Page 200 - Fundamentals of Air Pollution

166 12. Atmospheric Chemistry
form oxygenated products such as aldehydes, nitrogen dioxide (NO 2), and
sulfuric acid (H 2SO 4). These oxygenated species become the secondary
products formed in the atmosphere from the primary emissions of anthro-
pogenic or natural sources (Fig. 12-1).
Solar radiation influences the chemical processes in the atmosphere by
interacting with molecules that act as photoacceptors. Free radicals are
formed by the photodissociation of certain types of molecules. Free radicals
are neutral fragments of stable molecules and are very reactive. Examples
are O, atomic oxygen; H, atomic hydrogen; OH, the hydroxyl radical;
and HO 2, the hydroperoxy radical. In areas with photochemical smog, the
principal photoacceptors are aldehydes, NO 2, nitrous acid (HNO 2), and
ozone. The photodissociation process is energy dependent, and only pho-
tons with sufficient energy are capable of causing photodissociation. The
wavelength dependence of solar radiation is discussed in Chapter 17.
The reactivity of chemical compounds will differ because of their structure
and molecular weight. Hydrocarbon compounds have been ranked ac-
cording to their rate of reaction with various types of oxidizing species
such as OH, NO 3, and O 3 (1). The role of hydrocarbons, along with oxides
of nitrogen, in the formation of ozone is very complex. Ozone formation
is a function of the mixture of hydrocarbons present and the concentration
of NO X, [NOJ (= [NO] + [NO 2]). The concept of an incremental reactivity
scale permits accessing the increment of ozone formation per incremental
change in a single hydrocarbon component (2). Incremental reactivity is
determined by calculating the ozone formation potential in a baseline sce-
nario using a simple mixture of hydrocarbons representing an urban atmo-
sphere. Then for each hydrocarbon species of interest, the ozone formation
is recalculated with incremental hydrocarbons added to the mixture. From
this approach, the A[O 3]/A[HC] values represent the impact of a specific
hydrocarbon on urban photochemical smog formation.
The vapor pressure of a compound is important in determining the upper
limit of its concentration in the atmosphere. High vapor pressures will
permit higher concentrations than low vapor pressures. Examples of or-
ganic compounds are methane and benzo[fl]pyrene. Methane, with a rela-
tively high vapor pressure, is always present as a gas in the atmosphere;
in contrast, benzo[fl]pyrene, with a relatively low vapor pressure, is ad-

Fig. 12-1. Precursor-product relationship of atmospheric chemical reactions.