Ozone formation

More precisely, the rate of ozone formation depends closely on the chemical nature of the hydrocarbons present in the atmosphere. A reactivity scale has been proposed by Lowi and Carter (1990) and is largely utilized today in ozone prediction models. Thus the values indicated in Table 5.26 express the potential ozone formation as O3 formed per gram of organic material initially present. The most reactive compounds are light olefins, cycloparaffins, substituted aromatic hydrocarbons notably the xylenes, formaldehyde and acetaldehyde. Inversely, normal or substituted paraffins. 

Reactivities compared for selected organic compounds with respect to ozone formation. 

To estimate the effect of automobile traffic and motor fuels on ozone formation, it is necessary to know the composition of exhaust gas in detail. Figure 5.26 gives an example of a gas phase chromatographic analysis of a conventional unleaded motor fuel. 

For each type of component, its relative reactivity in ozone formation was taken into account which makes it possible to characterize by weighting the behavior of the overall motor fuel under the given experimental conditions. The overall reactivity is in fact governed by a limited number of substances ethylene, isobutene, butadiene, toluene, xylenes, formaldehyde, and acetaldehyde. The fuels of most interest for reducing ozone formation are those which contribute towards minimizing emissions of the above substances. 

Example of an analysis of exhaust gas by gas phase chromatography and j relative reactivity of effluents with respect to tropospheric ozone formation. I... 

Most ozone is formed near the equator, where solar radiation is greatest, and transported toward the poles by normal circulation patterns in the stratosphere. Consequendy, the concentration is minimum at the equator and maximum for most of the year at the north pole and about 60°S latitude. The equihbrium ozone concentration also varies with altitude the maximum occurs at about 25 km at the equator and 15—20 km at or near the poles. It also varies seasonally, daily, as well as interaimuaHy. Absorption of solar radiation (200—300 nm) by ozone and heat Hberated in ozone formation and destmction together create a warm layer in the upper atmosphere at 40—50 km, which helps to maintain thermal equihbrium on earth. 

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... 

Because ozone formation occurs only within these microdischarge channels, ozone-production efficiency for the most part depends on the strength of the microdischarges, which is influenced by a number of factors such as the gap width, pressure, properties of the dielectric and metal electrode, power... 

Ozone Generator Design. A better understanding of discharge physics and the chemistry of ozone formation has led to improvements in power density, efficiency, and ozone concentration, initiating a trend toward downsizing. 

Selective Catalytic Reduction of Nitrogen Oxides The traditional approach to reducing ambient ozone concentrations has been to reduce VOC emissions, an ozone precurssor. In many areas, it has now been recognized that ehmination of persistent exceedances of the National Ambient Air Qnality Standard for ozone may reqnire more attention to reductions in the other ingredients in ozone formation, nitrogen oxides (NOJ. In such areas, ozone concentrations are controlled by NO rather than VOC emissions. 

Approaches used to model ozone formation include box, gradient transfer, and trajectoty methods. Another method, the particle-in-cell method, advects centers of mass (that have a specific mass assigned) with an effective velocity that includes both transport and dispersion over each time step. Chemistry is calculated using the total mass within each grid cell at the end of each time step. This method has the advantage of avoiding both the numerical diffusion of some gradient transfer methods and the distortion due to wind shear of some trajectory methods. 

Gulf of Maine Oxidant Study (GOMOS) a study to investigate the sources and transport of pollutants contributing to ozone formation. 

A steady-state analysis of R13-R16 provides a means of understanding the role of peroxyl radicals such as HO2 in ozone formation ... 

Existence of the PSS was predicted theoretically by Leighton (61), and experimental studies of this relationship date back almost 20 years. These experiments have been accomplished in smog chambers (62), polluted urban air (63,64,65), rural environments (66), and in the free troposphere (67). The goal of these experiments has been to verify that our understanding of NOjj chemistry is fundamentally correct, and to ver the role of H02 and R02 in ozone formation. Studies in polluted air seem to confirm the dominance... 

Chemical kinetic analysis of these simplified reactions allows net ozone formation to be directly related to hydrocarbon consumption by HO on a time-independent basis... 

Moshiri, E. Computer Modeling of Photochemical Ozone Formation A Simplified Approach Ph.D. Thesis Portland State University Portland, OR, 1984 1-211. 

A long and difficult task will be confronted, in particular to bring ozone within the air quality standards, as the physics and chemistry of ozone formation in the MCMA are not sufficiently understood. 

Since ozone is generated by photolytic reactions, anything which affects available sunhght will affect the ozone concentration. Consequently, ozone levels are the highest in the summer months, when the days are longer and the sun is more intense [42]. Similarly, ozone levels are highest near midday and decrease almost to zero at night [43]. Temperature has little effect on ozone formation. 

O2 + A > ( i=180-240 nm) 0 + 0 Second, an oxygen molecule captures one of these oxygen atoms to form an ozone molecule 02+0 03+ Heat The second step occurs twice for each O2 fragmentation, giving the overall balanced process for ozone formation 3 O2 + h V( =l30-240 nm) Heat... 

C16-0012. Estimate the equilibrium constant for the ozone formation reaction at 825 °C. 

The extent to which this occurs depends on a number of issues (Finlayson-Pitts and Pitts 1997), including the reactivity of the hydrocarbon that is itself a function of many factors. It has been proposed that the possibility of ozone formation is best described by a reactivity index of incremental hydrocarbon reactivity (Carter and Atkinson 1987, 1989) that combines the rate of formation of O3 with that of the reduction in the concentration of NO. The method has been applied, for example, to oxygenate additives to automobile fuel (Japar et al. 1991), while both anthropogenic compounds and naturally occurring hydrocarbons may be reactive. 

Appreciation of interactive processes that have been outlined has been able to illuminate discussion of mechanisms for reactions as diverse as the acidification of water masses, climate alteration, ozone formation and destruction, and the possible enviromnental roles of trichloroacetic acid and nitroarenes. 

Denitrification involves the sequential formation of nitrite, nitric oxide, and nitrous oxide. Two aspects of nitric oxide have attracted attention (a) chemical oxidation of biogenic nitric oxide to Nq, in the context of increased ozone formation (Stohl et al. 1996) and (b) the physiological role in mammalian systems (Feldman et al. 1993 Stuehr et al. 2004), in parasitic infections (James 1995), and in the inhibition of bacterial respiration (Nagata et al. 1998). Nitric oxide may be produced microbiologically in widely different reactions such as... 

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. 

Returning to the ozone formation reaction example, when the photolysis reaction is producing O atoms the ArG for the reaction now changes due to the contribution from the reaction quotient. At a particular point the concentration of O atoms will reach that required by equilibrium and stop. If the O atom concentrations increase further the reaction is no longer spontaneous in the forward direction but reverses. What value of Q stops the reaction from being spontaneous ... 

Winer, A.M., Damall, K.R., Atkinson, R. Pitts, Jr., J.N. (1979) Smog chamber study of the correlation of hydroxyl radical rate constants with ozone formation. Environ. Sci. Technol. 7, 622-626. 

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