Why Halogen Chemistry?
Because halogen chemistry may influence the distribution of tropospheric ozone (O3), which 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.
Simplified schematic of reactive halogen chemical cycles in the troposphere. X = Cl, Br or I. After Wayne, R. P. et al., Halogen oxides: Radicals, sources and reservoirs in the laboratory and in the Atmosphere, Atmos. Environ., 29 , 2677 – 2884, 1995. (click to enlarge)
Until the 1970s 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. These reactions were first discussed in the 1970s 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. 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 bromine oxide (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 (kelp) in coastal regions initiates iodine radical chemistry that may substantially increase production of new particles. The impact of iodine emissions and chemistry over the open ocean remains speculative.
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