Photolysis Rate Constants

Photolysis direct photochemical transformation t,/2(calc) = 21 h, computed near-surface water, latitude 40°N, midday, midsummer and photolysis t,/2 = 160 d and 200 d in 5-m deep inland water body without and with sediment-water partitioning, respectively, to top cm of bottom sediment over full summer day, 40°N (Zepp Schlotzhauer 1979) t,/2 = 21 h, atmospheric and aqueous photolysis half life, based on measured sunlight photolysis rate constant in water adjusted for midday summer sunlight at 40°N latitude and t,/2 = 63 h after adjusting for approximate winter sunlight intensity (Howard et al. 1991) t,/2 = 160 d under summer sunlight in surface water (Mill Mabey 1985) ... 

Chen, J.W., Kong, L.R., Zhu, C.M., Huang, Q.G., Wang, L.S. (1996) Correlation between photolysis rate constants of polycyclic aromatic hydrocarbons and frontier molecular orbital energy. Chemosphere 33, 1143-1150. 

Ni(II) by strong oxidants, such as OH, Br and (SCN), produced by pulse radiolysis and flash photolysis. Rate constants are 10 M" s for oxidation by OH and Brf and = 10 M s for (SCN)f Ref. 259. The most popular means of production in both aqueous and nonaqueous solution is electrolytic, jjjg ligands which stabilize Ni(III) are cyanide, deprotonated peptides, amines and aminocarboxylates, a-diimines and tetraaza macrocycles, including porphyrins. Low spin d Ni(III) resembles low spin Co(II). The kinetics of the following types of reactions have been studied ... 

An expression for the number of A molecules dissociating per unit volume per unit time is developed in Box 3.2. Comparing Eq. (NN) in Box 3.2 to Eq. (CC), the photolysis rate constant kp must be given by... 

There are several approaches to measuring actinic fluxes and photolysis rate constants. One approach is to measure the rate of decay of a species such as N02 directly, so-called chemical actinometry (e.g. see Madronich et al., 1983). Another approach is to measure the light intensity and convert this to an actinic flux. 

RELATIONSHIPS BETWEEN RADIANCE, IRRADIANCE, ACTINIC FLUX, AND PHOTOLYSIS RATE CONSTANTS... 

Figure 3.29, for example, shows measurements of the photolysis rate of 03, J(03), made at the Mauna Loa Observatory on two different days, compared to model calculations of the photolysis rate constant (Shetter et al., 1996). The two model calculations use different assumptions regarding the quantum yield for 03 photolysis in the absorption tail beyond 310 nm (see Chapter 4.B). The measurements are in excellent agreement for the second day but somewhat smaller than the model calculations on the first. 

TABLE 3.19 3. SPECTROSCOPY AND PHOTOCHEMISTRY FUNDAMENTALS Calculated Photolysis Rate Constants for CH CHO Photolysis at 30°N Latitude Six Hours after Noon on July 1 ... 

Once the actinic fluxes, quantum yields, and absorption cross sections have been summarized as in Table 3.19, the individual products < .,v(A)wavelength interval can be calculated and summed to give kp. Note that the individual reaction channels (9a) and (9b) are calculated separately and then added to get the total photolysis rate constant for the photolysis of acetaldehyde. However, the rate constants for the individual channels are also useful in that (9a) produces free radicals that will participate directly in the NO to N02 conversion and hence in the formation of 03, etc., while (9b) produces relatively unreactive stable products. 

In this case, the photolysis rate constant for reaction (9b) is zero because the quantum yield drops off rapidly to zero above 290 nm, and at this large solar zenith... 

Calculate the photolysis rate constants, kp, for each of the two photolysis paths as well as the overall... 

While the relative importance of the various paths is not well established, it is expected that dissociation to the alkoxy radical, RO, and N02 will predominate. Luke et al. (1989) experimentally measured rates of photolysis of simple alkyl nitrates and compared them to rates calculated using the procedures outlined in Chapter 3.C.2. Figure 4.22 compares the experimentally determined values of the photolysis rate constants (kp) for ethyl and n-propyl nitrate with the values calculated assuming a quantum yield for photodissociation of unity. The good agreement suggests that the quantum yield for photodissociation of the alkyl nitrates indeed approaches 1.0. 

FIGURE 4.22 Experimental values of the photolysis rate constant, kp, for (a) ethyl nitrate and (b) n-propyl nitrate as a function of zenith angle compared to calculated values shown by the solid lines. Different symbols represent different measurement days (adapted from Luke et al., 1989). 

Figures 4.26, 4.27, and 4.28 show typical UV absorption spectra for some simple aldehydes and ketones (Rogers, 1990 Martinez et al., 1992 see also Cronin and Zhu, 1998, for n-pentanal). Formaldehyde stands out from the higher aldehydes and ketones in that it has a highly structured spectrum and furthermore, the absorption extends out to longer wavelengths. The latter difference is particularly important because the solar intensity increases rapidly with wavelength here (Chapter 3.C.1) and hence the photolysis rate constant for HCHO and the rate of production of free radicals...

The absolute values of the absorption cross sections of HCHO have been somewhat controversial. This appears to be due to a lack of sufficient resolution in some studies as discussed in Chapter 3.B.2, if the spectral resolution is too low relative to the bandwidth, nonlinear Beer-Lambert plots result. The strongly banded structure means that calculations of the photolysis rate constant require actinic flux data that have much finer resolution than the 2- to 5-nm intervals for which these flux data are given in Chapter 3 or, alternatively, that the measured absorption cross sections must be appropriately averaged. One significant advantage of the highly structured absorption of HCHO is that it can be used to measure low concentrations of this important aldehyde in the atmosphere by UV absorption (see Sections A.ld and A.4f in Chapter 11.). 

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 ... 

That is, in the presence of sufficient quantities of NO that H02 and CH302 react primarily with NO rather than with each other, the total concentration of peroxy radicals should vary directly with the photolysis rate constant for 03 rather than with its square root as was the case at low NO. 

This reaction is primarily a daytime reaction because most OH sources are photolytic in nature. As a result, the N02 reaction with OH competes with N02 photolysis, reaction (4). As discussed in Chapter 3, a typical value of the photolysis rate constant for N02 would be kp = k4 = 7 X 10 3 s-1 at a solar zenith angle of 50° (e.g., see Fig. 3.31). Thus, the reaction with OH is not usually a dominant loss process for N02, but it is still sufficiently fast to form significant amounts of HNO, during the day, particularly in polluted regions with relatively large N02 concentrations. 

Using the kinetics for the OH + NO reaction discussed in this chapter, estimate the steady-state concentration of HONO that would exist at noon at the earth s surface if the OH radical concentration is 5 X 106 radicals cm"3, the NO concentration is 1 ppb, and the photolysis rate constant for HONO is 1.4 X 10"3 s"1. 

FIGURE 16.4 Relative spectral distributions for a typical black lamp and the solar spectrum at zenith angles of 0 and 80° normalized to the same N02 photolysis rate constant (adapted from Carter el al., 1984). 

From measurements of the concentration C, of the compound i as a function of exposure time, the first-order photolysis rate constant, kp(/1), is then determined by calculating the slope of a plot of In C, /C,0 versus time (see Section 12.3). Since the... 

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