Ozone photolysis rate

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). 

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,... 

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. 

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... 

Fuglestvedt, J. S J. E. Jonson, and L S. A. Isaksen, 1994 Effects of reductions in stratospheric ozone on tropospheric chemistry through changes in photolysis rates. Tellus, 46B, 172-192. 

Zweiner, C., Weil, L., and Niessner, R., UV- and UV/ozone degradation of triazine herbicides in a pilot plant estimation of UV-photolysis rate constants and quantum yields, Vorm Wasser, 84, 47-60, 1995. 

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... 

Thus, it is important to differentiate between the perturbation effects caused on the atmosphere by light-induced heterogeneous processes on aerosols from those caused by photochemical gas-phase reactions induced or inhibited by aerosol light scattering. In the following subsections several examples of the former type will be discussed. The enhancement of homogeneous photolysis rates and the problem of ozone depletion will not be discussed, as they are outside the scope of the present chapter. 

The rate of ozone photolysis increased with increasing light intensity, ozone concentration and pH, and decreased with increasing inorganic carbon concentration. As the formation of HO is tied to ozone decomposition, this model can be extended to predict the oxidation rates of water contaminants by HO generated in the process. 

In the lower stratosphere, there is more ozone in model B than in model A due to an increase in the photolysis rate of O2 resulting from the greater solar flux. The net effect is an approximate 7% increase in column density of Os when the smaller cross-section for oxygen absorption (O2) ) is used (Model B). 

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