Variations of ice nuclei concentration induced by rain and snowfall within a local forested site in Japan

Biological ice nuclei (IN) such as certain species of bacteria and fungi are believed to have impacts on ice nucleation in mixed-phase clouds at temperatures warmer than −15 °C. Recent studies have indicated that rain is closely related to increases of biological IN in the near-surface atmosphere. However, variations of IN concentrations during rain and snowfall have not been compared. In the present study, field measurements of atmospheric IN were carried out under fine, cloudy, rain and snow at a local forested site in Japan. IN concentrations at −7 °C in spring were dramatically increased by rain, and concentrations associated with rain (0.86-2.2 m−3) were greater than 2.6 times higher than the mean concentration during fine weather (0.33 m−3). In winter, concentrations associated with rain (1.6 to >5.7 m−3) were also higher than those under cloudy sky (1.1 m−3), but increases were not observed during snowfall (0.21-0.4 m−3). Detectable IN concentrations associated with rain considerably decreased after heat treatment at 90 °C, indicating that IN increased during rain were likely biological substances such as heat-sensitive ice nucleation active proteins. Consequently, different types of precipitation may have varying effects on IN concentration associated with biological substances.