Investigating the potential of meltwater as a local source of ice nucleating particles in the central Arctic summer

October 15, 2024

Camille Mavis

Committee: Sonia Kreidenweis (Advisor); Jessie Creamean (Co-advisor); Jeffrey Pierce; Graham Peers (Biology)

Abstract

Due to climate change, the Arctic has crossed a threshold into positive feedbacks between sea-ice loss and increased absorption of solar radiation, causing warming up to four times the global average. Parameterizing the Arctic radiation budget to predict the new steady-state is paramount for guiding policies impacting future global socio-economics and Arctic livelihoods. Arctic mixed-phase clouds (AMPCs) are a pillar in the feedback systems by modulating the surface energy budget, depending on the partitioning of cloudwater between ice and liquid phases that is sensitive to the concentration of ice nucleating particles (INPs) in the atmosphere. However, current observational gaps of central Arctic INP concentrations and sources may contribute to current challenges in resolving the controls on Arctic cloud ice content. The year-long expedition aboard the RV Polarstern from 2019 - 2020, entitled The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), was a highly coordinated interdisciplinary effort that provided a unique opportunity to observe INPs in the central Arctic. The Arctic summer is a unique period characterized by pristine aerosol conditions, in which emissions from local sources have an increased influence, potentially impacting the ubiquitous low-lying AMPCs. Thus the summer is an ideal season for exploration of the potential importance of INPs from local sources, such as melt ponds.

In this study, we used the Colorado State University (CSU) Ice Spectrometer and chemical treatments to determine the INP concentration and inferred composition in source samples of bulk sea water and meltwater from ponds and leads over the month of July. In addition, ambient aerosol filters were deployed both on the ship and on the ice, downwind of these meltwater features. We found that the concentration of INPs in meltwater was 10 times higher than in the mixed layer of the ocean, a surprising result since previous studies did not see a difference in the two source samples. The INPs in meltwater were capable of freezing at temperatures (T) ≥ -10 °C and were predominantly biological, based on our heating assay. Biological INPs capable of freezing at T ≥ -10 °C were present in 80% of the on-ice aerosol samples. The alignment of slopes of the cumulative INP spectra between the meltwater and aerosol filter samples for T ≥ -15 °C suggested an influence from meltwater on the aerosol INPs at those temperatures. Similarities between aerosol INP sampled on the ice and on-board Polarstern suggested that the on-ice INP concentrations were likely influenced by a regional meltwater source signature, rather than being measurably impacted by a singular upwind pond. A relationship was observed between wind speed, supermicron particle counts, and on-ice aerosol INP populations active at warm (-15 °C) and cold (-25 °C) temperatures. A distinct on-ice aerosol sample containing no INPs active ≥ -15 °C was found to be influenced by southerly air over the ice-free ocean, emphasizing the potential impact meltwater may have as a unique source of warm temperature INPs in the central Arctic. These findings suggest that summertime central Arctic biological INP concentrations may increase if, as predicted, a spatio-temporal expansion of the melt season occurs in the near future. This increased INP concentration from local sources could impact central Arctic cloud microphysics, and thus their impact on the surface energy budget.