NOX molecules

One of the key questions in connection with studies of future ozone changes from NOx emissions in general is the non linearity in the ozone forming process with ozone formation becoming less efficient per NOx molecule emitted at high NOx levels. Ozone formation occurs via the following sequence of reactions in the troposphere, and in the lowermost part of the stratosphere ... 

A key question when future ozone perturbations from NOx emissions are studied is the question of non linearity. Will the ozone production become less efficient per NOx molecule emitted for high NOx emissions from aircraft, or in an environment with higher background levels of NOx which we will have in the future due to higher surface NOx emissions The non linearity in ozone production is shown in Tale 5 below... 

Table 5. Atmospheric ozone increases from aircraft emission per NOx molecule emitted, relative to the increase in 1992. The emissions of NOx are die same as given in Table l.The values represent increases in global ozone concentrations from the Earth s surface to 16 km and is the average increase for the models that participated in the model studies given in Table 4.

Other work has shown that Pt Rh are more S resistant than Pd [3,13], and also more Pb resistance than Pd [6,12-14], Our hypothesis proposes that Pb will form Pb-S-Pd compounds which deactivate the Pd activity, during aging. Because the NOx reduction is greatly dependent on the Pd dispersion, and is effective only when Pd particles or the available Pd surface active sites are close to each other, any steric hinderance generated on the Pd surface that would block the N-N bond formation will compromise the NOx reduction activity. When the Pb-S-Pd compound is formed, illustrated hypothetically below, the closeness of two NOx molecules required so that the N-N bond can be formed (N0+N0->N>+02) is no longer feasible. Consequently, the NOx reduction function of the Pd catalyst is hindered by the physical steric effect. Since neither HC or CO oxidations require formation of a C-C bond, or the proximity of two same molecules, to complete their reactions, the Pb-S-Pd compound intereference is therefore less predominant. 

The disadvantage of this diffusion barrier is that it slightly changes the gas composition at the electrode because of the different diffusion coefficients of the constituents of the raw exhaust gas. On average the rich gas components, especially the small H2, are faster than the bigger 02 or NOx molecules causing a lean shift that is typical for all sensors. The closer the exhaust gas is to equilibrium, the less lean shift is observed. Downstream of the catalytic converter the lean shift completely disappears. Furthermore, precatalytic layers in front of the electrode, or catalytic materials inside the protection layer, reduce the effect... 

Table 3.Catalytic activity (Rg, NOx molecules/s g and Rq, NOx molecules/s Cu-atom) at573 K and W/F=0.15 g cnj for the NOx ft duction ai Cu-HZSvI5 and Cu-NaY.

Reactive species such as NOx molecules, chlorine radicals, and hydrogen radicals present in the stratosphere can also cause the catalytic conversion of ozone to oxygen, in competition with the last reaction shown above. Reactions such as the following, therefore interfere with the equilibrium photochemical processes, reducing the concentration of ozone in the stratosphere. The net reaction is one where ozone is converted to oxygen and catalytic Cl species survives for further reaction. Particularly effective in this regard are halogen monoxides (CIO and BrO). 

Reactions 5 and 6 constitute a catalytic cycle because the radical NO that attacks O3 is regenerated by the reaction of NO2 with an O-atom. The net effect is the removal of one O3 molecule and one O-atom. Thus, although the concentration of NO and NO2 (or NOx) in the stratosphere is small, each NO molecule can destroy thousands of ozone molecules before being scavenged by a reaction such as the following ... 

The thermal stability of NOx adsorbed species and their reactivity in the presence of gaseous reductant molecules was addressed by thermal decomposition in He (TPD) or by heating in flowing H2/He mixtures [temperature-programmed surface reaction (TPSR)], respectively. In these cases, after NOx adsorption and He purge at the adsorption temperature (300 100oC), the samples were cooled to RT under flowing He. Then the samples were heated at 15°C/min up to 500-600°C in He (TPD) or in He + H2 (2000 ppm) (H2-TPSR). 

The simplest phenoxy radical, Cf)H,0, does not react with 02 (k < 5 X 1CT21 cm3 molecule-1 s-1) but does react with NO (k = 1.9 X 1CT12 cm3 molecule-1 s-1) and with N02 (k = 2.1 X 10-12 cm3 molecule-1 s-1), suggesting that reactions with NOx will be its primary fate in the troposphere (Platz et ai, 1998b). However, this may not be the case for the larger, hydroxylated phenoxy radicals from the OH-aromatic-02-NO reaction sequence. 

Ketones. Reactions of ketones are similar to those of alkanes, with abstraction by OH, N03, and Cl occurring from the alkyl chains. In the case of acetone, for example, these reactions generate the radical CH3C0CH202 in air. As for typical R02, this reacts rapidly with NO (k m = 8 X 10 12 cm3 molecule-1 s"1) and with N02 ( 29x = 6.4 X 10-12 cm3 molecule -1 s-1) (Sehested et al., 1998b), giving calculated lifetimes with respect to reaction with 100 ppt each of NO and N02 of about 1 min. In remote areas where the NOx concentrations can be much smaller (see later and Chapter 11), reactions of CH3C0CH202 with H02 and other R02 may also become important. 

It is noteworthy that the dimethylamino radical reaction with 02 is about a factor of 106-107 slower than its reactions with NO and N02. For example, Lindley et al. (1979) measured the ratio of rate constants /c81//c82 = 1.5 X 10-6 and /c81//c83a = 3.9 X 10-7. Thus, at 10 ppb NOx, reactions of the nitrogen-centered radical with NO and N02, in addition to 02, become important. This is perhaps not surprising, given that NH2 radicals also react extremely slowly with 02, with an upper limit of 6 X 10-21 cm3 molecule-1 s-1 (Tyndall et al., 1991). 

Figure 6.31 shows plots of measured peroxy radical concentrations for low-NOx conditions at Cape Grim, Tasmania (Penkett et al., 1997). Overlaid are plots of 7(0 D) and /(O D) 05. The plot of /(O D) 05 provides a better match. The slower decay in the peroxy radical concentration at dusk is due to the slow decay due to self-reactions, with some contribution from the CH302 + O, reaction (k 1 X 10"17 cnr1 molecule 1 s 1 Tyndall et al., 1998) and perhaps a small contribution from deposition (Monks et al., 1996). 

Infrared absorption bands attributable to methyl thioformate have been observed in the oxidation of DMS by OH in the absence of NOx but not when NOx was present (Barnes et al., 1996 Patroescu et al., 1999). Reaction (51) appears to be quite slow (k < 1 X 10 15 cm2 molecule-1 s-1), so that the dominant fate of CH3SCH20 is decomposition to HCHO + CH3S Turnipseed et al. (1996) measured the production of CH3S in the reaction of OH with DMS and also suggest, based on its relatively high yield, that the thermal decomposition (50) predominates over reaction (51) with 02 to form methyl thioformate. The methyl thioformate observed in laboratory systems in the absence of NO is thus likely due to cross reactions of CH3SCH200 with itself or other R02 (Barnes et al., 1994a), and the abstraction channel in the OH + DMS... 

Determination of the specific reactivity of the exhaust emissions requires accurate knowledge of both the types and amounts of compounds emitted as well as how each contributes to 03 formation. The latter factor, the ozone-forming potential, is treated in terms of its incremental reactivity (IR), which is defined as the number of molecules of ozone formed per VOC carbon atom added to an initial surrogate atmospheric reaction mixture of VOC and NOx ... 

The family of photo-oxidants includes tropospheric ozone, O3 (the bad ozone), ketones, aldehydes and nitrated oxidants, such as peroxy-acetylnitrate (PAN) and peroxybenzoylnitrate (PBN). The modeling of photo-oxidants is more complicated than that of acid deposition (NRC 1991). Here, the primary precursor is NOx, which as mentioned before, is emitted as a result of fossil fuel combustion. A part of NOx is the N02 molecule, which splits (photodissociates)... 

Figure 1 shows the non linear dependence of 03 production efficiency on NOx and non methane hydrocarbon (NMHC) levels. The 03 production efficiency is defined as the number of 03 molecules produced per molecule of NOx removed from the atmosphere. This non linear behavior of chemistry in the atmosphere reflects the occurrence of catalytic cycles. It implies the consideration of the spatial variability of short-lived 03 precursors which present important concentration gradients between continental and oceanic areas. 

In this study we investigated the reactivity of CH3S radical with 0 and NOx utilizing the photolysis of CH3SNO as a source of CH3S radical. N0180 was also used to check if the oxygen atom of NO2 transferred to a product SO2 molecule. 

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