Development and evaluation of a polarization lidar for pre-detection of aerosol hotspots

Abstract

Bioaerosols such as fungal spores, viruses, bacteria or pollen can cause irritation, allergic reactions and asthma in humans. In rare cases, more dangerous bioaerosols can also get into the air, where exposure can lead to severe or even fatal consequences. As a preventive measure, the Institute of Technical Physics at the German Aerospace Center is developing a bioaerosol detection system that will detect and characterize bioaerosols locally. High-power lasers are used for this purpose, which can cause injuries if they come into contact with humans. For this reason, a polarization lidar is being developed in this thesis as a pre-detection system, which is intended to detect and roughly characterize aerosol hotspots in an eye-safe manner. The polarization lidar developed in this thesis works with a 1550 nm laser with a pulse width of 8 ns. A lens arrangement consisting of aspherical lenses was selected for the receiving path in order to focus from a 3 ” receiving optic to a 200 µm detector. Furthermore, the optomechanical setup as well as the alignment and calibration of the system are described. For evaluation purposes, four experiments are described in this thesis, which are divided into atmospheric experiments and experiments with synthetic aerosols. In the atmospheric experiments, atmospheric measurements were conducted, one with a standard atmosphere for the Stuttgart area and one with Saharan dust in the air. This was used to test the sensitivity of the system and its ability to detect atmospheric aerosols such as Saharan dust. The ability to measure depolarization was also tested. For the measurements of synthetic aerosols, near-field and far-field measurements were conducted. Salt and water vapor were measured for the near-field measurements, while water vapor, methane flare and sand were measured for the far-field measurements. The influence of the measurement distance on the measurement results was investigated. In addition, the far-field measurement represents a practical application for the system. With the polarization lidar developed in this thesis, it is possible to measure synthetic aerosols at a greater distance and to characterize them on the basis of their depolarizing properties. The experiments conducted in this work made it possible to differentiate between liquid and solid aerosols.