Rainfall drives atmospheric ice nucleating particles in the maritime climate of Southern Norway

Ice nucleating particles active at warm temperatures (e.g. -8 °C; INP-8) can transform clouds from liquid to mixedphase, even at very small number concentrations (<10 m). Over the course of 15 months, we found very similar patterns in weekly concentrations of INP-8 in PM10 (median = 1.7 m, maximum = 10.1 m) and weekly amounts of rainfall (median = 28 mm, maximum = 153 mm) at Birkenes, southern Norway. Probably, INP-8 were aerosolised locally by the impact of 10 raindrops on plant, litter and soil surfaces. Major snowfall or heavy rain onto snow-covered ground were not mirrored by enhanced numbers of INP-8. Further, source receptor sensitivity fields obtained from transport model calculations for large and small numbers of INP-8 differed more in their likelihood to provide precipitation to southern Norway, than in the proportion of land cover or landuse type. In PM2.5 we found consistently about half as many INP-8 as in PM10. From midMay to midSeptember, INP-8 correlated positively with the fungal markers arabitol and mannitol, suggesting that INP-8 may 15 consist largely 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.