Photochemical destruction

I.Z. Shirgaonkar and A.B. Pandit, Sono-photochemical destruction of aqueous solution of 2,4,6-trichlorophenol, Ultrasonics Sonochemistry, 1998, 5, 53-61. 

The use of an Intense laser light source with biological materials Is accompanied by the concomitant problems of localized sample heating and the possibility of protein denaturetlon. A further complication Introduced by resonance Raman spectroscopy Is the Increased potential for photochemical destruction of chromo-phorlc metal centers as a result of the absorption of large amounts of Incident radiation. Both of these situations may be ameliorated by freezing samples to liquid nitrogen temperature ( 90 K), while the even lower temperatures made possible with a closed-cycle... 

The result of this chemistry is the photochemical destruction of O, and the formation of peroxides. 

This chemistry suggests that under low-NOx conditions, there should be net photochemical destruction of 03 accompanied by the formation of peroxides. That is,... 

The combination of NH2 to form N2H4 is a more favorable path than the reformation of NH3, (VIII-90), and NH3 is photochemically destructed. The N2H4 formed is partially converted into N2 and H2... 

The second loss mechanism for 03s is dry deposition at the surface (Figure 3c). It contributes significantly in winter and early spring when photochemical destruction is relatively inefficient, but is relatively unimportant during the rest of the year when it contributes relatively little to the total 03 removal. The 03s content in the NH troposphere maximizes in winter and early spring, contributing almost 50% to the tropospheric 03 content (Figure 3d). In the summer, as photochemical destruction of 03s and photochemical production of 03t are more efficient, it contributes about 30%, mainly in the upper troposphere. 

The importance of photochemical destruction in the 03s tropospheric budget implies that the lifetime of 03s is coupled to the chemical production and destruction of 03. Consequently, the simulated tropospheric budget of 03s may be affected directly by differences in the simulated chemistry. For example, simulations with a pre-industrial and a present-day emission scenario or with and without representation of NMHC chemistry will produce different estimates of the tropospheric oxidation efficiencies [39, 40]. However, our simulations indicate only small effects on the calculated 03s budget [6]. Figure 5 presents the simulated zonal distribution of 03s, the chemical destruction rate, of ozone (day"1) and the chemical loss of 03s (ppbv day 1) for the climatological April. The bulk of the 03s in the troposphere resides immediately below the tropopause, whereas the ozone chemical destruction rate maximizes in the tropical lower troposphere (Figures 5a and 5b). Hence, most 03s is photochemically destroyed between 15-25 °N and below 500 hPa. This region... 

The UV photons can directly excite the molecule of the organic contaminants, thus leading to their direct photochemical destruction. 

Demas et al. described optical oxygen sensors using analogous osmium(II) complexes that have intense red absorptions and that can be excited with low-cost, high-intensity red diode lasers [25]. The osmium(II) complexes are probably more photochemically robust than ruthenium(II) complexes because of the larger energy gap between emitting state and the photochemically destructive upper d-d state. In Table 2, the photochemical and photophysical properties of osmium(II) tris(l,4-diphenyl-l,10-phenanthroline) (Os(dpp)3+) and osmium(II) tris(l,10-phenanthroline) (Os(phen)3+) are indicated as examples of osmium(II) complexes. The luminescence lifetimes of Os(dpp)3+ and Os(phen)3+ are 4.6 and 6.0 ns in dichloromethane solution,... 

Irradiation at longer wavelengths and the use of an inert atmosphere are often useful in preventing the photochemical destruction of susceptible molecules. 

The probability of NH2 disappearance by (VIII-89) is about 10"3 sec-1 using/c89 = 10-10 cm3 molec-1 sec-1 and [NH2] = 107 molec cm-3, while that of NH2(A2/4,) production (VIII-94) is 10 4 sec-1 on the basis of a solar intensity of 10 6 photons cm-2 sec-1 and an absorption cross section of 10- 20 cm2 of NH2 (625a) in the region 4300 to 9000 A. Thus, (VIII-95) may not be significant in the lower stratosphere but may be important in the upper stratosphere. Strobel (945) believes that the NH3 density profile in the stratosphere deviates significantly from a mixing ratio of 7.6 x 10-7 because of photochemical destruction and slow mixing. 

For a long time, transport from the stratosphere to the troposphere was thought to be the dominant source of ozone in the troposphere. Early in the 1970s, it was first suggested that tropospheric ozone originated mainly from production within the troposphere by photochemical oxidation of CO and hydrocarbons catalysed by HO and NO c- These sources are balanced by in-situ photochemical destruction of ozone and by dry deposition at the earth s surface. Many studies, both experimental- and model-based have set about determining the... 

Case Study II — Photochemical control of ozone in the remote marine boundary layer (MBL) - An elegant piece of experimental evidence for the photochemical destruction of ozone comes from studies in the remote MBL over the southern ocean at Cape Grim, Tasmania (41 In the MBL, the photochemical processes are coupled to physical processes that affect the observed ozone concentrations, namely deposition to the available surfaces and entrainment from the free troposphere. The sum of these processes can be represented in the form of an ozone continuity equation (a simplified version of Equation 2.6), viz... 

The night-time replenishment of ozone is caused by entrainment of ozone from the free troposphere into the boundary layer. The overnight loss of peroxide is due to deposition over the sea surface (and heterogeneous loss to the aerosol surface), as peroxide has a significant physical loss rate, in contrast to ozone which does not. Therefore, the daytime anti-correlation of ozone and peroxide is indicative of the net photochemical destruction of ozone. 

We cannot say what the relative mix of photochemical and thermal effects is as yet. The literature suggests that significant photochemical reactions should occur due to 248nm irradiation of acridine (25,26), but these are not the massive bond-breaking type that characterize 193nm photoablation(16). The fluorescence yield of acridine in PMMA is known to be about 0.2 (26) so considerable heat is produced by the absorption of short pulses in the 100 mJ/cm2 range an estimate based on an approximate heat capacity formula (27) is about 300 0. The excited state properties of acridine in PMMA show a pronounced temperature dependence (26). It seems likely that the bleaching arises from a combination of photochemical destruction of the acridine chromophore and polymer ablation. 

Direct photolysis or, as it is sometimes called photochemical destruction, leads to the break of chemical bonds, absorbed by light, in macromolecule. Photochemical transformations may be caused by radiation in the region 100-400 nm. This energy is quite enough to break the bond C-C or C=O and also C-H [154]. Hence, in order for the destruction to flow according to the mechanism of direct photolysis, PETP macromolecule must absorb... 

Polyester fibre displays high stability at its protection from a part of spectrum, that causes its photochemical destruction. 

T.B. Boboev, Photochemical destruction of polymers. Author s abstract of doctor dissertation, Moscow (1992) (in Russian). 

The photochemical destruction of ortho, meta and para-nitrophenols induced by ultra-violet light illumination of aqueous slurries of titanium dioxide has been monitored by electronic absorption spectroscopy the products are said to have been identified as dihydroxynitrobenzene isomers by coupled gas chromatography-mass spectrometry although no details are supplied. Nitrophenols have also been identified in the fog shrouding the University of Bayreuth. They are presumed to be the products of photochemical nitration and the possible precursors (phenol, cresol and nitrate) were also detected. 

Since there is no source of methane in the atmosphere, the vertical distribution of CH4 results from an equilibrium between its photochemical destruction and transport upward from the surface. The continuity equation can therefore be written ... 

Figure 5.14 shows the photochemical lifetime for atmospheric methane as a function of altitude along with the time constants associated with transport by the winds and vertical mixing. Since the stratospheric lifetime of this compound against photochemical destruction is the same order of magnitude as the transport time, it provides an excellent tracer to study transport processes. 

Because of the stability of HF, the atmospheric densities of F and FO are very small and the effect of fluorine on odd oxygen is insignificant. The reaction of HF with 0(1D) is chemically possible, but is negligible due to the low abundance of this excited atom. The distribution of HF is therefore largely determined by the rates of the surface emission of fluorine containing gases, of photochemical destruction of these gases, and atmospheric dynamics. 

Time gated fluorescence spectroscopy of the tumour localizing fraction of haematoporphyrin derivative, used as a photochemically destructive sensitizer, in the presence of cationic surfactant... 

Korshak et al. [348] also investigated the effect of low molecular weight compounds from the photochemical destruction of polystyrene. 

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