Chemical tracers reactive

Some attempts have been made to use reactive hydrocarbons in conjunction with inert chemical tracers to deduce HO concentrations in urban plumes (139,140,141). Difficulties in deducing [HO ] from these experiments have been studied by McKeen et al (142), who conclude that such experiments can underpredict HO concentrations by a factor of 2 when more reactive hydrocarbons are used and parameterization of transport processes is not properly accounted for. 

Although there are three Rji isotopes in the U- and Th-decay series, only is sufficiently long lived tm= 3.8 days) to be a useful estuarine tracer. Radioactive decay of Ra continuously produces Rn, which because of its short half-life is generally in secular equilibrium in seawater. Being chemically non-reactive except for very weak Van der Waals bonding makes this isotope a unique marine tracer in that it is not directly involved in biogeochemical cycles. 

One can visualize RTD through an experiment with the use of chemical tracers, introducing them at a particular moment or since the beginning of the reaction. This chemical tracer should necessarily be a compound not reactive to the system under study, by measuring its concentration in the reactor outlet. In general, dye compounds are used, but also other materials can be used with conductive or radioactive material properties that can be measured quantitatively. 

Following the movement of airborne pollutants requires a natural or artificial tracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source. Limitations placed on the tracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emitted from high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable. On the basis of these criteria, rare earth isotopes, such as those of Nd, were selected as tracers. The choice of tracer, in turn, dictated the analytical method (thermal ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of... 

Be, and °Be. Radioisotopes of hydrogen and carbon are of particular interest because they are major biological elements, and thus can be used as both chemical and biological tracers. Recently, particle reactive Be = 53.3 day) has been utilized as a valuable coastal tracer of short-term particle deposition and remobilization. ... 

The time that a molecule spends in a reactive system will affect its probability of reacting and the measurement, interpretation, and modeling of residence time distributions are important aspects of chemical reaction engineering. Part of the inspiration for residence time theory came from the black box analysis techniques used by electrical engineers to study circuits. These are stimulus-response or input-output methods where a system is disturbed and its response to the disturbance is measured. The measured response, when properly interpreted, is used to predict the response of the system to other inputs. For residence time measurements, an inert tracer is injected at the inlet to the reactor, and the tracer concentration is measured at the outlet. The injection is carried out in a standardized way to allow easy interpretation of the results, which can then be used to make predictions. Predictions include the dynamic response of the system to arbitrary tracer inputs. More important, however, are the predictions of the steady-state yield of reactions in continuous-flow systems. All this can be done without opening the black box. 

Equations 11.1.33 and 11.1.39 provide the basis for several methods of estimating dispersion parameters. Tracer experiments are used in the absence of chemical reactions to determine the dispersion parameter )L this value is then employed in a material balance for a reactive component to predict the reactor effluent composition. We will now indicate some methods that can be used to estimate the dispersion parameter from tracer measurements. 

The experiments performed by Amdurer [847] were not intended to study chemical speciation or cycling of natural elements in the ecosystem. To do this, the tracers must be fully equilibrated with all of the reactive (i.e., nonmatrix) phases of the stable elements. This equilibration may require a time... 

Masclet and co-workers (1986) have also developed a relative PAH decay index. They used it, for example, to identify various major sources of urban pollution and developed a model for PAH concentrations at receptor sites. An interesting and relevant area that is beyond the scope of this chapter is the use of PAHs as organic tracers and incorporating their relative decay rates (reactivities) into such receptor-source, chemical mass balance models. Use of relative rates can significantly improve such model performances (e.g., see Daisey et al., 1986 Masclet et al., 1986 Pistikopoulos et al., 1990a, 1990b Lee et al., 1993 Li and Kamens,... 

Davis, J. A. et al., Multispecies reactive tracer test in an aquifer with spatially variable chemical conditions, Water Resour. Res., 36, 119, 2000. 

Transport (advection and diffusion) of tracers (both passive and reactive) is performed on-line at each meteorological time-step using WAF scheme for advection and a true (second order) diffusion, with diffusion coefficient carefully estimated from experiments (Tampieri and Maurizi 2007). Vertical diffusion is performed using ID diffusion equation with a diffusion coefficient estimated by means of an k-l turbulence closure scheme. Dry deposition is computed through the resistance-analogy scheme and is provided as a boundary condition to the vertical diffusion equation. Furthermore, vertical redistribution of tracers due to moist convection is parameterized consistently with the Kain-Frisch scheme used in the meteorological part for moist convection. Transport of chemical species is performed in mass units while gas chemistry is computed in ppm. 

In order to gauge the relative importance and possible interaction between turbulence and chemical reactions, it is necessary to evaluate the various processes involved in reactive mixing. When a fluid element of different component (tracer) is added to the turbulent flow field, molecular mixing (and reaction, if possible) proceeds through several steps/mechanisms, some of which are listed below ... 

Carrier gas here usually consists of a major component, chemically inert towards tracer(s) and of minor (still macroscopic) quantities of reactive gases or vapors (see below). 

When the necessary condition that the chemistry at point A does not change with time is met, inverse mass balance models are still applicable when the chemistry at point B changes with time. An example is a laboratory column study, in which the chemistry of influent is maintained in the experiments while the effluent chemistry continues to change. In this case, we are assured that the effluent is chemically evolved from the influent. The variation of chemistry with time in the effluent does not violate the steady-state assumption. Another example is field injection of reactive tracers, during which the injectate chemistry is constant. Actually, laboratory titration experiments would also fit into this category because we know the initial solution chemistry from which the final solution evolves. Inverse mass balance modeling should find applications in these situations. 

James G. Anderson is Philip S. Weld Professor of Atmospheric Chemistry at Harvard University. He received his B.S. in physics from the University of Washington and his Ph.D. in physics-astrogeophysics from the University of Colorado. His research addresses three domains within physical chemistry (1) chemical reactivity viewed from the microscopic perspective of electron structure, molecular orbitals, and reactivities of radical-radical and radical-molecule systems (2) chemical catalysis sustained by free-radical chain reactions that dictate the macroscopic rate of chemical transformation in the Earth s stratosphere and troposphere and (3) mechanistic links between chemistry, radiation, and dynamics in the atmosphere that control climate. Studies are carried out both in the laboratory, where elementary processes can be isolated, and within natural systems, in which reaction networks and transport patterns are dissected by establishing cause and effect using simultaneous, in situ detection of free radicals, reactive intermediates, and long-lived tracers. Professor Anderson is a member of the National Academy of Sciences. 

---