An Introduction to Environmental Chemistry

sources (Section 3.4.2). This natural source, however, accounts for only 25% of

the chlorine which is transported across the tropopause. By the early 1970s the

CFCs used as aerosol propellants and refrigerants had become widely distributed

through the troposphere. There appeared to be no obvious mechanism for the

destruction of these highly stable compounds in the lower atmosphere. However,

the knowledge that CFCs were being transported to the stratosphere raised

concern over their effect on the O 3 layer. These compounds, for example

CFCl 3 (Freon-11; Fig. 3.4b) and CF 2 Cl 2 (Freon-12), absorb UV radiation in the

190–220mm range, which results in the photodissociation reactions:

eqn. 3.43

eqn. 3.44

These reactions produce the free chlorine atoms that react with O 3 in the

catalytic manner, i.e.:

eqn. 3.45

eqn. 3.46

which sum:

eqn. 3.47

However, ClO produced on reaction with O 3 may not always react with atomic

oxygen (eqn. 3.46), but can interact with nitrogen compounds instead:

eqn. 3.48

This reaction is of considerable importance because it effectively removes the

nitrogen and chlorine species involved in the cycles that destroy O 3. If, however,

there are solid surfaces present, the chlorine sequestered in this reaction can be

released:

eqn. 3.49

eqn. 3.50

eqn. 3.51

eqn. 3.52

eqn. 3.53

eqn. 3.54

Equations 3.51–3.54 then sum to:

eqn. 3.55

The reaction sequence (eqns. 3.49–3.54) is particularly fast at low temperature.

Moreover, the square dependence on chlorine concentration implicit within

equation 3.50 makes the reaction very sensitive to chlorine concentration. It is

these low-temperature processes on particle surfaces that offer the best explana-

23 OO 32 ()ggÆ ()

ClO 22 ()gg+Æ +M Cl()O()g+M

Cl O 22 ()gg+Æhv ClO 2 ()+Cl()g

2 ClO()g+ÆM Cl O (^22) ()g +M
22 2 2Cl()gg+Æ +O 32 () ClO()ggO()
Cl 2 ()gg+Æhv 2 Cl()
ClONO 223 ()gsg+Æ+HCl() Cl() HNO()s
ClO()gg++ÆNO 22 () M ClONO()g+M
OO O 32 ()gg+Æ() (^2) ()g
ClO()gg+Æ +O() O 2 ()g gCl()
O 32 ()gg g+Æ +Cl() O()ClO()g
CF Cl 22 ()gg+Æhv CF Cl 2 ()+Cl()g
CFCl 32 ()gg+Æhv CFCl()+Cl()g
62 Chapter Three