in UV-B 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:
Jx = ò F(l) sx(l,T) fx(l,T) dl
In the above equation, F(l) represents the actinic flux and is independent of species, sx and fx 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 UV-B 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 Si:
Si = ln(Ji*/Ji) / ln(O3/O3*)
where Ji* and Ji are the photodissociation rate coefficients of a specific photolysis reaction corresponding to ozone column amounts O3* and O3, respectively. In essence, the value of Si gives the percent increase in Ji resulting from a 1% reduction of stratospheric ozone. Calculated values of Si are given in Table 6.1 for selected species of tropospheric importance. As shown in the table, the Si for O3 is the largest, while the Si for NO2 is very small. The response of JO3 to stratospheric ozone depletion is significant while that of JNO2 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 satellite-based instruments, and the corresponding calculated changes in globally averaged tropospheric JO3, the dissociation rate coefficient for O3 yielding O(1D). 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 JO3 increased by 12 percent from 1979 to 1993.
Table 6.1. Sensitivity (Si) 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.