Ozone production and destruction

In this section, we use another chain reaction to show the relation between the steady-state treatment and the quasi-equilibrium treatment. The former is more general than the latter, and leads to more complete but also more complicated results. Ozone, O3, is present in the stratosphere as the ozone layer, and in the troposphere as a pollutant. Ozone production and destruction in the atmosphere is primarily controlled by photochemical reactions, which are discussed in a later section. Ozone may also be thermally decomposed into oxygen, O, although... 

The results clearly show the dominance of in situ tropospheric ozone production and destruction over downward transpeat from the stratosphere. With the same model, estimates were also made of the present and pre-industrial ozone concentration distributions. The calculations indicate a clear increase in tropospheric ozone concentrations over the past centuries mainly due to higher amounts of fuel (CO and CH4) and enhancements in the NO catalysts from fossil fuel and biomass burning (19-25). 

Ozone in the stratosphere is present at a steady-state concentration resulting from the balance of ozone production and destruction by the above processes. The quantities of ozone involved are interesting. A total of about 350,000 metric tons of ozone are formed and destroyed daily. Ozone never makes up more than a minuscule fraction of the gases in the ozone layer. In fact, if all the atmosphere s ozone were in a single layer at surface temperature and pressure conditions of approximately 273 K and 1 atm, it would be only 3 mm thick ... 

Fig. 1.1 Ozone Production and destruction rates, including absolute and relative contributions by the Chapman reaction R4 (Do), NO catalysis Rll + R12 (Dn), HO catalysis R5 + R6 (Dh) and ClOx catalysis R21 + R22 (DCl)x. The calculations neglect the heterogeneous halogen activation which become very important below 25 km under cold conditions...

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. 

Figure 3.4. Long-range transport of aerosols and gases and effects on ozone production and destruction [4],...

Oxidation-reduction reactions in water are dominated by the biological processes of photosynthesis and organic matter oxidation. A very different set of oxidation reactions occurs within the gas phase of the atmosphere, often a consequence of photochemical production and destruction of ozone (O3). While such reactions are of great importance to chemistry of the atmosphere - e.g., they limit the lifetime in the atmosphere of species like CO and CH4 - the global amount of these reactions is trivial compared to the global O2 production and consumption by photosynthesis and respiration. 

For centuries, the production and destruction of ozone in Earth s ozone layer was constant. Explain why in recent decades the ozone has been depleted faster than it was replaced. 

Located several kilometres above the Earth s surface is the stratosphere. Here the ozone layer acts as a filter, protecting life on Earth from harmful low-wavelength ultraviolet radiation known as UV-C, which damages biological macromolecules such as proteins and DNA. In order to understand the effects of anthropogenic input into the stratosphere, the production and destruction of the ozone layer has been studied by a variety of photochemical models and experimental methods. 

The importance of photochemical destruction in the 03s tropospheric budget implies that the lifetime of 03s is coupled to the chemical production and destruction of 03. Consequently, the simulated tropospheric budget of 03s may be affected directly by differences in the simulated chemistry. For example, simulations with a pre-industrial and a present-day emission scenario or with and without representation of NMHC chemistry will produce different estimates of the tropospheric oxidation efficiencies [39, 40]. However, our simulations indicate only small effects on the calculated 03s budget [6]. Figure 5 presents the simulated zonal distribution of 03s, the chemical destruction rate, of ozone (day"1) and the chemical loss of 03s (ppbv day 1) for the climatological April. The bulk of the 03s in the troposphere resides immediately below the tropopause, whereas the ozone chemical destruction rate maximizes in the tropical lower troposphere (Figures 5a and 5b). Hence, most 03s is photochemically destroyed between 15-25 °N and below 500 hPa. This region... 

Figure 1.3 Chemical reactions involved in the production and destruction of ozone ...

The first issue is the possibility raised in the early 1970 s that human activities might add certain chlorine, nitrogen, and other catalytic substances to the stratosphere. These substances, in turn, would upset the balance between the production and destruction of ozone in a manner that could increase the intensity of ultraviolet light reaching the biosphere with... 

Ozone, in turn, can be destroyed by interaction with another photon that breaks it into an oxygen molecule (02) and an oxygen atom (O). Stratospheric ozone also can be destroyed by reaction with other species, such as nitric oxide (NO) — as in Eq. [4-35], and chlorine atoms (from CFCs). The net concentration of ozone is established by the rates of both the production and destruction reactions. 

The amount of ozone in the stratosphere is determined by a dynamic balance between the processes of production and destruction. Ozone is formed by a two stage process that begins with photodissociation of oxygen by wavelengths < 242 nm... 

The production and destruction processes above occur naturally to produce the dynamic photochemical equilibrium of ozone distribution described earlier. This equilibrium has been upset by the addition of extra species of M to the stratosphere, most notably chlorine (Cl), from human sources. 

Appreciation of the interactive processes outlined earlier has been able to illuminate discussion on mechanisms of problems as diverse as acidification of water masses, climate alteration, ozone formation, and destruction, and the possible environmental roles of trichloroacetic acid and nitroarenes. The analysis and distribution of these—and other—transformation products is therefore clearly motivated (Sections 2.5 and 3.6). 

If one accepts the classical viewpoint and assumes photochemical ozone production and loss reactions negligible on a global scale, the budget of ozone in the troposphere will be dominated by the injection of ozone from the stratosphere and its destruction at the ground surface. Clearly, this is a minimum budget. Injection and destruction rates are examined below. For steady-state conditions both rates must balance, and if they do not, one would have an indication for the importance of additional sources or sinks of tropospheric ozone. The following discussion will show, however, that within a rather wide margin of error, the two rates are indeed compatible. 

Terrestrial stratospheric chemistry is closely linked to the ozone (O3) layer at 15-35 km, which shields the Earth s surface from harmful UV sunlight (X<300 nm) and dissipates the absorbed solar energy as heat. The abundance of O3 in the stratosphere is a balance between production, destruction, and lateral transport. Production and destruction of O3 in the absence of other perturbing influences is described by the Chapman cycle given in Table V. 

In air, the O atoms produced photochemically combine with O2 in the presence of a third body to yield O3 by the reaction already discussed (Section 1.A.4). Ozone itself, however, is readily photodissociated, and also reacts with NO to reform O2 and NO2. The rates of production and destruction of O3 in an air-nitrogen oxide system are such that the concentrations of NO and O3 should be roughly equal. In practice, however, the combination of NO and hydrocarbons plus solar ultraviolet leads to a net production of O3 and other oxidants. 

In the actual remote atmosphere, O3 concentrations are determined mostly by long-range transport, and do not necessarily reflect the in situ production and destruction directly. However, the production and loss of O3 described above are reflected in the regional scale distribution of O3, and are important for the consideration of the global budget of tropospheric ozone. 

The daily production and destruction for odd oxygen below 50 km have been estimated for conditions at the Equator. The observed ozone distribution was taken from Diitsch (1964). The results of the computations are shown in Table 3.1. 

The term CFCs is a general abbreviation for ChloroFluoroCarbons. They have been extensively used since their discovery in the thirties, mainly as refrigerant, foam blowing agent, or solvent because of their unique properties (non toxic, non flammable, cheap). However, after the first warning of Rowland and Molina [1] in 1974 that CFCs could destroy the protective ozone layer, the world has moved rapidly towards a phase-out of CFCs. Because the destruction of stratospheric ozone would lead to an increase of harmful UV-B radiation reaching the earth s surface, the production and use of CFCs is prohibited (since January 1, 1995 in the European Union and since January 1, 1996 worldwide). 

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