A Novel Mechanism of Sea-Surface Microlayer Formation Driven by Terrestrial Runoff: A Source of Ice Nucleating...

Biological aerosols serve as efficient ice-nucleating particles (INP), driving the liquid-to-ice transition in clouds and thereby shaping cloud thickness, lifetime, albedo, and precipitation. Although atmospheric INP derive from terrestrial, freshwater, and marine environments, their specific sources and microbial producers remain largely unresolved, especially in the rapidly warming Arctic. Here, we examine terrestrial runoff as a driver of INP enrichment during summer in Young Sound fjord, Northeast Greenland. INP concentrations active at -10 °C were significantly elevated in the sea-surface microlayer (SML) and at river outlets relative to underlying seawater, correlating negatively with salinity and distance from freshwater input, highlighting terrestrial runoff as a dominant source of warm-temperature INP. We identify a novel runoff-driven mechanism of SML formation in Arctic coastal waters, whereby terrestrial particles and compounds are transported downstream and selectively accumulate at the air-water interface likely due to their surface-active properties combined with limited seawater mixing driven by salinity gradients. Biological INP were also observed in the atmosphere (1.3-25.3 INP_-10 m^-3), with sequencing data suggesting terrestrial origins. Our findings demonstrate that terrestrial ecosystems strongly enrich the coastal SML with biological INP, constraining the pool of potential atmospheric INP sources and underscore Arctic coastal interfaces as key regions for cloud-relevant processes under ongoing climate change.