In a groundbreaking study led by researchers at the University of California Riverside, conventional wisdom about cloud formation is being challenged by an unexpected factor: trace gases long dismissed as irrelevant. The new findings suggest that these minute volatile organic compounds present in the atmosphere play a critical and complex role in determining whether clouds form and, consequently, whether precipitation occurs. This research, published in the prestigious journal Science Advances, offers fresh insights into the microphysical interplay at the air-water interface that governs cloud droplet nucleation, a process essential to understanding weather patterns and climate dynamics.
Historically, cloud formation has been understood primarily as a function of relative humidity surpassing 100% and the presence of aerosol particles known as cloud condensation nuclei (CCN). These nuclei, consisting of salts, dust, or pollution particles suspended in the atmosphere, provide surfaces upon which water vapor condenses to form droplets. The supersaturation threshold necessary for droplet formation depends intricately on the size and chemical properties of these CCN. Scientists have largely modeled this as a particle-phase dominated phenomenon. However, the UC Riverside team’s meticulous observations reveal that the trace gas component of the atmosphere — often overlooked due to its low concentration — can dramatically influence the nucleation efficiency of these particles.
The study involved collecting air samples from the marine cloud layer of Southern California, near Mt. Soledad in La Jolla, a region known for its chemical complexity due to the admixture of oceanic and urban emissions. Through a novel experimental approach, the researchers employed a charcoal-based gas scrubber to selectively remove trace gases from these air samples. The results were startling: the ability of aerosol particles to activate into cloud droplets shifted significantly with the presence or absence of these trace gases. Such rapid and substantial effects were previously thought impossible, especially given the minor concentration of these compounds and the brevity—mere seconds—of their removal.
Markus Petters, an atmospheric chemist and co-author of the study, remarked that the magnitude of change resembled the impact one might expect from prolonged exposure to ultraviolet radiation or elevated temperatures on aerosol particles. Remarkably, a brief 20-second removal of trace gases elicited comparable shifts in cloud droplet formation thresholds. This finding challenges the classical paradigm, revealing a previously unknown sensitivity of the cloud microphysics system to the ambient trace gas milieu, igniting new scientific curiosity about the exact molecular and kinetic mechanisms involved.
While the precise identity of the responsible trace gases remains unresolved, the researchers hypothesize that organic acids such as formic and lactic acid could be major players. These compounds are known to exist ubiquitously in the atmosphere but had not been fully studied in the context of their influence on particle activation and cloud formation. The discovery opens an imperative pathway for atmospheric science to integrate gas-particle interfacial chemistry with traditional aerosol-cloud interactions, a step that may substantially enhance the fidelity of cloud and climate models.
The implications of this discovery extend far beyond meteorological curiosity. Clouds exert a powerful influence on the Earth’s radiation budget by reflecting incoming solar radiation and modifying surface temperatures. Moreover, the formation and behavior of clouds are pivotal for precipitation processes and, consequently, for freshwater availability and ecosystem health. Despite decades of advances, accurately predicting cloud development remains one of the thorniest and most uncertain aspects of climate simulation. The revelation that trace gases have a tangible, and sometimes counterintuitive, effect on droplet nucleation may finally illuminate a key missing variable that has eluded scientists.
Intriguingly, the study noted that the presence of certain trace gases did not always facilitate droplet formation as thermodynamic theories would predict. Instead, some gases appeared to suppress it, suggesting complexity in the surface chemistry at play. This paradox points to a gap in the current understanding of air-water interfacial dynamics and calls for revisiting the assumptions underpinning cloud microphysics models. The unexpected suppression effect highlights that the interplay between gas molecules and particle surfaces may involve more than just simple alterations in surface tension, hinting at deeper molecular interactions.
Co-author Elavarasi Ravichandran, a UCR doctoral student, emphasized the novelty and significance of these results, underscoring that this was an area ripe for further exploration. The findings do not just add a variable to cloud physics equations; they redefine the conceptual framework regarding the microenvironment where water vapor transitions into liquid droplets. The air-water interface, influenced by trace gases, emerges as a dynamic zone of complex chemical and physical exchanges rather than a passive surface, challenging the field to develop more advanced experimental and theoretical tools.
This research forms part of the larger efforts spearheaded by the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE), a collaborative initiative coordinated by the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Program. Alongside UCR, UC San Diego’s Scripps Institution of Oceanography is also a key participant, reflecting the interdisciplinary nature of atmospheric science today. These collaborations integrate field observations, lab experiments, and advanced modeling to decode the multifaceted processes shaping clouds, aerosols, and precipitation in dynamic coastal and marine environments.
The UC Riverside Center for Environmental Research and Technology (CE-CERT) plays a pivotal role in this project, aligning with its mission to tackle environmental challenges through rigorous scientific inquiry and technology development. CE-CERT’s expertise in air pollution research, emissions testing, and renewable energy innovation provides a rich backdrop for conducting such interdisciplinary and high-impact studies. The evolving understanding of trace gas influences on cloud formation also underscores the importance of comprehensive air quality monitoring in regions subjected to urban and marine air mass mixing.
As the researchers continue their quest to identify which gases specifically modulate droplet nucleation, the broader scientific community will be attentive to the potential integration of these findings into climate models. Improved parameterizations capturing trace gas effects could sharpen precipitation forecasts and enhance predictions of cloud feedback mechanisms, ultimately contributing to more reliable projections of climate change impacts. The discovery exemplifies how minute constituents of the atmosphere can yield outsized effects on fundamental Earth system processes, serving as a reminder of nature’s intricate interconnectedness.
In summary, the UC Riverside-led study overturns long-standing assumptions that trace gases play a negligible role in cloud droplet formation. By demonstrating that removing these gases significantly alters aerosol activation, sometimes paradoxically suppressing droplet nucleation, this research challenges existing thermodynamic models and heralds a new frontier in atmospheric chemistry and physics. Future research will need to decode the molecular-level interactions at the air-water interface to clarify the mechanisms at work and expand our understanding of how trace gases influence global weather and climate systems.
Article Title: Removal of trace gases can both increase and decrease cloud droplet formation
News Publication Date: 14-Jan-2026
Web References: 10.1126/sciadv.adx096
Image Credits: Markus Petters & Elavarasi Ravichandran, UC Riverside
Keywords: cloud formation, cloud condensation nuclei, trace gases, volatile organic compounds, aerosol activation, atmospheric chemistry, supersaturation, air-water interface, climate modeling, atmospheric physics, precipitation processes, VOCs
