Increases
in UVB Photodissociation Rate Coefficients
Photodissociation reactions are of the general form:
AB + hn ® A + B.
The photodissociation rate coefficient J of species x in the troposphere is calculated by evaluating the integral equation:
J_{x} = ò F(l) s_{x}(l,T) f_{x}(l,T) dl
In the above equation, F(l) represents the actinic flux and is independent of species, s_{x} and f_{x} denote the molecular absorption cross section and quantum yield (both of dependent on the species x), T is the temperature of the air parcel, and l is the wavelength of the radiation. The sensitivity of the response of J to increases of UVB radiation varies significantly for different species (Madronich and Granier, 1994; Krol and Van Weele, 1997). To quantify the response to ozone change, Madronich and Granier (1994) defined the sensitivity factor S_{i}:
S_{i} = ln(J_{i}*/J_{i}) / ln(O_{3}/O_{3}*)
where J_{i}* and J_{i} are the photodissociation rate coefficients of a specific photolysis reaction corresponding to ozone column amounts O_{3}* and O_{3}, respectively. In essence, the value of S_{i} gives the percent increase in J_{i} resulting from a 1% reduction of stratospheric ozone. Calculated values of S_{i} are given in Table 6.1 for selected species of tropospheric importance. As shown in the table, the S_{i} for O_{3} is the largest, while the S_{i} for NO_{2} is very small. The response of J_{O3 }to stratospheric ozone depletion is significant while that of J_{NO2} is negligible. Values given in Table 6.1 are similar to those computed by Madronich and Granier (1994), Madronich et al. (1995; 1998), and Granier et al. (1998). Small differences stem from difference in conditions (e.g. latitudes, solar zenith angles) as well as some model differences.
Fuglestvedt et al. (1995) calculated monthly J values (for the 15th of each month) of 16 photolytic reactions from 1979 to 1993. Figure 6.2 shows the changes in global total ozone observed by satellitebased instruments, and the corresponding calculated changes in globally averaged tropospheric J_{O3}, the dissociation rate coefficient for O_{3} yielding O(^{1}D). Both are given as annual averages and normalized to 1979 levels. As shown in figure 6.2, the global total ozone column densities decreased by 8 percent while the J_{O3} increased by 12 percent from 1979 to 1993.
Table 6.1. Sensitivity (S_{i}) of photodissociation coefficients of several molecules, to changes in total column ozone. Calculations for upper troposphere, total ozone column of 328 Dobson Units. From Ma, 1996.
Chemical Formula  Name  S_{i} 
O_{3}  Ozone  1.45 
HNO_{3}  Nitric acid  0.89 
CH_{3}CHO  Acetaldehyde  0.73 
CH_{3}COCH_{3}  Acetone  0.60 
HCHO  Formaldehyde  0.38 
H_{2}O_{2}  Hydrogen peroxide  0.31 
CH_{3}OOH  Methyl hydroperoxide  0.31 
N_{2}O_{5}  Dinitrogen pentoxide  0.30 
NO_{2}  Nitrogen dioxide  0.02 
HNO_{2}  Nitrous acid  0.01 
