EarthWeek ChemSplainer: Weichman, Deike, and “a thin layer of sunscreen for the planet”

Earlier this spring, Assistant Professor Marissa Weichman received a collaborative grant from Princeton Engineering’s Sustainability of Our Planet Fund for the project, “Ice Nucleation in Solar Radiation Management Science.”

Aimed at advancing fundamental knowledge on atmospheric aerosol science, the project like the science itself is still in the very early stages. But researchers are asking important questions about how to approach it in the lab.

As we continue to celebrate EarthWeek, we highlight this up-and-coming collaboration between the Weichman Lab and Mechanical and Aerospace Engineering’s Deike Lab as yet another way scientists are using fundamental research to mine solutions for the future.

Enjoy Weichman’s remarks below.

Q: What is solar radiation management?

A: Solar radiation management (SRM) is a broad class of proposed ideas for how one might dial down the amount of sunlight absorbed by Earth and its atmosphere as a means to counteract global warming and slow climate change. SRM would be a stop-gap measure to cool the planet in the near term and buy us more time to pivot towards greener energy technologies and scale up carbon capture efforts.

Q: What is stratosphere aerosol injection?

A: Stratosphere aerosol injection (SAI) is a specific SRM proposal to disperse tiny, inert inorganic aerosol particles in the stratosphere to scatter incoming sunlight away from Earth. It is basically a thin layer of sunscreen for the planet. While SAI, and SRM in general, is still (understandably) a very controversial topic, there is an increasing consensus that we should be doing basic research in this area to have a better sense of what would happen if these strategies were ever implemented by a rogue actor, or if we decide they need to be implemented in a climate emergency. As you can imagine, field experiments are not really possible here at Princeton, so we are focused on controlled lab work.

Q: What would we need to understand through basic research about the potential of these ideas?

A: There are still a lot of open questions about how one would actually implement SAI—including what kinds of particles would work best—and how to minimize unintended side effects. Our ongoing collaboration with Luc Deike’s group in Princeton’s Department of Mechanical and Aerospace Engineering is working to answer some of these questions.

For instance, say we disperse nanoscale calcite aerosols in the stratosphere for SAI. Those particles will stay aloft for some timescale on the order of years. During that residence time, the particles’ size, composition, and properties are not set in stone. They may undergo surface chemistry with vapor-phase molecules in the stratosphere; they may get coated with condensing water vapor or organic molecules; they may agglomerate through sticky collisions with other aerosol particles. We are interested in understanding how these various processes affect a given particle’s long-term efficiency for SAI.

Q: What are some immediate questions researchers might have?

A: How well does a particle continue to cool the atmosphere by scattering sunlight after it has undergone all this chemistry, coating, and agglomeration? What are the long-term knock-on effects?

Q: What are some of the concerns?

A: As mentioned above, a particle does not stay aloft in the stratosphere forever. Once it sediments down into the troposphere, where clouds live, it can do all kinds of other things that may have unexpected implications for the global climate. One important thing is that SAI particles can nucleate the growth of ice crystals and thereby impact cirrus cloud cover, which has much larger-scale effects on climate. So we are also very interested in the ice-nucleating propensity of different SAI candidates.

Q: What are the tools you’ll be using?

A: My lab has built a single-particle spectrometer that allows us to levitate and trap individual charged particles for hours up to days. We then deploy single-particle spectroscopy and imaging to track optical and chemical behavior over time. Cavity-enhanced spectroscopy is one of many tools we use to characterize how a single particle extinguishes light via both scattering and absorption.

In parallel, we are using a large-scale cloud chamber housed in Luc Deike’s lab to examine how ensembles of aerosols behave. We can trigger a pressure drop in the chamber that transiently brings the system to very low temperature and triggers the nucleation of ice on seeding aerosol particles. We then watch the changing shapes, size distributions, and trajectories of these growing ice particles using holographic imaging and other techniques. We are interested in how our measurement statistics change when the cloud is seeded with different SAI aerosol candidates to get a better sense of how these particles might influence clouds.

Q: Fundamental research can rule out, and rule in, possible approaches, right?

A: We are not necessarily advocating to deploy SAI anytime soon, though our experiments might ultimately inform policy decisions on when and how best to perform SAI if we really need to. In part, by testing how SAI particles like calcite, titania, alumina, or sulfate undergo chemistry and interact with clouds, we might find fatal flaws that would rule out a particular candidate from safe or practical use.