Ice nucleating particles active at warm temperatures (e.g. &amp;minus;8&amp;thinsp;°C; INP_&amp;minus;8) can transform clouds from liquid to mixed-phase, even at very small number concentrations (<&amp;thinsp;10&amp;thinsp;m^&amp;minus;3). Over the course of 15 months, we found very similar patterns in weekly concentrations of INP_&amp;minus;8 in PM₁₀ (median = 1.7&amp;thinsp;m^&amp;minus;3, maximum = 10.1&amp;thinsp;m^&amp;minus;3)

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... d weekly amounts of rainfall (median = 28&amp;thinsp;mm, maximum = 153&amp;thinsp;mm) at Birkenes, southern Norway. Probably, INP_&amp;minus;8 were aerosolised locally by the impact of raindrops on plant, litter and soil surfaces. Major snowfall or heavy rain onto snow-covered ground were not mirrored by enhanced numbers of INP_&amp;minus;8. Further, source receptor sensitivity fields obtained from transport model calculations for large and small numbers of INP_&amp;minus;8 differed more in their likelihood to provide precipitation to southern Norway, than in the proportion of land cover or landuse type. In PM_2.5 we found consistently about half as many INP_&amp;minus;8 as in PM₁₀. From mid-May to mid- September, INP_&amp;minus;8 correlated positively with the fungal markers arabitol and mannitol, suggesting that INP_&amp;minus;8 may consist largely of fungal spores. In the future, warmer winters with more rain instead of snow may enhance airborne concentrations of INP_&amp;minus;8 during the cold season in southern Norway and in other regions with a similar climate.