These data maps from Aura's Microwave Limb Sounder depict levels of
hydrogen chloride (top), chlorine monoxide (center), and ozone (bottom)
at an altitude of approximately 19 kilometers (490,000 feet) on selected
days during the 2004-05 Arctic winter. White contours demark the boundary
of the winter polar vortex.
The maps from December 23, 2004, illustrate vortex conditions shortly
before significant chemical ozone destruction began. By January 23, 2005,
chlorine is substantially converted from the "safe" form of hydrogen
chloride, which is depleted throughout the vortex, to the "unsafe" form of
chlorine monoxide, which is enhanced in the portions of the region that
receive sunlight at that time of year. Ozone increased over the month as
a result of dynamical effects, and chemical ozone destruction is just
beginning at this time. A brief period of intense cold a few days later
promotes further chlorine activation and consequent changes in hydrogen
chloride and chlorine monoxide levels on January 27, 2005. Peak chlorine
monoxide enhancement occurs in early February.
By February 24, 2005, chlorine deactivation is well underway, with
chlorine monoxide abundances dropping and hydrogen chloride abundances
rising. Almost all chlorine monoxide has been quenched by March 10, 2005.
The fact that hydrogen chloride has not fully rebounded to December
abundances suggests that some of that chemical was recovered into another
chlorine reservoir species.
Ozone maps for January 27, 2005, through March 10, 2005, show indications
of mixing of air from outside the polar vortex into it. Such occurrences
throughout this winter, especially in late February and early March,
complicate analyses, and detailed calculations are required to rigorously
disentangle chemical and dynamical effects and accurately diagnose
chemical ozone destruction.
Based on various analyses of Microwave Limb Sounder data, we estimate
that maximum local ozone loss of approximately 2 parts per million by
volume (approximately 60 percent) has taken place at this level during
the period from January 23, 2005, to March 10, 2005, with vortex-averaged
loss of approximately 1.5 parts per million by volume.