Source Tracking of Ice Nucleating Particles from an Active Thermokarst Region

The Arctic is warming at a rate nearly 4 times that of lower latitudes; a phenomenon called Arctic Amplification. This, in turn, leads to significant landscape changes, especially in permafrost regions where thawing permafrost and development of thermokarst lead to subsidence and surface collapse. Permafrost has previously been shown to be a rich reservoir of ice nucleating particles (INPs), that catalyze ice formation at temperatures up to -5 °C, in high concentrations of approximately 10⁸ g^-1 at -15 °C. If these particles can be aerosolized, lofted into the atmosphere, and transported to cloud level, they can alter the lifetime and radiative effects of clouds. This has implications for Arctic mixed-phase clouds that are known to persist for days, potentially due to low abundances of INPs. A permafrost source of INPs is not currently included in climate models. Measurements of permafrost and other potential sources (e.g., sediment, seawater, vegetation) in a thermokarst landscape were made in September 2021 at peak summer season thaw (active layer development) around Utqiaġvik, Alaska, as part of the ARCtic Study of Permafrost Ice Nucleation (ARCSPIN) project. A significant finding from ARCSPIN was that the entire thermokarst landscape is an active source of INPs, from the vegetation, to the seasonally thawed active layer, to the thermokarst lake water itself.

Aerosol samples were collected upwind and downwind of several thermokarst lakes and at a fixed site with aim to uncover which potential sources may contribute the most to the observed INPs. The low abundances of INPs necessitate indirect methods for characterization to infer bioaerosol types. We developed a heat-curve test, which was applied to source and aerosol samples to determine the impacts on INP concentrations after heating to 50, 95, 121, and 180 °C. Results from a representative site indicated high sensitivity to 50 °C (a proxy for ice nucleation active bacteria) from the ice wedge, thermokarst lake, vegetation, and aerosol samples warmer than -20 °C, but relative insensitivity to this modest heat for samples from the active layer, sediment, and permafrost. The 180 °C treatment showed large concentration reductions in the sediment, permafrost, active layer, thermokarst lake water, and in vegetation samples across the temperature spectrum. No effect of heating to 180 °C was detected in the ice wedge and aerosol samples for INPs active at -25 °C, indicating a potential common INP among these samples. Results from additional samples will be presented, as well as other indirect methods under development, including principal component analysis to define INP spectral types. Combined with information on bacterial and fungal sequences found in the air, terrestrial, and aquatic samples, some inferences as to the most likely aerosolization pathways for INPs can be made.