Photolysis Processes

Photochemical processes in the atmosphere are initiated by the absorption of a photon (quantum of electromagnetic energy) by a molecule XY, leading to promotion of the molecule to an electronically-excited state (conventionally denoted by the superscript )  

The excited state is usually unstable, and is therefore followed by dissociation 

The excited state molecule can also be quenched by a third body M 

The quantum yield of a given pathway following the light absorption is defined as the number of reactant molecules which decompose along this pathway relative to the number of photons absorbed. The sum of quantum yields for all possible pathways is then unity. In this section, we focus on photodissociation processes (also called photolysis), as described by Reactions (2.75) and (2.76a). 

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Applicabdity Limitations Photolysis is appropriate for difficult-to-treat chemicals (e.g., pesticides, dioxins, chlorinated organics), nitrated wastes, and those chemicals in media which permits photolyzing the waste. The waste matrix can often shield chemicals from the light (e.g., ultraviolet light absorbers, suspended solids, solid wastes). The photolysis process typically requires pretreatment to remove suspended materials, and the by-products formed may be more toxic than the parent molecules. 

If the photolysis process is not immediate, but instead has some finite lifetime r, the parent molecule may have time to rotate during dissociation and so wash out the angular distribution. In this case a reduced effective anisotropy parameter (/3eff) is used to characterize the observed angular distribution. The relationship between [3eg and parent lifetime can be expressed as52,54... 

Chemicals will undergo photolysis if they can absorb sunlight. Photolysis can occur in air, soil, water, and plants. The rate of photolysis is dependent upon the pH, temperature, presence of sensitizers, sorphon to soil, depth of the compound in soil and water. Lyman et al. (1982) present an excellent overview of the photolysis process. 

Direct photolysis processes involve the direct adsorption of light by the substrate... 

Ciani A. (2003). Photolysis processes in soils and other porous media. Ph.D. dissertation, Swiss Federal Institute of Technology, Ziirich, Switzerland. Available at http // e-collection.ethbib.ethz.ch/ecol-pool/diss/fulltext/ethl5306.pdf... 

Benzene and some halogenated compounds including carbon tetrachloride, trichlorotriflu-oroethane (Freon 113), and dichloroethane absorb light weakly and thus photolyze relatively slowly. Therefore even shorter wavelengths than those available from the current technology are needed to create a commercially viable, direct photolysis process for these compounds. 

The time-resolved spontaneous emission from an electronically excited halogen atom X(np5 2Py2) produced in a flash photolysis process in a static system is found experimentally to be described by an over-all first-order kinetic process for a long interval following the photolytic pulse.11 Thus the intensity of emission (Iemm) is given by... 

Batch processes using a number of photochemical reactors of optimal size in a parallel arrangement with a central unit in which optimal reaction conditions (temperature, gas saturation, mixing, etc.) can be maintained and classical operations of product separation can be performed (Figure 20). Such a production unit also permits maintenance of photochemical reactors without interruption or strong disturbance of an ongoing production or photolysis process. 

Previous workers had used the molecular beam TOF technique (134) and the VUV flash photolysis LIF technique (135). Ling and Wilson (136) had suggested that either the A(2n) state of CN is produced in the original photolysis process or that I atoms were produced in the Pi/2 and 3/2 states. It had been previously shown (135), by collisional quenching studies, that the A state of CN was not produced. This earlier work has been reviewed by Baronvaski (137) but recently both he and others have done further work on this molecule using excimer laser sources in both static gases and pulsed molecular beams. 

Since L- values are averaged over 24 hours and over each 3-month season, the accuracy of rate estimates depend on the rate of the photolysis process compared with the averaging interval. For example, a compound which photolyzes with a half life of 1 hour will photo-lyze three times faster at solar noon than at 800 or 1600 hrs. However, if the half life is one week near mid-season, diurnal variations make little difference. 

In the atmospheric gas phase the main reactive species are OH, N03, 03, and sunlight itself which can be involved in direct photolysis processes. In the latter case a sunlight-absorbing molecule reaches an electronically and vibra-tionally excited state after absorption of a photon of appropriate wavelength. The surplus energy can be dissipated by vibrational relaxation (i.e., thermally lost), fluorescence, phosphorescence, or chemical reactivity. The latter is often in the form of bond breaking (photolysis), induced by the excess of vibrational energy that can sometimes increase vibration amplitude beyond the threshold where the atoms involved in the bond (B and C in Equation 17.1) are permanently separated [7]. 

Direct photolysis processes on the surface of airborne particulate matter can be important sinks of sunlight-absorbing compounds (see [28] for a recent review by our group on this subject), and in particular of polycyclic aromatic hydrocarbons (PAHs) [11]. The particles can protect adsorbed substrates against reaction with species such as OH and N03 from the gas phase, and enhance the relative role of direct photolysis. However, it should be considered that black carbonaceous... 

A number of photochemical reactions can also take place in clouds and fog [27]. Direct and sensitized photolysis processes can be operational, the latter (see... 

The olefin can be regarded as a scavenger for the radicals produced during the photolysis process, and its reaction with the carbamoyl radical leads to the 1 1 adduct which is the higher amide. Telomers of different degrees of telomerization (mainly 2 1 to 4 1) were also formed in this reaction and supply additional proof for the free radical nature of the reaction, which is summarized in the following scheme ... 

Scheme 1 Photochemistry of CpRe(CO)2(H)2 in Nujol. (i) Photolysis processes in Nujol, 12 K (ii) secondary process in N2 matrix (iii) secondary process in CO matrix, (iv) thermal reversal in Nujol, 298 K. Ref. 114...

The important primary processes expected to result from the absorption of near UV by the three polymers are listed in Table I. Also shown are the estimated absorptions due to chromophores and the efficiencies of the primary photolysis processes. Efficiencies are expressed as quantum yields for the formation of certain primary products, and are collected from published values (, 6, The... 

Comparison with Direct Photolysis Process. The Ti02-mediated photocatalytic oxidation reaction involves a complex free-radical reaction mechanism in which OH radicals are responsible for the oxidation of 4-chlorophenol. The initial reaction step produces 4-chlorocatechol as the main product. In contrast, the direct photolysis of 4-chlorophenol produces a different set of reaction products. Figure 8 shows that the direct photolysis of... 

In addition to the difference in forms and the distribution of reaction products, the photocatalysis and photolysis processes exhibit different extents of mineralization. As shown in Figure 1C, the generation of carbon dioxide in direct photolysis of 4-chlorophenol is almost negligible. In contrast, pho-tocatalytic oxidation leads to total mineralization of 4-chlorophenol to carbon dioxide. There is, however, no evidence that further ring-cleavage reaction takes place in direct photolysis of 4-chlorophenol. 

In atmospheric models currently used, quantum yields are often assumed to be unity, which is seldom the case, as is shown in this study. This leads to overestimation of the calculated photodissociation rates and the associated radicals formed in the photolysis processes. These new data will reduce the present uncertainties associated with photolysis processes, so that future assessments can be made with more confidence. 

The rates of photolysis processes depend on the intensity and spectral distribution of the light source, which vary from one chamber to another. Spectral distributions (based on spectrometer measurements) and absolute light intensities (based on NO2 actinometry experiments) are documented for all the SAPRC chambers (e.g. Carter et al., 1995a, Carter et al., 1995b). The rates of photolysis processes for application in MCM v3 were thus determined using this spectral information in conjunction with absorption cross sections and quantum yield data previously used in conjunction with the MCM, based on the soiuces summarised by Jenkin et al., (1997). Furthermore, the rates of a number of photolysis reactions were updated during the course of the present study, as discussed fiulher below. 

The MCM v3 mechanism for butane was found to provide an acceptable reaction framework for describing the NOx-photo-oxidation experiments on the above systems, although a number of parameter modifications and refinements were identified and introduced which resulted in an improved performance. These generally relate to the magnitude of sources of free radicals from carbonyl photolysis processes, which are currently under review in MCM development activities. Specifically, recommendations are made to update the photolysis parameters for HCHO, MEK, either in line with data reported since the MCM mechanism development protocol (Jenkin et ai, 1997), or on the basis of optimization in the current study. 

In addition to the technical developments the scientific applications of chambers are being expanded, for example, the current understanding of secondary organic aerosol formation has been developed principally from environmental chamber studies and chambers have been used even more recently to investigate photolysis processes under real atmospheric conditions. Such new scientific applications are often linked to new technical analytical and other technical developments and environmental chambers offer a perfect platform for testing environmental monitors. 

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