Total bacterial number concentration in free tropospheric air above the Alps

Over a period from June to October 2010, we carried out four short campaigns on the northern alpine ridge (High Altitude Research Station Jungfraujoch, 3,450 m above sea level) to determine bacterial number concentrations by collecting aerosol with liquid impingers, followed by filtration, fluorescent staining and counting with a microscope. Impinger liquid was also subjected to drop freeze tests to determine the number of ice nucleators. Parallel measurements of Rn-222 enabled us to distinguish air masses with no, or little, recent land surface contact (free troposphere, Rn-222 a parts per thousand currency sign 0.50 Bq m(-3)) from air masses influenced by recent contact with land surface (Rn-222 > 0.50 Bq m(-3)). In free tropospheric air, concentration of total bacteria was on average 3.4 x 10(4) cells m(-3) (SD = 0.8 x 10(4) cells m(-3)). When wind conditions preceding sampling were calm, or when the station was in clouds during sampling, there was no detectable difference in bacterial number concentrations between free tropospheric air and air influenced by recent land surface contact. One campaign was preceded by a storm. Here, recent land surface contact had enriched the air in bacterial cells (up to 7.5 x 10(4) cells m(-3)). Very few of these bacteria may act as ice nucleators in clouds. The median ratio of ice nucleators to the number of bacterial cells in our study was 1.0 x 10(-5). We conclude that injection of bacterial cells into the free troposphere is an intermittent process. Conditions controlling the release of bacteria into near surface air are probably more of a limiting factor than vertical transport and mixing of near surface air into the free troposphere.