Drifting off to sleep on top of floating clouds and waking up a PhD student, by Ariana Gray Bé
A post by Geiger group student Ariana Gray Bé summarizing her two recent papers on atmospheric cloud formation namely
Bé, A. G.; Upshur, M. A.; Liu, P.; Martin, S. T.; Geiger, F. M.; Thomson, R. J. Cloud activation potentials for atmospheric α-pinene and β-caryophyllene ozonolysis products. ACS Central Science2017, 3(7), 715–725.***
Bé, A. G.; Chase, H. M.; Liu, Y.; Upshur, M. A.; Zhang, Y.; Tuladhar, A.; Chase, Z. A.; Bellcross, A. D.; Wang, H.-F.; Wang, Z.; Batista, V. S.; Martin, S. T.; Thomson, R. J.; Geiger, F. M. Atmospheric β-caryophyllene-derived ozonolysis products at interfaces. ACS Earth and Space Chemistry 2019, 3(2), 158–169.
*** ACS Central Science is an open access journal, meaning anyone at any time can access its publications for free!
Figure 1. Table of contents (TOC) graphics for the two papers discussed in this post.
Key takeaway: Clouds are an enormous crux in the puzzle of climate system, and it is important that we learn more about how they form so that climate change predictions can in turn be improved.
When I was five years old, I covered the ceiling of my bedroom with glow-in-the-dark stickers shaped like stars and clouds. The night sky was so beautiful to me and I wanted to sleep under it (even if not outside). I used to lie bundled under my covers fantasizing about drifting off to sleep on top of floating clouds made of fluffy pillows. I actually remember the exact memory when I learned that clouds were not actually solid, white blobs high up in the sky that I could sit on, but actually collections of tiny water droplets and ice crystals suspended above Earth. Before that moment, I never gave much thought to the definition of a cloud: What are clouds made of? What are they composed of? How do they form? Do they have a purpose, besides blocking the sun and creating gloomy, gray days? At the time, I would never have guessed that 20 years later these questions would be so central in my mind today.
It is easy to forget about the complex processes taking place above and around us in our climate system at large. The climate system is extremely interactive in nature and numerous interconnected factors influence how it evolves in time. Determining the extent to which these diverse components impact the climate system is a crucial first step toward cracking the puzzle of climate change, one of the toughest threats humanity has faced for sustaining life on Earth to date.Among the factors influencing Earth’s climate, the effects of clouds turn out to be the least understood of all. Questions behind cloud formation and its role in the climate system still continue to puzzle scientists, hindering predictions of the scope and impacts of climate change. Motivated by this lack of understanding, I set out to study the culprits driving this “cloudy” uncertainty — tiny balls, invisible to the naked eye, that are situated in the atmosphere, named atmospheric aerosol particles.
From the lingering ash produced by the ongoing Northern California wildfires, to the noticeable haze that gives the Great Smoky Mountains their name, atmospheric aerosol particles are pervasive airborne solid specks and liquid droplets that collectively impact the climate system, both by directly interacting with the sun’s rays and through their ability to form clouds. A big challenge arises when studying the influence that aerosols have on cloud formation simply because there are soooooooooo many kinds of aerosol particles that exist with different sources, compositions, and properties.
To simplify the inherent complexity of the target problem at hand, my research has focused on a key class of the most abundant aerosol particles that form in nature from organic molecules (chemical substances whose atomic building block is the element carbon) that are made by trees and other plants. In fact, a whiff of forest air during a walk in the woods gives a good impression of what these molecules are: Fragrant members of the diverse class of molecules called terpenes. Terpenes are produced by different types of trees, flowers, and other plants, and many of them contribute to the smell of natural fragrances such as pine, orange, lavender, cloves, and hops. Many of these terpenes easily vaporize, which allows them to escape from trees and rise into the atmosphere. Once in the atmosphere, the terpenes chemically react with ozone (familiar as a cause of urban smog or from the ozone “hole”) to generate aerosol particles. It is here, suspended up in the atmosphere, that aerosol particles form clouds. But how does this process work?
Clouds form when water vapor in the atmosphere condenses from a gas to liquid droplets that are small and light enough to stay suspended in the air. Much like droplets collecting on the surface of a glass of ice cold water on a hot summer day, atmospheric water vapor forms cloud droplets more readily when there is a surface onto which it can condense. Aerosol particles act as these surfaces, serving as tiny “seeds” onto which water vapor can condense, forming the clusters of droplets we observe as white clouds in the sky. However, not all molecules within aerosols are created equal, and my goal is to probe specific “surface-active” molecules, which concentrate on the surface of aerosol particles where they have an outsized impact on the water droplet formation process that leads to clouds. For this project, I was particularly interested in targeting the molecular components in aerosol particles that are derived from a terpene with a woody, spicy aroma that is commonly found in black pepper, cloves, rosemary, and of course, trees. This molecule is called β-caryophyllene. Of the terpenes in trees that are known to form aerosol particles in the atmosphere, β-caryophyllene has been the underdog because it hasn’t yet been extensively studied and therefore its importance may be underestimated.
The overarching question I set out to address was: Of the molecules believed to form when β-caryophyllene reacts with ozone in the atmosphere, which are the most surface active and therefore have a greater potential to promote cloud droplet condensation? As a joint PhD student in the Geiger and Thomson labs at Northwestern, this research endeavor was achieved by uniquely bridging expertise from organic synthesis, surface spectroscopy, and aerosol science and engineering. Using organic synthesis, I made practical quantities of the molecules derived from β-caryophyllene that we predicted might be surface-active in order to study their individual properties within the difficult to measure mixtures in aerosol particles. Similar to the approach taken when the desired molecules are medicines, plastics, or pesticides, we synthesized our β-caryophyllene products by strategically designing roadmaps composed of chemical reactions that ultimately lead to the desired molecular structures.
With these molecules in hand, I used them as a benchmark to learn about the molecular surface chemistry that occurs in actual aerosol particles composed of them. Through a collaboration with the Martin lab at Harvard University, I was able to use large chambers housed in their laboratories to simulate similar conditions to those found in the atmosphere to produce aerosol particles with our molecules. As is often done in science, we chose to make and collect the particles under controlled atmospheric conditions in a lab, rather than out in nature, in order to reduce complexity that would make it difficult to draw universally applicable conclusions about the particles being studied.
After producing the particles in the chambers and collecting them on filters for handling, it was time to carry out my analysis of these particles and the synthesized molecules using a combination of two methods that distinctly probe the chemistry taking place at surfaces. These two techniques are called pendant drop tensiometry and sum frequency generation spectroscopy. We predicted that the surface might be simpler than the center, or bulk, of a particle, because it can only contain the subset of molecules that are sufficiently surface active to localize there. With this prediction in mind, I first measured the entire series of molecules that I synthesized to determine the degree to which each one favored the surface of water droplets, and then I compared each sample to the surfaces of the aerosol particles I collected (Figure 1).
Ultimately, I distinguished two molecules, named β-caryophyllene aldehyde and β-caryophyllonic acid, that exhibited significantly higher surface activity than any other molecules our labs have studied to date. These molecules likely reside at the surface of aerosol particles, within the molecular layer that first comes into contact with surrounding water vapor to seed the formation of clouds. This a finding brings the scientific community one step closer to pinning down the molecular origins of cloud activation. Looking up at the sky today, I no longer see clouds as the fluffy, white pillows that I pictured when I was five, but instead I hold a deep curiosity for probing the intricate mechanisms driving their formation and climate impacts. The key takeaway: Clouds are an enormous crux in the puzzle of climate system, and it is important that we learn more about how they form so that climate change predictions can in turn be improved.