Key messages from "Overview of the Chemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) Field Campaign,” by Jose D. Fuentes (The Pennsylvania State University), Sara Lance, Kerri A. Pratt, Paul B. Shepson, William R. Simpson, Izabella Antczak, Katja Bigge, Nathaniel Brockway, Natasha Garner, Kristian D. Hajny, Daun Jeong, Robert Kaeser, Peter K. Peterson, Miranda Serratos, Tim Starn, Brian H. Stirm, and Sarah Woods. Published online in BAMS, November 2025. For the full, citable article, click the link above.
The Arctic, one of the coldest regions on Earth, is warming nearly three times faster than the global average. As temperatures rise, the once-thick sea ice is thinning, and cracks in the ice, called leads, are becoming more common. Leads release heat, aerosols, and water vapor into the atmosphere, altering the structure of the lowest atmosphere and potentially influencing cloud cover—an important climate feedback. Warming snow and ice surfaces affect sunlight-driven chemical reactions involving halogens (chlorine, bromine, and iodine) that influence the composition of the atmosphere in snow- and ice-covered regions. Increasing areas of open water in the Arctic Ocean weaken the temperature gradient between the equator and the pole, with consequences for weather patterns. To study how these cryospheric changes impact the atmosphere, scientists from Stony Brook University, Penn State University, the University of Alaska Fairbanks, the University of Michigan, and the University at Albany conducted the Chemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) field campaign from mid-February to mid-April 2022. The project investigated how changes in sea ice and Arctic industrial emissions affect the chemistry and physics of the Arctic boundary layer and formation of clouds. The team studied how sea ice leads affect aerosols that in turn impact cloud cover, as well as the evolution of halogen chemistry in the Arctic. They also investigated how emissions from oil and gas extraction in the Prudhoe Bay region modify halogen chemistry, alter multiphase halogen recycling, and influence the fate of nitrogen oxides and ozone downwind of oil fields.
The CHACHA field campaign was based in Utqiaġvik, Alaska, the northernmost city in the United States, situated at the interface between the Chukchi and Beaufort Seas. The team deployed two instrumented aircraft: the University of Wyoming King Air and Purdue University’s Airborne Laboratory for Atmospheric Research (ALAR). Both aircraft functioned as airborne laboratories equipped with instruments to measure trace gases, aerosols, cloud microphysics, winds, and turbulence. Ground-based measurements near Utqiaġvik provided ozone observations and samples of airborne particles and snow, linking surface conditions with vertical profiles collected from aircraft. The King Air completed 33 flights, sampling air over tundra, snow, ice, and open leads. The ALAR conducted 24 flights, including 9 over the Prudhoe Bay oil fields. Several flights focused on clouds forming over leads, during which dropsondes (a small instrument package to measure temperature, winds, and humidity) were released from altitudes up to 3.5 km. These coordinated observations examined the coupled interactions among sea ice, clouds, aerosols, and halogens. Reactive halogens released from salty snow and ice can catalyze chemical reactions that remove ozone from the near-surface atmosphere during late winter and early spring. By measuring halogen compounds together with aerosols and cloud properties, CHACHA sought to determine how Arctic chemical cycles are evolving as sea ice breaks up earlier in the year and industrial activity in the region expands.
During March and April 2022, CHACHA scientists observed several episodes when near-surface ozone concentrations dropped nearly to zero. These “ozone depletion events” occurred after sunrise, when sunlight triggered reactions involving reactive forms of bromine that are released from snow and sea ice. Bromine atoms react rapidly with ozone, effectively removing it from the lower atmosphere. The team documented this relationship directly: where ozone was lowest, levels of reactive bromine were highest. Flights downwind of Prudhoe Bay revealed substantial emissions of nitrogen oxides, with nitrogen dioxide reaching 60–70 ppb—hundreds of times greater than background levels in clean Arctic air. These emissions interacted with bromine compounds, altering halogen chemistry and slowing ozone depletion while generating products such as nitric acid. The findings demonstrate that industrial emissions are altering the Arctic’s natural chemical balance near sources and throughout the broader region. Analyses of particles collected near open leads showed organic-coated sea salt aerosols, which act as cloud condensation nuclei and promote cloud formation. Flights over and downwind of leads documented shallow Arctic clouds, typically 200–400 meters thick, forming directly above open water. Warm, moist air rising from leads generated small convective plumes that evolved into mixed-phase clouds containing ice crystals and supercooled liquid droplets. The aircraft measured updrafts and downdrafts of 2–3 m s–1—remarkably strong turbulence for such a cold environment. Over the leads, the atmosphere became convective and moist, allowing vertical mixing through several hundred meters that transported heat, moisture, and reactive species such as sea spray aerosols, bromine, and ozone throughout the boundary layer. Over solid sea ice, the atmosphere was cold—reaching temperatures as low as −35 °C—and strongly stably stratified, with marked temperature inversions that inhibited turbulent mixing.
The CHACHA project produced some of the first vertically resolved measurements of reactive bromine, ozone, aerosols, and lead cloud microphysics in the Arctic, especially above leads. These observations are helping scientists understand how the region’s chemical cycles are being reshaped by a warming climate, and how these changes feed back into the climate system. The CHACHA datasets will improve our understanding of Arctic atmospheric chemistry and cloud processes, clarify how industrial emissions interact with the polar environment, and help anticipate future changes as the region continues to warm and sea ice thins. The results remind us that the Arctic is not isolated. Exchanges of gases, particles, and energy among the ocean, ice, and atmosphere influence weather and climate patterns far beyond the polar regions. CHACHA is also a story of collaboration and discovery. Researchers, engineers, and students from across the United States worked together during a challenging time, amid the pandemic, in one of the harshest environments on Earth. They endured subzero temperatures and fierce winds to collect data essential for understanding our planet. Their dedication reflects how curiosity and teamwork drive scientific progress. Every flight, every sample, and every measurement brings us closer to understanding the Arctic’s role in the Earth’s climate system. The Arctic is both a laboratory and a warning—what happens there today offers a glimpse of changes that may unfold elsewhere tomorrow.
METADATA
BAMS: What would you like readers to learn from this article?
The CHACHA Project Team: The Arctic is undergoing rapid transformation driven by atmospheric and oceanic warming and human activities. In this new Arctic, thick, perennial sea ice is disappearing and being replaced by thinner, seasonal ice that fractures more easily, forming open-water channels known as leads. With continued warming, these leads are increasing in frequency and forming earlier in the season. They release heat, water vapor, and sea spray aerosols—key ingredients for cloud formation that can feed back on the regional climate. Over leads, relatively deep (300–800 meters) convective currents develop, allowing gases such as reactive halogens emitted from snow to mix throughout the boundary layer and ozone from aloft to mix down to replenish ozone-depleted air. This process influences the oxidative capacity of the Arctic atmosphere. Industrial activities, particularly oil extraction, further affect both surface and atmospheric conditions. For example, oil extraction activities in Prudhoe Bay emit substantial amounts of nitrogen oxides, leading to ambient nitrogen dioxide mixing ratios of 60–70 parts per billion—levels comparable to those in major urban centers of the United States. As the combustion pollutants are transported, they undergo reactions to form pernicious secondary pollutants such as nitric acid, disrupt regional ecosystems, alter natural chemical cycles (including snowpack-initiated halogen chemistry), and modify cloud properties.
BAMS: How did you become interested in the topic of this article?
CPT: As scientists, we are driven by the pursuit of new knowledge and the desire to understand how natural and human-driven processes shape our planet. The Arctic is one of the last frontiers on Earth, and its natural beauty is awe-inspiring! Flying in that environment is also very exciting! We are passionate about understanding how change in the Arctic is disrupting the functioning of life in the Arctic—from the algae underneath the sea ice that is at the bottom of the food chain, through the top of the food chain often represented by polar bears and bowhead whales. Personally, we are intrigued by what will happen to them if the sea ice is gone due to climate change. Scientifically, we have mostly sampled air from coastal surface sites, leaving the air and clouds above the fascinating leads largely as a mystery. In the CHACHA project, our objectives were twofold. First, we sought to determine how leads influence the release of heat, water vapor, and sea spray aerosols, and how these exchanges affect cloud formation and properties. Using aircraft flights over open leads, sea ice, and tundra, we examined the spatial variability of gases and aerosols and the chemical processes involving reactive halogens. Second, we aimed to characterize air pollutants from Arctic industrial activities, focusing on the types, quantities, and spatial extent of emissions from Alaska’s North Slope oil fields. Local Arctic pollution is often forgotten and ignored, with little known—including in the Alaskan Arctic, despite early development of the Prudhoe Bay oil field over 50 years ago. Ultimately, we sought to determine how these emissions modify the region’s natural chemical cycles and influence the evolving Arctic atmosphere. CHACHA is advancing our understanding of the combined effects of natural and anthropogenic emissions in the Arctic that remain poorly understood. We are driven to expand this knowledge that is essential for defining the current state and functioning of the Arctic system, assessing how these changes affect both the regional ecosystem and the people who depend on it, and improving simulations of future change in the new Arctic. The resilient Iñupiat Peoples, who have lived in the North American Arctic for thousands of years, stand at the forefront of global climate change. We feel privileged to study the microphysical and chemical processes of this beautiful region that the Iñupiat Peoples call home.
BAMS: What got you initially interested in meteorology or the related field you are in?
CPT: We became interested in meteorology because it provides a framework for developing and testing new theories that quantify environmental processes. The field offers unique opportunities to design and conduct experiments in the natural environment, enabling us to directly observe and measure the atmosphere to test our hypotheses. With advanced technologies, we can, in a sense, “touch and feel” the state of the environment. Most importantly, whether through theoretical analyses or applied research, our work contributes to a deeper understanding of the Earth System and supports efforts to improve environmental conditions for ecosystems and human well-being. Now more than ever, this type of science is essential, as human activities continue to alter the environment in ways that threaten the long-term sustainability of organisms, ecosystems, and societies. On the atmospheric chemistry side, there are chemical processes involving halogens derived from sea salt that are accelerated above saline snow, and as a result, the composition of the air, and how it cleans itself, is very unique in the Arctic. We want to know how all that will change if all sea ice is removed due to climate change. Whether we each get excited about meteorological or chemical variables, we are excited by interdisciplinary, collaborative, purpose-driven science that also includes the element of discovery!
BAMS: What surprised you the most about the work you document in this article?
CPT: Several unexpected findings emerged from the CHACHA field project. One notable surprise was the development of convective layers reaching depths of up to 800 meters over open leads. These convective air parcels promoted the efficient vertical transport of heat, moisture, and aerosols, resulting in the formation of mixed-phase cloud layers 300–400 meters thick. Another significant finding involved the magnitude and spatial extent of air pollutants emitted from the Prudhoe Bay oil fields, and their impacts on the natural halogen chemistry. These industrial emissions affected not only the air directly above the sources but also extended laterally, influencing regions spanning hundreds of square kilometers. It was particularly surprising to find that the impacted areas were far larger than previously assumed, given the prevailing expectation that strong winter and springtime atmospheric stability limits dispersion. Under more unstable conditions, which enhance both vertical and horizontal mixing, the affected areas are likely to expand even further. The quantities of pollutants produced by human activities disrupt natural chemical cycles—such as those involving reactive halogens—and can give rise to highly noxious compounds, including acids.
BAMS: What was the biggest challenge you encountered while doing this work?
CPT: During winter and spring, the Arctic presents a harsh environment for fieldwork, characterized by extreme cold. Under such conditions, it is logistically challenging to deploy ground-based observing systems, such as instrumented flux towers, and to operate research aircraft. It was sometimes often very difficult to operate our own bodies! They seem to sometimes object to the cold! Data-gathering missions had to be carefully planned around weather conditions to ensure not only the safety of the research teams but also the quality of the collected data. Compounding these natural challenges was the Covid-19 pandemic, which required strict health and safety precautions in all operations. In the end, the CHACHA project was a resounding success, made possible by collaboration among all participants, and their great determination and resolve to succeed. We are indeed proud of what we achieved! We measured that success through the health and safety of the team and the exceptional quality and quantity of the datasets collected during the campaign. Indeed, the large suite of samples and data available have the opportunity to motivate science for many years to come!
BAMS: What’s next? How will you follow up?
CPT: The CHACHA project provided the first opportunity to investigate the chemistry and physics in the context of the thermodynamic characteristics of the atmospheric boundary layer immediately above sea-ice leads—fractures in the ice revealing open water—using two instrumented research aircraft. The campaign produced rare and valuable data sets describing the horizontal and vertical distributions of aerosols and trace gases—including halogens, nitrogen oxides, and ozone—over snow-covered sea ice, frozen tundra, and industrial areas including oil fields. These data are currently being analyzed and interpreted to report new scientific findings in peer-reviewed journals and other outlets. Our next goal is to use these observations to develop and test numerical models that simulate the turbulent transport and chemistry of reactive gases and aerosols. We really do not know if/how well we understand the atmosphere, until we try to simulate our observations using complex three-dimensional photochemical models of the Arctic atmosphere. We also plan to apply such models to better understand and quantify how industrial emissions, particularly those associated with oil exploration and extraction, alter regional atmospheric chemistry. Recent year-round measurements in the central Arctic and on the ground within the North Slope of Alaska oil fields provide complementary data to build a more detailed understanding of this rapidly changing environment. Looking ahead, a follow-up CHACHA field campaign during the fall season would be invaluable for contrasting seasonal differences in chemical cycling and cloud formation and properties in response to changing atmospheric conditions and delayed sea ice freeze-up. The urgency to study the Arctic during this rapid change cannot be understated. The Alaskan Arctic has completely transformed over the course of just the satellite record, and this change is only accelerating!
