Page 159 - Basic physical chemistry for the atmospheric sciences

Photochemistry 1 4 �

j
N0 (g) + h v � NO(g) + O(g) (7.20)
2

(7. 2 1 )

(7.22)
where the symbols above the arrows indicate the rate coefficients.
Each of the above three reactions is rapid , and as NO(g) accumulates
NO z(g) is formed by Reaction (7 .22) as rapidly as it is depleted by the
photolytic Reaction (7 .20 . Therefore,
)
j[NOz(g)] = k 2 [0 (g)][NO(g)]
3
or,

7
( . 2 3)
Equation (7 2 3) determines the ratio of the concentration of N02(g) to
.
NO(g) when this system is in a photostationary steady state.
It is worthwhile noting here that Reactions (7. 2 0) and (7. 2 1 ) are the
only definitely established chemical mechanism for producing ozone in
the troposphere ; the other source for tropospheric ozone is downward
transport from the stratosphere . Together these two sources maintain
a background concentration of ozone in the troposphere of about 0.03
to 0 . 05 parts per million of the air by volume (ppmv). Ozone is of
critical importance in the c h emistry of the troposphere because, not
only is it a powerful oxidant itself, it is the primary source of the
hydroxyl radica/ 2 (OH), which is highly reactive and of paramount
importance in tropospheric chemistry. Also, as can be seen from Eq.
(7. 2 3), the concentration of ozone in the air determines the ratio of
[NOz(g)] to [NO(g)] . Nitric oxide, NO(g), is also a very reactive gas
and of great importance in atmospheric chemistry. 3

7.6 Stratospheric ozone and photochemistry; depletion of
stratospheric ozone
In recent years a great deal of scientific attention and public concern
has been directed at the problem of ozone depletion in the Earth' s
stratosphere caused by anthropogenic chemicals. Ozone is toxic , and