Box 3.3 Concentrations and Lifetimes
The tropospheric concentration of a halon or a replacement compound is dependent on the rate of its emission into the atmosphere and its atmospheric lifetime. Measurements of halons 1211 and 1301 show that their current global mixing ratios (the ratio of volume density or concentration to the volume density of air) are about 2.5 and 2.0 parts per trillion by volume (pptv), respectively, and are currently increasing at about 2 and 6% per year, respectively.1,2,3 These rates of increase have slowed appreciably in recent years, consistent with the reduction in production and emission of these compounds. Despite such small concentrations, production of these compounds has been halted because of the capability of the bromine they contain to destroy ozone. Atmospheric models indicate that halons 1211 and 1301 are essentially nonreactive in the troposphere and are destroyed through photolysis in the stratosphere, resulting in atmospheric lifetimes of about 20 and 65 years, respectively.4,5 Because of their long atmospheric lifetimes, the destruction of these halons generally releases their bromine into the stratosphere, where the bromine has the greatest impact on ozone. The long atmospheric lifetimes also imply that halons already emitted will be releasing bromine into the stratosphere for several decades to come.