Ozone steady-state concentrations

In order to calculate the steady-state concentration of ozone in the stratosphere, we need to balance the rate of production of odd oxygen with its rate of destruction. Chapman originally thought that the destruction was due to the reaction O + 03 —> 2O2, but we now know that this pathway is a minor sink compared to the catalytic destruction of 03 by the trace species OH, NO, and Cl. The former two of these are natural constituents of the atmosphere, formed primarily in the photodissociation of water or nitric oxide, respectively. The Cl atoms are produced as the result of manmade chlorofluorocarbons, which are photodissociated by sunlight in the stratosphere to produce free chlorine atoms. It was Rowland and Molina who proposed in 1974 that the reactions Cl + 03 —> CIO + O2 followed by CIO + O —> Cl + O2 could act to reduce the concentration of stratospheric ozone.10 The net result of ah of these catalytic reactions is 2O3 — 3O2. 

Substitution of the steady-state concentrations of the two intermediates into the equation for the rate of disappearance of ozone gives... 

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 ... 

The possibility that free radicals, particularly hydroperoxy, have significant effects on biologic surfaces exposed to the irradiated atmosphere should be investigated. Sticking coefficients are needed. In experiments in which the observed biologic effects cannot be attributed to the measured ozone and PAN concentrations, the possibility of damage by the steady-state concentrations of free radicals in the atmosphere should be considered. 

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

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. 

Chapman (1930) first proposed the fundamental ozone-forming and destruction reactions that lead to a steady-state concentration of O, in the stratosphere. These reactions are now known as the Chapman cycle ... 

For sunlit surface waters and drinking water treated by ozonation, carbonate radical steady-state concentrations were estimated to be typically two orders of magnitude higher than HO concentrations (see Fig. 16.1). Thus, this process may become important for compounds that react with CO by less than a factor of 100 to 1000 more slowly as compared to HO . Such compounds include the more easily oxidiz-... 

Modeling in drinking water applications is largely confined to describing chemical processes. The mathematical models used in this area are based on the reaction rate equation to describe the oxidation of the pollutants, combined with material balances on the reaction system to calculate the concentrations of the oxidants as a function of the water matrix. As noted above, the reaction rate equation is usually simplified to pseudo-first order. This is based on the assumption of steady-state concentrations for ozone and the radicals involved in the indirect reaction. 

The assumption of a steady-state ozone concentration for the direct reaction is based on the relatively large concentration of ozone compared to the micropollutants, which means the change in the ozone concentration over time is negligible. Several authors have shown that the indirect reaction of OH° with organic compounds is pseudo-first order due to the steady-state concentration of the hydroxyl radicals (e. g. Yao and Haag, 1992 von Gunten et al., 1995). Further assumptions are that the concentrations of the intermediates, e. g. 02°, 0,°-, H0,° and organic radicals, are also at steady-state (Peyton, 1992). 

The numerator contains all hydroxyl radical forming reactions and all initiating reactions are summarized (Sk c(/()). The denominator contains all hydroxyl radical consuming reactions. The second term includes all reactions with intermediates (EkPi c(P,)), the third the reactions with scavengers(Lksi c(S,)). Similarly, the steady-state concentrations of ozone and hydrogen peroxide can be calculated from the liquid phase mass balances. 

Equations (VIII-41) to (VI11-44) give the steady state concentrations of ozone molecules as a function of altitude as follows. The rate of ozone change, r/[03]/[Pg.111]

Ozone is one of the indicators of polluted air. Suppose the steady-state ozone concentration is 2.0 x 1CT8 mol/L, and the hourly production of O3 by all sources is estimated to be 7.2 x 10-13 mol/L. Assume the only mechanism for the destruction of O3 is the second-order reaction 2O3 -> 3O2. Calculate the rate constant for the destruction reaction defined by the rate law for — A[C>3]/Af to maintain the steady-state concentration. 

The choice of the saturation gas is critical. When Ar and Kr were sparged in water irradiated at 513 kHz, an enhancement in the production of OH radicals of between 10% and 20%, respectively, was observed, compared with 02-saturated solutions [22]. The higher temperatures achieved with the noble gases upon bubble collapse under quasi-adiabatic conditions account for the observed difference. Because the rate of sonochemical degradation is directly linked to the steady state concentration of OH radicals, the acceleration of those reactions is expected in the presence of such background gases. The use of ozone as saturation gas (in mixtures with 02) provided new reaction pathways in the gas phase inside the bubbles, which also increase the measured reaction rates (see Sect. IV.G.l). 

These three reactions result in a fast cycle where ozone is produced through reactions (a) and (b), but destroyed through reaction (c). As a result, concentrations of ozone quickly reach a steady-state concentration during the daytime, with no net production of ozone. 

These NO emissions reduce the steady-state concentration of ozone due to reaction (c). However, cars also emit carbon monoxide and a variety of hydrocarbons (HC) as a result of incomplete combustion. These emissions react with the hydroxyl radical to produce peroxy radicals ... 

Ozone absorbs uv radiation from 200 to 360 nm. This leads partly to a reversal of reaction 11-2 and thus a steady state concentration is established. The net result of all these processes is absorption and conversion to heat of considerable solar uv radiation that would otherwise strike the earth s surface. Destruction of any signifi-... 

The source strength for CFC induced stratospheric ozone removal is, of course, the tropopause mixing ratio for each of the CFC compounds. This must be known to extremely high precision at selected positions across the globe in order to predict steady state concentrations corresponding to a given release rate. Intensive analysis of the time dependence of these mixing ratios over at least a decade is critical in order to establish a lower limit on the atmospheric lifetime. 

Since Reactions 1, 2, and 3 are much faster than competing reactions involving olefins, the equilibrium relationship 4 must hold even in the presence of a-pinene (15). The buildup of ozone in Figure 1 above the steady state concentration it shows when no a-pinene is present is therefore accompanied by an increase in the N02. N0 ratio. This increase is effected by reactions which convert NO to NO2 as shown by a few reactions from Wayne s (11) mechanism for the N02-initiated photo-oxidation of an olefin ... 

Production rates are much slower in the free troposphere, and loss usually exceeds production. However, NO concentrations of 100 ppt, which are much too small to allow the formation of episodic high ozone levels, would still allow ozone to remain at a steady-state concentration of —80 ppb. As of early 2000s, the level of background ozone in the lower troposphere (20-40 ppb) is closely related to the photochemical steady state, achieved over several months, based on concentrations of NO t and organics in the remote troposphere. 

Absorption of ultraviolet radiation by O3 causes it to decompose to O2. In the upper atmosphere, therefore, a steady-state concentration of ozone is achieved, a concentration ordinarily sufficient to provide significant ultraviolet protection of the Earth s surface. However, pollutants in the upper atmosphere such as nitrogen oxides (some of which occur in trace amounts naturally) from high-flying aircraft and chlorine atoms from photolytic decomposition of chlorofluorocarbons (from aerosols, refrigerants, and other sources) catalyze the decomposition of ozone. The overall processes governing the concentration of ozone in the atmosphere are extremely complex. The following reactions can be studied in the laboratory and are examples of the processes believed to be involved in the atmosphere ... 

Figure 11.5. Oxidation of S(IV) by ozone (a) the a values for the S02(aq) system and (b) the k pne (equation xi) and half-life for an assumed steady-state concentration of [03(aq)] = 10 M. Above pH = 5, the oxidation of S(IV) is sufficiently fast that the resupply of O3 into the water droplets (from the atmosphere) may not be sufficiently fast to maintain a steady-state concentration of 03(aq).

It was established that nitrogen dioxide is consumed in these reactions (Figure 24). It was established that the photolysis of nitrogen dioxide alone in oxygen produces a steady-state concentration of ozone (Figure 24). 

In these equations, (N02)s and (Oa) are the steady-state concentrations of nitrogen dioxide and ozone, respectively, while (N02)o and (63)0 are their concentrations in the influent stream. Q is the air flow rate through the reactor, and V is the reaction volume. The results of this study are given in Table I. 

Ozone is formed and destroyed in a series of stratospheric reactions. Its steady state concentration is described by the Chapman cycle. 

Since ROS are formed from the absorption of UVR by DOM and its subsequent photochemical decay, any changes in the atmosphere such as tropospheric warming or stratospheric ozone depletion should affect steady state concentrations of ROS in the water column. Initial studies with H2O2 suggest that the formation of an ozone hole will increase production rates by 20-50%. Changes in atmospheric ozone levels are also expected to affect production rates of other... 

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). 

Reactions 2 and 3 regulate the balance of O and O3, but do not materially affect the O3 concentration. Any ozone destroyed in the photolysis step (3) is quickly reformed in reaction (2). Tlie amount of ozone present results from a balance between reaction (1), which generates the O atoms that rapidly form ozone, and reaction (4), which eliminates an oxygen atom and an ozone molecule. The concentrations of O and O3 that result under conditions of constant sunlight (constant Ji and Ji) are termed steady-state concentrations. These are the concentrations of O and O3 defined by the equations d[0]/df = 0 and d[03]/df = 0. Two equations result that may be solved for [O] and [O3] ... 

Yields of Ozone from Mixtures of C02 and 02. Steady-state concentrations of ozone obtained in the presence of added 02 are given in Figure 1, and we have also measured the initial yield of ozone in C02/02 mixtures in a gas flow system. At a dose rate of 7 X 1016 e.v. cc.-1 sec."1 and a high gas flow rate (0.6-0.9 volume changes per sec.) yields calculated from the energy absorbed in the C02 fraction were G(03) = 4.48 and 4.92 for 5 and 10% concentrations of Oo in C02. Some ozone will arise from the energy absorbed by the 02 fraction of the gas, and published values (18, 22) for G(03) in pure 02 range from 1-12. If a value... 

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