New Insights Into the Composition and Origins of Ultrafine Aerosol in the Summertime High Arctic

The summertime high Arctic atmosphere is characterized by extremely low aerosol abundance, such that small natural aerosol inputs have a strong influence on cloud formation and surface temperature. The physical sources and the mechanisms responsible for aerosol formation and development in this climate‐critical and changing region are still uncertain. We report time‐resolved measurements of high Arctic Aitken mode (∼20–60 nm diameter) aerosol composition during August–September 2018. During a significant Aitken mode formation event, the particles were composed of a combination of primary and secondary materials. These results highlight the importance of primary aerosol sources for high Arctic cloud formation, and they imply the action of a poorly understood atmospheric mechanism separating larger particles into multiple sub‐particles. Plain Language Summary Clouds are an important part of Earth's climate system, in part because they play a role in controlling how much energy passes from the sun to Earth's surface and from Earth's surface out into space. Aerosols, or disperse atmospheric particles, act as seeds onto which water can condense to form cloud droplets. The high Arctic region, near the North Pole, is a unique environment because it often has very low numbers of aerosols, which impacts the formation of clouds and their energy trapping and releasing properties. We set out to learn more about how aerosols form in this remote region, where climate is rapidly changing, with likely impacts on aerosol sources. We measured the composition of small, so‐called ultrafine particles and found that they are made of a combination of sea spray and molecules added onto them as gases from the air. Surprisingly, it seems that sometimes large numbers of particles are formed from the breakup of larger composite particles, a process which has been hypothesized but, if it is occurring, is still not understood. Key Points Aitken mode (20–60 nm) particles in the summertime high Arctic were composed of aged, organic‐rich sea spray and secondary compounds An intense Aitken mode number increase was driven by the atmospheric breakup of pre‐existing aerosol by an unknown mechanism