Ozone equilibrium concentration

Thus, the mean temperature of the atmosphere, which is about 20°C at sea level, falls steadily to about —55° at an altitude of 10 km and then rises to almost 0°C at 50 km before dropping steadily again to about —90° at 90 km. Concern was expressed in 1974 that interaction of ozone with man-made chlorofluorocarbons would deplete the equilibrium concentration of ozone with potentially disastrous consequences, and this was dramatically confirmed by the discovery of a seasonally recurring ozone hole above Antarctica in 1985. A less prominent ozone hole was subsequently detected above the Arctic Ocean. The detailed physical and chemical conditions required to generate these large seasonal depletions of ozone are extremely complex but the main features have now been elucidated (see p. 848). Several accounts of various aspects of the emerging story, and of the consequent international governmental actions to... 

The Henry s law constant in water was used in the McJilton et al. uptake model to determine the equilibrium concentration of ozone and sulfur dioxide at the surface of a simulated mucus film along the airways in Weibel s symmetric model.It is also used to determine the concentration of absorbed gas at the surface of the mucus when the pollutant gas undergoes a homogeneous or heterogeneous chemical reaction within the mucus layer. 

Ozone production in the stratosphere is another example. Ozone concentration in the atmosphere is not the equilibrium concentration (which is negligible). The mechanism of ozone production is as follows ... 

The maximum concentration of ozone in the stratosphere (or the ozone layer) is about 9 ppm at an altitude of about 35 km. That is, the concentration of ozone in the so-called ozone layer is still very low. Transport of ozone in the atmosphere modifies ozone concentration levels at each altitude and latitude. It is emphasized that the steady-state concentration of O3 in the stratosphere is not the thermodynamic equilibrium concentration, but is established by kinetics of photochemical reactions. 

Not only have various names been used for the same concept, Morris (1988) found a variety of values for the equilibrium concentration of ozone in water reported in the literature. Based on the data from more than nine authors, he suggested that a linear correlation between the solubility ratio of ozone,, 5 (which is the inverse of the dimensionless Henry s Law constant Hc), and the temperature can be used as a first estimate for the solubility of ozone in water ... 

At T= 20 °C the aqueous solubility of ozone is about one third of the gas-phase concentration seven times higher than that of oxygen However, we must consider that the gas does not contain pure ozone, but rather approximately 20 % wt O-, in 02 (corresponding to cG = 320.1 g nrf3 at STP, cf. Table B 1-5), which can be achieved with modern electrical discharge ozone generators at standard pressure. Thus, the equilibrium concentration of ozone in the liquid is usually less than cL = 108 g m ... 

A more comprehensive analysis of the influences on the ozone solubility was made by Sotelo et al., (1989). The Henry s Law constant H was measured in the presence of several salts, i. e. buffer solutions frequently used in ozonation experiments. Based on an ozone mass balance in a stirred tank reactor and employing the two film theory of gas absorption followed by an irreversible chemical reaction (Charpentier, 1981), equations for the Henry s Law constant as a function of temperature, pH and ionic strength, which agreed with the experimental values within 15 % were developed (Table 3-2). In this study, much care was taken to correctly analyse the ozone decomposition due to changes in the pH as well as to achieve the steady state experimental concentration at every temperature in the range considered (0°C [Pg.86]

Another problem is the equilibrium concentration cL. Use of the correct value of cL is essential for calculating a correct A , a. The value for cL is derived from the gas phase ozone concentration by applying Henry s Law. It is evident that the value calculated for the effluent gas will be lower than that calculated for the influent gas. In small laboratory scale ozonation reactors, such as STRs, a mean value can be calculated ... 

The relevance of the pH-value was already seen in the chain reaction of ozone, especially in the initiation step. It also plays an important role in all the acid-base equilibrium by influencing the equilibrium concentrations of the dissociated/nondissociated forms. This is especially important for the scavenger reaction with inorganic carbon, which will be discussed further in Section B 4.4.4. 

The equilibrium concentration of ozone is established with a maximum at an altitude of about 25 km, depending on the intensity of the solar flux, the rates of formation [processes (Vl-98) and (VI-99)] and those of destruction of ozone [processes (VI-100) and (Vl-101)]. The concentration profile of ozone as a function of altitude is given in Fig. VI-14. It has been recognized, however, that the four reactions (VI-98) to (VI-101) are not sufficient to account for the global ozone balance. About 80% of the ozone produced by sunlight must be destroyed by reactions other than those proposed by Chapman. 

Data on three successful flights by Naval Research Laboratory personnel are reproduced in Figure 11 24). While data obtained by a rocket spectrograph in this manner cannot be considered highly accurate, the large variations in the vertical distribution of ozone at high altitudes are indicative of a certain amount of air movement at these levels. It appears that air motion upsets the ozone equilibrium conditions by the equivalent of altitude changes at times of as much as 5 or 6 km. Furthermore, it has been established by these experiments that ozone extends up to and sometimes alDove 70 km. The maximum relative concentration of ozone has been... 

FIGURE 2.10 Outline of representative normal processes and some interfering reactions which affect the equilibrium concentration of stratospheric ozone. 

The reactions controlling the lifetime of ozone at the surface depend on the chemical composition of the aqueous film. If this is not specified, it is therefore not possible to estimate the steady-state concentration for this photooxidant in the aqueous phase. However, the chemical effect of ozone in films or atmospheric droplets can be estimated by assuming that the steady-state concentration of ozone is still in equilibrium with the concentration of the ozone in the atmosphere. An atmospheric concentration of ozone of 1012 molecules per cubic centimeter would result in an aqueous equilibrium concentration of about 1 nanomolar (20°C) as the environmental factor to be considered in the aqueous film. Figure 7 gives examples for the rate constants for different types of compounds and a scale for the corresponding half-lives of these compounds exposed to the estimated concentration of ozone. (For further rate constants see Ncta et al., 1988 or Hoigne and Bader, 1983 and Hoigne et al., 1985). 

It is evident that the equilibrium ozone concentration also depends on the temperature profile and on the zenith distance of the Sun. For this reason the equilibrium concentration at a given height decreases with increasing altitude. It... 

The reaction of Cl atoms and O3 is the direct dissipation reaction of O3 molecules in the ClOx cycle (Sect 8.2.3), and is important as a rate-determining step to reduce the equilibrium concentration of ozone by CH3CI in the natural stratosphere. 

A 0.10-mol sample of pure ozone, 03, is placed in a sealed 1.0-E container and the reaction 2 03(g) 3 0,(g) is allowed to reach equilibrium. A 0.50-mol sample of pure ozone is placed in a second 1.0-L container at the same temperature and allowed to reach equilibrium. Without doing any calculations, predict which of the following will be different in the two containers at equilibrium. Which will be the same (a) Amount of 02 (b) concentration of 02 (c) the ratio [0, / 0.] (d) the ratio [02 3/[03 2 (e) the ratio [03 2/[02 Explain each of your answers. 

This is the main reaction for the formation of ozone although, under equilibrium conditions, the concentrations of NO2, NO, and O3 are interdependent and no net synthesis of O3 occurs. When, however, the equilibrium is disturbed and NO is removed by reactions with alkylperoxy radicals (reactions 1+2+3), synthesis of O3 may take place. 

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 ... 

An important consequence of this rapid turnover is the establishment of a steady-state concentration of ozone. One can express this dynamic equilibrium as follows ... 

Equation 2-4, [Oj] = kJNOjJ/JlTj [NO], should be tested in the real atmosphere, as well as in laboratory experiments. Simultaneous measurements of the concentrations of ozone, nitric oxide, and nitrogen dioxide and of the intensity of sunlight for a variety of conditions will provide a much-needed check on this dynamic equilibrium. 

If photosynthetic and respiratory changes cannot account for the increases in adenylate concentration, which system is responsible It has been reported that ADP and ATP concentrations of Ehrlich ascites tirnior cells increase in the presence of adenine (15), Whether this wo ild hold true for plant cells is not known, but it seems plausible that equilibrium shifts would initiate similar responses. An increase in adenine concentrations could occur if there was any breakdown of nucleic acids. There is one report that the number of ribosomes in the chloroplast does decrease in response to ozone (16). An increase in synthesis of purines is also possible but there is no evidence to either support or refute this hypothesis. 

Eventually, an equilibrium is reached between the production and dissociation of ozone that results in the steady-state concentration currently observed in the stratosphere. 

Ozone in the atmosphere is a good example of photochemical reactions. Atmospheric ozone is not due to equilibrium. The production and decomposition of ozone are largely by photochemical process, and the concentration of ozone in the stratosphere is at steady state, controlled by the kinetics of photochemical production and decomposition. 

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