An Introduction to Environmental Chemistry

The Atmosphere 51

Box 3.6 Reactions in photochemical smog

Reactions involving nitrogen oxides (NO and
NO 2 ) and ozone (O 3 ) lie at the heart of
photochemical smog.

eqn. 1

eqn. 2

eqn. 3

where M represents a ‘third body’ (Section
3.10.1)
It is conventional to imagine these
processes that destroy and produce nitrogen
dioxide (NO 2 ) as in a kind of equilibrium,
which is represented by a notional
equilibrium constant relating the
partial pressures of the two nitrogen oxides
and O 3 :

eqn. 4

If we were to increase NO 2 concentrations (in
a way that did not use O 3 ), then the
equilibrium could be maintained by
increasing O 3 concentrations. This happens in
the photochemical smog through the
mediation of hydroxyl (OH) radicals in the
oxidation of hydrocarbons. Here we will use
methane (CH 4 ) as a simple example of the
process:

eqn. 5

CH 32 ()gg+ÆO() CH O 32 ()g eqn. 6

OH()gg gg+Æ +CH 42 () H O()CH 3 ()

K=ppNOpNO◊O^3

2

ONO ONO 32 ()g+Æ+()gg() 2 ()g

OO M O M()ggg gg++Æ+ 23 () () () ()

NO 2 ()gg+<hv( 310 nm)Æ+O()NO()g

eqn. 7

eqn. 8

eqn. 9

These reactions represent the conversion of

nitric oxide (NO) to NO 2 and a simple alkane

(see Section 2.7) such as CH 4 to an aldehyde

(see Table 2.1), here formaldehyde (HCHO).

Note that the OH radical is regenerated, so

can be thought of as a kind of catalyst.

Although the reaction will happen in

photochemical smog, the attack of the OH

radical is much faster on larger and more

complex organic molecules. Aldehydes such

as acetaldehyde (CH 3 CHO) may also undergo

attack by OH radicals:

eqn. 10

eqn. 11

eqn. 12

eqn. 13

The methyl radical (CH 3 ) in equation 13 may

re-enter at equation 6. An important branch

to this set of reactions is:

eqn. 14

leading to the formation of the eye irritant

peroxyacetylnitrate (PAN).

CH COO NO CH COO NO

PAN

32 ()gg+Æ 2 () 322 ()g

( )

CH CO 32 ()gggÆ+CH 3 ()CO 2 ()

CH COO (^32) ()g+Æ +NO()ggNO 2 ()CH CO 32 ()g
CH CO 3232 ()gg+ÆO() CH COO()g
CH CHO 33 ()gg+ÆOH() CH CO()g g+H O 2 ()
HO 22 ()gg+Æ +NO() NO()ggOH()
CH O 32 ()gg+ÆO() HCHO()g g+HO 2 ()
CH O 32 ()g+Æ +NO()gggCH O 3 ()NO 2 ()
There is no fog when Los Angeles smog forms, and visibility does not decline to
just a few metres, as was typical of London fogs. Of course, the Los Angeles smog
forms best on sunny days. London fogs are blown away by wind, but the gentle
sea breezes in the Los Angeles basin can hold the pollution in against the moun-
tains and prevent it from escaping out to sea. The pollution cannot rise in the
atmosphere because it is trapped by an inversion layer: the air at ground level is
cooler than that aloft, so that a cap of warm air prevents the cooler air from rising
and dispersing the pollutants. A fuller list of the differences between Los Angeles-
and London-type smogs is given in Table 3.5.