Trace species

J. B. Edwards, Combustion The Formation and Emissions of Trace Species, Ann Arbor Science Pubhshers, Ann Arbor, Mich., 1974, p. 16. 

In order to estimate the extent of ozone depletion caused by a given release of CFCs, computer models of the atmosphere are employed. These models incorporate information on atmospheric motions and on the rates of over a hundred chemical and photochemical reactions. The results of measurements of the various trace species in the atmosphere are then used to test the models. Because of the complexity of atmospheric transport, the calculations were carried out initially with one-dimensional models, averaging the motions and the concentrations of chemical species over latitude and longitude, leaving only their dependency on altitude and time. More recently, two-dimensional models have been developed, in which the averaging is over longitude only. 

The interstitial air trapped during this process preserves a largely unaltered record of the composition of past atmospheres on time scales as short as decades and as long as several hundred thousand years. Such records have provided critical information about past variations in carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), and the isotopic composition of some of these trace species. In addition, studies of the major elements of air nitrogen, oxygen, and argon, and their isotopic composition, have contributed... 

Table 19-1 Radiatively important trace species in the atmosphere. Percent change in flux measured relative to the pre-industrial time...

In order to calculate the steady-state concentration of ozone in the stratosphere, we need to balance the rate of production of odd oxygen with its rate of destruction. Chapman originally thought that the destruction was due to the reaction O + 03 —> 2O2, but we now know that this pathway is a minor sink compared to the catalytic destruction of 03 by the trace species OH, NO, and Cl. The former two of these are natural constituents of the atmosphere, formed primarily in the photodissociation of water or nitric oxide, respectively. The Cl atoms are produced as the result of manmade chlorofluorocarbons, which are photodissociated by sunlight in the stratosphere to produce free chlorine atoms. It was Rowland and Molina who proposed in 1974 that the reactions Cl + 03 —> CIO + O2 followed by CIO + O —> Cl + O2 could act to reduce the concentration of stratospheric ozone.10 The net result of ah of these catalytic reactions is 2O3 — 3O2. 

It is important to be aware of some tricks of the trade and unusual phenomena that can occur in electrospray MS. B oth positive and negative ions may be observed by electrospray but these ionic species may not be the predominant species in solution, they may in some cases only be trace species, as the electrospray technique is very sensitive to small concentrations of ions. In general the solutions suitable for electrospray are dilute, 1CT3-K) 5 M. As the solution droplet evaporates the concentration may increase and if rapid polymerization processes are possible in solution, then dimers and trimers may be observed that may not have been present in the original solution. 

All effluents must be characterized in detail when treating agents contaminated with metals from disassembled chemical weapons (i.e., potential trace species and reaction by-products, such as nitrated hydrocarbons, partially oxidized products, and metals, must be identified) and their environmental impacts evaluated. 

The Demo II test evaluated all of the major effluent streams for a full suite of trace species and reaction byproducts. At the time the committee was preparing its Demo II report, not all of the data were available and the impact of trace species, particularly in brines and atmospheric releases, on facility permitting remains to be determined. This information was still unavailable to the committee as of the time the present report was being prepared. 

ERC, "Effects of Coal-Derived Trace Species on the Performance of Molten Carbonate Fuel Cells," topical report prepared for U.S. DOE/METC, DOE/MC/25009-T26, October 1991. 

Coal Gas Trace Species MCFCs to date have been operated on reformed or simulated natural gas and simulated coal gas. Testing is being conducted with simulated coal gas has involved the expected individual and multi-trace constituents to better understand coal operation (45). 

D. Rastler, EPRI, G. Devore, Destec Engineering, R. Castle, Haldor Topsoe, C. Chi, ERC, "Demonstration of a Carbonate Fuel Cell Stack on Coal-Derived Gas," in Fuel Cell Seminar. "Effects of Coal-Derived Trace Species on the Performance of Molten Carbonate Fuel Cells," Topical Report prepared by Energy Research Corporation for US DOE/METC, DOE/MC/25009-T26, October, 1991. 

Edwards J.B., Combustion - Formation and Emission of Trace Species, Ann Arbor Science Publ. Inc., Michigan (USA), (1977). 

Most of the chemical analyses are performed after melting of the snow samples which subdivides the species into water-soluble and insoluble ones. The concentrations of trace species can vary by at least two orders of magnitude and the lowest values are in the range of 1-10 pg/1 for major ions such as Ca ", NH4, NOs , S04, and 10-100 ng/1 for trace metals. Thus, the snow and ice samples are particularly sensitive with respect to contamination and require special care in sample handling. 

Various concentration records of a number of chemical trace species and gases obtained from the different Alpine ice cores have been published, see for example [17-22]. These records clearly demonstrate the impact of anthropogenic emissions... 

Fig. 5 Concentration records of different trace species in the ice core from Colle Gnifetti glacier as 5-year averages °...

These results are consistent with previous measurements which showed that CO concentration was lowest at the combustor operating conditions that most efficiently reduced the overall emission of toxic gases. Thus a measurement of CO concentration can serve as an effective indicator of combustor performance. The results demonstrate the applicability of multiplexed diode laser sensors for rapid, continuous measurements and control of multiple flowfield parameters, including trace species concentrations, in high-temperature combustion environments. 

The principal limitations of ESCA include the inability to detect elements present at trace concentrations within the analytical volume, and insufficient lateral resolution to characterize single micrometer-sized particles. The inability to characterize trace species is illustrated in Figure 10 for a sample of coal fly ash particles (11). The fly ash results from the noncombustible mineral components of the coal and consists largely of fused iron oxides and aluminosilicates (42). In addition, most elements are present in at least trace concentrations (22, 42), and many of these elements are highly enriched in the surface region of the particles (evidence for this will be discussed in the next section). However, the ESCA spectrum acquired over several hours of counting time indicates only the presence of detectable surface S and Ca in addition to the fly ash matrix constituents. 

The in situ spectroscopies and the signal processing have limitations. Therefore, the set of observable species is a proper subset of all liquid phase species S. The validity of Eq. (4), namely, that the number of observable species is less than the number of species, is easily verified. Regardless of the instrument, the sensitivity is finite, and some dilute and most trace species must be lost in the experimental noise. In addition, numerous experimental design shortcomings further contribute to the validity of Eq. (4). 

For simple tracer experiments, the trace species should be conserved in the sense that the amount eventually eluted from the system is the same as the quantity entering this means, for instance, that the tracer... 

Moyers, J. L., Ranweiler, L. E., Hopf, S. B. and Korte, N. E. (1977). "Evaluation of Particulate Trace Species in Southwest Desert Atmosphere," Environmental Science Technology, 11, 789. 

Fe, and trace species such as V, Ni, and Zn. Detection limits range from about 20 to 200 ng/cm area density on each filter, corresponding to about 0.2 to 2 yg/filter. Typical uncertainties in the mass determination of a particular element were 15-20% or 2 yg/filter, whichever was larger. 

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