A year in the central Arctic: Evaluating aerosols and their impacts on clouds

Aerosols can have a significant impact on surface temperature through their interactions with clouds, especially in remote regions such as the central Arctic. Our study focuses on evaluating the abundances and sources of aerosols that affect cloud ice formation for an entire year over the sea ice.
A year in the central Arctic: Evaluating aerosols and their impacts on clouds

The Arctic is changing faster than anywhere else on Earth. As it warms rapidly, melting glaciers and sea ice, and thawing permafrost cause feedback effects that amplify atmospheric warming, affecting weather and climate globally. Arctic clouds play a substantial role in regulating surface temperatures, but the formation and lifetimes of these clouds are poorly constrained in models. This is in part due to the limited understanding of interaction processes between aerosols and clouds, specifically, aerosols that serve as seeds for cloud ice formation.

Cloud ice is important on a global scale. It is a key ingredient for initiating precipitation and it affects the lifetime of clouds, and thus, resulting effects on the surface energy balance. In the Arctic, clouds help regulate this balance over dynamic and evolving surfaces such as the central Arctic sea ice. As the sea ice has been declining steadily over time, the need to understand the role of clouds in modulating light and heat reaching the sea ice surface is imperative. 

The sea ice changes over the year due to the dichotomy of the seasons -- from the 24-hour-sunlight summer to the 24 hours of dark and cold conditions in the winter. These contrasting seasons, in addition to the spring and fall transitions between freeze up and melt, also encompass different sources of air masses, introducing variable levels of moisture, atmospheric dynamics, and aerosols that fuel cloud particle formation. 

In our paper published in Nature Communications, we evaluate the first ever annual cycle of aerosols called ice nucleating particles (INPs) in the central Artic. Aerosols are typically required to form cloud droplets, which can freeze into ice when the optimal subzero temperatures are met. This process is called "immersion freezing" and is arguably the most important pathway for cloud ice formation. However, only a subset of aerosols can facilitate cloud ice crystal formation by serving as INPs. These INPs can include mineral particles from terrestrial locales or organic materials from sea spray that form ice at relatively cold temperatures (< –15 °C), to primary biological particles such as bacteria, fungi, pollen, viruses, and phytoplankton (just to name a few) from a host of marine and terrestrial sources that form ice at relatively warm temperatures (> –15 °C and even up to –2 °C).

The overarching goal of our research was to assess how the quantities and sources of INPs change over the course of a full year, to ultimately link to cloud properties and help guide modelling efforts focused on clouds and their radiative impacts over the Arctic sea ice. To achieve this goal, we participated in the largest polar research expedition in history -- the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. This expedition involved scientists from all over the world, and from many walks of life and scientific expertise. Our research team, led by Colorado State University, entailed an international collaboration and interdisciplinary approach to understanding the "story" of central Arctic INPs.

We used a combination of in situ and remote sensing aerosol, meteorological, sea ice, and ecological datasets, and airmass modelling simulations, to help explain where these INPs originated from throughout the year. Our results revealed a strong seasonality of INPs, with lower concentrations in the winter and spring controlled by transport from lower latitudes, to enhanced concentrations of INPs during the summer melt, likely from marine biological production in local open waters such as melt ponds or cracks in the ice called "leads". Colder and higher clouds in the winter may be affected by INPs that form ice at colder temperatures, specifically mineral dust INPs from continental sources. Given the wintertime Arctic atmosphere is highly stratified, and the surface is mostly frozen over, these long-range sources that travel at higher levels in the atmosphere become increasingly important. In the spring and fall, the lower Arctic atmosphere can be mixed, meaning the near-surface sea spray sources of INPs from south of the pack ice in the larger Arctic region can be advected poleward and lofted to the clouds above. The Arctic summer is unique, in that it typically has lower and warmer clouds, where local biology that thrives in the sunlit, newly-opened waters serves as a source of INPs that can form ice at relatively warm temperatures (up to –6 °C as we observed during MOSAiC).

As aerosol pollutant emissions decrease, amplified warming leads to sea ice retreat and enhanced biological productivity in the ice and ocean. Understanding the sources of natural aerosols that seed clouds from within the Arctic then becomes essential as the Arctic becomes potentially rainier and cloudier. Our new results raise the possibility that strong seasonal variability in local marine, and episodic long-range transported terrestrial emissions largely control the central Arctic INP population and its subsequent influence on cloud formation and phase.

While the presence of cloud liquid water significantly increases cloud radiative effects on the surface, converting some of that liquid to ice reduces the radiative impact. Ultimately, INP-modulated changes in cloud phase partitioning can significantly impact the energy budget at the sea ice surface. We anticipate that information gleaned from our INP work can help guide models of all scales to improve their representation of Arctic clouds and the resulting surface energy budget.

Poster photo credit: Lianna Nixon (https://liannanixon.com/)

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