Ozone, photolysis

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,... 

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-... 

FIGURE 6.31 Measured peroxy radical concentrations ([H02 ] + [ROj]) and ozone photolysis rate /(O D) (a) or /(O D) 05 (b) in clean marine air at Cape Grim, Tasmania (adapted from Penkett et al., 1997). 

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 ... 

FIGURE 6.33 Measured concentrations of H02 + R02 as a function of (a) ozone photolysis rate, /(O D), or (b) /(O D))05 under polluted conditions at Mace Head, Ireland. Lines are guides for the eye (adapted from Carpenter et al., 1997). 

Penkett, S. A., P. S. Monks, L. J. Carpenter, K. C. Clemitshaw, G. P. Ayers, R. W. Gillett, I. E. Galbally, and C. P. Meyer, Relationships between Ozone Photolysis Rates and Peroxy Radical Concentrations in Clean Marine Air over the Southern Ocean, J. Geophys. Res., 102, 12805-12817(1997). 

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... 

The question now arises as to whether O A,) is a primary product of ozone photolysis at wavelengths between 2660 and 3100 A ... 

The advanced oxidation process with ozone and UV-radiation is initiated by the photolysis of ozone. The photodecomposition of ozone leads to hydrogen peroxide (Peyton, 1988). Ultraviolet lamps must have a maximum radiation output at 254 nm for an efficient ozone photolysis. 

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... 

Substitution of isobutane for a small fraction of the airstream increases the quenching of spurious HO fluorescence in the background channel by about 1%, so exact cancellation is not achieved in subtraction and a false positive signal is introduced (80, 103, 107). This situation applies not only to ozone photolysis but also to all other photolytic HO sources. 

A. excitation could greatly reduce photolytic interference, nonresonant fluorescence, and scattering backgrounds. The method can be calibrated by fundamental calculation or by ozone photolysis by a shortwave UV source. The successful 2-A determination of NO provides confidence in the fundamental principles of this technique and shows its adaptability for nonsimul-taneous measurements of NO, N02, and HN03. 

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. 

Precise determinations of quantum yields in dry ozone photolysis would be of considerable value in clarifying the possible mechanisms. If an energy or photon chain exists, there is no reason why the quantum yield should be limited to about 6. It should be possible to use experimental conditions where it would be much higher. If an energy or photon chain does not exist, the quantum yield should not exceed 4 or, perhaps, 6. It should be emphasized that the most precise work in the ultraviolet yields a maximum quantum yield of 6 and that an error of 2 in this value would be possible. 

Prousek (1996) showed that hydrogen peroxide is in fact the primary product of ozone photolysis and also summarized the chemistry involved in the generation of HO radicals by the UV/ozone process as follows ... 

Mokrini et al. (1997) proposed that the photolytic ozonation kinetics of substrates is a linear combination of purging, ozonation, photolysis, and photolytic ozonation ... 

From the literature, the rate constants between p-hydroxybenzonic acid and ozone, UV photon, and hydroxyl radical are known (Gurol and Nekouinaini, 1984). Therefore, contribution of ozonation, photolysis, and hydroxyl radical... 

A similar exchange has been shown to occur rapidly in the ozone photolysis system40 408, viz. 

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