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
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
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.
AIRMAP is a UNH air quality and climate program unraveling fundamental
chemistry-climate connections in the rural atmosphere of New England,
directly downwind of major urban/industrialized emissions.