Ozone, atmosphere photolysis

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 ... 

Gijs, A., A. Koppers, and D.P. Murtagh, Model studies of the influence of O2 photodissociation parameterizations in the Schumann-Runge bands on ozone related photolysis in the upper atmosphere. Ann Geophys If, 68, 1997. 

Atmospheric photolysis provides the predominant source of free radicals in the atmosphere. Photolysis of NO2 produces O atoms which form ozone ozone photolysis in the near UV produces 0( D) which reacts with H2O to produce OH radicals. A number of organic species absorb UV light and dissociate to yield organic peroxy and HO2 radicals in the presence of O2,... 

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 ... 

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. 

To investigate the role of water photolysis in ozone production in a prebiotic atmosphere, the following mechanism can be explored ... 

Air t1/2 = 6 h with a steady-state concn of tropospheric ozone of 2 x 10-9 M in clean air (Butkovic et al. 1983) t/2 = 2.01-20.1 h, based on photooxidation half-life in air (Howard et al. 1991) calculated atmospheric lifetime of 11 h based on gas-phase OH reactions (Brubaker Hites 1998). Surface water computed near-surface of a water body, tl/2 = 8.4 h for direct photochemical transformation at latitude 40°N, midday, midsummer with tl/2 = 59 d (no sediment-water partitioning), t,/2 = 69 d (with sediment-water partitioning) on direct photolysis in a 5-m deep inland water body (Zepp Schlotzhauer 1979) t,/2 = 0.44 s in presence of 10 M ozone at pH 7 (Butkovic et al. 1983) calculated t,/2 = 59 d under sunlight for summer at 40°N latitude (Mill Mabey 1985) t,/2 = 3-25 h, based on aqueous photolysis half-life (Howard et al. 1991) ... 

Hexachloroethane is quite stable in air. It is not expected to react with hydroxyl radicals or ozone in the atmosphere or to photodegrade in the troposphere (Callahan et al. 1979 Howard 1989). Degradation by photolysis may occur in the stratosphere. 

Tropospheric chemistry is strongly dependent on the concentration of the hydroxyl radical (OH), which reacts very quickly with most trace gases in the atmosphere. Owing to its short boundary layer lifetime ( 1 s), atmospheric concentrations of OH are highly variable and respond rapidly to changes in concentrations of sources and sinks. Photolysis of ozone, followed by reaction of the resulting excited state oxygen atom with water vapour, is the primary source of the OH radical in the clean troposphere ... 

Quantum yields for formaldehyde photolysis have not received the same attention as those for ozone photolysis and are clearly important even in an unpolluted environment. The absorption spectmm is highly structured and more detailed measurements, under atmospheric conditions, are needed. In this work the uncertainty in HCHO measurements was es-... 

A knowledge of the kinetics of the decomposition of ozone is essential for the understanding of the chemistry of some important processes which occur in earth s atmosphere. Yet, in spite of numerous studies and the structural simplicity of ozone, the mechanism of its ultraviolet photolysis is still uncertain. Electronically and vibrationally excited species are involved in ozone decomposition and the current knowledge of the chemical behavior of such intermediates is still in its infancy. 

Laser-induced electronic fluorescence. Two devices reported recently look very promising for continuous atmospheric monitoring. Sensitivities of 0.6 ppb for nitrogen dioxide and ppb for formaldehyde are claimed. Careful attention to possible interference from other species is necessary. Detection of the hydroxyl radical in air ( 10 molecules/cm ) has been claimed for this technique, but it has been pointed out that this concentration seems much too high, especially because the air had been removed fix>m the sunlight 6 s before analysis spurious effects, such as photolysis of the ozone in the air by the laser beam and two-photon absorption by water vapor, might have been responsible for the hydroxyl radical that was observed. 

Photolytic. Based on data for structurally similar compounds, acenaphthylene may undergo photolysis to yield quinones (U.S. EPA, 1985). In a toluene solution, irradiation of acenaphthylene at various temperatures and concentrations all resulted in the formation of dimers. In water, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, 1,2-epoxyacenaphthylene, and 1-naphthoic acid. In methanol, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, methyl 8-formyl-1-naphthoate, and dimethoxyacetal 1,8-naphthalene dialdehyde (Chen et al., 1979). Acenaphthylene reacts with photochemically produced OH radicals and ozone in the atmosphere. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with OH radicals (500,000/cm ) at 25 °C are 8.44 x lO " cmVmolecule-sec and 5 h, respectively. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with ozone at 25 °C are... 

Photolytic. Photolysis products include carbon monoxide, ethylene, free radicals, and a polymer (Calvert and Pitts, 1966). Anticipated products from the reaction of acrolein with ozone or OH radicals in the atmosphere are glyoxal, formaldehyde, formic acid, and carbon dioxide (Cupitt,... 

Irradiation of gaseous formaldehyde containing an excess of nitrogen dioxide over chlorine yielded ozone, carbon monoxide, nitrogen pentoxide, nitryl chloride, nitric and hydrochloric acids. Peroxynitric acid was the major photolysis product when chlorine concentration exceeded the nitrogen dioxide concentration (Hanst and Gay, 1977). Formaldehyde also reacts with NO3 in the atmosphere at a rate of 3.2 x 10 cmVmolecule-sec (Atkinson and Lloyd, 1984). 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

Ozone is main component in many oxidation processes assembled imder the term ozonation processes. In these processes ozone is applied either alone (O3 process) or with the addition of oxidant, e.g. H2O2 (O3/H2O2 process), UV radiation (explained in above subchapter), catalyst, activated carbon, ultrasoimd etc. Ozone is inorganic molecule constituted by three atoms of oxygen. It is present in nature in upper atmosphere in the form of stratospheric layer aroimd the earth, and it is formed by the photolysis of diatomic oxygen and further recombination of atomic and diatomic oxygen, shown by equations (25) and (26) [35] ... 

Oxidation in the atmosphere begins photolytically with radiation from the sun rather than thermolytically thus, atmospheric chemistry differs between day and night. In the daytime, the most common initiation step for VOC degradation involves photolysis of ozone by the sun s ultraviolet light, leading to hydroxyl (HO ) radical generation ... 

The ozone layer in the atmosphere is an important protective layer for life on the Earth. Ozone is photochemically produced from O2 in the atmosphere. The following account is from Pilling and Seakins (1995). First, oxygen atoms are generated by short-wavelength U V photolysis (at wavelengths below 242 nm) in the stratosphere. That is, UV photons split the oxygen molecule as follows ... 

Junkermann, W., U. Platt, and A. Volz-Thomas, A Photoelectric Detector for the Measurement of Photolysis Frequencies of Ozone and Other Atmospheric Molecules, . /. Atmos. Chem., 8, 203-227 (1989). 

In the early 1950s, the major ingredients in photochemical air pollution had been identified by Haagen-Srnit and co-workers as VOC and NO, and the photolysis of N02 had been identified by Blacet as the source of the high ozone levels (see Chapter 1.B.3). Initially, the atmospheric conversion of emitted NO to N02 was thought to be due to its reaction with 02 ... 

TABLE 10.36 Calculated Atmospheric Lifetimes of Selected PAHs and Nitro-PAHs Due to Gas-Phase Reactions with the OH Radical, the N03 Radical, and Ozone and from Photolysis (from Arey, 1998a)... 

In summary, reactions (43a)-(45) have generally been taken to represent the chemistry occurring in the ozone hole. However, reduced efficiency of chlorine atom production in the photolysis of (C10)2, reaction (44), and hence ozone destruction, needs to be modeled and tested against the atmospheric observations. 

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