Many CBRNE threat materials are optimally implemented as aerosols. However, aerosol threats present unique challenges for sensor development, test and evaluation since particles may disburse in a turbulent atmosphere differently from tracer gases. This presentation addresses the need for tracer aerosol particles with known size distributions to be released with agent target simulant aerosols to provide ground truth for sensor test and evaluation. A novel approach for achieving uniquely identifiable individual aerosol particles is described based on utilization of quantum dots (QDs) and/or other luminescent nanocrystals (NCs), to create a multiplexed spectral barcode in tracer aerosol particles. QDs are small, typically nanometer scale, compared to micron-sized polymer beads as host aerosol particles. They also possess desirable optical properties of narrow, efficient emission bands, and are typically long-lived compared to organic dye molecules that photodegrade in sunlight. Multiple QD subpopulations, each with a narrow emission band at a distinct peak wavelength, can be encapsulated in a polymer microbead, conferring a superposition emission profile having multiple narrow peaks. The relative intensities of the emission peaks can be controlled by adjusting the number of QDs in each subpopulation. This spectral emission profile effectively becomes an individual particle barcode. Multiple polymer bead samples can be prepared each with different emission pattern (barcode). These samples can be mixed with target materials to be simultaneously released as aerosols to provide test ground truth for the simulant. Proof-of-principle experiments assessing the feasibility for using combinations of embedded NC populations in micron-sized droplets, as well as potential challenges to practical implementation will be discussed.
One of the uses of the Ambient Aerosol Test Facility (AATF) at the U.S. Naval Research Laboratory is to operate as a test facility for developmental chemical aerosol sensors. The facility draws ambient air from outside the building, with or without HEPA filtration, then introduces aerosols by various means within a 30 cm diameter, 14 m long wind tunnel at flow velocities ranging from 2 to 20 m/s. The turbulent flow generated provides a uniform distribution to a few percent across 90% of the tube diameter to sampling ports in a 3 m long test section at the end. The test section allows sampling and analysis by various sensors to determine aerosol size, concentration, and chemical composition. For the current program of interest, we generate simulant aerosols representing various classes of chemicals of interest. A range of referee instruments to characterize the aerosol in terms of size, number and composition is planned. Commercially available particle sizers and counters, a gas analyzer and an aerosol mass spectrometer are part of the suite of referee instruments. We use a high-resolution, time-of-flight aerosol mass spectrometer (HR-ToF-AMS) from Aerodyne, Inc., which is configured with an aerosol focusing lens to transmit between 50 nm and 3.5 μm diameter particles, and provides size, mass loading and chemical composition information. A calibration system consisting of a scanning mobility particle sizer and a water-based condensation particle counter is used to validate the operation of the AMS instrument. We describe the AMS instrument and its use at the AATF for the assessment of other instruments.
Degraded visual environments (DVEs) are the result of a high concentration of obscurants in the air, and occur often during rotary wing aircraft landings. When the obscurants are comprised of sand this leads to a condition called “brownout.” Current DVE penetrating technology is severely limited by a weak return signal in severe brownout conditions. One method of overcoming this weak return signal is to use light tuned to the Christiansen wavelength, the wavelength at which the refractive index of the scatterer matches that of the surrounding medium, eliminating the scattering effect associated with the obscurant particles. We have previously shown a novel method for determining the optical constants of particulate samples using spectroscopic ellipsometry and determined that, for several different sand samples, the Christiansen wavelength is approximately 8 microns. We present data from falling sand of several different types using Fourier transform infrared (FTIR) spectroscopy showing a higher transmission percentage at the Christiansen wavelength. We explain features in this data using previous measurements of the optical properties of these sands.
The most accurate insight to how aerosolized material responds to UV radiation is obtained by performing experiments on freely suspended particles, absent from the shadowing that deposition on a surface may impose. For this purpose we have developed a linear electrodynamic particle trap to confine suspended particles using a contact-free technique. The trap allows us to challenge and study aerosols under controlled environmental parameters such as temperature, humidity and radiation exposure. We present the results of a quantitative study on the changes in viability of Bacillus anthracis Sterne strain spores confined within this trap and illuminated by either simulated sunlight or a UV light source at 253.7 nm. Up to 500 same-size particles, (that is, consisting of approximately the same number of spores), were created from a droplet-on-demand injector, trapped and irradiated with varying time scales. Illumination times ranged from 5 to 300 seconds with a maximum fluence of 500 J/m2 using the UV source, and particle clusters containing as little as 1 up to as many as 55 spores were used. As will be discussed, the viability of spores decreased as total fluence increased as expected, and for the same fluence, viability improved as the number of spores in each particle increased.
The use of light with orbital angular momentum (OAM) is being investigated in a wide range of fields that include material interrogation, light propagation, sensing and communication1 . The defining characteristic of the electric field of OAM light is an angular term described by an azimuthal component such as e -imφ , which produces helical phasefronts and an angular momentum equal to mħ2 . For the cases where m ≠ 0 the beams can have an intensity minima along the central axis creating a beam of light which is tube-like in structure. This, together with the angular component of the energy flux, can cause the scattering interactions with materials to be different than that from plane waves. Theoretical angular scattering calculations show that the light scattering maxima can occur at different angles from the forward direction of zero degrees. In this work we investigate the scattering properties of OAM light from single, micron sized spherical particles that are suspended in a linear electrodynamic trap. Using phase plates we generate OAM beams (wavelength of 532 nm) that are incident on a single suspended particle. Using three separate CCDs we capture the scattered light intensity over a total range across 40 degrees in the forward, back and side scattering planes. Comparisons between angular scattering measurements from Gaussian beams and OAM mode equal to 3 is presented.
Knowledge of the optical constants of particulate samples is critical in order to accurately model their optical behavior. For example, the dispersion and attenuation of a silicate sand are required to model scattering through a dust cloud. Most methods of measuring these quantities, however, require a polished solid sample and are therefore not suited to particulates. We present a novel method of measurement based on spectroscopic ellipsometry that can be applied to any particulate material. First, an adhesive compound is prepared and polished, and its optical constants are extracted. Then, a mixture of the adhesive and a particulate sample is prepared, and, treating the mixture as a Bruggeman effective medium, the optical constants of the particulate material are determined. We test the method’s effectiveness using pure silica powder, demonstrating that the results match literature values. The method is then applied to real sand samples. We present data for several types of sand and show that it is possible to accurately determine their optical properties and to observe the Christiansen effect in these samples.
We report on the advances made in the basic research to label specific chemical or biological aerosols on-the-fly using an electrospray technique. Fluorescent biomarkers that have been created for specific targets, and that produce a detectable change in emission characteristics only upon binding, will be used to coat all aerosols in an air stream. Aerosols with appropriate receptors will be labeled in this manner, allowing them to be identified in near real-time using a simple laser-induced fluorescence technique. In effect, an immunoassay is quickly performed on the surface of single chemical or biological particles as they flow in an air stream, labeling specific ones for rapid, single-particle interrogation and identification among a diverse and dynamic background. This method permits the use of solutions containing mixtures of different biomarkers to simultaneously identify multiple types of chemical or biological aerosols. Some issues that are currently being investigated include the kinetics of biomarker surface binding to an aerosol in flight and the control of charged aerosols for efficient single particle interrogation.
We examine how aggregation affects the light-scattering signatures, especially the polarization in the near-backward-scattering direction. We use the discrete dipole approximation (DDA) to study the backscatter of agglomerate particles consisting of oblong monomers. We examine the effects of monomer number and packing structure on the resulting negative polarization branch at small phase angle. We find large a dependence on the orientation of the monomers within the agglomerate and a smaller dependence on the number of monomers, suggesting that the mechanism producing the negative polarization minimum depends strongly on the interactions between the individual monomers. We also examine experimental measurements of substrates composed of biological cells. We find that the light-scattering signatures in the backward direction are not only different for different spore species, but for spores that have been prepared using different methodologies. These signatures are reproducible in different substrates composed of the spores from the same batches.
We are developing a novel method to fluorescently label specific biological aerosols on-the-fly using an in-line electrospray technique. Fluorescently labeled biomarkers such as molecular beacons, aptamer beacons, or those constructed from antibodies, will be used to coat aerosol particles in an air stream. Single biological particles with appropriate receptors will be tagged with biomarkers that fluoresce at a particular wavelength allowing the particle to be identified in near real time using a simple laser induced fluorescence technique. The fluorescent markers are normally quenched in the absence of their target analyte, permitting the use of mixtures of different biomarkers for simultaneously identifying multiple types of biological particles. The technique can also be applied to inorganic particulate with a molecular surface composition that lends itself to epitopic binding. Some of the issues that are currently being investigated include the kinetics of biomarker binding in an aerosol stream, optimal electrospray geometries and the nondestructive charging of biological particles on the fly.
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