Sea Ice as a Source of Ice Nucleating Particles: Biogenic Origin and Atmospheric Relevance
Arctic clouds strongly modulate surface energy balance, yet the seasonal sources of warm-temperature ice-nucleating particles remain poorly constrained. Here we show that first-year sea ice is a concentrated reservoir of highly active, predominantly particulate INP that can be efficiently transferred to the atmosphere during spring melt. Across Greenland sea-ice cores collected in 2016-2023, INP concentrations active at -10 degrees celcius were typically 10-100-fold higher than in under-ice seawater. Laboratory cold-finger freezing experiments and in-situ nilas formation experiments showed no preferential enrichment of INP from seawater into growing ice, indicating that incorporation is largely stochastic. In contrast, sea-ice INP activity co-varied with specific microbial taxa, consistent with in-ice production and retention within particulate or aggregated material. Enrichment and cultivation experiments showed a loss of ice-nucleating activity under phototrophic growth, and isolated Melosira cultures and filtrates were inactive, implicating heterotrophic microbial consortia rather than algae as the dominant source. Sea-spray simulations demonstrated strong aerosolization of sea-ice-derived INP, yielding sea-salt-normalized INP levels comparable to peak summer Arctic seawater emissions. These results identify seasonal sea ice as a previously overlooked springtime INP source with implications for mixed-phase cloud glaciation in a rapidly changing Arctic.