Articles | Volume 17, issue 18
Atmos. Chem. Phys., 17, 11065–11073, 2017
Atmos. Chem. Phys., 17, 11065–11073, 2017

Research article 19 Sep 2017

Research article | 19 Sep 2017

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

Franz Conen1, Sabine Eckhardt2, Hans Gundersen2, Andreas Stohl2, and Karl Espen Yttri2 Franz Conen et al.
  • 1Department of Environmental Sciences, University of Basel, Basel, Switzerland
  • 2NILU – Norwegian Institute for Air Research, Kjeller, Norway

Abstract. 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.

Short summary
Observation of ice nuclei active at −8 °C show that rainfall drives their abundance throughout all seasons and that they are equally distributed amongst coarse and fine fraction of PM10. Concurrent measurements of fungal spore markers suggest that some fraction of INP-8 may consist of fungal spores during the warm part of the year. Snow cover suppresses the aerosolisation of ice nuclei. Changes in snow cover and rainfall may affect atmospheric concentrations of ice nuclei in future.
Final-revised paper