Photodissociation rate coefficients

At night there is no OH production, and n(oH), along with all photodissociation rate coefficients, goes to zero, resulting in... 

Radiative transfer calculations were performed in the context of modeling the photochemistry of sulfur in the early Earth atmosphere. Rather than present an in-depth discussion of fhe many photochemical possibilities [29], my emphasis here is on the dissociation rates of SO2 isotopologues. However, because absorption by CO2, H2O and other possible compounds will alter the dissociation rates of SO2, particularly at short wavelengths (<200 nm), the radiative transfer and photochemistry are necessarily linked. The photodissociation rate coefficient for the S02 isotopologue (x = 32, 33,34 or 36) from 190 to 220 nm is given by... 

The photodissociation rate coefficients are included as source and sink terms in a system of time-dependent continuity equations for the atmosphere. Modem values for vertical (eddy) diffusion and solar photon flux are utilized. The system of 2nd-order ordinary differential equations is solved by integration, and yields chemical species abundances as a function of time and altitude. The isotope atmospheric chemistry includes only SO2 isotopologue photodissociation reactions and production of SO isotopologues. Additional isotopic reactions such as SO2 oxidation by OH, SO photolysis, SO disproportionation during self-reaction, and SO dimmer formation, have been neglected. My objective here is to focus only on SO2 photolysis as a S-MIF mechanism. 

As indicated, the quantity on the RHS multiplying nA isjA. The spectral actinic flux is then the radiative quantity that drives the photodissociation, that is, the quantity that multiplies photodissociation rate coefficient. 

Figure 4.14 shows the photodissociation rate coefficient for 02, namely, j0l, as a function of altitude above 30 km. From 30 to 60 km, the Herzberg continuum provides the dominant contribution to j0l At about 60 km, the contribution from the Schumann-Runge bands equals that from the Herzberg continuum above 60 km, the Schumann-Runge bands predominate until about 80 km, where the Schumann-Runge continuum takes over. In the mid-to upper stratosphere, at solar zenith angle = 0°, an approximate value of jo2 is jo2 — 10 9 s. ... 

FIGURE 4.14 Photodissociation rate coefficient for 02, y o2, and 02 photolysis rate as a function of altitude at solar zenith angle 0q = 0°. (Courtesy Ross J. Salawitch, Jet Propulsion Laboratary.)... 

Figure 4.18 shows the N02 photodissociation rate coefficient, jN0, as a function of altitude. The majority of N02 photolysis occurs for X > 300 nm. Since in this region of the... 

Madronich, S., and Weller, G. (1990) Numerical integration errors in calculated tropospheric photodissociation rate coefficients, J. Atmos. Chem., 10, 283-300. 

The studies of Hasinoff [53] on the recombination rate of carbon monoxide and the heme units after photodissociation of carboxy ferrous microperioxidase come close to satisfying the requirements for observing the effects of anisotropic reactivity and rotational diffusion on the rate of a translational diffusion-limited reaction. In Chap. 2, Sect. 5.6, the details of this study were briefly mentioned. Hasinoff found that the rate of recombination was substantially diffusion-limited in all three aqueous solvents used at 260 K, but at higher temperatures, the rate of reaction of the encounter pair, feact, was a significant factor in determining the overall rate of recombination (see Fig. 9). The observed rate coefficient of recombination, feobs, was separated into the rate coefficient of diffusive formation of encounter pairs, feD, and the rate coefficient of reaction of encounter pairs, fcact, with the Collins and Kimball expression, eqn. (26)... 

The reactions and rate coefficients used in later calculations in this chapter are listed in Table VI, and the noontime altitude profiles for the photodissociation rates for the same calculations, in Table VII. 

The conclusion was at variance with the findings of Jackson " , who could not detect CO2 formation, but found ozone to be the only product. He concluded that the reaction of O atoms with CO must be at least 150 times slower than with O2. In his experiments, O atoms were formed by photodissociation of O2 with an aluminum spark (A, > 1719 A). Groth ", in similar experiments but with a Xe lamp (X = 1470, 1296 A), found the rate coefficient for reaction of O atoms with CO to be 1/40 of that with O2. 

Photolysis reactions are central to atmospheric chemistry, since the source of energy that drives the entire system of atmospheric reactions is the Sun. The general expression for the first-order rate coefficient j for photodissociation of a species A is given by (4.39). Because the rate of a photolysis reaction depends on the spectral actinic flux / and because... 

Figure 6.8 shows the first-order loss rate of PAN as a function of altitude by thermal decomposition and photodissociation. The thermal decomposition rate coefficient for PAN can be obtained from the forward rate coefficient for reaction 3, 3, and the equilibrium constant for reactions 3 and 4, 3,4 (Sander et al. 2003),... 

Maricq et al. [180] have studied the self-reaction of FCO radicals. Although the self-reaction will be of minor importance for upper atmospheric chemistry, this reaction is critical in laboratory studies involving FCO radicals. For example, in the measurement of quantum yields from CFjO, CFCIO, and HFCO photodissociation, the self reaction could lead to a gross underestimation of the quantum yields. The rate coefficient of 1.9 0.2 X 10 cm s suggests that the self-reaction is quite rapid. In the self-reaction, the FCO radicals recombine to form oxylfluoride, which is produced with excess energy above the barrier for molecular dissociation to carbonyl fluoride and carbon monoxide [192]. 

CiSj. Photodissociation of CS2 at wavelengths X > 185 nm could lead to formation of both SCF) and S( D). The principle absorption system 200 nm populates the Sj( yl ) electronic state, which lies below the lowest state (in contrast to all the other molecules in this group apart from CSe2), and in consequence it correlates with CS(AT S" ") + S( D) (see Fig. 3.5.10). Flash photolysis of CS at X > 185 nm produced vibrationally excited CS(2f 2 ) and S( P) as the only detectable transients in absorption, but the detection of S( D) would have been prevented through rapid quenching by the diluent gas, N2. At the pressures employed, S( D) atoms would decay within a few nanoseconds, since the rate coefficient for quenching by N2 is 1.5 X 10 cm molecule" s ... 

---