Ozone photolysis reaction

The major photochemical losses of tropospheric ozone are provided by reactions of O3 with OH (Reaction (5.90)) and HO2 (Reaction (5.91)), as well as by ozone photolysis (Reaction (5.16)), if it is followed by the irreversible loss of 0(1D), for example, by the reaction of this tome with water vapor. 

Here the rate constants k refer to the rates of the numbered reactions above the value ho2/ro2 an average for different R02 entities. The A term accounts for HOjj production via ozone photolysis R1-R3, the Bj term accounts approximately for the source from aldehyde photolysis (R12 plus higher aldehydes), and the B2 term is a composite source from formaldehyde (RIO) and dicarbonyls (Cj) less the HOjj sink from PAN formation (R22) B2=Ci-C2). Values for Bj,... 

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. The 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. Under conditions of constant sunlight, which implies constant /i and /s, the concentrations of O and O3 remain constant with time and are said to correspond to the steady state. Under steady-state conditions the concentrations of O and O3 are defined by the equations d[0]/df = 0 and d[03]/df = 0. Deriving the rate expressions for reactions 1-i and applying the steady-state condition results in the equations given below that can be solved for [O] and [O3]. 

Wallington TJ, O Sokolov, MD Hurley, GS Tyndall, 11 Orlando, 1 Barnes, KH Becker, R Vogt (1999) Atmospberic cbemistry of metbylcyclopentadienyl manganese tricarbonyl photolysis, reaction with bydroxyl radicals and ozone. Environ Sci Technol 33 4232-4238. 

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 ... 

Photolysis of the N02, producing O, which then forms ozone by reaction with 02 ... 

The relationship between the peroxy radical concentration and the ozone photolysis rate constant for these higher NO conditions can be again approximated using steady-state analysis (Penkett et al., 1997 Carpenter et al., 1997). While OH is recycled in its reactions with CO and CH4 via H02, it is permanently removed at higher NOx concentrations by the reaction of OH with N02, forming nitric acid ... 

Both 02 and 03 absorb light below 242 nm (see Chapter 4.A and 4.B). When stratospheric 03 is removed by reactions described in detail in this chapter, there is more light available for photolysis of Oz and the formation of more ozone via reactions (1) and (2). This so called self-healing effect in part mitigates the destruction of ozone by other species such as CFCs, halons, and NOx. 

Secondary reactions of 0(1D) could also give rise to excited molecular oxygen in the ozone photolysis [energy transfer to 02 from 0(JD) is considered in Sect. IV-E]. The reaction... 

Two spurious HO cases may be recognized when excitation is with a pulsed laser—both involving ozone photolysis to produce O (lD)—and its subsequent reaction with ambient water vapor to produce HO. In the first case, this spurious HO is detected by the same laser pulse, whereas in the second case it is detected by a subsequent laser pulse. The latter problem can be more significant, because the spurious HO grows rapidly in time following the initial production of O ( D). These two types of behavior make laser temporal pulse width, repetition rate, and air velocity important in... 

The photochemical processes of triatomic molecules have been extensively studied in recent years, particularly those of water, carbon dioxide, nitrous oxide, nitrogen dioxide, ozone, and sulfur dioxide, as they are important minor constituents of the earth s atmosphere. (Probably more than 200 papers on ozone photolysis alone have been published in the last decade.) Carbon dioxide is the major component of the Mars and Venus atmospheres. The primary photofragments produced and their subsequent reactions are well understood for the above-mentioned six triatomic molecules as the photodissociation involves only two bonds to be ruptured and two fragments formed in various electronic states. The photochemical processes of these six molecules are discussed in detail in the following sections. They illustrate how the knowledge of primary products and their subsequent reactions have aided in interpreting the results obtained by the traditional end product analysis and quantum yield measurements. 

Three dominant reactions during ultraviolet (UV)/ozone (03) treatment processes that effectively decompose organic pollutants are photolysis, ozonation, and reactions of hydroxyl radicals. The generation of hydroxyl radicals is essential in this oxidation process as it is the reaction between these radicals and organic compounds that can ultimately destroy organic pollutants. Physical parameters, such as temperature, pH, initial compound and ozone concentrations, UV intensity, and ozone partial pressure will also have considerable effects on the kinetic rate constants and removal efficiency of any compound. 

Ozonation processes are rather complex, due to the high instability of ozone in aqueous solutions. Ozone absorbs UV photons with the maximal absorption at 253.7 nm. The decomposition of ozone under UV radiation typically occurs through three reactions direct photolysis, direct ozonation, and reactions between hydroxyl radicals and hydrogen peroxide as shown in the following reactions ... 

Gas- and particle-phase organic compounds can undergo chemical change by a number of routes (Atkinson, 1995, 1996). For gas-phase chemicals, these involve photolysis, reaction with ozone (03), reaction with the hydroxyl (OFI) radical, and reaction with the nitrate (N03) radical (Atkinson, 1995). (For a discussion of photolysis, see Chapter 15). 

Ozone Photolysis The most important sets of photochemical reactions volve the photolysis of ozone, 03 which occurs over a Table 3 Absorption cross sections of 03 at 273 K [106] in the troposphere in-very broad wavelength... 

Gurol and Akata [43] studied the kinetics of ozone photolysis following a conceptual model based on possible reaction pathways. They obtained experimentally the primary quantum yield of ozone photolysis at 254 nm (0.48). 

From the preceding discussion of atmospheric photochemistry and NO chemistry, it can be seen that the fate of the peroxy radicals can have a marked effect on the ability of the atmosphere either to produce or to destroy ozone. Photolysis of NO2 and the subsequent reaction of the photoproducts with O2 (reactions (2.4) and (2.20)) are the only known way of producing ozone in the troposphere. In the presence of NO the following cycle for the production of ozone can take place ... 

As both reactions (2.37) and (2.19) lead to the oxidation of NO to NO2 the subsequent photolysis leads to the formation of ozone (see reactions (2.2) and (2.20)). The individual reaction mechanism depends on the identity of the organic compounds and the level of complexity of the mechanism. Although OH is the main tropospheric oxidation initiator, reaction with NO3, O3, 0( P) or photolysis may be an important loss route for some NMHCs or the partially oxygenated products produced as intermediates in the oxidation (see reaction (2.38)). 

Ripperton and Vukovich (213) suggested the existence of an atmospheric sink for ozone, possible reactions with NC. Recently Crutzen (43) suggested that reactions involving peroxy radicals may be an indirect source of ozone in the lower troposphere and that the photolysis... 

McGrath and Morrow " studied the reactions of both 0( D) and 0( P) with cyanogen at room temperature by flash photolysis. The 0( D) was produced by the photolysis of ozone. The reaction was monitored by absorption spectroscopy. At first, they attributed the previously unidentified ultraviolet absorption bands at 3250-3330 A to the fulminate radical (CNO), concluding that the initial step of the reaction was... 

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