In this work we present the setup and first tests of our new BIO IN detector (Bundke et al., 2010). This detector was constructed to classify atmospheric ice nuclei (IN) for their biological content. It is designed to be coupled to the Fast Ice Nucleus CHamber FINCH. If one particle acts as ice nucleus, it will be at least partly covered with ice at the end of the development section of the FINCH chamber. The device combines an auto-fluorescence detector and a circular depolarization detector for simultaneous detection of biological material and discrimination between water droplets, ice crystals and not activated large aerosol particles. The excitation of biological material with UV light and analysis of auto-fluorescence is a common principle used for flow cytometry, fluorescence microscopy, spectroscopy and imaging. The detection of auto-fluorescence of airborne single particles demands some more experimental effort. However, expensive commercial sensors are available for special purposes, e.g. size distribution measurements. But these sensors will not fit the specifications needed for the FINCH IN Counter (e.g. high sample flow of up 10 LPM). The newly developed - low cost - BIO IN sensor uses for the electronic excitation a single high-power UV-LED instead of much more expensive UV lasers. Other key advantages of the new sensor are the low weight, compact size, and the little effect on the aerosol sample which allows it to be coupled with other instruments for further analysis.

             The instrument will be flown on one of the first missions of the new German research aircraft “HALO” (High Altitude and LOng range).

          Method: The single particle fluorescence detector for the in situ measurement of biological ice nuclei is based on the optical detector of the Fast Ice Nucleus CHamber (FINCH), which has been previously described in detail (Bundke et al., 2008). The UV source used for the excitation of the bioaerosol particles is a fiber-coupled high-power LED operating at 365 nm in cw-mode. The UV light is focused by an aspheric two-lens collimating focusing assembly with 360 nm band-pass emission filter to a circular spot of about 2.5 mm diameter, which overlaps with both the 2 mm diameter cross section of the gas stream and the elliptical spot of the circular polarized 635 nm scattering laser (see Figure 1). The fluorescence light is collected perpendicular to the UV beam axis by an aspheric condenser lens and an additional spherical reflector. Two optical filters separate the fluorescence light from the scattered light of both light sources. After this, a second lens focuses it to a photomultiplier detector.

Figure 1 Schematic diagram of the Bio IN detector optics. The aerosol flow intersects the paper plane vertically at the point of intersection of the laser and UV-LED beam.

          References:

Bundke U., Nillius, B., Jaenicke, R., Wetter,T., Klein, H. and Bingemer, H., (2008). The Fast Ice Nucleus Chamber FINCH, Atmospheric Research, doi:10.1016/j.atmosres.2008.02.008

Bundke, U., Reimann, B., Nillius, B., Jaenicke, R. and Bingemer, H. (2010), Development of a bioaerosol single particle detector (BIO IN) for the fast ice nucleus chamber FINCH, Atmos. Meas. Tech.., 3, 263-271.

             Acknowledgements: This work has been performed within Project A1 of the Collaborative Research Centre (SFB) 641 “The Tropospheric Ice Phase”, funded by the German Science Foundation and by the Grant Bu 1432/3-1 (Development of an ice nucleus (IN) counter for HALO) within the framework of SPP 1294: Atmospheric and Earth system research with the "High Altitude and Long Range Research Aircraft" and by the virtual institute Aerosol Cloud Interaction VI233 funded by the Helmholtz society.