Ozone production in the troposphere

Ozone production in the troposphere occurs almost exclusively through the reaction... 

Ozone Production In the Troposphere The Tropospheric Ozone Cycle and Ozone Distribution... 

Ozone production in the troposphere results from the formation of 0( P)-atoms following the photolysis of NO2. The reaction of HO2 and organic peroxy radicals with NO reforms the NO2 thereby completing the cycle and enabling more ozone production ... 

In summary, biomass burning is a major source of many trace gasses, especially the emissions of CO2, CH4, NMHC, NO,, HCN, CH3 CN, and CH3 Cl (73). In the tropics, these emissions lead to local increases in the production of O3. Biomass burning may also be responsible for as much as one-third of the total ozone produced in the troposphere (74). However, CH3 Cl from biomass burning is a significant source for active Cl in the stratosphere and plays a significant role in stratospheric ozone depletion (73). 

Photolytic. Irradiation of vinyl chloride in the presence of nitrogen dioxide for 160 min produced formic acid, HCl, carbon monoxide, formaldehyde, ozone, and trace amounts of formyl chloride and nitric acid. In the presence of ozone, however, vinyl chloride photooxidized to carbon monoxide, formaldehyde, formic acid, and small amounts of HCl (Gay et al, 1976). Reported photooxidation products in the troposphere include hydrogen chloride and/or formyl chloride (U.S. EPA, 1985). In the presence of moisture, formyl chloride will decompose to carbon monoxide and HCl (Morrison and Boyd, 1971). Vinyl chloride reacts rapidly with OH radicals in the atmosphere. Based on a reaction rate of 6.6 x lO" cmVmolecule-sec, the estimated half-life for this reaction at 299 K is 1.5 d (Perry et al., 1977). Vinyl chloride reacts also with ozone and NO3 in the gas-phase. Sanhueza et al. (1976) reported a rate constant of 6.5 x 10 cmVmolecule-sec for the reaction with OH radicals in air at 295 K. Atkinson et al. (1988) reported a rate constant of 4.45 X 10cmVmolecule-sec for the reaction with NO3 radicals in air at 298 K. 

The relative magnitude of the R02 + NO reaction compared to the R02 + H02 or R02 reactions is a critical factor in ozone formation in the troposphere. As discussed in more detail in Section J, if R02 + NO predominates, N02 is formed and through its photolysis to 0(3P), 03 is ultimately generated. On the other hand, the R02 + H02/R02 reactions can lead to the formation of stable products such as ROOH, without conversion of an NO to N02. In this case, no 03 is formed and indeed, through its photolysis to form OH radicals and subsequent reactions with H02 and OH, destruction of 03 occurs. 

Based on this chemistry, the production rate of 03 is expected to be very sensitive to the NO concentration, increasing with NO (see also Chapter 16 for a discussion of the dependence of 03 generation on NOx). In this context, Folkins et al. (1998) suggest that acetone is likely the major contributor to enhanced ozone production in the upper troposphere, since increased CH3OOH and H202 concentrations at 9- to 12-km altitude were observed only at very small NO concentrations, indicative of clean marine boundary layer air under such low NOx conditions, destruction rather than production of 03 is expected. 

Also shown are the separate contributions by ozone originating from the stratosphere (03s) and from photochemical production in the troposphere (03t). The data are summarized in Table 2. We note that both the downward flux of 03s and the tropospheric content of 03t in the NH are about 10% larger compared to the previous model version [18] due to model changes mentioned in Section 3. 

In the literature a wide range of estimates regarding the influx of ozone from the stratosphere into the troposphere is presented, derived by many different methods. Some studies report an annual flux between 200-870 Tg 03 yr 1 globally, whereas other studies report a flux between 500 and 1000 Tg 03 yr 1 for the NH only. With our model we estimate for the NH a net downward flux from the stratosphere of 580 Tg 03 yr1, which is partly balanced by an upward flux of ozone from photochemical production in the troposphere of 210 Tg 03 yr1, yielding a net downward cross-tropopause ozone transport of 370 Tg 03 yr 1. Globally, these values are 950, 370 and 580 Tg 03 yr 1, respectively. (Note that the troposphere-to-stratosphere ozone flux in the... 

Increases of ozone in the troposphere are mainly a result of increases in NOx, which is an important agent in tropospheric ozone production. In the lower tropical stratosphere, another factor is the increase in the photolysis of molecular ozone, resulting from the reductions in overhead ozone. 

The peak of the O2 photodissociation occurs in the stratosphere (near 35 km for an overhead sun) where the total number of 02 molecules pho-todissociated is of the order of 107 cm-3 sec-1. Below the ozone peak (<25 km) the photodissociation rate decreases rapidly, particularly when the solar zenith angle increases. Below 20 km, the atomic oxygen production becomes very small and there is no atomic oxygen production in the troposphere by the 02 photodissociation. The ozone photodissociation is the result of the absorption of solar radiation in the visible and the ultraviolet ... 

On the basis of ratios of C and C present in carbon dioxide, Weinstock (250) estimated a carbon monoxide lifetime of 0.1 year. This was more than an order of magnitude less than previous estimates of Bates and Witherspoon (12) and Robinson and Robbins (214), which were based on calculations of the anthropogenic source of carbon monoxide. Weinstock (250) suggested that if a sufficient concentration of hydroxyl radical were available, the oxidation of carbon monoxide by hydroxyl radical, first proposed by Bates and Witherspoon (12) for the stratosphere, would provide the rapid loss mechanism for carbon monoxide that appeared necessary. By extension of previous stratospheric models of Hunt (104), Leovy (150), Nicolet (180), and others, Levy (152) demonstrated that a large source of hydroxyl radical, the oxidation of water by metastable atomic oxygen, which was itself produced by the photolysis of ozone, existed in the troposphere and that a chain reaction involving the hydroxyl and hydroperoxyl radicals would rapidly oxidize both carbon monoxide and methane. It was then pointed out that all the loss paths for the formaldehyde produced in the methane oxidation led to the production of carbon monoxide [McConnell, McElroy, and Wofsy (171) and Levy (153)1-Similar chain mechanisms were shown to provide tropospheric... 

As illustrated in Figure 2, the production of hydroxyl radicals is initiated by the photolysis of ozone. Ozone, present in the troposphere at concentrations ranging from 10 to 100 ppbv, has a bond energy of 26 kcal/mole. Solar photons having wavelengths between 315 and 1200 nm can dissociate ozone and produce an oxygen atom in its ground electronic state ... 

Ozone is of major interest to tropospheric chemists for two reasons It leads to the production of hydroxyl radicals, and it is a greenhouse gas. For many years, tropospheric ozone was believed to be chemically inert. Scientists held that the ozone present in the troposphere was formed initially in the stratosphere — where ultraviolet radiation is of high enough energy to dissociate oxygen — and mixed down into the troposphere. It was postulated further that ozone was removed from the atmosphere primarily by reacting with Earth s surface. 

Photochemical ozone creation potential (POCP) [50-56] Emissions to air which lead to ozone production in the lower troposphere (close to ground level) Hydrocarbons... 

The ozone concentration in the troposphere during the daytime is typically about 1 pphm (parts per hundred million parts of air by volume) [20], Values up to 100 pphm were measured in some photochemical smog areas. The molecular mechanism of the ozone aging of diene based elastomers was studied in detail and is well understood [19,21], Products or intermediates different from those arising in autoxidation or photo-oxidation of polymers were identified ozonides (3), zwitterions (4), diperoxides (5), polyperoxides (6), polymeric ozonides (7) and terminal aldehydes (8). Reactivity of aminic antiozonants (AOZ) with these species accounts for the protection of rubbers against atmospheric 03. AOZ must also possess antioxidant properties, because the free radical processes are concerted with ozonation due to the permanent presence of oxygen. 

In many respects the key reaction that determines the effect of an N J, injection on stratospheric chemistry is reaction 4.36, which controls the (NOal/lNOj ratio. If released at about 20 km altitude, NO, leads to ozone depletion through an enhancement of the NO, cycles. Release at a lower altitude, at about the tropopause, 10 to 12 km, such as by the current fleet of subsonic aircraft, is predicted actually to lead to a slight increase in ozone. The reason for this behavior is the presence of hydrocarbons in the upper troposphere that interact with the emitted NO, to generate ozone by the same reactions responsible for ozone production in the lower troposphere. (We will consider this chemistry in Chapter 5.) A crossover point is predicted at a particular altitude (about 18 km) where NO, emissions go from ozone enhancing to ozone depleting. 

Although we wish to preserve the ozone in the stratosphere, we want to minimize its production in the troposphere, that part of the atmosphere where we live. Photochemical smog produced as a result of the action of solar radiation on the effluents from automobiles is the primary source of ozone in the troposphere. The trigger for photochemical smog is based on nitrogen oxide chemistry, so we defer our discussion of this problem until Chapter 16 on Group 5A chemistry. 

Now this is the same ozone that in the stratosphere shields the biosphere from dangerous uv radiation. (See Section 11.5 for an extended discussion of ozone and the stratospheric ozone layer. Chapter 18 discusses threats to the ozone layer from chlorine and bromine compounds.) But this ozone is in the troposphere, the layer of air we breathe and live in. It causes a number of health problems including respiratory irritation, choking, coughing, and fatigue and has also been implicated in damage to forests and crops. It also attacks rubber products and causes cracks in tires. 

In the years 1972-1974 Cmtzen proposed that NO and NO2 could catalyse ozone production in the background troposphere by reactions occurring in the CO and CH4 oxidation chains. Additional photochemical reactions leading to ozone loss were likewise identified. These gross ozone production and destruction terms are each substantially larger than the downward flux of ozone from the stratosphere, which until then had been considered the main source of tropospheric ozone. 

Considerable attention has been given to the persistence and fate of organic compounds in the troposphere, and this has been increasingly motivated by their possible role in the production of ozone by reactions involving NO. ... 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

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