Chamber radical source

Glasson and Dunker (1989) used the oxidation of CO by OH to remove the chamber radical source. When an excess of CO is used, the OH is converted to H02 and then reacted with NO to give N02 ... 

Because there is no primary radical source during butane photo-oxidation, this parent system was the most sensitive to changes in the chamber radical source parameter. In conjunction with the optimizations described above, it was found that the best fit to the butane-NOx photo-oxidation data was obtained by multiplying the Carter chamber radical source values used in the SAPRC evaluation (Carter, 2000) by a factor of 1.35 for the ITC and ETC chambers, and by a factor of 1.2 for the DTC, CTC and XTC chambers. 

As indicated above, the butane, HCHO, CH3CHO and MEK systems were considered iteratively, such that the optimized chamber radical sources and the changes made to the photolysis parameters for HCHO and MEK led to a self-consistent description of all the systems. 

Zador, J., Zsely, I.G., Turanyi, T., Ratto, M., Tarantola, S., Saltelli, A. Local and global uncertainty analyses of a methane flame model. J. Phys. Chem. A 109, 9795-9807 (2(X)5b) Zador, J., Turanyi, T., Wirtz, K., Pilling, M.J. Uncertainty analysis backed investigation of chamber radical sources in the European Photoreactor (EUPHORE). J. Atmos. Chem. 55, 147-166 (2006a)... 

Molecular beam epitaxy (MBE) is a radically different growth process which utilizes a very high vacuum growth chamber and sources which are evaporated from controlled ovens (15,16). This technique is well suited to growing thin multilayer stmctures as a result of very low growth rates and the abihty to abmpdy switch source materials in the reactor chamber. The former has impeded the use of MBE for the growth of high volume LEDs. 

Temperature control has an additional advantage with respect to the problem of chamber contamination. After a smog chamber has been used, some hydrocarbons and nitrogen compounds may remain adsorbed on the chamber walls. These may desorb in subsequent runs and, in some cases (e.g., HCHO), act as free radical sources to accelerate the photooxidation processes. The ability to bake out smog chambers while pumping to low pressures is therefore useful in reducing chamber contamination effects. 

The OH concentration is then calculated from the rate of loss of NO, the CO concentration, and the known rate constants. In contrast to the observations of Carter et al. (1982), the chamber OH radical source was found to depend on light intensity and temperature, but not on N02. 

Figure 16.13, for example, shows the concentration-time profiles for a run in the evacuable chamber shown in Fig. 16.3 and for one in the evacuable chamber of Akimoto et al. (1985). The calculation, which assumes no radical source, curve A, clearly underpredicts Oa by a large margin. However, inclusion of a photoenhanced production of HONO via reaction (14), curve B, matches the observations quite well (Sakamaki and Akimoto, 1988). 

However, without knowledge of the source of the increased OH flux, extrapolation of the concentration-time profiles of both the primary and secondary pollutants observed in such smog chamber studies to real atmospheres becomes less certain. For example, the reactions leading to the unknown precursor(s) to OH may occur only in smog chambers. Extrapolation to ambient air would thus require subtracting out this radical source. On the other hand, the same reactions may occur in ambient air where surfaces are available in the form of particulate matter, buildings, the earth, and so on if this is true, then the rates would be expected to depend on the nature and types of surfaces available and may thus differ quantitatively from the smog chamber observations. 

Once a chemical submodel has been developed, it must be tested extensively prior to its application in comprehensive computer models of an air basin or region. This is done by testing the chemical submodel predictions against the results of environmental chamber experiments. While agreement with the chamber experiments is necessary to have some confidence in the model, such agreement is not sufficient to confirm that the chemistry is indeed correct and applicable to real-world air masses. Some of the uncertainties include those introduced by condensing the organic reactions, uncertainties in kinetics and mechanisms of key reactions (e.g., of aromatics), and how to take into account chamber-specific effects such as the unknown radical source. 

Akimoto, H H. Takagi, and F. Sakamaki, Photoenhancement of the Nitrous Acid Formation in the Surface Reaction of Nitrogen Dioxide and Water Vapor Extra Radical Source in Smog Chamber Experiments, lnt. J. Chem. Kinet., 19, 539-551 (1987). 

Carter, W. P. L R. Atkinson, A. M. Winer, and J. N. Pitts, Jr., Evidence for Chamber-Dependent Radical Sources Impact on Kinetic Computer Models for Air Pollution, Int. J. Chem. Kinet., 13, 735-740(1981). 

Glasson, W. A., and A. M. Dunker, Investigation of Background Radical Sources in a Teflon-Film Irradiation Chamber, Environ. Sci. Technol., 23, 970-978(1989). 

Sakamaki, F., and H. Akimoto, HONO Formation as Unknown Radical Source in Photochemical Smog Chamber, lnt. J. Chem. Kinet, 20, 111-116(1988). 

The photolytic radical source in a chamber might be less controllable (e.g. wall source of HONO). 

The radical source is a crucial point in using reaction chambers for photooxidation... 

Comparison of Radical Source and NOx Offgasing with Other Chambers... 

Figure 4. Plots of ratios of NOx offgasing or radical source to flie NO2 photolysis rates derived fi om modelling characterization rans for various chambers.

Therefore, the radical source and NOx offgasing rates indicated by the characterization data for the first series of experiments for this chamber is probably as low as one can obtain for reactors constructed of FEP Teflon film, which is generally believed to be the most inert material that is practical for use as chamber walls. Although die radical source and NOx offgasing rates for the second series of experiments is higher (see also Figure 3), they are still about an order of magnitude lower than observed for the UCR and UNC chambers previously used for mechanism evaluation. 

Moartgat, G.K. (Ed.) Evaluation of radical sources in atmospheric chemistry through chamber and laboratary studies. Final Report of the EC Project ENV4-CT97-0419, Mainz (2000). 

Moortgat, G. K. Evaluation of Radical Sources in Atmospheric Chemistry trough Chamber and Laboratory Studies, RADICAL , Final Report on EU Project, EC contract ENV4-CT97-0419, 2000. 

However, the new UCR EPA chamber dataset also includes experiments where the effect of adding CO to aromatics - NOx irradiations was determined. These experiments were carried out because model calculations indicated that the addition of CO would cause a significant enhancement in O3 formation, due to die NO to NO2 conversions caused when CO reacts with the radicals produced in the aromatic photooxidation reactions, and we wanted to test this prediction. In this regard, CO acts as a radical amplifier , enhancing the effects of radicals on ozone formation. CO addition is also useful because CO has the simplest possible mechanism to represent other VOCs present in ambient mixtures its reactions only cause NO to NO2 eonversions and its reactions result in formation of no other products or direct radical sources or sinks. Therefore, added CO experiments should provide a test of an aspect of the aromaties mechanisms that is applicable to its effects in ambient simulations, and that is different than the tests provided by aromatic NOx experiments in the absence of other VOCs. 

Fig. 18. Diagram of reactive scattering apparatus for the study of non-metal reactions A, scattering chamber B, source chambers C, liquid nitrogen cooled cold shield D, detector E, source bulkheads G, liquid nitrogen trap H, oil diffusion pumps N, free radical source P, nozzle source Q, skimmer E, ion source H, liquid He trap I, ion lenses P, photomultiplier Q, quadrupole rods R, light baffle S, slide valve T, radial electric field pumps (from C. F. Carter et al. 02 by permission of the Chemical...

Akimoto, H., K. Takagi and F. Sakamaki (1987) Photoenhancement of the nitrous acid formation in the surface reaction of nitrogen dioxide and water vapor Extra radical source in smog chamber experiments. International Journal of Chemical Kinetics 19, 539-551. 

The chamber was first evacuated to a pressure of under LOxlO" Pa using a rotary pump and a turbo molecular pump, and the chamber was then filled with nitrogen plasma (>99.99995 vol,% purity) from an RF radical source (model RF-4.5, SVT Associates, USA). Both continuous pumping and introduction of N plasma were carried out during the deposition. The thin films were prepared under a ft xed total pressure ofl.5xlO Pa. 

The uptake and reaction of NO2 on soot is very important as a process to release nitrous acid (HONO) into the gas phase. The formation of HONO by the heterogeneous surface reaction and its enhancement by light irradiation has been found for the first time by Akimoto et al. (1987) relevant to the unknown radical source in a smog chamber (see column on p.278). The uptake of NO2 and photocatalytic reaction on soot has been interested in as a model reaction of such heterogeneous process to elucidate the characteristics of HONO formation in the atmosphere. 

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