In the swirling dance of atmospheric molecules—where invisible actors choreograph the fate of our climate, air quality, and even human health—there lies a group of fleeting yet powerful players known as Criegee intermediates (CIs). These high-energy compounds emerge when ozone collides with alkenes, a class of hydrocarbons emitted both naturally and by human activity. For decades, scientists have glimpsed the shadows of these intermediates, aware of their importance but never fully capturing their behavior. Now, an astonishing new discovery is rewriting the rules of atmospheric chemistry and uncovering a reaction pathway that may be far more influential than we ever imagined.
The Atmospheric Puppeteers: Criegee Intermediates in Action
To understand the impact of Criegee intermediates, one must first appreciate their role in the atmospheric symphony. When ozone, a potent oxidizer, reacts with alkenes such as those released by plants or vehicle exhaust, it creates a range of highly reactive compounds, among them Criegee intermediates. These are not long-lived molecules—they’re ephemeral, persisting for milliseconds—but during their brief existence, they wield immense influence.
CIs, particularly a species called syn-CH₃CHOO, are instrumental in forming hydroxyl (OH) radicals, often referred to as the “detergents” of the atmosphere. These OH radicals clean the air by breaking down pollutants and greenhouse gases like methane. CIs also initiate the formation of secondary organic aerosols, tiny particles suspended in the air that affect cloud formation, climate regulation, and human respiratory health.
Among all CIs, syn-CH₃CHOO stands out. Depending on the season and atmospheric conditions, it constitutes a staggering 25% to 79% of all Criegee intermediates. That means any new understanding of its behavior has ripple effects across models of climate change, pollution, and even public health policies.
The Old Story: A Decomposition Dominance
For years, scientists believed they had a handle on how syn-CH₃CHOO disappeared from the atmosphere. The consensus was that its fate was largely sealed by a process called unimolecular decomposition—essentially, the molecule destabilizes and falls apart on its own, without needing to react with anything else. This idea helped atmospheric chemists plug important values into global climate and air quality models. But it now appears that this framework may have missed a crucial piece of the puzzle.
In a landmark study published in Nature Chemistry, a team of researchers from the Dalian Institute of Chemical Physics (DICP), part of the Chinese Academy of Sciences, has challenged this long-standing view. Led by Profs. Yang Xueming, Zhang Donghui, Dong Wenrui, and Fu Bina, the team discovered that water vapor—ubiquitous in the atmosphere—is reacting with syn-CH₃CHOO far more aggressively than anyone had anticipated.
The Stunning Discovery: Water Speeds Things Up
Using ultra-sensitive laser spectroscopy techniques, the researchers directly measured the reaction rate between syn-CH₃CHOO and atmospheric water vapor. The results were nothing short of shocking: the reaction occurred nearly 100 times faster than theoretical models had predicted. This wasn’t just a tweak to existing data—it was a bombshell that suggested the water pathway could rival or even exceed self-decomposition as a dominant sink for syn-CH₃CHOO.
But why were the models so wrong? Why did this particular reaction fly under the radar for so long? The answer lies not just in experimental technique but in the realm of computational chemistry, where a revolution in how we simulate molecular interactions is yielding extraordinary insights.
Enter the 27-Dimensional Puzzle
To probe the underlying cause of this unexpected acceleration, the DICP researchers turned to an emerging tool: full-dimensional quantum dynamics. They constructed a 27-dimensional potential energy surface (PES)—a kind of hyper-detailed map of all possible movements and interactions between atoms in the reacting molecules. But they didn’t rely on traditional methods to build this surface. Instead, they used a cutting-edge fundamental invariant-neural network (FI-NN) approach that fuses machine learning with quantum chemistry to generate ultra-accurate simulations of molecular behavior.
With this detailed PES in hand, the team ran full-dimensional dynamical simulations of the reaction between syn-CH₃CHOO and water. What they found astonished even seasoned chemists.
The Roaming Revolution: A New Reaction Mechanism Emerges
Contrary to expectations, the reaction didn’t follow the usual “minimum energy path”—the path of least resistance that most chemical reactions take. Instead, the molecules engaged in what’s now being called a “roaming mechanism.” Rather than crashing together directly, the Criegee intermediate and the water molecule drift near each other, bound not by a tight embrace but by long-range dipole-dipole interactions—the same kinds of attractions that help water molecules cling together in a droplet.
These interactions allow the molecules to loiter in close proximity, forming a sort of reactive cloud. In this cloud, the molecules have time and flexibility to reorient and exchange energy, making it far more likely that a successful reaction will occur, even though the molecules aren’t in direct contact.
This roaming behavior represents a paradigm shift in how we understand certain chemical reactions. It’s like discovering that two dancers don’t need to collide on stage to influence each other’s movements—they can perform an intricate duet just by swirling in each other’s gravitational pull.
Rewriting Atmospheric Models
The implications of this discovery are profound. If the water-induced roaming reaction is as significant as self-decomposition, then current atmospheric models are missing a major sink for syn-CH₃CHOO. That miscalculation could cascade into errors in how we predict everything from smog formation and air pollution episodes to the efficiency of atmospheric cleansing and the life span of greenhouse gases.
Moreover, this finding suggests that the influence of water vapor—a ubiquitous atmospheric component—has been vastly underestimated. Water isn’t just a passive participant in atmospheric chemistry; under the right conditions, it’s a highly reactive agent capable of driving critical transformations.
Beyond the Sky: Roaming Across the Cosmos and Engines
While this research is rooted in atmospheric science, the ramifications stretch far beyond Earth’s troposphere. The idea that roaming mechanisms driven by long-range electrostatic interactions can speed up or dominate chemical reactions has potential consequences for other fields where such interactions are common.
In combustion chemistry, for example, fuel molecules mix with oxidizers in high-energy environments, often producing transient species similar to Criegee intermediates. If roaming mechanisms are at play here, they could inform the design of more efficient and cleaner-burning engines.
In astrochemistry, the interstellar medium is full of cold, sparse gases where traditional collisional reactions are rare. Yet molecules somehow manage to assemble into more complex compounds. Could roaming reactions help explain how chemistry unfolds in the vacuum of space? The answer may lie in research yet to be conducted—but the spark has been lit.
A New Lens on Invisible Chemistry
At its heart, this discovery is a story about the power of observation—and the dangers of assumptions. For years, our understanding of atmospheric chemistry rested on the belief that certain pathways were dominant, others negligible. But molecules, it seems, don’t always behave the way we think they should. Sometimes, they wander. Sometimes, they find each other in the fog, not with a bang, but with a subtle electric whisper.
The fusion of state-of-the-art experimental techniques and neural network-driven quantum simulations is not just refining our models—it is revealing a previously invisible dimension of chemistry. By capturing how real molecules interact in real environments, we’re opening doors to more accurate weather predictions, better climate models, and new frontiers in chemical science.
The Road Ahead: Embracing Complexity
The path forward is both exciting and challenging. As scientists incorporate this new understanding of syn-CH₃CHOO and its interactions with water, atmospheric models will need updating. Policies based on air pollution predictions may require revision. More broadly, chemistry itself must evolve to embrace non-traditional mechanisms, like roaming, that fall outside the neat frameworks of old.
But there’s also a philosophical lesson here. Nature is subtle, intricate, and often surprising. The invisible forces shaping our world are not always brute collisions but delicate dances governed by faint electromagnetic tugs. To truly understand our atmosphere—and by extension, our planet—we must be willing to look deeper, question assumptions, and listen to what the molecules are trying to tell us.
And they are whispering, it seems, of new paths, of hidden forces, and of the marvelous complexity that lies just beyond the limits of our models.
Reference: Yiqiang Liu et al, Reactivity of syn-CH3CHOO with H2O enhanced through a roaming mechanism in the entrance channel, Nature Chemistry (2025). DOI: 10.1038/s41557-025-01798-9
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