Detecting and tracking thin aerosol clouds

Biological warfare agents are dangerous even if inhaled in low concentration. Thus, for reconnaissance purposes there is much interest in detecting and tracking the emission of small amounts of biological material as an aerosol cloud. An aerosol is unlikely to radiate high-enough levels of electromagnetic waves for passive detection and therefore has to be the target of an active emitting source such as radar or a laser beam from a light detection and ranging (lidar) system. Since the droplet size of biological material in a respirable aerosol ranges from 0.5μm to 5μm, only high concentrations of material can be detected by radar.

There have been many approaches to using laser remote sensing devices, and these are well documented in a 2010 NATO technical report (RTG-55).¹ Most of these systems, including our biological aerosol lidar (BALI),² concentrate on getting the most information out of a biological aerosol at a short or mid-range distance. For us it proved to be a big constraint that our BALI is not eye-safe and needs secured ranges even for testing simulants. We also lacked robust and efficient laser sources for the IR and UV region.

Whereas most other systems primarily analyze aerosols hit by a fixed detection beam, we switched our attention first to detecting the aerosol.^{3, 4} Previous experiments with our BALI system proved that elastic backscattering at 1064nm was a sensitive measurement for respirable aerosols.² Hence we decided to aim to detect aerosol clouds with a very low concentration of particles that evolve in space and time.⁵ This time evolution of the covered space should hint at the emitting source and at the aerosol type.

As a starting point for further investigations, we decided to build an aerosol detecting and tracking system with an eye-safe operated fiber laser working in the near-IR (1540nm) as a scanning system (see Figure 1). At the chosen wavelength, the atmospheric transmission allows a mid- to long-range setup. The scanner consists of a lidar module, a turn actuator to alter the detector angle, and a computer-controlled steering unit. The optical and mechanical structure fits inside a box sized 0.8×0.6×0.6m, leaving space for additional sensor heads.

Figure 1. Design of the eye-safe scanning unit, which consists of a light detection and ranging (lidar) module, a turn actuator, and a steering unit.

In contrast to the BALI system, we designed the lidar so that the sending and receiving beams use the same optical path (coaxial). This, together with a range resolution of 5m, allows short-range measurements. We took special care in the optic design to avoid irradiation of a strong signal (blooming) in the short range. Such blooming could blind the sensitive receiver element. Although the laser is a commercially available system, the main board of the lidar module is an adapted version of a lidar platform from the German optical product company Jenoptik (see Figure 2).

Figure 2. The coaxial IR lidar has a main board adapted from a commercial lidar platform.

Our lidar system does not allow us to take into account the backscatter contribution of the air molecules. Also, the setup of our fiber laser does not allow depolarization measurements, which would detect a change in polarization between the sending and receiving beam. Such measurements would in theory allow us to distinguish between water droplets, which preserve the original polarization plane, and other small particles such as agglomerations of spores. We are further investigating the potential of depolarization.

In summary, we have built a new coaxial lidar aerosol detection system that is eye-safe and therefore suitable for use in the field. Next, we plan to test the scanner design and its sensitivity when measuring low particle concentrations in an aerosol. The emphasis is on developing an adequate mathematical description and modeling of the measurement of thin aerosol clouds with the aim of identifying best fitting strategies to find and track aerosol clouds. To support these measurements and acquire additional information on the thermal and gaseous background, we will use a new Fourier transform IR hyperspectral imager (Bruker Optics), which has a focal plane array that operates in the long-wave IR spectral range.⁶

We have just finished the experimental setup of our BALI system. The first results are expected by the end of 2013. If we succeed with aerosol detection, we intend to add a UV laser for laser-induced fluorescence and depolarization measurements. Depending on the results of our experiments with the BALI system, we may add a second UV channel. We are currently investigating mathematical methods to determine the potential of this method for classifying aerosol types. Testing dispersion models and assimilating the measurement data into models promises to be interesting and rewarding work.