Ozone formation, steady-state

Because of the formation of nitrogen oxides, a steady-state ozone concentration cannot be obtained instead, due to the buHdup of nitrogen oxides, an increase in residence time in the discharge results in a decrease in ozone concentration beyond the maximum value. Thus, there is an optimum residence time for maximum ozone production. 

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

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. 

The concentration of ozone taken up by the media containing linolenic acid is plotted against time after addition in Figure 8. The rate of ozone breakdown is constant (ozone uptake linear with time) for the first two min until about 0.12 ml ozone are absorbed and then the rate decreases sharply, reaching a steady-state rate of ozone uptake between 10-12 min. This first break in the curve corresponds to an ozone uptake of 0.12 ml + (24 moles/liter) = 0.005 millimoles (or 10 M). This is equivalent to 1 mole of linolenic acid added per mole ozone absorbed. Thiobarbituric acid reactant production is also plotted on the same axis. This compound (TBA reactant) probably arises by formation of a three-carbon fragment (malondialdehyde) from the ozone-induced oxidation of linolenic acid (23). The rate of TBA reactant formation is also linear for the first 2 min at which point the curve undergoes a less pronounced break. Malondialdehyde formation ceases immediately when the ozone is shut off (Scrub 1 on). An oxygen control sample produced no malondialdehyde. 

There are several important points with respect to the effects of any future HSCT emissions. First, ozone concentrations at a particular location and time depend not only on the local chemistry but on transport processes as well. In the lower stratosphere, transport processes occur on time scales comparable to the rates of ozone formation and loss so that taking into account such transport is particularly important. However, in the middle and upper stratosphere, production and removal of 03 are much faster than transport so that a steady state exists between these two processes. 

The absorption cross sections for NO2 and the corresponding quantum yields are given in Table 8 and 9, respectively. The photolysis of NO2 has been investigated intensively over the last 40 years because of its critical role in the formation of ozone in the polluted tropospheric boundary layer [56-63]. The three reactions of Eqs. 33 and 34 form the basis for the photochemical production of ozone. If one considers only these three reactions, then the photo-stationary state (or photochemical steady-state approximation) can be invoked around the oxygen atom as follows ... 

The dominant loss of OH radicals is reaction with CO and organic compounds such as CH4, both reactions produce peroxy radicals. Peroxy radicals play a key role in atmospheric chemistry. They are intimately involved in the formation and destruction of ozone and in the photooxidation of all organic compounds in the atmosphere [4], The lifetime of OH radicals with respect to reactions Eq. 3 and Eq. 8 is of the order of a second and in the day-time a steady state condition is established. The OH radical concentration in the atmosphere varies with location, time of day, season, and meteo-... 

In contrast to the water phase the HO radicals can have a much longer lifetime in gaseous media, i.e. up to 1 s for the OH and 60 s for the HO radical, respectively (Fabian, 1989). Despite the low concentration of OH radicals of about 10 molecules per cm in the sunlit troposphere (Ehhalt, 1999) they play an important role in controlling the removal of many organic natural and manmade compounds from the atmosphere (Eisele et al., 1997, Eisele and Bradshaw, 1993). Even in indoor environments, the formation of hydroxyl radicals is possible by ozone/alkene reactions (Atkinson et al., 1995). Steady-state indoor hydroxyl radical concentrations of about 6.7x10 ppb equivalent to 1.7x10 molecules cm were calculated at an ozone concentration of 20 ppb (Weschler and Shields, 1996). 

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. 

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... 

NiO can decompose back to NO, and NO2 either photolytically or thermally. Since its formation does not represent a permanent loss of NO, N Os is a reservoir species for NO,. N2OS is itself photolyzed in the 200 to 400 nm region. Since this wavelength region overlaps that of the strongest O, absorption, the photolysis lifetime of N Os varies depending on the overhead column of ozone. The lifetime of N O., is typically on the order of hours at 40 km and many days near 30 km. At night, NO, concentrations reach an approximate steady state as a result of reactions 4.23 and 4.26/,... 

The experiments were made with ammonia using an all-glass ozonizer the surface of the inner electrode of which was deposited with a thin film of a metal. The metals used were Pt, Pd, Ni, Au and Ag. With recirculation of the gas mixture at a pressure close to atmospheric steady-state conversions into hydrazine as high as 13.5% were attained with Pd which is an increase in the N2H4 yield by a factor of 4 (Fig. 18). The overall conversion of ammonia and the formation of N2 and H2 were almost independent of the metal coatings (Fig. 18) suggesting that hydrazine is not an intermediate in the d omposition of NH3 into Nj and Hj. Therefore, it seems that there are two independent reaction pathways the formation of hydrazine which is a surface reaction that is dependent on the state of the electrode surface... 

Reaction (5.7) does not lead to a net destruction of ozone. Instead, O is almost exclusively converted back to O3 by reaction (5.14). However, because O2 dissociates to free oxygen atoms above about 30 km, below 30 km reaction (5.145) results in a net loss of odd oxygen (if the odd oxygen concentration is defined as the sum of the O3 and O concentrations). The budget between the photodissociation of O3 and its formation via (5.14) is zero (steady state). Because the rate of reaction (5.14) decreases with altitude, whereas that for reaction (5.7) increases, most of the odd oxygen below 60 km is in the form of O3, whereas above 60 km it is in the form of O. Odd oxygen is produced by reaction (5.16). It can be seen that reactions (5.14) and (5.7) do not affect the odd oxygen concentrations but merely define the ratio of O to O3. 

Spectroscopy in the ultraviolet and visible range has been used to follow the evolution of NO2 in order to study the autoxidation kinetics (29), and symmetric NO3 has been characterized by the method already in the early twentieth century (5,48) it can be prepared in easily detectable quantities by the reaction of N2O5 or NO2 with ozone. It is, however, not very fikely an intermediate of NO autoxidation, because its formation would require the splitting of02, and because the electrode potential ofits reduction to NOs" has been estimated to be higher than 2 V, which would lead to side reactions that would have been hardly overlooked. In contrast, the electronic spectrum of ONOO is not known, and it would most likely not be helpfijl in detecting it at steady-state concentrations, since known extinction coefficients of N-O compounds in the visible and near ultraviolet spectrum are all below 1000 cm Thus, the absorption of ONOO during autoxidation would vanish under the contribution of the product, NO2. ... 

In the absence of anthropogenic influences, the concentration of ozone in the stratospheric ozone layer had maintained a steady state (a state in which the net rate of destruction equals the net rate of formation) for millions of years. On the average, ozone molecules were destroyed by a variety... 

A slightly simplified version of the steady-state process for ozone formation (reactions 1 and 2) and destruction (reactions 3 and 4) is known as the Chapman cycle (shown in left margin) after the scientist who proposed it. 

Chapman Cycle The set of four reactions that represents the steady-state formation and destruction of ozone in the stratosphere. 

As so far described, photolysis of nitrogen dioxide can give rise to small steady-state concentrations of ozone, which are limited by the reaction with nitric oxide. Concentrations of ozone in city centers tend to be lower than those in adjacent rural areas due to fresh emissions of NO from traffic reacting with ozone, as above. Equating the rate of NO2 loss by photolysis to the rate of NO2 formation leads to the following expression for the concentration of ozone ... 

The kinetic curve of the changes in the ozone concentration at the bubbling reactor outlet (Fig. 10) is characterized by three different regions AB - fast ozone consumption after the addition of pyrocatechol, BC -steady-state part, when the rate of the chemical reaction becomes equal to the rate of ozone supply, and CD - the ozone concentration begins to rise up due to the p Tocatechol consumption. The BC part of the curve allows calculation of the rate constant, and based on the area below the curve ABCD - evaluation of the stoichiometry of the reaction. The straight line designated [03] is the ozone concentration at the reactor inlet. Curve 2 presents the o-quinone formation in the course of the reaction time. Its profile suggests the intermediate formation of o-quinone. 

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