Seasonal Dynamics of Arctic Marine Ice-Nucleating Particles
Authors: Castenschiold Christian Ditlev Funder , Ellebæk Anne , Finster Kai , Šantl-Temkiv Tina 
Published: March 2024

There is growing evidence that the oceans constitute an important source of ice-nucleating particles (INPs) in the atmosphere, aerosolized through sea spray. These particles play a crucial role in cloud formation and cloud properties by inducing ice crystal formation. Microorganisms, in particular, can produce ice-nucleating proteins which are efficient catalysts in the formation of ice, triggering heterogenous freezing between -1&#176;C and -15&#176;C. INPs have been measured in sea bulk water and sea surface microlayer, and specifically Arctic waters have been shown to exhibit ice-nucleation activity at high temperatures. In addition, terrestrial environments have long been recognized as substantial reservoirs of INPs. The runoff from these terrestrial environments, facilitated by meltwater and rivers, could have the potential to contribute a substantial influx of INPs to coastal marine environments. Our understanding of the extent of this input, the properties, and concentrations of INPs, and their connection with the microbial community in sea bulk water and sea surface microlayer remains limited. Furthermore, there is a lack of investigation into the temporal and spatial distribution of INPs in sea water. This information, coupled with atmospheric INP measurements, is needed to improve predictions of INP emissions from the ocean to the atmosphere. Therefore, we conducted a sampling campaign at Disko Island, Greenland, and collected sea bulk water, sea surface microlayer, and air samples from May to September 2023. Freshwater samples were collected from a river in continuation of a marine transect spanning eight km offshore to investigate the impact of terrestrial runoff on the coastal marine microbial community and INPs. Our investigation reveals distinct seasonal variations in INP concentrations, ice-nucleation activity, and microbial community at a regularly visited marine site throughout the sampling campaign. Air samples were collected simultaneously at this marine site, enabling the measurement of INP concentrations and the exploration of the microbial community present in the immediate overlaying air masses. Additional air samples were consistently collected from a foreland, located approximately five km from the marine sampling site, using a high-flow-rate impinger with a specific focus on capturing sea spray emissions and facilitating the integration of the data to the marine water samples. Our results further demonstrate a pronounced input of INPs originating from terrestrial runoff into the sea surface microlayer within coastal marine waters. Notably, this was not observed in the bulk water due to the stratification resulting from the introduction of freshwater. Our study unveils seasonal dynamics of INPs and microbial communities and a prominent impact of terrestrial runoff in Arctic marine waters. The study emphasizes the importance of considering the marine environment as a major source of atmospheric INPs and, further, contributes valuable insights to improve predictions of INP emissions from the ocean to the atmosphere.