Ozone forming reaction

The conversion of NO into NO2 is the characteristic step that leads to ozone formation, and the rate of conversion of NO to NO2 is often used to represent the ozone formation rate. The process does not remove either OH or NO from the atmosphere, so that the OH and NO may initiate additional ozone-forming reactions. 

The search for definitive proxies of atmospheric chemistry had its first big success in recognizing a unique isotopic signature in ozone. Analyses of samples collected by high-altitude balloons found large enrichments in and in ozone (25). Laboratory experiments confirmed that it was the ozone-forming reaction, itself, that confers a measurable excess of and on ozone (26). 

Ozone can react rapidly with NO to produce NO2, which re-enters the ozone formation cycle O3 + NO — O2 + NO2. This is the main ozone-depleting reaction in the absence of sunlight. Ozone also reacts with NO2 (to form NO, which in turn reacts with NO2 to form N20 ), as... 

Several theories have appeared in the Hterature regarding the mechanism of protection by -PDA antiozonants. The scavenger theory states that the antiozonant diffuses to the surface and preferentially reacts with ozone, with the result that the mbber is not attacked until the antiozonant is exhausted (25,28,29). The protective film theory is similar, except that the ozone—antiozonant reaction products form a film on the surface that prevents attack (28). The relinking theory states that the antiozonant prevents scission of the ozonized mbber or recombines severed double bonds (14). A fourth theory states that the antiozonant reacts with the ozonized mbber or carbonyl oxide (3) in Pig. 1) to give a low molecular weight, inert self-healing film on the surface (3). 

The relinking (14) and self-healing film (3) theories require chemical interaction between the antiozonant and ozonized mbber. The evidence for these interactions is meager (35,36). Overall, there seems to be no clear evidence in the Hterature for PDA derivatives becoming attached to mbber chains as a result of ozone attack. Much fundamental work in this area remains to be done, however. It seems clear at this point that any antiozonant—mbber interaction must be much less important than the scavenging effect of the antiozonant. In summary, the scavenger model is beheved to be the principal mechanism of antiozonant action. Ozone—antiozonant reaction products form a surface film that provides additional protection (37). 

In the schemes considered to this point, even the complex ones, the products form by a limited succession of steps. In these ordinary reaction sequences the overall process is completed when the products appear from the given quantity of reactants in accord with the stoichiometry of the net reaction. The only exception encountered to this point has been the ozone decomposition reaction presented in Chapter 5, which is a chain reaction. In this chapter we shall consider the special characteristics of elementary reactions that occur in a chain sequence. 

Reactions 2 and 3 regulate the balance of O and O3, but do not materially affect the O3 concentration. Any ozone destroyed in the photolysis step (3) is quickly reformed in reaction 2. The amount of ozone present results from a balance between reaction 1, which generates the O atoms that rapidly form ozone, and reaction 4, which eliminates an oxygen atom and an ozone molecule. Under conditions of constant sunlight, which implies constant /i and /s, the concentrations of O and O3 remain constant with time and are said to correspond to the steady state. Under steady-state conditions the concentrations of O and O3 are defined by the equations d[0]/df = 0 and d[03]/df = 0. Deriving the rate expressions for reactions 1-i and applying the steady-state condition results in the equations given below that can be solved for [O] and [O3]. 

With carbon monoxide, if the temperature is greater than -78°C, or with nitrogen oxide, even at -189°C, ozone forms expiosive oxidation reactions. 

In air, in the absence of additives, NO removal takes place by oxidation [56-58], Wu et al. [56] observed that the concentration of ozone generated in the corona discharge in the presence of NO is significantly smaller as compared to the ozone formed in air without NO under the same conditions. Thus, they concluded that NO oxidation by ozone and atomic oxygen is an important reaction path for the NO conversion. 

In the present-day atmosphere ozone forms into layers and this was first explained by Chapman who proposed a photolysis mechanism for ozone formation. Chapman s mechanism is a simple steady-state production of ozone and led to the concept of odd oxygen. The odd-oxygen reaction scheme is shown in Table 7.4. 

Photolysis of the N02, producing O, which then forms ozone by reaction with 02 ... 

In this study, synthetic aqueous solutions of phenol were treated with ozone. The reaction of ozone with phenol was investigated at several conditions, such as different phenol and ozone concentrations, and contact times. Total Organic Carbon (TOC) and UV analysis of the aromatic by-products formed during and after the ozonation reaction were employed. The reaction rates calculated from TOC analysis were investigated. 

Much evidence has been accumulated that the ozone-olefin reaction has a predominant role in aerosol formation from alkenes, cyclic olefins, diolefins, and other unsaturated compounds. Free radicals are formed in the reaction and can react further, along with nitric oxide and nitrogen dioxide, either with the various intermediates or with the olefin itself (see the recent review by Pitts and Finlayson ). 

Ozone formed in the stratosphere is unstable. It can be broken down by ultraviolet light shorter than 320 nm (UV-C and UV-B) according to the reaction ... 

The principal cause of acid rain is the combustion of fossil fuels that produce sulfur and nitrogen emissions. The primary sources are electrical power plants, automobiles, and smelters. Power plants produce most of the sulfur emissions and automobiles most of the nitrogen emissions. Other sources of acid rain include nitrogen fertilizers, jet aircraft, and industrial emissions. Just as in our discussion of ozone, numerous reactions are involved in the formation of acid rain. The process can be understood by considering the transformation of sulfur and nitrogen oxides into their respective acidic forms sulfuric acid and nitric acid. Sulfur, present up to a few percent in fuels such as coal, is converted to sulfur dioxide when the fuel is burned. The sulfur dioxide reacts with water to produce sulfurous acid, H,SO ,, that is then oxidized to sulfuric... 

Oxides of nitrogen play a central role in essentially all facets of atmospheric chemistry. As we have seen, N02 is key to the formation of tropospheric ozone, contributing to acid deposition (some are toxic to humans and plants), and forming other atmospheric oxidants such as the nitrate radical. In addition, in the stratosphere their chemistry and that of halogens interact closely to control the chain length of ozone-destroying reactions. 

Chapman (1930) first proposed the fundamental ozone-forming and destruction reactions that lead to a steady-state concentration of O, in the stratosphere. These reactions are now known as the Chapman cycle ... 

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 ... 

Note that since the atomic chlorine consumed in reaction 8.5 is regenerated in 8.7, it acts as a catalyst for ozone decomposition. Reaction 8.6 represents the decomposition of ozone by ultraviolet light, so that reactions 8.6 and 8.2 form a dynamic system that accounts for the absorption of ultraviolet light by the ozone layer. 

The numerator contains all hydroxyl radical forming reactions and all initiating reactions are summarized (Sk c(/()). The denominator contains all hydroxyl radical consuming reactions. The second term includes all reactions with intermediates (EkPi c(P,)), the third the reactions with scavengers(Lksi c(S,)). Similarly, the steady-state concentrations of ozone and hydrogen peroxide can be calculated from the liquid phase mass balances. 

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 ... 

Through the last sequence H02 is reformed to react with NO. The main point here is that nitrogen oxides are cycled through reactions R1 - R2, and therefore, this cycle will not limit the ozone forming potential. However, the formation of the other compound involved in the initial step in the ozone formation, H02, requires that CO be oxidised. The number of ozone molecules formed is therefore determined by the amount of CO present. H02 molecules can in a similar way be formed through the oxidation of CH4 and other hydrocarbons. The initial methane oxidation mainly through the reaction with OH ... 

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