Page 201 - Fundamentals of Air Pollution 3E

II. Role of Solar Radiation in Atmospheric Chemistry 167

sorbed on particulate matter and is therefore not present as a gas. Vapor
pressure also affects the rate of evaporation of organic compounds into the
atmosphere and the conversion of atmospheric gases to particulate matter,
e.g., SO 2 to the aerosol H 2SO 4 (3).
Atmospheric chemical reactions are classified as either photochemical or
thermal. Photochemical reactions are the interactions of photons with spe-
cies which result in the formation of products. These products may undergo
further chemical reaction. These subsequent chemical reactions are called
thermal or dark reactions.
Finally, atmospheric chemical transformations are classified in terms of
whether they occur as a gas (homogeneous), on a surface, or in a liquid
droplet (heterogeneous). An example of the last is the oxidation of dissolved
sulfur dioxide in a liquid droplet. Thus, chemical transformations can occur
in the gas phase, forming secondary products such as NO 2 and O 3 ; in the
liquid phase, such as SO 2 oxidation in liquid droplets or water films; and
as gas-to-particle conversion, in which the oxidized product condenses to
form an aerosol.

II. ROLE OF SOLAR RADIATION IN ATMOSPHERIC CHEMISTRY
The time required for atmospheric chemical processes to occur is depen-
dent on chemical kinetics. Many of the air quality problems of major metro-
politan areas can develop in just a few days. Most gas-phase chemical
reactions in the atmosphere involve the collision of two or three molecules,
with subsequent rearrangement of their chemical bonds to form molecules
by combination of their atoms. Consider the simple case of a bimolecular
reaction of the following type:

B + C -* products (12-1)
Rate of reaction = Jt[B][C] (12-2)
where k = A e\p[-E a/RT] and k, the rate constant, is dependent on the
frequency factor (A), the temperature (T), the activation energy of the
reaction (£ a) and the ideal gas constant (K). The frequency factor, A, is of
the same order of magnitude for most gas reactions. For T = 298 K the
rate of reaction is strongly dependent on the activation energy £ a as shown
in Table 12-1 (4). When E a is > 30 kj/mol, the rates become very small,
limiting the overall rate of reaction. Table 12-2 contains the activation ener-
gies for bimolecular collisions of different molecular species. For the first
three reactions between molecular species, E a is > 100 kj, but for the last
three reactions £ a < 10 kj. The last three reactions involve the participation
of free radical or atomic species. The activation energies of reactions involv-
ing atomic or free radicals are very small, permitting chemical transforma-
tions on a short time scale.