Shape Discrimination of Individual Aerosol Particles Using Light Scattering

We established an experimental apparatus by combining polarized light scattering and angle-resolved light scattering measurement technology to rapidly identify the shape of an individual aerosol particle. The experimental data of scattered light of Oleic acid, rod-shaped Silicon dioxide, and other particles with typical shape characteristics were analyzed statistically. To better study the relationship between the shape of particles and the properties of scattered light, the partial least squares discriminant analysis (PLS-DA) method was used to analyze the scattered light of aerosol samples based on the size screening of particles, and the shape recognition and classification method of the individual aerosol particle was established based on the analysis of the spectral data after nonlinear processing and grouping by particle size with the area under the receiver operating characteristic curve (AUC) as reference. The experimental results show that the proposed classification method has a good discrimination ability for spherical, rod-shaped, and other non-spherical particles, which can provide more information for atmospheric aerosol measurement, and has application value for traceability and exposure hazard assessment of aerosol particles.

Classification of Aggregates Using Multispectral Two-Dimensional Angular Light Scattering Simulations

Airborne particulate matter plays an important role in climate change and health impacts, and is generally irregularly shaped and/or forms agglomerates. These particles may be characterized through their light scattering signals. Two-dimensional angular scattering from such particles produce a speckle pattern that is influenced by their morphology (shape and material composition). In what follows, we revisit morphological descriptors obtained from computationally generated light scattering patterns from aggregates of spherical particles. These descriptors are used as inputs to a multivariate statistical algorithm and then classified via supervised machine learning algorithms. The classification results show improved accuracy over previous efforts and demonstrate the utility of the proposed morphological descriptors.

Measurement of circular intensity differential scattering (CIDS) from single airborne aerosol particles for bioaerosol detection and identification

The circular intensity differential scattering (CIDS), i.e. the normalized Mueller matrix element -S₁₄/S₁₁, can be used to detect the helical structures of DNA molecules in biological systems, however, no CIDS measurement from single particles has been reported to date. We report an innovative method for measuring CIDS phase functions from single particles individually flowing through a scattering laser beam. CIDS signals were obtained from polystyrene latex (PSL) microspheres with or without coating of DNA molecules, tryptophan particles, and aggregates of B. subtilis spores, at the size of 3 μm in diameter. Preliminary results show that this method is able to measure CIDS phase function in tens of microseconds from single particles, and has the ability to identify particles containing biological molecules.

Imaging atmospheric aerosol particles from a UAV with digital holography

The lack of quantitative characterization of aerosol particles and their loading in the atmosphere is one of the greatest uncertainties in climate-change science. Improved instrumentation capable of determining the size and shape of aerosol particles is needed in efforts to reduce this uncertainty. We describe a new instrument carried by an unmanned aerial vehicle (UAV) that images free-floating aerosol particles in the atmosphere. Using digital holography, the instrument obtains the images in a non-contact manner, resolving particles larger than ten micrometers in size in a sensing volume of approximately three cubic centimeters. The instrument, called the holographic aerosol particle imager (HAPI), has the unique ability to image multiple particles freely entering its sensing volume from any direction via a single measurement. The construction of HAPI consists of 3D printed polymer structures that enable a sufficiently low size and weight that it may be flown on a commercial-grade UAV. Examples from field trials of HAPI show images of freshly emitted tree pollen and mineral dust.

Effects of the shape distribution of aerosol particles on their volumetric scattering properties and the radiative transfer through the atmosphere that includes polarization

In this paper, we investigate the effects of shape distribution of aerosol particles on the volumetric scattering properties, as well as the radiance and polarization distributions of skylight, by numerical simulations. The results demonstrate that the shape distribution indeed exerts a significant influence on the skylight degree of linear polarization. The skylight polarization calculated assuming the microscope-measured shape distributions is distinct from that using the inversion-based shape distributions. The significant effects will influence the retrieval of the sphericity of aerosols based on the sun-sky radiometer measurements. Our results suggest that using representative shape distributions obtained by direct microscopic observations of aerosol samples captured in the natural atmosphere has a high potential to improve the retrieval of the aerosol shape parameter.

Solving the inverse problem for coarse-mode aerosol particle morphology with digital holography

Coarse mode atmospheric aerosol particles are abundant in agricultural, desert, and urban environments. Accurate characterisation of these particles' morphology is an important need in scientific and applied contexts, especially to advance our understanding for how such aerosols influence solar radiative forcing of the atmosphere. Elastic light scattering is a standard method to study aerosol particles in a contact-free manner, wherein measured scattering patterns are interpreted to infer particle morphology. Due in part to the absence of wave-phase information in these measurements, the inference is not unique, a difficulty generally known as the inverse problem. An alternative approach is digital holography where wave-phase information is encoded in the measurements. We show that digital holography and spatial filtering can solve the inverse problem for free-flowing aerosol particles in the sense that a measured scattering pattern can be uniquely associated with the particle size, shape, and orientation producing it.

Studying aerosol light scattering based on aspect ratio distribution observed by fluorescence microscope

Particle shape is crucial to the properties of light scattered by atmospheric aerosol particles. A method of fluorescence microscopy direct observation was introduced to determine the aspect ratio distribution of aerosol particles. The result is comparable with that of the electron microscopic analysis. The measured aspect ratio distribution has been successfully applied in modeling light scattering and further in simulation of polarization measurements of the sun/sky radiometer. These efforts are expected to improve shape retrieval from skylight polarization by using directly measured aspect ratio distribution.

Measurement of back-scattering patterns from single laser trapped aerosol particles in air

We demonstrate a method for measuring elastic back-scattering patterns from single laser trapped micron-sized particles, spanning the scattering angle range of θ=167.7°-180° and φ=0°-360° in spherical coordinates. We calibrated the apparatus by capturing light-scattering patterns of 10 μm diameter borosilicate glass microspheres and comparing their scattered intensities with Lorenz-Mie theory. Back-scattering patterns are also presented from a single trapped Johnson grass spore, two attached Johnson grass spores, and a cluster of Johnson grass spores. The method has potential use in characterizing airborne aerosol particles, and may be used to provide back-scattering data for lidar applications.

Simultaneous holographic imaging and light-scattering pattern measurement of individual microparticles

This work combines digital holography with spatial filtering at two wavelengths to record the hologram and light-scattering pattern for a single particle using a color sensor. Particles 30-100 μm in size and with various shapes are considered. The results demonstrate the ability to unambiguously associate a complicated scattering pattern with the particle size, shape, and orientation.

Real-time detection of airborne asbestos by light scattering from magnetically re-aligned fibers

Inadvertent inhalation of asbestos fibers and the subsequent development of incurable cancers is a leading cause of work-related deaths worldwide. Currently, there is no real-time in situ method for detecting airborne asbestos. We describe an optical method that seeks to address this deficiency. It is based on the use of laser light scattering patterns to determine the change in angular alignment of individual airborne fibers under the influence of an applied magnetic field. Detection sensitivity estimates are given for both crocidolite (blue) and chrysotile (white) asbestos. The method has been developed with the aim of providing a low-cost warning device to trades people and others at risk from inadvertent exposure to airborne asbestos.

Measurement and autocorrelation analysis of two-dimensional light-scattering patterns from living cells for label-free classification

We incorporate optics and an ICCD to record the two-dimensional angular optical scattering (TAOS) patterns retrieved from single aerosolized cells. We analyze these patterns by performing autocorrelations and demonstrate that we are able to retrieve cell size from the locations of the secondary maxima. Additional morphological information is contained in the autocorrelation functions and decay rate of the heights of the autocorrelation peaks. We demonstrate these techniques with C6 and Y79 cells, which are readily distinguishable. One key advantage of this methodology is that there is no requirement for antibody and fluorescent labeling molecules.

Scattering of singular beams by subwavelength objects

In recent years, there has been a mounting interest in better methods of measuring nanoscale objects, especially in fields such as nanotechnology, biomedicine, cleantech, and microelectronics. Conventional methods have proved insufficient, due to the classical diffraction limit or slow and complicated measuring procedures. The purpose of this paper is to explore the special characteristics of singular beams with respect to the investigation of subwavelength objects. Singular beams are light beams that contain one or more singularities in their physical parameters, such as phase or polarization. We focus on the three-dimensional interaction between electromagnetic waves and subwavelength objects to extract information about the object from the scattered light patterns.

Pollen characterization and identification by elastically scattered light

The authors recorded the elastic light-scattering pattern of pollen over a large spatial angle range to investigate the potential light scattering for pollen identification. The scattering from elm, hazel, birch, chestnut, willow, sunflower, ragweed and pine was measured. The scattering patterns show distinct differences that can be used for the classification of pollen with simple algorithms.

Two-dimensional Guinier analysis: application to single aerosol particles in-flight

This work presents an apparatus that measures near-forward two-dimensional elastic scattering patterns of single aerosol particles and proposes a two-angle extension of the Guinier law to analyze these patterns. The particles, which approximately range from 2 to 8 micrometers in size, flow through the apparatus in an aerosol stream. A spatial filtering technique separates the near-forward portion of the patterns from the illumination light. Contours intended to represent the geometrical profile of the particles are generated from the patterns using the extension of the Guinier law. The analysis is applied to spherical and nonspherical particles, and the resulting contours are found to be consistent with particle shape only for spherical particles.

Spatial filtering technique to image and measure two-dimensional near-forward scattering from single particles

This work describes the design and use of an optical apparatus to measure the far-field elastic light-scattering pattern for a single particle over two angular-dimensions. A spatial filter composed of a mirror with a small through-hole is used to enable collection of the pattern uncommonly close to the forward direction; to within tenths of a degree. Minor modifications of the design allow for the simultaneous measurement of a particle's image along with its two-dimensional scattering pattern. Example measurements are presented involving single micrometer-sized glass spherical particles confined in an electrodynamic trap and a dilute suspension of polystyrene latex particles in water. A small forward-angle technique, called Guinier analysis, is used to determine a particle-size estimate directly from the measured pattern without a priori knowledge of the particle refractive index. Comparison of these size estimates to those obtained by fitting the measurements to Mie theory reveals relative errors low as 2%.

Evaporation kinetics of a non-spherical, levitated aerosol particle using optical resonance spectroscopy for precision sizing

We describe how a time series of optical resonance spectra of an evaporating, non-spherical, irregular aerosol particle levitated in an electrodynamic balance exhibits patterns which are related to its evaporation kinetics. Simulated spectra of an evaporating, model aerosol particle show comparable features. If these patterns are used to deduce the particle size change with time, the resulting vapor pressures and enthalpies of vaporization compare favorably with literature data for both crystalline ammonium nitrate and succinic acid particles.

Simultaneous forward- and backward-hemisphere elastic-light-scattering patterns of respirable-size aerosols

Two-dimensional angular optical scattering (TAOS) patterns of aerosols are measured simultaneously from the forward hemisphere 15 degrees < theta < 90 degrees as well as the backward hemisphere 90 degrees < theta < 165 degrees (detecting 63% of the 4pi sr of scattered light) by using an ellipsoidal reflector and an intensified CCD detector. TAOS patterns were obtained from polystyrene-latex spheres (individuals and aggregates) and from single Bacillus subtilis spores. These information-rich patterns, measured with a single laser pulse for individual particles on the fly, suggest that forward-TAOS and backward-TAOS measurements may be used for rapid classification of single aerosol particles.