Socioeconomic Data and Applications Center Environmental Effects of Ozone Depletion 1998 Assessment

Increases 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:

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 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 S_i:

S_i = ln(J_i*/J_i) / ln(O₃/O₃*)

where J_i* and J_i are the photodissociation rate coefficients of a specific photolysis reaction corresponding to ozone column amounts O₃* and O₃, 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 O₃ is the largest, while the S_i for NO₂ 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 satellite-based instruments, and the corresponding calculated changes in globally averaged tropospheric J_O3, 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 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.

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