Search

The COVID-19 pandemic has illuminated the lack of knowledge regarding the airborne transmission pathway of disease. The pathway consists of pathogens contained in small particles ejected when speaking and coughing. A crucial characteristic of these particles is their size that is connected to the their suspension longevity in the air as well as the location of generation and deposition within a subject's respiratory system. Sizing of particles launched from the respiratory system is a challenge for a number of reasons: (1) the size of ejected particles varies over a wide range, between sub- to several hundreds of microns, (2) particles are ejected at various speeds in different directions, (3) each single event is unique where for example the number of particles can vary greatly between two occurrences of the same event and subject, (4) the size of the particles vary significantly as they evaporate over time. To overcome these challenges and categorize full coughing and speaking events, new measuring methods are needed. In this work, we present, in detail, high-speed scattered light imaging to size liquid particles (droplets) in unique respiratory events. A high-speed camera records scattered laser light at 16 000 frames per second in a semi-forward scattering direction. The illumination is close to the ejection source which means that the particles are sized before evaporation. The measurement can size stationary droplets from 3.4 to 44 μ m and moving droplets from 4 to 80 μ m resolved in both time and space. To get a reliable estimation, careful calibration of scattering angles and calibration uncertainty has been performed showing a general uncertainty of 8%. Thus, the approach proposed in this article can provide valuable and accurate data to improve the understanding of the airborne transmission pathway. • Measurable size range is between 3.4–44 μ m for stationary and 4–80 μ m for moving droplets. • The sizing prediction uncertainty is generally around 8%. • Both spatial and temporal information of a respiratory event is resolved. • Semi-forward detection angle halved the min size compared to perpendicular detection. • The scattering volume must be thick enough relative the droplet speed and recording frame rate for particle sizing. [ABSTRACT FROM AUTHOR]

The purpose of this tutorial is to teach the method of digital in-line holography (DIH) as a means to characterize aerosol particles in an in-situ manner. The scope is limited to the simplest, yet powerful, lens-less implementation, which is the most intuitive and forgiving of the varieties of digital holography. By illuminating free-flowing aerosol particles with a pulsed, expanded, collimated laser beam, one can record the interference pattern of scattered and unscattered light on an image sensor. The resulting digital hologram can then be used in a diffraction calculation to render a silhouette-like image of the particle, or particles, present in the beam. A key advantage of imaging with DIH over conventional imaging, e.g. microscopy, is that particles located at different positions can be brought into focus from a single hologram measurement. Useful information can be obtained from such images including the number of particles present, their size, shape, and orientation. Notably, this is all achieved without the need to collect or otherwise trap particles. The inherent motion of aerosol particles imposes specific constraints on the design and performance of DIH imaging, and in particular, restricts the resolution in comparison to applications involving stationary or otherwise controlled particles. As such, DIH is currently most effective for particles larger than several micrometers in size. The tutorial will explain the theoretical basis of DIH, illustrate how particle images are generated computationally from hologram measurements, provide code in the Mathematica language to do so, and discuss several limitations imposed by hardware. Also reviewed will be a method to approximate a particle's extinction cross section and two-dimensional angular light scattering pattern. [Display omitted] • Digital in-line holography (DIH) is a simple method to characterize aerosols. • Particle size and shape is provided from DIH images of flowing particles. • The theory of DIH is reviewed and its computational implementation explained. • Extinction and scattering can be measured from digital holograms. • Example code is provided to perform image reconstruction from holograms. [ABSTRACT FROM AUTHOR]

Abstract: We develop a new method of measuring the time-dependent solid angle scattering cross section for detection of individual particles flowing across a Gaussian laser beam. This method is based on the principle that the normalized derivative of the scattering signal measured by an instrument can be decomposed into the normalized derivative of the incident irradiance and that of the scattering cross section . For evaporative particles, holds true until evaporation starts at a certain position in the laser beam. The for individual particles is determined from , which is extracted from . is derived from and . We apply the method to particles with and without evaporation using a single-particle soot photometer. The robustness of the method is confirmed through results for non-evaporative particles. For evaporative particles, derived values before the onset of evaporation are compared to Mie theory. [Copyright &y& Elsevier]

Abstract: The intrinsic nature of elastic light scattering by aerosol particles—the non-monotonic dependence of the scattered intensity vs. size—influences the performance of practically all single optical particle spectrometers resulting frequently in substantial erroneous sizing of particles. As a possible solution for this problem we developed a measuring system and data reduction procedure allowing simultaneous determination of particle size and its complex refractive index. The system contains two laser sources with different wavelengths and four detectors collecting scattered light in four spatially separated angular ranges. The targeted size and refractive index ranges of this device cover the size from 0.1 to , the real part of the refractive index from 1.1 to 2, and the imaginary part of refractive index between 0 and 1 corresponding to the properties of ambient aerosol particles. The numerical performance study shows that the sizing error of the method remains less than 10% independent of the refractive index of the particles. The assessment of the real and imaginary part of the refractive index is possible within about 15%. Experimental results verify the feasibility of this new instrumental approach showing the potential of the method to determine in real-time the size and complex refractive index of single aerosol particles without the necessity of instruments’ recalibration for varying particle materials. [Copyright &y& Elsevier]

Abstract: This comment adds performance data for a modified version of one of the optical particle counters investigated in , namely the WELAS 2100. The new version was found to have a counting efficiency much closer to unity for larger particle sizes as well as some improvement in the lower 50% detection limit. [Copyright &y& Elsevier]

Bipolar charging and neutralization of aerosol particles with diameter below 10 nm in a circular tube with uniform ion-pair generation have been studied theoretically to determine the conditions required to reach the stationary charge distribution. In order to reach the stationary distribution the initial ion-to-aerosol number concentration ratio, N i n / n i n , and the N i n τ product (τ is the mean aerosol residence time in the charger) must both be sufficiently large. These parameters can be combined in a single dimensionless parameter, β N i n 2 τ / n i n , where β is the attachment rate coefficient between a charged particle and an ion of opposite polarity. Attainment of the stationary distribution requires that this dimensionless number be larger than about 800 for neutralization of charged particles, and larger than about 500 for charging of uncharged particles. Experiments have shown that the stationary charge distribution cannot be reached neither using low cost, low activity 241Am sources nor a commercially available and widely used 2 mCi 85Kr neutralizer. When particle sizing by mobility analysis is carried out employing a charger in which the stationary charge distribution is not reached, inversion of the mobility distribution assuming that the charge distribution on the particles is the stationary one yields correct values of the mean particle diameter and the standard deviation, but fails to predict the aerosol number concentration. • Attainment of the stationary charge distribution is controlled by a single dimensionless coefficient. • Stationary charging state is not reached with a low activity radioactive source. • Stationary charge distribution cannot be achieved with a 2 mCi Kr85 commercial neutralizer. • Size distribution mean and width are accurately retrieved even with a non-stationary charger. [ABSTRACT FROM AUTHOR]