The stratospheric aerosol is a critical yet understudied component of the earth-atmosphere system. Stratospheric particles provide the seeds for stratospheric cloud formation which drives ozone depletion, they provide surfaces for heterogeneous chemical reactions, and they can substantially modify the earth's climate following volcanic eruptions.

Interest has grown in recent years in the concept of solar radiation management (SRM), a form of geoengineering that is envisioned to be used temporarily (for a few decades) to reduce global temperatures while CO₂ emissions and atmospheric burdens are reduced. New efforts are being made to better understand the processes governing the current state of the stratospheric aerosol so that such proposals can be evaluated objectively and quantitatively. Recently, airborne measurements were made at altitudes up to 20 km from the Northern Hemisphere subtropics to the polar vortex using new instruments to measure the composition, optical properties, and size distribution of particles from 3 nm to 10 µm in diameter, along with key reactive and trace gas species such as OCS, SO₂, N₂O, and SF₆.

The results show the following: sub-100 nm tropospheric particles made of mixtures of organic and inorganic matter enter the lower stratosphere in the tropics and subtropics and grow by coagulation and condensation of sulfuric acid produced from the oxidation of OCS. Larger particles are removed by sedimentation, leading to a narrow standard deviation of the size distribution. Aircraft emissions, the suspected source of a layer of SO₂ in the lower mid- and high-latitude troposphere, may supplement these small tropospheric particles. In the polar vortex, descending air from the upper stratosphere and mesosphere carries sub-100 nm particles produced from the condensation of sulfuric acid onto nano-scale particles composed of smoke from ablated meteors and rocket and satellite re-entry. Most stratospheric chemistry models do not include all of these processes, and mis-classify and mis-size the aerosol as a result, though they do simulate the main characteristics of the main stratospheric aerosol (Junge) layer with reasonable fidelity. While not the focus of this talk, the satellite and rocket particles may have unexpected consequences for heterogenous chemistry and polar stratospheric cloud nucleation when considering the expected order-of-magnitude increase in rocket launches in the next decade.

Together, these observations point to the need to more thoroughly investigate and understand the existing stratospheric aerosol, and its volcanic state, prior to considering geoengineering efforts via stratospheric aerosol injection. Surprises abound.