An unrecognized mode of small particles in the lower stratosphere

[Geoengineering News]

https://www.science.org/doi/10.1126/science.adw8939

Authors: Ming Lyu, Adam T. Ahern et al.

23 April 2026

Editor’s summary

The lower stratosphere is rife with a class of extremely small aerosols rich in organic compounds that have been largely underappreciated until now. Lyu et al. showed that these particles, most of which originate in and are transported from the troposphere, dominate the surface area available for heterogeneous chemistry and constitute the major sink for condensable vapors. The consequences of these particles on heterogeneous chemistry and aerosol microphysical processes in the lower stratosphere may be considerable, particularly for geoengineering proposals that involve injecting aerosol precursors into the stratosphere. —Jesse Smith

Structured Abstract

INTRODUCTION

The lower stratosphere is a complex and fascinating region of the atmosphere where air from the troposphere below enters and mixes with air that has been in the stratosphere for years. This region is particularly important to climate and atmospheric chemistry because aerosol particles scatter solar radiation and provide surfaces for heterogeneous reactions that affect stratospheric ozone. Further, many proposed albedo modification by stratospheric aerosol injection (SAI) efforts, a form of “geoengineering,” would emit aerosol particles or precursors in this region of the stratosphere to scatter more sunlight to space and cool Earth’s surface. A thorough understanding of the processes that govern aerosol sources, sinks, and characteristics in the lower stratosphere is essential to understand these effects.

RATIONALE

Both heterogeneous reactions and the condensation sink for gas-phase species are more closely related to the aerosol surface area than to number or mass. Past measurements by aircraft and balloons have demonstrated that there is a population of nanoparticles present in the lower stratosphere. Because such particles are too small to scatter much visible light, satellite sensors and most in situ optical measurements on balloons are relatively insensitive to them. Therefore, previous measurements have not accurately determined the surface area of these nanoparticles nor quantified their evolution in the stratosphere.

RESULTS

Using multiple instruments on a high-altitude research aircraft, we measured the size distribution and composition of particles in the lower stratosphere at altitudes up to 19 km. Particles with diameters <150 nm dominated the aerosol surface area. These small particles mixed and coagulated with the larger stratosphere background particles, resulting in a bimodal size distribution in the lower stratosphere. A chemistry-climate model did not replicate this bimodal size distribution. Our measurements show two distinct sources for the small-particle mode. In older stratospheric air they are sulfuric acid with metals from meteors. In younger stratospheric air influenced by the troposphere, the small-particle mode is mostly tropospheric particles with high organic content, which will affect their reactivity with gas-phase species.

CONCLUSION

This work provides well-resolved measurements of sub–150-nm particles in the lower extratropical stratosphere over a wide range of stratospheric ages. Because the small particles were observed previously in limited observations in stratospheric air at altitudes <13 km, as well as in earlier test flights for this study, this bimodal structure is likely a consistent feature of the extratropical lower stratosphere. The proper representation of these small, mostly organic, particles in chemistry-climate models may affect the predicted aerosol surface area density and reactivity, thus influencing heterogeneous chemistry and ozone abundance. Despite their importance to both chemistry and climate, previous measurements and models do not resolve the small-particle mode. A simple box model showed that condensable species, such as those from SAI efforts, would condense on and grow the smaller particles, affecting their light-scattering properties. Because the climate and chemistry effects of aerosol particles in the stratosphere are ultimately evaluated using global-scale models, it is essential that they replicate the observed aerosol characteristics and the processes that govern them.

Source: Science