Chlorine monoxide, ozone depletion

The Airborne Submillimeter SIS Radiometer (ASUR), operated on-board the German research aircraft FALCON, measures thermal emission lines of stratospheric trace gases at submillimeter wavelength. Measurement campaigns with respect to ozone depletion in the Arctic winter stratosphere were carried out in yearly intervals from 1992-97 to investigate the distributions of the radical chlorine monoxide (CIO), the reservoir species hydrochloric acid (HC1), the chemically inert tracer nitrous oxide (N20), and ozone (O3). The high sensitivity of the receiver allowed to take spatially well resolved measurements inside, at the edge, and outside of the Arctic polar vortex. This paper focuses on the results obtained for CIO from... 

The immediate cause of the depletions of stratospheric ozone is thought to involve atoms of chlorine or simple compounds such as chlorine monoxide (CIO). However, these chemicals are thought to have an indirect origin through human activities, especially the emission of CFCs to the atmosphere. Once formed in the stratosphere by the degradation of a CFC molecule, a single chlorine atom is capable of destroying as many as 100,000 ozone molecules before it is removed from the upper atmosphere. 

Shindell, D.T., J.M. Reeves, L.K. Emmons, and R.L. de Zafra, Arctic chlorine monoxide observations during spring 1993 over Thule, Greenland, and implications for ozone depletion. J Geophys Res 99, 25,697, 1994. 

The chlorine not only destroys the ozone that is produced from reaction (2) in the Chapman mechanism, but it also inhibits the formation of ozone by sequestering oxygen atoms in the form of chlorine monoxide (CIO). Under certain meterological conditions the CIO is itself stored in the form of chlorine nitrate, to be re-released when the meterology changes. This is what makes the ozone depletion process so effective in forming the antarctic ozone hole, for example. 

Chlorine monoxide CIO, bromine monoxide BrO, and iodine monoxide lO are the main chain carrier of stratospheric and tropospheric halogen chain reactions. CIO is partially photolyzed only in the stratosphere, and the photolytic rates of BrO and lO are large also in the troposphere. Their photolysis rates have to be considered in the evaluation of ozone depleting chain reactions. 

Regeneration of atomic chlorine, essential for catalysis, does not take place by means of reaction 8 because of the very low concentration of atomic oxygen (reaction 1 does not happen in the dark ). However, reaction 14 with bromine monoxide does occur instead. Furthermore, dimerization (equation 18) of chlorine monoxide18 is followed by photolysis (equation 19) and dissociation (equation 20) of the chloroperoxy radicals which serve to release chlorine atoms19. Thus arises the now famous Antarctic ozone hole, a very significant depletion in ozone concentrations at altitudes of 15-20 km during the austral... 

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