Re-emission

The HVAC system also acts as a pollutant source when it is not maintained properly. Microorganisms breed in various environments present within components (e.g., cooling coils, ducts) of the system and may be distributed throughout the building. Improper maintenance of filters leads to loss of efficiency and re-emission of contaminants. 

Fig. 2.1 Nuclear resonance absorption of y-rays (Mossbauer effect) for nuclei with Z protons and N neutrons. The top left part shows the population of the excited state of the emitter by the radioactive decay of a mother isotope (Z, N ) via a- or P-emission, or K-capture (depending on the isotope). The right part shows the de-excitation of the absorber by re-emission of a y-photon or by radiationless emission of a conversion electron (thin arrows labeled y and e , respectively)...

Resonant y-ray absorption is directly connected with nuclear resonance fluorescence. This is the re-emission of a (second) y-ray from the excited state of the absorber nucleus after resonance absorption. The transition back to the ground state occurs with the same mean lifetime t by the emission of a y-ray in an arbitrary direction, or by energy transfer from the nucleus to the K-shell via internal conversion and the ejection of conversion electrons (see footnote 1). Nuclear resonance fluorescence was the basis for the experiments that finally led to R. L. Mossbauer s discovery of nuclear y-resonance in ir ([1-3] in Chap. 1) and is the basis of Mossbauer experiments with synchrotron radiation which can be used instead of y-radiation from classical sources (see Chap. 9). 

So far, we have discussed only the detection of y-rays transmitted through the Mossbauer absorber. However, the Mossbauer effect can also be established by recording scattered radiation that is emitted by the absorber nuclei upon de-excitation after resonant y-absorption. The decay of the excited nuclear state proceeds for Fe predominantly by internal conversion and emission of a conversion electron from the K-shell ( 90%). This event is followed by the emission of an additional (mostly Ka) X-ray or an Auger electron when the vacancy in the K shell is filled again. Alternatively, the direct transition of the resonantly excited nucleus causes re-emission of a y-photon (14.4 keV). 

The plot of CE = Pout/Ps (from Eqs (5.10.33) and (5.10.37)) versus Ag for AM 1.2 is shown in Fig. 5.65 (curve 1). It has a maximum of 47 per cent at 1100 nm. Thermodynamic considerations, however, show that there are additional energy losses following from the fact that the system is in a thermal equilibrium with the surroundings and also with the radiation of a black body at the same temperature. This causes partial re-emission of the absorbed radiation (principle of detailed balance). If we take into account the equilibrium conditions and also the unavoidable entropy production, the maximum CE drops to 33 per cent at 840 nm (curve 2, Fig. 5.65). 

The development of an SCR system for vehicle applications requires precise calibration of the amount of urea injected as a function of the quantity of NO emitted by the engine, exhaust temperature and catalyst characteristics. Although model simulations can help in the control, it is necessary to use specific NO sensors which, however, still have problems of sensitivity and transient response. Installing a clean-up catalyst for ammonia would provide more latitude and obtain higher NO conversion ratios without re-emission of ammonia into the atmosphere. 

Re-emission of a photon in the reversal of the excitation step photodissociation is unimportant. 

Not all excited atoms will relax by the re-emission of X-rays and the... 

Apart from anthropogenic emissions, heavy metals enter the atmosphere of Europe due to re-emission of previously deposited substances and from natural sources. These types of sources are taken into account on the basis of expert estimates made in MSC-E (Ryaboshapko and Ilyin, 2001 Travnikov and Ryaboshapko, 2002). 

Natural emission and re-emission processes are particularly important for the mercury cycle in the environment. The distribution of mercury re-emission from soil in Europe is illustrated in Figure 4. The most significant re-emission fluxes are in Central Europe... 

Figure 4. Spatial distribution of mercury re-emission from soils in Europe.

In 2002 anthropogenic emissions of lead in Europe amounted to 8 x 103 tons per year (kt/yr). This is about 11% less than in 2001. In addition, natural emissions and re-emissions made up 1 kt/yr. The total depositions to Europe in 2002 were 6.7 kt. 

In 2002 anthropogenic emission of cadmium in Europe amounted to 257 t/yr that is 5% lower than in 2001. Emission caused by natural processes (natural emission and re-emission) add up 55 t/yr. Depositions to Europe in 2002 were 240 t/yr. Spatial distribution of cadmium deposition in Europe is shown in Figure 9. The regions... 

Mercury emissions from European anthropogenic sources in 2002 totaled 180 tons this is 11 % lower than those in 2001. The input from natural emission and re-emission from European soils and the marginal seas is estimated at about 150 tons. More than 65% of emitted mercury was transported beyond the boundaries of Europe. The total mercury depositions to Europe were about 100 tons. Of this amount, 50 tons originated from anthropogenic sources of European countries the rest was the input from natural sources, re-emission and global anthropogenic sources. 

For the evaluation of long-range atmospheric transport and deposition of POPs, a multi-compartment transport model EMEP/MSCE-POP is used (Mantseva et al 2004). It includes such media as the atmosphere, soil, seawater and vegetation (Figure 1). A multi-compartment approach is conditioned by the ability of POPs to be accumulated in soil, seawater and vegetation with subsequent re-emission. Apart from atmospheric transport the model also takes into account the transport of pollutants by sea currents. 

On the basis of calculations the reductions of depositions and air concentrations of all selected POPs in each particular European countries are evaluated. The relative and absolute reduction of PCDD/Fs deposition over Central and Eastern European countries in comparison with relative emission changes is given in Figures 5(a) and (b). For the considered period the maximum relative decrease in PCDD/Fs depositions was obtained for Bulgaria and Belarus (about 3 times) (Figure 5(a)). At that the character of relative decrease in PCDD/Fs total depositions did not completely coincide with that of emissions. It can partly be explained by the influence of trans-boundary transport of PCDD/Fs and by the impact of re-emission flux in these countries. The highest absolute reduction was observed in Russia, Ukraine and Poland (Figure 5(b)). 

PCDD/F re-emission flux from soil, which slows down the tendency for a decrease in PCDD/F content in the atmosphere. 

Re-emission of toxic congeners from soil to the atmosphere can affect the levels of PCDD/F air pollution. 

Model calculations allow one to evaluate POP trans-boundary transport between European countries. The contributions of external sources to PCDD/Fs air concentrations in some European countries including Eastern European countries (given in black) are shown in Figure 9(a). In spite of the general decrease of contamination in Europe, the role of trans-boundary transport is yet essential. The fraction of air concentrations caused by external sources amounts approximately to 60% for Hungary, Romania and Slovakia. As it was mentioned above air contamination by PCDD/Fs is partly explained by accumulation in media with subsequent re-emission. The fraction of concentrations caused by this process can reach 10-15% in some countries (Figure 9(b)). 

Figure 9. Contributions of trans-boundary transport (a) and re-emission (b) to the POPs air pollution in some European countries in 2001 (data on Eastern European countries are given in black).

Ryaboshapko, A., Ilyin, I. (2001). Mercury re-emission to the atmosphere in Europe. In Proceedings of EUROTRAC Symposium 2000 Transport and Chemical Transformation in the Troposphere . 

In the case of radiative transfer between identical molecules, the fluorescence decays more slowly as a result of successive re-absorptions and re-emissions. A simple kinetic model has been proposed by Birks (1970). It is based on the assumption of a unique value for the average probability a that an emitted photon is absorbed, i.e. without distinction between the generations of photons (a photon of generation n is emitted after n successive re-absorptions). This model leads to the following expressions for the effective lifetime and the macroscopic fluorescence quantum yield ... 

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