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.