Study: Thickness and Phase of Organic Particles Affect Cloud Formation, Climate
In the upper troposphere, organic aerosols shape the formation of cirrus clouds, those wispy, high-altitude formations that are vital regulators of Earth’s energy balance.
February 6, 2025 I By Dave DeFusco
In the intricate dance of Earth’s climate system, few elements are as critical yet enigmatic as aerosols. These microscopic particles, suspended in the atmosphere, wield a dual influence: they scatter sunlight, cooling the planet, and seed clouds, indirectly altering weather patterns and climate. Among these, organic aerosols have emerged as pivotal players, particularly in the upper troposphere, where they shape the formation of cirrus clouds, those wispy, high-altitude formations that are vital regulators of Earth’s energy balance.
Cirrus clouds trap outgoing infrared radiation while reflecting incoming sunlight, with their net effect on climate hinging on their microphysical properties. A cornerstone of cirrus cloud formation is ice nucleation, a process heavily influenced by aerosols acting as ice-nucleating particles. While mineral dust and sea salts have long been recognized as efficient ice-nucleating particles, the role of organic aerosols has remained shrouded in mystery due to their complex chemical composition and variable physical states.
An Environmental Science and Technology study, “Quantifying and Modeling the Impact of Phase State on the Ice Nucleation Abilities of 2-Methyltetrols as a Key Component of Secondary Organic Aerosol Derived from Isoprene Epoxydiols,” co-authored by J.D. Surratt, professor of chemistry and of environmental sciences and engineering, illuminates a groundbreaking aspect of organic aerosols—their phase state and viscosity—shedding light on their role as ice-nucleating particles.
This discovery not only fills a crucial gap in our understanding of aerosol-cloud interactions but also paves the way for improved climate models. At the heart of the research lies the phase state of organic aerosols, which can range from liquid to semisolid to glassy. This state determines their viscosity, a property that directly impacts their ability to nucleate ice.
“Until now, a direct parameterization linking aerosol viscosity to ice nucleation rates has been elusive, leaving a significant gap in our understanding of how organic aerosols influence cloud formation and, by extension, climate,” said Dr. Surratt.
In the study, researchers delved into the ice nucleation behavior of 2-methyltetrols (2-MT), a key component of secondary organic aerosols derived from isoprene-epoxydiol (IEPOX). These secondary organic aerosols, abundant in regions like the Amazon rainforest, are major contributors to atmospheric organic aerosol mass. By experimentally measuring the ice nucleation rate of 2-MT aerosols across varying viscosities, the study revealed a striking trend: as the phase state shifted from liquid to semisolid, the ice nucleation rate surged by two to three orders of magnitude.
“This finding underscores the critical role of viscosity in governing the ice nucleation potential of organic aerosols under cirrus cloud conditions,” said Dr. Surratt. “The semisolid state, characterized by intermediate viscosity, appears to provide an optimal balance for water vapor deposition and ice crystal formation, making these aerosols highly effective ice-nucleating particles.”
To translate these experimental insights into actionable knowledge, the researchers developed an innovative parametric model based on classical nucleation theory. This model quantifies the impact of viscosity on heterogeneous ice nucleation rates and is simple enough to integrate into regional and global climate models. Its accuracy was validated against laboratory data, showing excellent agreement across a wide range of conditions.
By applying this model to real-world scenarios, such as the Amazon rainforest’s upper troposphere, the researchers predicted INP concentrations ranging from 1 to 10s per liter in cirrus-forming regions. These predictions align with field observations, reinforcing the model’s robustness and its potential to enhance climate simulations.
The implications of this research are profound. First, it highlights the underestimated role of organic aerosols in cloud microphysics, particularly in regions where they dominate aerosol populations. Second, it provides a critical tool for reducing uncertainties in climate models, especially concerning aerosol-cloud interactions—a major source of variability in climate projections.
“The study also emphasizes the interconnectedness of natural ecosystems and atmospheric processes,” he said. “For instance, the Amazon rainforest, a hotspot for isoprene emissions, indirectly influences global climate by modulating the properties of cirrus clouds through IEPOX-derived secondary organic aerosols.”
While this study marks a significant leap forward, it also opens new avenues for research. Future work could explore the interplay between the chemical composition of organic aerosols, environmental factors and ice nucleation efficiency. Additionally, extending this parameterization to other types of organic aerosols could further refine our understanding of their climatic impacts.
As the world grapples with the twin challenges of climate change and air quality, such research underscores the need for holistic approaches that consider the multifaceted roles of aerosols. By unlocking the secrets of organic aerosols and their role in ice nucleation, scientists are not only advancing our understanding of Earth’s climate system but also equipping policymakers with the knowledge to make informed decisions in the fight against climate change.
“This research is a testament to the power of interdisciplinary research, blending experimental science, theoretical modeling and field observations to unravel one of the most complex puzzles in atmospheric science,” said Dr. Surratt. “In doing so, it brings us one step closer to a more accurate and comprehensive understanding of our planet’s climate dynamics.”