Chorine and Bromine Chemistry

Contact Lucien Froidevaux

Atmospheric measurements of chlorine and bromine compounds are important because chemically reactive chlorine and bromine species have been the major culprits involved in chemical ozone depletion over the past few decades.

The source of chlorine and bromine compounds in the upper atmosphere is the release and slow decomposition of long-lived gases emitted at the Earth's surface, from (primarily) industrial products since the 1950s. The most striking consequence of these emissions was the discovery of an ozone hole over Antarctica in the early 1980s.

Once the long-lived source gases (chlorofluorocarbons or CFCs, containing chlorine, and halons, containing bromine), used in refrigeration, aerosol propellants, foam material, solvents, dry cleaning, agriculture, or fire extinguishers, have been transported into the stratosphere (above about 15 km), they decompose, mainly from slow destruction by sunlight. This leads to reservoirs of chlorine (such as HCl, hydrogen chloride, measured by Aura MLS) and bromine (such as HBr), as well as more reactive ozone-depleting gases such as ClO (chlorine monoxide) and BrO (bromine monoxide), both measured by Aura MLS.

Global measurements of ClO were also carried out from 1991 to 2000 by the Upper Atmosphere Research Satellite (UARS) MLS experiment; other very useful data have been provided by aircraft and ground-based measurements. A more minor species (HOCl, hypochlorous acid) is also measured by Aura MLS. The abundance of these gases is measured in amounts typically below a few parts per billion, down to parts per trillion, but the reactive radicals (ClO, BrO) can destroy ozone rapidly (at the rate of 1 percent/day in the lower stratosphere, during polar winter/spring) via catalytic reactions whereby one atom of chlorine (or bromine) can lead to the destruction of more than 100000 molecules of ozone.

The total chlorine abundance at the Earth's surface has been decreasing since about 1993, and the sum of bromine abundances at the surface has decreased since the late 1990s. It is important to track the decrease (at a rate of about -0.5 to -1 %/yr) of total upper atmospheric chlorine (as seen in HCl near 50 km) as well as bromine; such measurements help to confirm the beneficial impact of the Montreal Protocol (which curtailed harmful surface emissions), and provide a database for comparison with atmospheric models and for improved projections of future change.

For more specifics about chlorine and bromine scientific publications tied to the MLS observations, see the MLS publications list.

For an overview (targeted to the general public) of the impact of chlorine and bromine species on ozone destruction, see (for example) https://www.theozonehole.com/ozonedestruction.htm. See also the MLS website information regarding Global Ozone and Polar Ozone.