The findings, published in Nature Geoscience, provide the first field evidence for a mechanism previously seen only in controlled laboratory experiments at CERN’s CLOUD chamber. An international team led by the University of Birmingham detected a mixture of iodine, sulfur and organic compounds in the atmosphere around Greenland and the Davis Strait during an expedition in spring and summer 2022. Under sunlight, these gases reacted to form new atmospheric particles, some of which grew large enough to act as cloud condensation nuclei—the microscopic seeds on which water vapor condenses to form cloud droplets.

The most dramatic changes occurred in the marginal ice zone, a narrow and highly dynamic region where open ocean meets melting sea ice. During one observed event, the concentration of cloud-seeding particles rose from approximately 50 to 1,500 particles per cubic centimeter. Researchers found that new particle formation occurred on more than 80 percent of sunny days in the study area, suggesting that the process is not an isolated chemical curiosity but a recurring feature of the Arctic atmosphere during the brighter months.

The chemistry begins with emissions from several natural sources. Iodine compounds are released by seawater, sea ice and coastal environments, while dimethyl sulfide—a sulfur-containing gas commonly abbreviated as DMS—is produced by marine microorganisms, algae and other forms of ocean life. Organic vapors also enter the atmosphere from the ocean and surrounding land. Sunlight transforms these gases through a sequence of oxidation reactions, generating highly reactive molecules that can cluster together and form new particles only a few nanometers across.

At that size, however, the particles are too small to reliably affect clouds. Their climatic importance depends on whether they can continue growing before they are removed from the atmosphere. The researchers identified a previously unknown group of compounds called iodine-containing oxygenated organic molecules, or I-OOMs. These molecules appear to contribute to the growth and stabilization of newly formed particles, helping them reach the size required to activate as cloud condensation nuclei. Their detection points to new pathways in atmospheric iodine chemistry that were not included in existing descriptions of Arctic aerosol formation.

Clouds are among the most influential and uncertain components of the climate system. By reflecting incoming sunlight, they can cool the surface, but by trapping outgoing infrared radiation, they can also produce a warming effect. The balance depends on cloud altitude, thickness, droplet size, water and ice content, and the season. More cloud condensation nuclei can lead to clouds containing greater numbers of smaller droplets, potentially changing how much sunlight the clouds reflect and how long they persist. In the Arctic, even modest shifts in cloud properties can have consequences for sea ice and ocean temperatures.

The researchers emphasize that the discovery does not yet establish whether the newly formed particles will produce an overall warming or cooling effect. That outcome will depend on how the particles interact with different cloud types and atmospheric conditions. During the Arctic’s sunlit season, brighter clouds could reflect additional solar energy and cool exposed ocean surfaces. Yet clouds can also act as an insulating blanket, retaining heat near the surface and potentially accelerating sea-ice loss. Their influence may vary from one location and season to another, making the newly identified process important but difficult to translate into a single climate prediction.

The finding is particularly significant because the Arctic is warming more than three times faster than the global average, while its sea ice is retreating and the marginal ice zone is expanding. As more productive ocean water becomes exposed near the ice edge, marine algae and other organisms may release greater quantities of iodine-, sulfur- and carbon-containing compounds. This creates the possibility of a feedback involving sea-ice loss, biological activity, atmospheric particle formation and cloud evolution. Whether that feedback amplifies or moderates warming remains unknown, but its omission from climate models means current projections may be missing an important piece of Arctic atmospheric chemistry.

The team is now working to determine how widespread the process is beyond the waters surveyed during the expedition and how efficiently the particles become cloud droplets under different conditions. Incorporating iodine chemistry, marine sulfur emissions and I-OOM formation into climate models could improve simulations of Arctic clouds and radiation. The study also highlights how rapidly changing environments can expose chemical interactions that are difficult to reproduce through theory alone. In a region already undergoing profound transformation, a previously invisible population of particles may help determine what the Arctic sky looks like—and how much heat the planet keeps or reflects.

Subject of Research: Atmospheric particle formation and Arctic cloud condensation nuclei

Article Title: Arctic cloud condensation nuclei enhanced by iodine, sulfur and organic precursors

News Publication Date: 5 August 2026

References: Mao Du, James Brean, Douglas R. Worsnop et al., “Arctic cloud condensation nuclei enhanced by iodine, sulfur and organic precursors,” Nature Geoscience.

Keywords: Arctic, climate change, sea ice, marginal ice zone, cloud condensation nuclei, atmospheric chemistry, iodine, sulfur, dimethyl sulfide, marine algae, cloud formation, aerosols, Arctic warming, climate models

For decades, the scientific community has grappled with the intricacies of how atmospheric pollutants, especially reactive nitrogen species like nitrate, travel thousands of kilometers from industrialized regions to the pristine Arctic. Traditionally, researchers focused on emission sources and atmospheric transport models, but many observed discrepancies called for a deeper insight into the chemical transformations taking place en route. The newly described acidity-dependent partitioning process addresses these longstanding enigmas by highlighting how the physical and chemical properties of aerosols interact dynamically with the surrounding gaseous environment.

At the crux of this research lies the phenomenon of gas-particle partitioning — the reversible distribution of chemical species such as nitrate between the gaseous phase and particulate matter suspended in the atmosphere. The study demonstrates that the acidity of these particles plays a decisive role in controlling how much nitrate exists in particle form versus gaseous form. Essentially, when aerosols become more acidic, nitrates are more likely to transfer from gas to particle phases. This shift significantly impacts the particles’ atmospheric lifetime, transport distances, and eventual deposition patterns.

The team, led by Iizuka, Matsumoto, Kawakami, and colleagues, employed a combination of long-term atmospheric measurements, advanced chemical transport modeling, and laboratory simulations to recreate the complex Arctic chemical environment. By integrating data spanning over a century, the researchers chronologically mapped nitrate concentrations and speciation changes, effectively linking them to industrialization timelines and global emission patterns. Their findings compellingly indicate that increasing aerosol acidity during the industrial revolution fundamentally altered how nitrate behaves in the atmosphere.

One of the study’s most striking insights is the identification of acidity as a regulatory "switch" that dictates nitrate’s phase partitioning, thus influencing its atmospheric fate. In less acidic conditions, nitrate remains predominantly gaseous, limiting its ability to condense onto particles and be transported over long distances. Conversely, higher acidity conditions facilitate the formation of particulate nitrate, which can hitch rides on aerosols, traveling farther and depositing in remote Arctic environments. This mechanism accounts for observed trends in Arctic nitrate levels that previously resisted explanation by standard transport models.

Understanding this gas-particle partitioning mechanism provides critical clarity on Arctic pollution trends, especially given recent concerns about the increasing influx of anthropogenic nitrogen compounds in the region. These nitrogen compounds contribute to climate forcing, influence atmospheric radiative balance, and affect terrestrial and marine ecosystems through nutrient deposition. The implications extend beyond pure chemistry, offering predictive insights relevant to policymakers aiming to evaluate the effectiveness of emission control strategies.

Moreover, the study underscores how industrial activities over the past century have unintentionally transformed the Arctic atmosphere’s chemical landscape. Emissions of sulfur dioxide (SO₂) and nitrogen oxides (NOx) from fossil fuel combustion increase aerosol acidity via sulfate formation, thereby indirectly boosting particulate nitrate formation and transport. This interconnected chemical feedback loop highlights the nuanced interplay between multiple pollutants, challenging the conventional approach to mitigating Arctic contamination by focusing on individual substances.

Laboratory experiments recreated artificial acidic aerosols, establishing the thermodynamic equilibrium constants governing nitrate partitioning under varying pH levels, temperature, and humidity. These controlled studies confirmed that even subtle changes in aerosol acidity markedly shift the balance between gaseous and particulate nitrate species. Coupling laboratory insights with atmospheric observations empowered the researchers to validate their models and project future nitrate transport scenarios under different emission trajectories.

Additionally, this research contributes a vital piece to the puzzle of atmospheric nitrogen cycles in cold climates, where photochemical reactions slow down, and deposition processes dominate. It highlights that chemical partitioning, mediated by aerosol acidity, influences not only where nitrates end up but also the rates at which they deposit onto surfaces, shaping nutrient availability and acidification processes within Arctic ecosystems.

The investigation did not stop at identifying mechanisms but delved into the temporal evolution of nitrate transport. By analyzing ice core samples alongside atmospheric data, the researchers reconstructed historical deposition rates and correlated them with known industrial milestones. This temporal perspective confirms that nitrate deposition surges align closely with periods of increased aerosol acidity, pointing toward the industrial era as a pivotal phase in altering atmospheric transport chemistry.

Importantly, the findings prompt a reevaluation of how global climate models represent reactive nitrogen transport and deposition in high-latitude areas. Incorporating acidity-driven partitioning mechanisms will enhance model fidelity, improving predictions related to aerosol radiative forcing and nitrogen-driven ecological effects under different future emission scenarios. This research thus opens new avenues for improving environmental assessment tools targeting Arctic preservation.

The study also emphasizes the need for continued and expanded atmospheric monitoring in polar regions, employing advanced chemical sensors capable of differentiating nitrate species and measuring aerosol acidity in situ. Such data will be essential to track ongoing shifts in pollution dynamics driven by global industrialization, climate change, and evolving emission regulations, helping scientists discern anthropogenic influences from natural variability.

From a broader perspective, the work highlights an often-overlooked aspect of atmospheric chemistry—the delicate balance of acidity in governing pollutant behavior far from primary emission sources. The interconnectedness of chemical species, aerosol microphysics, and climatic conditions exemplifies the complexity of Earth’s atmosphere and underscores why addressing environmental challenges requires multidisciplinary approaches.

In summary, Iizuka and colleagues have elucidated a fundamental chemical process controlling nitrate transport to the Arctic, fundamentally shaped by aerosol acidity changes over the industrial era. Their findings offer a robust framework to understand past and present atmospheric nitrogen trends and provide essential insights for future climate and pollution modeling efforts. As Arctic environments continue to evolve in response to human activities and climate change, comprehending these intricate chemical pathways remains paramount for safeguarding the region’s ecological integrity.

Scientists and policymakers alike now have a valuable new tool to interpret Arctic nitrogen cycles and anticipate how anthropogenic emissions influence this critical and sensitive region. With climate change accelerating and industrial activities persisting, continuous advancements in atmospheric chemistry will be indispensable for crafting effective responses to protect Earth’s northernmost frontiers.

Subject of Research: Atmospheric Chemistry and Transport of Nitrate to the Arctic with a Focus on Acidity-Driven Gas-Particle Partitioning

Article Title: Acidity-driven gas-particle partitioning of nitrate regulates its transport to Arctic through the industrial era

Article References:
Iizuka, Y., Matsumoto, M., Kawakami, K. et al. Acidity-driven gas-particle partitioning of nitrate regulates its transport to Arctic through the industrial era. Nat Commun 16, 4272 (2025). https://doi.org/10.1038/s41467-025-59208-0

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