Rainfall drives atmospheric ice-nucleating particles in the coastal climate of southern Norway

Ice-nucleating particles (INPs) active at modest supercooling (e.g. −8 C; INP−8) can transform clouds from liquid to mixed phase, even at very small number concentrations (< 10 m−3). Over the course of 15 months, we found very similar patterns in weekly concentrations of INP−8 in PM10 (median= 1.7 m−3, maximum= 10.1 m−3) and weekly amounts of rainfall (median= 28 mm, maximum= 153 mm) at Birkenes, southern Norway. Most INP−8 were probably aerosolised locally by the impact of raindrops on plant, litter and soil surfaces. Major snowfall and heavy rain onto snow-covered ground were not mirrored by enhanced numbers of INP−8. Further, transport model calculations for large (> 4 m−3) and small (< 4 m−3) numbers of INP−8 revealed that potential source regions likely to provide precipitation to southern Norway were associated with large numbers of INP−8. The proportion of land cover and land use type in potential source regions was similar for large and small numbers of INP−8. In PM2.5 we found consistently about half as many INP−8 as in PM10. From mid-May to mid-September, INP−8 correlated positively with the fungal spore markers arabitol and mannitol, suggesting that some fraction of INP−8 during that period may consist of fungal spores. In the future, warmer winters with more rain instead of snow may enhance airborne concentrations of INP−8 during the cold season in southern Norway and in other regions with a similar climate.