Time resolution, aerosol instruments

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.)...

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. 

The adaptive spectrometer described in this paper produces either non-imaging, onedimensional or two-dimensional multispectral radiance datasets ( datacube in the case of two-dimensional spectral mapping) for gas or aerosol discrimination and classification. The spectral, temporal and spatial resolution of the data collected by the instrament are adjustable in real time, making it possible to keep the tradeoff between sensor parameters at optimum at all times. The instrument contains no macro-scale moving parts making it an excellent candidate for the development of a robust, compact, lightweight and low-power-consumption device suitable for field operation. 

EMSL. Mass Spectromer Aerosol, Time-of-flight, High Resolution, http //www. emsl.pnl.gov/emslweb/instruments/mass-spectrometer-aerosol-time-flight-high-resolution (accessed March 27, 2015). 

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