The three following reactions describe the basic ozone cycle in
the lower atmosphere:
(1) NO 2 + Sunlight (wavelength
< 430 nm) --> NO + O
NO 2 is a strong absorber
of sunlight (blue and near UV). (see note 1 below)
(2) O + O 2 + M --> O
3 + M
This reaction is a three-body process. The M molecule is necessary
to carry off energy released in forming the new bond.
(3) NO + O 3 --> NO 2 + O 2
Call k1 the rate constant for reaction (1)
as written. Similarly, k2 and k3 are the rate constants for reactions
(2) and (3). These three reactions form a complete cycle; for
NO 2 the cycle takes only
a few minutes. This creates a dynamic equilibrium, with the steady-state
concentration of O 3 given
by:
[O 3 ] = (k1/k2) [NO 2 ]/[NO] = 21 ppb [NO 2
]/[NO]
As long as the [NO 2 ]/[NO]
ratio is small, the ozone levels will remain low.
However, these three reactions cannot completely explain the buildup
of ozone. If only these reactions were involved, a low steady-state
O 3 level would build up
in a few minutes, and then no further changes would occur. During
this build up, the NO 2 concentration
would decrease. However, the data indicate that the ozone buildup
occurs more slowly over several hours accompanied by an increase
in the NO 2 concentration.
The dominant factor is the ratio of [NO 2 ]/[NO].
As discussed below, other reactions can increase this ratio and
thus the levels of O 3 .
One set of reactions which can oxidize NO without decomposing
O 3 :
OH + CO --> H + CO 2
H + O 2 +
M --> HO 2 + M
HO 2 + NO --> OH + NO
2
Another set involves the methyl radical:
CH 3 + O 2
+ M --> CH 3 O
2 + M
CH 3 O 2
+ NO --> CH 3 O
+ NO 2
CH 3 O
+ O2-- > CH 2 O + HO 2
The above cycle can occur with many organic
free radicals. Simply replace CH 3 with
R.
The processes above oxidize NO to NO 2 .
Reactions (1) and (2) continue to produce O 3
, in the presence of these reactions, but
these other reactions convert NO to NO 2 before
the NO can react to destroy the O 3 .
The free radicals (CH 3 ,
OH, and R) are generated from hydrocarbons by photochemical reactions
that require sunlight. Most free radicals have odd numbers of
electrons. Most stable molecules have even numbers of electrons.
When a radical reacts with a stable molecule, usually another
radical is generated. This process can be halted by one of three
reactions:
Radical-radical e.g. HO2 + CH 3 O
2 --> CH3O2H + O 2
Radical-NOx e.g.CH 3 OO
2 + NO 2
--> CH 3 OOO
2 NO 2 (PAN)
Radical-surface e.g. HO 2 +
surface --> absorbed radical
Free radicals can also be formed by decomposition of aldehydes:
CH 2 O + sunlight -->
CHO + H
CH 3 CHO + sunlight -->
CH 3 + CHO
In the polluted atmosphere, most of the oxidation of NO to NO
2 is carried out by HO 2 and RO 2 .
Oxidizable pollutants (hydrocarbons, aldehydes, CO) generate free
radicals that react with O 2 to
form RO 2 and HO 2
. They become degraded to other compounds,
which are often still reactive. As O 3 levels
build up, it becomes more probable that the O 3
will be destroyed by reacting with the hydrocarbons
or free radicals, and the rate of ozone increase levels off. (See
note 2 below.)
1 Nanometers (nm) are a measure of distance. One billion (10^9)
equal one meter. When used to describe radiation this distance
refers to wavelength. Radiation with a wavelength of 500 nm is
blue-green visible light; radiation at 250 nm is a type of ultraviolet
light called Ultraviolet-C.
2 Lambert J. L.; Paukstelis J. V.; Chiang Y. C. Environ. Sci Technol.
23:241 (1989)