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(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 satellite-based
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 |

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