The MIT Technology Review news item Harvard Scientists Moving Ahead on Plans for Atmospheric Geoengineering Experiments discusses a proposed experiment to study in a carefully controlled way the reflectivity induced by various kinds of particles released into the stratosphere. The underlying idea being explored is that increased atmospheric reflectance could partially offset warming effects due to greenhouse gases.
Sometime next year, Harvard professors David Keith and Frank Keutsch hope to launch a high-altitude balloon, tethered to a gondola equipped with propellers and sensors, from a site in Tucson, Arizona. After initial engineering tests, the “StratoCruiser” would spray a fine mist of materials such as sulfur dioxide, alumina, or calcium carbonate into the stratosphere. The sensors would then measure the reflectivity of the particles, the degree to which they disperse or coalesce, and the way they interact with other compounds in the atmosphere.
The researchers first proposed these balloon experiments in a 2014 paper. But at a geoengineering conference in Washington, D.C., on Friday, Keith said they have begun engineering design work with Arizona test balloon company World View Enterprises. They’ve also started discussions about the appropriate governance structure for such an experiment, and they plan to set up an independent body to review their proposals.
This is an effort to explore how best to do a small, controlled test and how to establish proper guidelines. This type of atmospheric experiment is very different than the long established practice of cloud seeding for rain, so this thoughtful approach is taken.
A short review of the background concept and associated issues can be found in the video shown in this article, after the first minute or so it gets more serious.
While Alumina (Al2O3) and calcium carbonate (CaCO3) might be in the form of solid particles for such an experiment, and particle size can be engineered to various controlled size distributions ahead of time, sulfur dioxide (SO2) is a gas at STP. At one atmosphere of pressure, it would become a liquid at about -10C. Would the plan be to spray or atomize liquid SO2 at the low ambient temperature at altitude?
If I understand correctly, SO2's mechanism would be to 'chemically scavenge' water vapor from the air, forming reflective droplets of sulfuric acid. Would the alumina and calcium carbonate also seed water droplet formation as their reflective mechanism, or would only the particles themselves be reflective?