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

The color of aerosol particles

Digital in-line holography (DIH) is an established method to image small particles in a manner where image reconstruction is performed computationally post-measurement. This ability renders it ideal for aerosol characterization, where particle collection or confinement is often difficult, if not impossible. Conventional DIH provides a gray-scale image akin to a particle's silhouette, and while it gives the particle size and shape, there is little information about the particle material. Based on the recognition that the spectral reflectance of a surface is partly determined by the material, we demonstrate a method to image free-flowing particles with DIH in color with the eventual aim to differentiate materials based on the observed color. Holograms formed by the weak backscattered light from individual particles illuminated by red, green, and blue lasers are recorded by a color sensor. Images are reconstructed from the holograms and then layered to form a color image, the color content of which is quantified by chromaticity analysis to establish a representative signature. A variety of mineral dust aerosols are studied where the different signatures suggest the possibility to differentiate particle material. The ability of the method to resolve the inhomogeneous composition within a single particle in some cases is shown as well.

Backscatter multiple wavelength digital holography for color micro-particle imaging

This work applies digital holography to image stationary micro-particles in color. The approach involves a Michelson interferometer to mix reference light with the weak intensity light backscattered from a distribution of particles. To enable color images, three wavelengths are used, 430, 532, and 633 nm, as primary light sources. Three separate backscattered holograms are recorded simultaneously, one for each wavelength, which are resolved without spectral cross talk using a three-CMOS prism sensor. Fresnel diffraction theory is used to render monochrome images from each hologram. The images are then combined via additive color mixing with red, green, and blue as the primary colors. The result is a color image similar in appearance to that obtained with a conventional microscope in white-light epi-illumination mode. A variety of colored polyethylene micro-spheres and nonspherical dust particles demonstrate the feasibility of the approach and illustrate the effect of simple speckle-noise suppression and white balance methods. Finally, a chromaticity analysis is applied that is capable of differentiating particles of different colors in a quantitative and objective manner.

Atmospheric Deposition on the Southwest Coast of the Southern Basin of Lake Baikal

A precipitation monitoring station in Listvyanka was set up to determine the potential impact of the coastal area on the state of the adjacent air environment above Lake Baikal on its southwest coast. This article presents the results of studying the chemical composition of atmospheric deposition (aerosols and precipitation) at this station in 2020, and of their comparison with the data from previous years (from 2000 to 2019). In 2020, the ionic composition of atmospheric aerosols and precipitation had changed compared to previous years. In the modern period, the total amount of ions in aerosols, accounting for 0.46 ± 0.40 μg∙m−3, was lower by an order of magnitude than between 2000 and 2004. The average annual total amount of ions in precipitation in Listvyanka was almost unchanged from the average values in 2000–2010 and was 10% lower than that from 2011 to 2019 (7.3 mg/L). The ratio of major ions of sulphates and ammonium changed in the aerosol composition: compared to the period from 2000 to 2004, in 2020, the contribution of ammonium ions had decreased significantly, from 32% to 24%; the contribution of sulphates had increased to 43%, and the contribution of calcium had increased from 8 to 13%. Since 2010, the contribution of K+ ions has increased to 8–10%, indicating the effect of smoke aerosols from wildfires. In precipitation, despite the dominance of sulphates (26%) and calcium (18%) throughout the year, the contribution of nitrates increases to 19% during the cold season (from October to March), while the contribution of ammonium ions and hydrogen ions increases to 13% and 17%, respectively, in the warm season (from April to September). In 2020, as in previous research years, the acidity of precipitation at the Listvyanka station was elevated (pH 5.1 ± 0.5); 50% of precipitation in 2020 had pH ˂ 5. We quantified ions in atmospheric aerosols and precipitation on the underlying surface of the coastal southwestern part of Lake Baikal. Ion fluxes with precipitation were the highest in the warm season, which corresponds to the annual maximum precipitation. Unlike previous years (from 2000 to 2010 and from 2011 to 2019), wet deposition of most ions—especially calcium, ammonium and nitrates—had decreased in 2020. There was a 35-fold decrease in nitrogen fluxes and a 5-fold decrease in sulphur fluxes in aerosols, as well as 1.6-fold and 1.3-fold decreases, respectively, in precipitation.

Multispectral small-angle light scattering from particles

Using a supercontinuum laser, reflective optics, and a spatial filter, we measure two-dimensional small-angle light-scattering patterns for a variety of microparticles including spheres, salt, sand, and volcanic dust. The measurements are done at 13 wavelengths from 450-850 nm, where the absence of refractive optical elements minimizes the effects of chromatic aberration. Qualitative particle-material sensitivity is demonstrated by layering differently colored patterns. Last, the multispectral capability of our device demonstrates a new possibility to probe different q-space regimes for a given particle in a single measurement.

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.

Two-dimensional small-angle scattering from single particles in infrared with a lensless technique

An experiment is described where two-dimensional small-angle light scattering (2D-SALS) patterns from single particles are measured in the infrared through a lens-free approach. Spatial filtering is employed to separate scattered light from unscattered light to within approximately one degree from the forward direction. Non-planar reflective elements are used in the filtering process, permitting 2D-SALS measurements to be done without chromatic aberrations over a broad spectral range and from 0.8 to 8 degrees in the polar scattering angle and zero to 360 degrees in the azimuthal angle. Patterns from spherical microparticles are presented along with nonspherical particles including volcanic ash and salt. An asymmetry analysis is applied to demonstrate an ability to differentiate spherical from nonspherical particles from the 2D-SALS patterns.

A Comparison of Different Counting Methods for a Holographic Particle Counter: Designs, Validations and Results

Digital Inline Holography (DIH) is used in many fields of Three-Dimensional (3D) imaging to locate micro or nano-particles in a volume and determine their size, shape or trajectories. A variety of different wavefront reconstruction approaches have been developed for 3D profiling and tracking to study particles’ morphology or visualize flow fields. The novel application of Holographic Particle Counters (HPCs) requires observing particle densities in a given sampling volume which does not strictly necessitate the reconstruction of particles. Such typically spherical objects yield circular intereference patterns—also referred to as fringe patterns—at the hologram plane which can be detected by simpler Two-Dimensional (2D) image processing means. The determination of particle number concentrations (number of particles/unit volume [#/cm3]) may therefore be based on the counting of fringe patterns at the hologram plane. In this work, we explain the nature of fringe patterns and extract the most relevant features provided at the hologram plane. The features aid the identification and selection of suitable pattern recognition techniques and its parameterization. We then present three different techniques which are customized for the detection and counting of fringe patterns and compare them in terms of detection performance and computational speed.

Design and Validation of a Holographic Particle Counter

An in-line holographic particle counter concept is presented and validated where multiple micrometer sized particles are detected in a three dimensional sampling volume, all at once. The proposed Particle Imaging Unit is capable of detecting holograms of particles which sizes are in the lower μm- range. The detection and counting principle is based on common image processing techniques using a customized Hough Transform with a result directly relating to the particle number concentration in the recorded sampling volume. The proposed counting unit is mounted ontop of a Condensation Nucleus Magnifier for comparison with a commercial TSI-3775 Condensation Particle Counter (CPC). The concept does not only allow for a precise in-situ determination of low particle number concentrations but also enables easy upscaling to higher particle densities (e.g., >30.000#ccm) through its linear expandability and option of cascading. The impact of coincidence at higher particle densities is shown and two coincidence correction approaches are presented where, at last, its analogy to the coincidence correction methods used in state-of-the-art CPCs is identified.

Digital holographic particle volume reconstruction using a deep neural network

This paper proposes a particle volume reconstruction directly from an in-line hologram using a deep neural network (DNN). Digital holographic volume reconstruction conventionally uses multiple diffraction calculations to obtain sectional reconstructed images from an in-line hologram, followed by detection of the lateral and axial positions, and the sizes of particles by using focus metrics. However, the axial resolution is limited by the numerical aperture of the optical system, and the processes are time consuming. The method proposed here can simultaneously detect the lateral and axial positions, and the particle sizes via a DNN. We numerically investigated the performance of the DNN in terms of the errors in the detected positions and sizes. The calculation time is faster than conventional diffracted-based approaches.

Generation of aerosol-particle light-scattering patterns from digital holograms

The similarity between the light-scattering pattern of a particle in the near-forward direction and diffraction from the particle's silhouette is investigated. Images of irregularly shaped free-flowing aerosol particles are obtained from digital hologram measurements, which are then binarized to yield a silhouette. Application of Huygens's principle to the silhouette generates an approximate scattering pattern, which when compared to the true measured pattern shows good agreement for particles much larger than the wavelength of light.