Aerosol instruments chemical composition

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

Calibration of the instruments to measure the size distribution and chemical composition requires methods of generating aerosols of well-defined sizes and known composition. Generating aerosols of known... 

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. 

An important advance in continuous analyzers uses both particle size data and single-particle chemical composition. These instruments employ a method of rapid depressurization of the aerosol that produces a particle beam and irradiation of particles to generate ions that are analyzed by mass spectroscopy. The single particle analyzers have been employed in atmospheric research recently but have not reached the stage where they are used routinely in air monitoring. 

Aerosols present a special case in that the investigator needs to measure the mass concentration of the chemical, the chemical composition as a function of particulate size, and the particle-size distribution of the aerosol. No continuous sampling instruments are available to measure both particle-size and chemical concentration. Particle detection can be accomplished using both forward- and back-scatter detectors. A typical back-scatter allows for non-invasive determinations over a range from 6 to 10 000 mg m . In the test, the aerosol is drawn through an orifice and articles impact on a surface positioned between a source and a counter. 

For example, many methods are available for the chemical analysis of deposited aerosol particles. Individual particles can be analyzed as well as heavier deposits. A serious gap in aerosol instrumentation is the lack of instrument.s for on-line measurement of aerosol chemical constituents without removing them from the gas. Very large amounts of information on multicomponent, polydi.sperse aerosols would be generated by an instrument capable of continuously sizing and chemically analyzing each particle individually, thereby permitting the determination of the size-composition probability density function, g (Chapter I), From this function, in principle, many of the chemical... 

In the fourth type of identification the chemical composition of particles is studied in situ. By suitable chemical aerosol instruments the concentration and the size distribution of certain elements can be continuously monitored. The flame photometry of sodium containing particles (e.g. Hobbs, 1971) is a good example for such a method. Recently flame photometric detectors have also been developed to measure aerosol sulfur in the atmosphere (e.g. Kittelson et at., 1978). 

A reasonably complete analysis of the inorganic chemical composition of the aerosol requires much effort and involves, in addition to wet chemical methods, instrumental techniques such as neutron activation analysis, atomic absorption spectroscopy, or proton-induced X-ray emission (PIXE). These latter techniques yield the elemental composition. They furnish no direct information on the chemical compounds involved, although auxiliary data from mineralogy, chemical equilibria, etc. usually leave little doubt about the chemical form in which the elements occur. Thus, sulfur is present predominantly as sulfate, and chlorine and bromine as Cl- and Br-, respectively, whereas sodium potassium, magnesium, and calcium show up as... 

Maenhaut, W., and W. H. Zoller (1977). Determination of the chemical composition of the south pole aerosol by instrumental neutron activation analysis. J. Radioanal. Chem. 37, 637-650. 

The CCN behavior of ambient particles can be measured by drawing an air sample into an instrument in which the particles are subjected to a known supersaturation, a so-called CCN counter (Nenes et al. 2001). If the size distribution and chemical composition of the ambient particles are simultaneously measured, then the measured CCN behavior can be compared to that predicted by Kohler theory on the basis of their size and composition. Such a comparison can be termed a CCN closure, that is, an assessment of the extent to which measured CCN activation can be predicted theoretically [see, for example, VanReken et al. (2003), Ghan et al. (2006), and Rissman et al. (2006)]. The next level of evaluation is an aerosol-cloud drop closure, in which a cloud parcel model, which predicts cloud drop concentration using observed ambient aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against direct airborne measurements of cloud droplet number concentration as a function of altitude above cloud base. The predicted activation behavior can also be evaluated by independent measurements by a CCN instrument on board the aircraft. Such an aerosol-cloud drop closure was carried out by Conant et al. (2004) for warm cumulus clouds in Florida. 

Aerosol mass spectrometry (AMS) is used to monitor the chemical composition of particulate matter in the atmosphere. Commercial AMS instruments can provide size and chemical mass loading data on aerosol particles in real time [176]. Such instruments integrate sampling and MS analysis sub-systems. They can be installed permanently or used as components of mobile laboratories [176]. Both quadrupole and TOF AMS devices can provide quantitative data on the chemical composition of volatile/semi-volatile submicrometer aerosols [177]. Importantly, AMS can provide non-refractory aerosol mass... 

In response to this need, aerosol mass spectrometry has developed rapidly and it is now possible to determine both the size (over a limited size range) and qualitative chemical composition of most gas-phase aerosols, with a response time of less than 1 s (see Suess and Prather (1999)). Most of the instruments described in the literature use laser ablation and ionization of the aerosol particles to characterize their chemical composition, but other methods, including thermal vaporization with electron impact ionization, are also used. Here, we first briefly sketch the development of instruments based on laser ablation/ ionization techniques and then describe some of the work that has been done using an aerosol TOP mass spectrometer. 

Both liquid and solid material can be suspended in a gas by a variety of mechanisms. Aerosols produced under laboratory conditions or by specific generating devices may have very uniform properties that can be investigated relatively easily by physical and chemical instrumentation. Natural aerosols found in the atmosphere are mixtures of materials from many sources that are highly heterogeneous in composition and physical properties. Their characterization has required the application of a variety of measurement techniques and has been a major activity in modern aerosol science. 

Trace element compositions of airborne particles are important for determining sources and behavior of regional aerosol, as emissions from major sources are characterized by their elemental composition patterns. We have investigated airborne trace elements in a complex regional environment through application of receptor models. A subset (200) of fine fraction samples collected by Shaw and Paur (1,2) in the Ohio River Valley (ORV) and analyzed by x-ray fluorescence (XRF) were re-analyzed by instrumental neutron activation analysis (INAA). The combined data set, XRF plus INAA, was subjected to receptor-model interpretations, including chemical mass balances (CMBs) and factor analysis (FA). Back trajectories of air masses were calculated for each sampling period and used with XRF data to select samples to be analyzed by INAA. 

There are instruments capable of measuring the composition of individual particles (Chapter 6)t and eventually it will be possible to determine the size-composition p.d.f. Currently, such measurements are made on a research basis in practice the average composition of the particles in a discrete size interval is determined by collecting an aerosol sample over a period of several hours using a cascade impactor (Chapter 6) and analyzing the material on each stage chemically. The concentration measured in this way is related to g(u, wj,. rt. r, f) as follows ... 

Available Instrumentation One of the most obvious constraints on chamber design and experimental programme is the availability of instrumentation to probe the quantities of interest. As outlined above, it is frequently necessary to simultaneously monitor aerosol physical and chemical properties in addition to gas phase ehemical composition. A wide range of instruments are available for deployment in ground and aircraft-based field measurement within the Atmospheric Sciences Group at the University of Manchester. These will be available for deployment in the ehamber in scheduled campaign mode and otherwise periodically when not in field use. In addition, there are a range of core instruments available for all experiments. The instrumentation suite is broadly classified as follows ... 

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