understanding the contribution of upwelling ultrafine particles formed near the tropical tropopause and their subsequent evolution in the stratospheric (Brewer-Dobson) circulation.

In the  Stratospheric Aerosol processes, Budget and Radiative Effects (SABRE) mission from January to March 2023, we used three particle sizing instruments on the NASA WB-57 high-altitude aircraft to measure the size distribution of aerosol particles in the size range from 0.003 to ~4.0 μm in the lower stratosphere from middle to high latitudes at altitudes up to 19.6 km. The composition of individual aerosol particles >0.1 µm was also measured, along with O₃, N₂O, SF₆ and OCS. Together, these measurements were used to investigate dynamical and chemical processes in the stratosphere that determine the evolution of the stratospheric aerosol as a function of stratospheric age up to several years. We observed a clear bimodal size distribution structure, with a small mode (< 0.1 μm diameter) originating from the troposphere and a larger mode (>0.2 µm diameter) originating from the photolysis of OCS (the classic Junge, or stratospheric, aerosol layer). These two modes evolve as a function of stratospheric age in a manner consistent with coagulation, condensation, and sedimentation. These are the first reported observations of the presence and evolution of this bimodal aerosol structure deep into the stratosphere. The small (tropospheric) particle mode provides a significant condensation sink at young stratospheric ages. SO₂ from modest volcanic eruptions emitted into the lower stratosphere may condense on these smaller particles, reducing the amount of light scattering per unit mass. Proposed geoengineering efforts must also account for this tropospheric mode of particles. Models simulating aerosol processes in the stratosphere need to accurately represent tropospheric particles in the lower stratosphere and their evolution with stratospheric age.