J.M. Clark

Airborne particle shape and size classification from spatial light scattering profiles

Original article can be found at: http://www.sciencedirect.com/science/journal/00218502 Copyright Elsevier B.V. [Full text of this article is not available in the UHRA]

An instrument for the classification of airborne particles on the basis of size, shape, and count frequency

Abstract A laser light scattering instrument for the study of airborne particulate material is described. The instrument examines individual particles (primarily in the size range 1–10 μm equivalent diameter) at rates of up to 10,000 s −1 . Each particle is ascribed an asymmetry factor, or asphericity factor, based upon the spatial intensity distribution of the scattered laser radiation. This factor ranges from 0 for perfect spheres to 100 for long fibres. Particles are also ascribed a size parameter based upon the total equivalent scattering from a perfect sphere. Data may be displayed graphically in terms of size vs count frequency, asymmetry factor vs count frequency, or as an axonometric plot of size vas asymmetry factor vs count frequency. The latter three-dimensional dimensional surface is a characteristic signature of the aerosol under examination and preliminary results indicate the possibility of employing this form of data analysis to monitor small changes in aerosol composition, such as, for example, the presence of low concentrations of spheres (droplets) or fibres. Results are presented for incompressible solid spheres, liquid droplets, and mineral fibres.

Procedures for the characterisation of bioaerosol particles. Part I: aerosolisation and recovery agent effects

The sampling and assay of bioaerosols are important ina number of industrial and health-care applications. Airborne microorganisms are notoriously difficult toenumerate accurately under such conditions and nosingle procedure is suitable for all applications. Problems are compounded by the differences in assaymethod or sampler type selected, making theinterpretation of results difficult.Understanding the airborne behaviour of microorganismsover a range of environmental conditions is vital ifprocedures are to be defined and recommended for theassessment of bioaerosols. Microorganisms that arerobust over a wide range of conditions are ideal astracer particles. Unfortunately, the large majorityof non-fungal bioaerosols are susceptible to damage. A predictable assessment procedure is required whichwill not affect the viability of the collectedsample.This paper examines how aerosolisation may affect the characteristics of two speciesof microorganism (Pseudomonas fluorescens andMS2 coliphage). It forms part of a larger programmeto develop standards for the assessment of biologicalparticles. The aim of the work was to develop procedures toexamine the effects of aerosolisation onmicroorganisms, with particular reference topre-aerosolisation protocol (spray suspension age) andpost-sampling handling protocol (aerosol age incollection solution). These procedures were then usedto examine the effect of recovery agents, addedto the spray suspension prior to aerosolisation, onthe culturability of E.coli.Aerosolisation reduces the culturability of P. fluorescensand the viability of viability of MS2coliphage. Pre-sampling and post-collection handlingand storage of these aerosolised microorganisms werealso found to have an effect. This and earlierstudies have shown that the culturable fraction ofmicroorganisms can be affected by the same factorsdescribed above. Of five microorganisms tested so farin the main programme, only Penicillium expansumspores were shown to be robust and stable with aconstant culturable fraction. Therefore, recommendinga particular microorganism (apart from P. expansum) as an airborne biological standard foraerosol studies is not advised. It is recommendedthat a microorganism, representative of the envisagedapplication, be characterised it in terms of theaerosolisation parameters, storage time and conditionsin the manner reported in this study. This can beachieved using the experimental equipment described.The addition of 0.1 mM concentrations of the sugarsinositol, trehalose and raffinose to spray suspensionsof Escherichia coli, prior to aerosolisation,made no significant difference to the culturablefraction of the aerosol.

The application of neural networks to particle shape classification

Abstract This paper describes work undertaken to demonstrate the applicability of artificial neural network technology to the classification of aerosol particles in terms of shape and size. The neural network demonstrator software has been developed by EDS-Scicon, with experimental data and expert domain knowledge provided by the Chemical and Biological Defence Establishment. Data on a range of aerosol particles with well defined shapes, and characteristic sizes, has been generated using a prototype light scattering instrument. This instrument monitors the cylindrical symmetry of laser light scattered by single aerosol particles using three photomultipliers placed around the optical axis. It also uses the narrow forward scattered light to obtain a measure of the size of the particles. Graphical representation of the data enables classification of the particles by visual inspection of the plots. The pattern classification capabilities of neural networks make them well suited to the shape classification of aerosol particles based on this light scattering data. This paper describes work undertaken to demonstrate the ability of neural networks to perform the classification task automatically when trained on samples of the photomultiplier output. The background to the application is described, along with details of the network architecture and the performance achieved by the fully trained network.

Particle shape discrimination using the aerosol shape analysis system (ASAS)

Abstract An instrument has been developed which can characterise airborne particles in terms of both size and shape. The instrument analyses the spatial distribution of light scattered by single particles as they pass through the focus of a laser beam. The instrument has been used extensively to characterise particles of well defined size and shape. Many of the applications of the system will involve the recognition of particles with a particular shape characteristic against a heterogeneous background. To determine how effective it is in this role the instrument has been used to analyze mixtures of particles with a range of sizes and shapes. The results so far indicate that the system is effective in discriminating specific particles in mixed aerosols.

The laboratory generation of test aerosols with well defined morphologies

Abstract There is a need for particles with well defined characteristics for the calibration and assessment of aerosol instrumentation. Currently the only well characterised particles that are readily available are spheres. The responses of instruments to non-spherical particles are usually unquantified. A number of techniques for the production of particles with a variety of shapes have been investigated. This paper outlines some of these techniques and in particular examines the growth of caffeine needles and a number of inorganic salt crystals with specific shapes.

Non-spherical particles for assessment of aerosol shape analysis systems

Abstract Monosize prolate spheroids of spindle-type iron oxide, produced with aspect ratios of up to 3:1, have proved to be a valuable tool in assessing the performance of a prototype real-time Aerosol Shape Analysis System (ASAS).

Optimising the performance of aerosol instruments and sampling systems using computational fluid dynamics

Abstract Computational Fluid Dynamics has been used in the design and optimisation of a number of types of aerosol equipment. The use of this technique in the design of a high volume, high ratio aerosol concentrator and the optimisation of the performance of the particle delivery system in a light scattering instrument are described here.