Page 206 - Fundamentals of Air Pollution 3E

172 12. Atmospheric Chemistry

Direct reaction
Photodissociation
In the case of NO 2, for each photon absorbed below 400 nm, photodissocia-
tion occurs. For other photoabsorbers, HNO 2 and aldehydes, the photodis-
sociation process leads to the formation of free radicals.

B. Nitric Oxide, Nitrogen Dioxide, and Ozone Cycles
Three relatively simple reactions can describe the interrelationships
among these components.

Reaction (12-9) shows the photochemical dissociation of NO 2. Reaction (12-
10) shows the formation of ozone from the combination of O and molecular
O 2 where M is any third-body molecule (principally N 2 and O 2 in the
atmosphere). Reaction (12-11) shows the oxidation of NO by O 3 to form
NO 2 and molecular oxygen. These three reactions represent a cyclic path-
way (Fig. 12-4) driven by photons represented by hv. Throughout the
daytime period, the flux of solar radiation changes with the movement of
the sun. However, over short time periods (—10 min) the flux may be
considered constant, in which case the rate of reaction (12-9) may be ex-
pressed as

where fc } is a function of time of day. Expressions for the time rate of change
for each of the components may be written. If this cycle reaches a steady

Fig. 12-4. Photochemical cycle of NO, NO 2, O 3, and free radicals.