Ozone photolysis ozonation

A recent study of the vibrational-to-vibrational (V-V) energy transfer between highly-excited oxygen molecules and ozone combines laser-flash photolysis and chemical activation with detection by time-resolved LIF [ ]. Partial laser-flash photolysis at 532 mn of pure ozone in the Chappuis band produces translationally-... 

We saw in Section 9.3.8 that spectroscopy, in the form of LIDAR, is a very important tool for measuring ozone concentration directly in the atmosphere. A useful indirect method involves the measurement of the concentration of oxygen which is formed from ozone by photolysis ... 

Aqueous Phase. In contrast to photolysis of ozone in moist air, photolysis in the aqueous phase can produce hydrogen peroxide initially because the hydroxyl radicals do not escape the solvent cage in which they are formed (36). 

Monomeric neutral SO4 can be obtained by reaction of SO3 and atomic oxygen photolysis of S03/ozone mixtures also yields monomeric SO4, which can be isolated by inert-gas matrix techniques at low temperatures (15-78 K). Vibration spectroscopy indicates either an open peroxo Cj structure or a closed peroxo C2v structure, the former being preferred by the most recent study, on the basis of agreement between observed and calculated frequencies and reasonable values for the force constants ... 

Despite their instability (or perhaps because of it) the oxides of chlorine have been much studied and some (such as CI2O and particularly CIO2) find extensive industrial use. They have also assumed considerable importance in studies of the upper atmosphere because of the vulnerability of ozone in the stratosphere to destruction by the photolysis products of chlorofluorocarbons (p. 848). The compounds to be discussed are ... 

Oxidant Formation. The role of HO. in controlling the time-scale and severity of tropospheric oxidant pollution may be seen from the parameterization of O Brien and co-workers (75,76). The simplest possible mechanism for oxidant (Le. ozone, PAN, H2O2, etc.) formation consists simply of the reaction of an individual NNlHCj with HO. to convert the NMHCj to a generic product(s) PRODj, followed by removal of the product by HO. (PROD photolysis may be important, but is ignored here)... 

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]. 

Nitrogen oxides also play a significant role in regulating the chemistry of the stratosphere. In the stratosphere, ozone is formed by the same reaction as in the troposphere, the reaction of O2 with an oxygen atom. However, since the concentration of O atoms in the stratosphere is much higher (O is produced from photolysis of O2 at wavelengths less than 242 nm), the concentration of O3 in the stratosphere is much higher. 

Considering natural stratospheric ozone pro-duction/destruction as a balanced cycle, the NO.v reaction sequence is responsible for approximately half of the loss in the upper stratosphere, but much less in the lower stratosphere (Wennberg et al, 1994). Since this is a natural steady-state process, this is not the same as a long term O3 loss. The principal source of NO to the stratosphere is the slow upward diffusion of tropospheric N2O, and its subsequent reaction with O atoms, or photolysis (McElroy et ai, 1976). 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

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. 

Gas-phase reactions have been carried out in 160 mL quartz vessels, and the products analyzed online by mass spectrometry (Brubaker and Hites 1998). Hydroxyl radicals were produced by photolysis of ozone in the presence of water ... 

Atomic oxygen was generated by photolysis of ozone which passed through a quartz capillary tube (0 2 mm and lehgth 60 mm), using the 253.7 nm line from a low-pressure mercury lamp (LI). 

The quantum yield of ozone decomposition at 334 nm (L2) is 4, indicating that one of the products must be an excited species capable of decomposing 0 further. The primary process of the 0 photolysis at 334 nm occurs according to the reactions ... 

The bimodal velocity distribution of the 0(3Pj) fragments produced via the triplet channel in the UV photodissociation of ozone has also been observed by Syage41,43>46 and Stranges et al.AA at photolysis wavelengths of 226 and 193 nm, respectively. Both authors measured anisotropy parameters for the fast and slow product pathways separately. 

Our determined anisotropy parameters for 226 nm photolysis agree favorably with the reported values of Syage, where a (3 value of 1.2 was measured for the fast 0(3P2) products. Syage observed a less anisotropic distribution for the slow 0(3P2) atoms, with a reported (3 value of 0.4. The f3 value of 1.2 for the high velocity component was rationalized by a prompt dissociation from the equilibrium ground state of ozone following an B I >2 <— X A i transition. 

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 ratio is clearly pressure dependent in the lower stratosphere [02] and [M] are fairly large and /3 is small (due to absorption above the required wavelengths), so the dominant odd-oxygen species is ozone. At higher altitudes both [02] and [M] fall and the photolysis rate increases so that O is the dominant species in the atmosphere. The net flux of radiation in the band 240-290 nm is nearly zero at the surface of the Earth, which is then shielded from this radiation. 

The rate of photolysis, J, depends on the absorption cross-section, a, the number density, the scale height and the angle, all of which are unique properties of a planetary atmosphere. For the Earth and the Chapman mechanism for ozone the O3 concentration maximum is 5 x 1012 molecules cm-3 and this occurs at 25 km, shown in Figure 7.12, and forms the Chapman layer structure. 

Consider the water photolysis mechanism for the formation of oxygen atoms and ozone, shown below ... 

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