How would this experiment's different particle types produce atmospheric reflectivity?

Question

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?

Answer 1

foreword: Particles and Droplets

Commonly, atmospheric particles can be considered as wet particles. E.g. when we talk about sulfate particles ($SO_4^{2-}$), these particles are actually droplets with dissolved sulfuric acid (or with a dissolved sulfate-cation compound).

Mechanism involving $SO_2$

$SO_2$ is released as gas and it reacts to $SO_4^{2-}$ (sulfate). Basically, there are different reaction pathways for the formation of sulfate from $SO_2$:

• $SO_{2,g}$ --(gas phase reaction)--> $H_2SO_{4,g}$ --($H_2O$ accumulates)--> "sulfate particle"
• $SO_{2,(g)}$ --(goes into solution)--> $SO_{2,solution}$ --(formation)--> $H_2SO_{4,solution}$ (deprotonizes directly)

Commonly, the second pathway is dominant. The relevant aspect is that it leads to the formation of ultra fine sulfate particles (sulfate droplets).

Sulfate particles are cloud condensation nuclei. That means that they support the formation of clouds. There are satellite pictures available, in which you can identify shipping lanes by lines of clouds. These clouds were formed because ships emit large amounts of $SO_2$ (at least outside of sulfur emission control areas).

Most clouds above the ocean reflect more solar radiation than the sea surface. In general, clouds above the ocean are considered as "good clouds" with respect to cooling the Earth. Since a large proportion of the Earth is covered by sea surface, one might wonder to generate clouds above it - e.g. by releasing $SO_2$. However, as far as I know, this process is relevant in the troposphere.

Releasing $SO_2$ into the stratosphere (as proposed in the linked article) is not meant to support cloud formation (correct me, if I am wrong). Nevertheless, the released $SO_2$ leads to ultra-fine sulfate particles. These sulfate particles themselves effectively scatter solar radiation and, thus, prevent it from reaching the Earth's surface. There is also a Wikipedia article available here.

Unfortunately, sulfate indirectly degrades the ozone layer: sulfate leads to the release of radicals that (and their successors) catalytically destroy ozone. See Keith et al. (2016) for a more detailed overview.

Why $CaCO_3$?

The basic idea is to add salts of alkali metals that neutralize sulfuric acid and nitric acid (generated in the presence of sulfuric acid) when we emit $SO_2$. This will reduce the negative impact on the ozone layer. $CaCO_3$ is such a salt (Keith et al., 2016).

Why $Al_2O_3$?

Foreword: I am not expert on this and just read some articles on this topic.

According to Weisenstein et al. (2015), fine aluminum particles (as well as diamonds) are an alternative to sulfate particles. According to them, aluminum is favourable. However, as studies on emissions from missiles and space crafts indicate, $Al_2O_3$ also has negative impacts on the ozone layer (SomeReport; DoD, 2004): $Cl$ radicals are released that degrade ozone. $CaCO_3$ also counteracts here, too.