The study of air pollution begins with the study of chemicals that make up the air. These chemicals include molecules in the gas, liquid, or solid phases. Because the air contains so many different types of molecules, it is helpful to become familiar with important ones through the history of their discovery. Such a history also gives insight into characteristics of atmospheric chemicals and an understanding of how much our knowledge of air pollution today relies on the scientific achievements of alchemists, chemists, natural scientists, and physicists of the past. This chapter starts with some basic chemistry definitions, then proceeds to examine historical discoveries of chemicals of atmospheric importance. Finally, types of chemical reactions that occur in the atmosphere are identified, and chemical lifetimes are defined.

BASIC DEFINITIONS

Air is a mixture of gases and particles, both of which are made of atoms. In this section, atoms, elements, molecules, compounds, gases, and particles are defined.

Atoms, Elements, Molecules, and Compounds

In 1913, Niels Bohr (1885–1962), a Danish physicist, proposed that an atom consists of one or more negatively charged electrons in discrete circular orbits around a positively charged nucleus. Each electron carries a charge of –1 and a tiny mass. The nucleus consists of 1–92 protons and 0–146 neutrons. Protons have a net charge of +1 and a mass 1,836 times that of an electron. Neutrons have zero net charge and a mass 1,839 times that of an electron.