Find in Library

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 PM_2. 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.