Can spores transported from distant terrestrial regions act as cloud seeds over the Arctic Ocean?
―Insights from microscopic analysis of aerosol particles―

1. Key Points

• Ice-nucleating particles (INPs^※1) active in Arctic low-level clouds remain poorly understood. During R/V Mirai observations over the Arctic Ocean, the team identified spores^※2 originating from land more than 100 km away from the coast, and they may act as seeds for cloud ice crystals.
• Single-particle analyses showed that spores transported over the ocean could serve as effective INPs, but some of them mixed with sea-salt particles^※3 would be less effective as INPs.
• This study provides the first in-situ observational evidence that terrestrial ecosystems in Alaska and Canada may supply INPs over the Arctic Ocean, and that INP properties can change during atmospheric transport.

Figure 1　Schematic diagram of spore release and sea salt particle production, their trasnport to the Arcitc Ocean, and changes in their ice nucleating ability through mixing.

2. Overview

Dr. Takeshi Kinase and colleagues at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), in collaboration with Dr. Yutaka Tobo from the National Institute of Polar Research (NIPR) and Dr. Kouji Adachi from the Meteorological Research Institute (MRI), have revealed that spores emitted from distant land areas could be transported to the Arctic Ocean and contribute to the population of ice nucleating particles (INPs), potentially affecting Arctic climate through ice formation in the Arctic low-level clouds. Additionally, they found that mixing with sea salt particles during transport over the ocean reduced the ice-nucleating efficiency of spores (Figure 1).

Aerosol particles^※4 suspended in the atmosphere affect climate by acting as nuclei for cloud droplets and ice crystals, influencing cloud reflectivity and precipitation. In the Arctic, where temperatures frequently fall below freezing, ice formation in low-level clouds plays a particularly important role. However, the sources and characteristics of INPs over the Arctic Ocean have remained poorly understood due to limited observations.

During an Arctic research cruise conducted in the summer of 2022 aboard the R/V Mirai, aerosol samples were collected over the open ocean^※5. Individual particles collected on substrates were examined using optical and electron microscopy to identify those that formed ice crystals. In addition, aerosol samples collected on filters were used to analyze the INP number concentration. The results showed that, in cases where INP concentrations were high, more than 14% of the INPs originated from biological particles, primarily spores. Model simulations estimated that these spores originated mainly from terrestrial regions in Alaska and Canada, suggesting that ecosystems in these areas can serve as important sources of INPs over the Arctic Ocean.

The study also found that the ice-nucleating efficiency of spores decreased when they were attached or coated with sea-salt particles. This result indicates that their ice-nucleating efficiency can be changed as they are transported over the ocean, and that such changes should be taken into account when assessing their impact on cloud formation.

As Arctic warming progresses, it is expected that snow-free areas on land and open-water areas over the ocean will expand. These changes may increase emissions of both biological particles from land and sea-salt particles from the ocean, potentially altering cloud formation processes in the Arctic. The findings of this study provide important insights for improving understanding of Arctic climate change.

This research was published in npj Climate and Atmospheric Science on February 16, 2026 (JST). This study was supported by the Arctic Challenge for Sustainability II (ArCS II), Program Grant Number JPMXD1420318865, the Arctic Challenge for Sustainability III (ArCS III), JPMXD1720251001, and JSPS KAKENHI grant numbers JP21K21342 and JP25K15435.