To fully appreciate photochemical smog formation, one must first recognize that nitrogen is transformed between many different substances in the atmosphere. Automobile exhausts release nitrous oxide (NO) along with small amounts of nitrogen dioxide (NO2). These two substances form the starting materials for a vast array of chemical reactions that lead to products with higher oxidation states. In a straight forward process, nitrogen oxide (NO) is converted to nitrogen dioxide (NO2), nitrogen dioxide is transformed to nitrogen trioxide (NO3), and nitrogen trioxide in transformed into dinitrogen pentoxide (N2O5). Each of these four simple nitrogen oxides then reacts through a photochemical process, or direct physical contact, with atmospheric substances to form and an impressive list of biological irritants. The list of irritants includes: (1) alkyl nitrite; (2) peroxyalkyl nitrate; (3) alkyl nitrate; (4) peroxyacetyl nitrate; (5) nitrous acid; (6) peroxynitric acid; (7) nitric acid, and; (8) ammonium nitrate. These chemical transformations are shown graphically in Figure 40.1.
Figure 40.1 Chemical Transformations of Nitrogen Oxides in the Troposphere.
The substances that react with nitrogen oxides to form oxidants (the final product of photochemical smog) are trace hydrocarbons (from incomplete combustion) and the hydroxyl radical. The necessary ingredients for photochemical smog formation are (1) nitrogen oxides, (2) sunlight, and (3) hydrocarbons. Photochemical smog formation proceeds through a sequence of reactions, all involving a free radical mechanism. Free radicals are generated by photodissociation of nitrogen dioxide, a process that generates ozone and oxygen atoms. Oxygen atoms react with water to form hydroxyl radicals, which in turn react with hydrocarbons to form hydrocarbon radicals. Oxidation of hydrocarbons by the hydroxyl radical leads to the formation of aldehydes. The aldehydes are oxidized further to form aldehyde peroxides and aldehyde peroxyacids. These final substances are the compounds that are so irritating to sensitive biological tissues and cause most of the health problems associates with photochemical smog. Figure 40.3, shown below, is a diagram showing the changes in atmospheric concentrations for nitrogen oxide, nitrogen dioxide, non-methane hydrocarbons, aldehydes, and oxidants. Analysis of the data shown in Figure 40.3 led to the equations for photochemical smog formation (equations 1-8, shown below).
Figure 40.2 Cartoon illustrating the main components of Photochemical Smog formation.
Figure 40.3 Daily Chemical Transformations Occurring in the Formation of Photochemical Smog.
Generalized Reaction Scheme for Photochemical Smog Formation
This is a cyclic process that needs light and nitrogen oxides to generate oxygen atoms. Once formed, the oxygen atoms react with water to form hydroxyl radicals. The hydroxyl radicals then react with hydrocarbons according to equations 5 through 8 to form peroxyacyl nitrates (or PAN).
In summary, this is what happens in photochemical smog formation
1) Nitrogen oxides generate oxygen atoms
2) Oxygen atoms form hydroxyl radicals
3) Hydroxyl radicals generate hydrocarbon radicals
4) Hydrocarbon radicals form hydrocarbon peroxides
5) Hydrocarbon peroxides form aldehydes
6) Aldehydes form aldehyde peroxides
7) Aldehyde peroxides form peroxyacylnitrates
Original page created by Dr.Richard Foust, Professor of Chemistry and Environmental Science, Northern Arizona University