Water vapor, photolysis

Participating OH radicals form in the atmosphere during water vapor photolysis. As a result, the simplest diagram of methane oxidation in the atmosphere is the following ... 

In the upper atmosphere today, a small amount of O2 is produced through photolysis of water vapor. This process is the sole source of O2 to the atmospheres on the icy moons of Jupiter (Ganymede and Europa), where trace concentrations of O2 have been detected (Vidal et ai, 1997). Water vapor photolysis may also have been the source of O2 to the early Earth before the evolution of oxygenic photosynthesis. However, the oxygen formed by photolysis would have been through reactions with methane and carbon monoxide, preventing any accumulation in the atmosphere (Kasting et al., 2001). 

Box 2. The Content of CO2 in the Prebiotic Atmosphere, after Fenchel et al, 1998 Perhaps the most interesting question on the early atmosphere is related to the abundance and oxidation states of carbon. Current models predict a CO2-rich atmosphere, which would contain a trace amount of methane and a slightly greater amount of carbon monoxide. We can try to explain it on the basis of the following speculations. It is known that even today volcanic activity is a dominant process in releasing CO2 and it should have been so in the past. It is related to the reasonable suggestion that the oxidation state of the upper mantle was about the same as today. Furthermore, the by-products of water vapor photolysis were enabled to oxidize both CH4 and CO. The possible reaction pathway could be the following... 

H2 oxidation is most rapid) and an increasing mixing ratio at higher altitudes due to the production of H2 from water vapor photolysis. 

During SOAPEX-2, measurements of the free-radicals OH, HO2, HO2+XRO2, NO3, IO and OIO were supported by measurements of temperature, wind speed and direction, photolysis rates (j D) and j(N02)), water vapor, O3, HCHO, CO, CH4, NO, NO2, peroxyacetyl nitrate (PAN), a wide range of NMHCs, organic halogens, H2O2, CH3OOH and condensation nuclei (CN). 

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. The major photolysis and hydrolysis products identified in distilled water were pentachlorocyclopentenone and hexachlorocyclopentenone. In mineralized water, the products identified include cis- and /ra/3s-pentachlorobutadiene, tetrachlorobutenyne, and pentachloro-pentadienoic acid (Chou and Griffin, 1983). In a similar experiment, irradiation of hexachlorocyclopentadiene in water by mercury-vapor lamps resulted in the formation of 2,3,4,4,5-pentachloro-2-cyclopentenone. This compound hydrolyzed partially to hexachloroindenone (Butz et ah, 1982). Other photodegradation products identified include hexachloro-2-cyclopentenone and hexachloro-3-cyclopentenone as major products. Secondary photodegradation products reported include pentachloro-as-2,4-pentadienoic acid, Z- and A-pentachlorobutadiene, and tetrachloro-butyne (Chou et ah, 1987). In natural surface waters, direct photolysis of hexachlorobutadiene via sunlight results in a half-life of 10.7 min (Wolfe et al, 1982). 

The major source of OH in remote areas is the photolysis of O, to electronically excited O( D), followed by its reaction with water vapor ... 

It should be noted that only a portion of the O( D) formed generates OH via reaction (2a) the remainder is deactivated to ground-state 0(3P), reaction (2b), which then re-forms O,. For example, at 50% RH and 300 K at the earth s surface, about 10% of the O( D) formed generates OH. As a result, as discussed later in this chapter, the relative importance of (2a) decreases at higher altitudes due to the decrease in water vapor. This is also an important source in polluted areas, where, however, there are additional sources as well. These include the photolysis of gaseous nitrous acid (HONO) and hydrogen peroxide (H202) ... 

The most universal characteristic of remote regions compared to those clearly subject to anthropogenic influences is the low NOx (see Crutzen, 1995, for a review). Under these conditions, OH is generated by the photolysis of O, to O( D), followed by its reaction with water vapor, which occurs in competition with deactivation to 0(3P) ... 

The major problem with LIF measurements in the past has been what might be called the atmospheric uncertainty principle i.e., in the act of carrying out the measurement, the system is perturbed and artifact formation of OH can occur (e.g., see Smith and Crosley, 1990 and Hard et al., 1992b). This is primarily due to the photolysis of 03 to generate O( D), which in the presence of water vapor forms OH ... 

At 298 K and atmospheric pressure with 50% relative humidity, about 0.2 HO" are produced per O( D) atom formed. Photolysis of 03 in the presence of water vapor is the major tropospheric source of HO", particularly in the lower troposphere where water vapor mixing ratios are high (for an explanation of the term mixing ratio see below). Other sources of HO" in the troposphere include the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other carbonyls in the presence of NO, and the dark reactions of 03 with alkanes. Note that all these processes involve quite complicated reaction schemes. For a discussion of these reaction schemes we refer to the literature (e.g., Atkinson, 2000). 

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... 

Fig. 13.4 Dependence of the yield of gas-phase water photolysis on water vapor pressure over 1.3 wt% and 10 wt% NaOH-coated Rh/Ti02. Psat is the saturated vapor pressure at ambient temperature. H2/02 ratio is stoichiometric in all runs.

Fig. 13.6 Dependence of the yield of water photolysis over Pt/Ti02 in NaOH solution on the solution amount. The reaction was started in 0.5 ml solution involving 1.1 mmol of NaOH. After the yield measurement at a given amount of solution, the solution amount was reduced by the method described in the text and the yield measured. After a series of measurements for the solution involving 1.1 mmol NaOH, NaOH was added to increase its amount to 2.0 mmol. Values in parentheses denote water vapor pressure (Torr) in the reaction system.

Very little experimental work has been done on the photolysis of water vapor. In 1939, Rollefson and Burton (67) stated that no photochemical evidence of any kind on the subject has been produced since 1931. Although this is an overstatement, nevertheless, it has been a relatively neglected subject even to the present time. 

The photolysis of water vapor has been studied in the region from 1295 to 1650 A. The results of a recent and reliable study by Watanabe and Zelikoff (102) of the absorption coefficients in this region are shown in Figure 6. They have also obtained the spectrum at shorter wave-... 

There would appear to be no difference in the reactions expected photo-chemically as the primary products are H atoms and OH radicals in both the electric discharge and on irradiation. Chen and Taylor (22) state that there is no evidence for oxygen atoms either in the photolysis or in the decomposition of water vapor in an electric discharge. However, the secondary formation of O atoms (2) and the formation of ozone (31, 50) in an electric discharge through water vapor have been demonstrated. It might be expected that under the proper experimental conditions similar results could be obtained photochemically. 

In a gaseous phase enriched with water vapor, the mechanism of photolysis involves the release of a molecule of oxygen and an atom of oxygen (1D). The latter may react with water to produce hydroxyl radicals ... 

The presence of 03 in the troposphere leads to the formation of OH radicals through the photolysis of 03 at wavelengths 290-350 nm to form the electronically excited oxygen atom, 0(JD), which either reacts with water vapor or is deactivated by reaction with 02 and N2 to the ground state oxygen atom, (03P) (Atkinson, 1995 Atkinson et al., 1997). 

Apart from photosynthesis, photolysis can be a source of oxygen in the atmosphere (i.e., the decomposition of water vapor under the influence of UV radiation in the upper layers of the atmosphere). However, the intensity of this source under present conditions is negligible. Nevertheless, let us denote this flux by // = aH WA, where WA is water vapor content in the atmosphere and aH is an empirical coefficient. If we assume that in the upper layers of the atmosphere a constant share of WA can reside, then at H° = 0.0039102 km-2 yr and WA= 0.025m, we have aH = 1.56 10 7 per year. 

Vinogradov, I.P. and Vilesov, F.I. (1976). Luminescence of the OH(j42E+) radical during photolysis of water vapor by vacuum uv radiation, Opt. Spectrosc. 40, 32-34. 

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