Ozone formation cycle

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... 

The chemical production term is given by the OH—HO2 conversion reaction shown in Fig. 5.7 of the whole ozone formation cycle as the rate-determining step ... 

NO and NO2 play crucial roles in the ozone formation cycle. The former provides the source of atomic oxygen (5.20) and the latter cycles the HO2 radical back to OH (5.35) for the continuous burning of the ozone precursors CO, CH4 and NMVOC (Fig. 5.7). 

Because SO2 is produced (and emitted) from fuel sulfides in combustion processes, four O atoms are needed to produce one sulfate. This is one atom of oxygen more than for nitrate (HNO3) formation from air nitrogen (N2). It has always been out of consideration that SO2 plays a role in the ozone formation cycle in transferring OH into HO2 (reactions 5.278 and 5.279) however, the rate of OH HO2 conversion through SO2 is small compared with the other O3 precursors of interest. There is also another irony of fate when SO2 concentration in urban areas has been so significant in the past (Table 2.73) that it contributed to O3 formation, nothing was known about the mechanism of tropospheric ozone formation. 

This is a major chain propagation step in the overall reaction mechanism for ozone formation in photochemical air pollution. Because H02 is intimately tied to OH through reaction (17) and cycles such as that in Fig. 1.4, when NO is present the sources and sinks of H02 are, in effect, sources or sinks of the OH radical. 

Analogous to chlorine chemistry, the formation of bromine nitrate represents the major short circuit in its ozone destruction cycle ... 

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 ... 

The pulp and paper industry is another industry where life-cycle assessment methodology has been applied. One example is a paper by Lopes et al.,130 who compared the two major fuels used in the pulp and paper industry fuel oil and natural gas. The environmental categories were the same categories listed by Allen and Shonnard.53 The use of methane in place of fuel oil decreases all of the environmental parameters except photochemical ozone formation, which does not vary between fuel options. 

It is important to note that certain cycles can lead to ozone formation this phenomenon is readily observed in the troposphere (see Box 5.4), which is rich in anthropogenic hydrocarbons and nitrogen oxides. In the free troposphere and lower stratosphere, the conversion of NO to NO2 by peroxy radicals (HO2 and CH3O2) produced by the oxidation of methane and carbon monoxide, followed by the photodissociation of NO2 leads to the formation of O3. The complete chains are the following (Crutzen, 1974) ... 

The essential role of NOx in ozone formation is evident in the CO oxidation mechanism (Section 6.4). For example, in the low NOx limit (NOx-limited), the rate of O3 formation increases linearly as [NO] increases and the rate is independent of [CO]. In the high NOx limit (NOx-saturated), the rate of O3 formation increases with [CO], but decreases as [NOx] increases. The explanation for the behavior in the high NOx limit is that, with ample NOx available, as NOx increases, the rate of the OH + N02 termination reaction increases, removing both HOx and NOx from the system, limiting OH - H02 cycling, and thereby decreasing the rate of 03 formation. 

The dominant removal process for FCO radicals is also the major formation reaction of FC(0)0j radicals, that is, reaction (101). It has been suggested that the interconversion reaction of FC(0)Oj radicals with ozone could lead to the possible involvement of FC(0)02 and FC(0)0 radicals in catalytic ozone destruction cycles. 

The photodissociation of ozone reverses the reaction that forms it. We thus have a cycle of ozone formation and decomposition, summarized as follows ... 

The reactions of the ozone cycle account for some, but not all, of the facts about the ozone layer. Many chemical reactions occur that involve substances other than oxygen. We must also consider the effects of turbulence and winds that mix up the stratosphere. A complicated picture results. The overall result of ozone formation and removal reactions, coupled with atmospheric turbulence and other factors, is to produce the upper-atmosphere ozone profile shown in FIGURE 18.4, with a maximum ozone concentration occurring at an altitude of about 25 km. This band of relatively high ozone concentration is referred to as the ozone layer or the ozone shield. ... 

A slightly simplified version of the steady-state process for ozone formation (reactions 1 and 2) and destruction (reactions 3 and 4) is known as the Chapman cycle (shown in left margin) after the scientist who proposed it. 

An animation of the Chapman cycle is found at a NASA website. Click on Atmospheric Chemistry under the Atmosphere heading. Then click on ozone and follow by clicking on the panel showing ozone creation to watch a movie of ozone formation, http // visibleearth.nasa.gov/... 

Ozone in the stratosphere also acts as a filter of high-eneigy ultraviolet Ught. Human activities have depleted the ozone layer by introducing chemicals into the stratosphere that perturb the natural cycle of ozone formation and decomposition. Notable among ttiese are the cMorc uorocathom (CFCs). 

In 1930, the English geophysicist Sydney Chapman (1888-1970) worked out the cycle of ozone formation and destruction in the stratosphere. There are four chemical reactions in the cycle ... 

Short-wavelength ultraviolet radiation (light with a wavelength less than 210 run) is the pump that drives the process. The single-oxygen atom species (O) that form are extremely reactive. They can collide either with an molecule and form ozone (as shown in the second chemical reaction in the cycle), or they can colUde with an O3 molecule and destroy ozone (as shown in the fourth chemical reaction in the cycle). Because the number of molecules is significantly greater than the number of O3 molecules, ozone formation is favored over ozone destruction. 

This formation mechanism is quite different from that described previously for the troposphere and summarized in cycles Cl and C2 of Box 5.2. Whereas oxides of nitrogen promote ozone formation in the troposphere, in the stratosphere, where the chemical composition and light spectmm are quite different, the effect of oxides of nitrogen is to catalyze ozone destmction via the reactions ... 

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