Halogen Chemistry

Chemical reactions involving halogen radicals significantly influence the composition of the Earth’s atmosphere. These reactions were first discussed in the 1970’s in connection with stratospheric ozone loss, especially within the polar vortices during spring. More recently it has been recognized that halogen chemistry may have important roles in the troposphere.

Tropospheric ozone plays a central role in regulating Earth’s environment. Photolysis of O₃ in the presence of water vapor produces the OH radical, which is the principal oxidant for many important atmospheric compounds including methane, other hydrocarbons, carbon monoxide, nitrogen oxides, and chlorofluorocarbon substitutes. At the Earth’s surface, high concentrations of O₃ can be toxic to humans and vegetation; it is one of the principal components of smog. In the middle and upper troposphere, O₃ is a major greenhouse gas. Until the 1970’s it was thought that tropospheric O₃ was mainly supplied by transport from the stratosphere, and removed by deposition involving reactions with organic materials at Earth’s surface. Research since then has shown that tropospheric O₃ is in fact largely controlled by chemical production and loss within the troposphere via processes that in many cases involve halogens. For example, the photochemical activation of Cl and Br during polar sunrise episodically enhances oxidation of hydrocarbons and destruction of O₃ in near-surface marine air. High concentrations of BrO and associated O₃ destruction have also been observed over salt flats near the Dead Sea and elsewhere. In contrast, reactions of Cl atoms with hydrocarbons can enhance O₃ production in polluted urban air.

Natural and anthropogenic aerosols also play key roles in chemical and radiative atmospheric processes. Aerosols influence gas phase chemistry by acting as sources or sinks of reactive species and by decreasing or increasing actinic flux. As such, they affect the oxidizing capacity of the atmosphere, which determines the chemical lifetimes of many trace gases and of aerosols themselves. Photolysis of iodine-containing organic compounds emitted by macroalgae in coastal regions initiates iodine radical chemistry that may substantially increase production of new particles. The impact of this chemistry over the open ocean remains speculative.