An Introduction to Environmental Chemistry

carries. It requires ultraviolet (UV) radiation of wavelength less than 242 nm to

have sufficient energy to split the oxygen molecule (O 2 ) apart:

eqn. 3.32

The UV photon here is symbolized by hv. Once oxygen atoms (O) have been

formed, they can react with O 2.

eqn. 3.33

The production of O 3 by this photochemical process can be balanced against the

reactions that destroy O 3. The most important is photolysis:

OOO 32 ()g+Æhv ()g +()g eqn. 3.34

OO O 23 ()gg+Æ() ()g

OOO 2 ()ggg+Æ +hv () ()

60 Chapter Three

140

Year

Total ozone (Dobson units)

340

1960 1970 1980 1990

320

300

280

260

240

220

200

180

160

180 °

90 °W 90 °E

0 °

Halley Bay^60 °S

Antarctica

South

Pole

0

pO

(mPa) 3

10

1985

2

8

4

6

Year

1990 1995 2000

Fig. 3.6Mean October levels of total ozone above Halley Bay (76°S), Antarctica, since 1957.

The 1986 value is anomalous due to deformation of the ozone hole, which left Halley Bay

temporarily outside the circumpolar vortex (a tight, self-contained wind system). Dobson units

represent the thickness of the ozone layer at sealevel temperature and pressure (where 1

Dobson unit is equivalent to 0.01 mm). Data courtesy of the British Antarctic Survey. Inset

shows seasonally averaged (Sep.–Nov.) ozone partial pressure at about 17 km at 70°S. Data

courtesy of G. König-Langlo.