Size resolution, aerosol instruments

A related technique that is suitable for measurement of aerosols at lower mass loadings is the aerodynamic particle sizer (3, 10). In this instrument the aerosol is rapidly accelerated through a small nozzle. Because of their inertia, particles of different aerodynamic sizes are accelerated to different velocities, and the smallest particles reach the highest speeds. The particle velocity is measured at the outlet of the nozzle. From the measurements of velocities of individual particles, particle size distributions can be determined. The instrument provides excellent size resolution for particles larger than about 0.8 xm in diameter, although sampling difficulties limit its usefulness above 10 xm. 

Chemical Composition Aerosol composition measurements have most frequently been made with little or no size resolution, most often by analysis of filter samples of the aggregate aerosol. Sample fractionation into coarse and fine fractions is achieved with a variety of dichotomous samplers. These instruments spread the collected sample over a relatively large area on a filter that can be analyzed directly or after extraction Time resolution is determined by the sample flow rate and the detection limits of the analytical techniques, but sampling times less than 1 h are rarely used even when the analytical techniques would permit them. These longer times are the result of experiment design rather than feasibility. Measurements of the distribution of chemical composition with respect to particle size have, until recently, been limited to particles larger than a few tenths of a micrometer in diameter and relatively low time resolution. One of the primary tools for composition-size distribution measurements is the cascade impactor. 

Aerosol Instrument Classification. Friedlander (34) classified the range of aerosol instrumentation in terms of resolution of particle size, time, and chemical composition. This classification scheme is illustrated in Figure 3. The ideal instrument would be a single-particle counter-sizer-analyzer. Operating perfectly, this mythical instrument would fully characterize the aerosol, with no lumping of size or composition classes, and would make such measurements sufficiently rapidly to follow any transients occurring in the aerosol system. 

Figure 3. Classification of aerosol instruments in terms of their inherent time, size, and composition resolution. (Adapted from reference 34.)...

The electrical aerosol analyzer and the optical counter are used to measure particle size distributions. Describe the size range and resolution characteristics of each of these instruments. 

There are two types of Mastersizer instruments the Mastersizer Micro and the Mastersizer E, which are low cost instruments for repetitive analyses and the modular series of Mastersizer S and Mastersizer X, the ultimate in resolution and dynamic size range, which are required when samples in the form of aerosols, suspensions and dry powders need to be measured. Mastersizer X provides a selection of small size ranges using a variety of interchangeable lenses whereas the Mastersizer S provides a wider dynamic size range covered in a single measurement. For powders which are to be suspended in a solvent, emulsions, suspensions and particles in liquids there are small volume cells which require as little as 15 ml of dispersant. Where a material is either valuable or toxic the Malvern Small Volume Flow Cell, with a sample volume of 50-80 ml and full sample recovery, can be used. The X-Y sampler is a 40-sample accessory... 

A newly developed aerosol mass spectrometer (Jayne et ah, 2000 Allan et ai, 2003) deposits size-selected particles on a heated surface so that volatile and semi-volatile materials are vaporized. These are then analyzed quantitatively on line. The instrument is able to provide quantitative, short-time-resolution measurements of the compositions of particles in selected size ranges. However, the instrument has poor sensitivity for refractory materials such as mineral dust and provides no information on particle-to-particle variability in composition. 

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