'Double Record Breaker' 2006
Antarctic Ozone Hole seen by MLS
The stratospheric ozone layer shields life on Earth from harmful ultraviolet
radiation emitted by the sun. Certain chemicals in the stratosphere (~15 - 50 km
altitude) can act to deplete ozone (O3), most notably in the polar
winter/spring, giving rise to the Antarctic 'ozone hole'. The ozone hole is the
result of a series of complex interactions between chemistry, cloud formation,
winds, sunlight and temperature.
Chemical depletion of ozone occurs mainly through reactions involving chlorine (Cl).
Most of the chlorine in the stratosphere originates from man-made
chlorofluorocarbon (CFC) gases released into the atmosphere. CFCs are chemically
very stable so, unlike most chemical pollutants, they can survive the several
year journey that air takes to travel from the lower atmosphere into the
stratosphere. Once in the stratosphere, the CFCs are eventually broken down by
the strong sunlight at these altitudes, and the chlorine atoms they contain form
the molecule HCl, itself also fairly stable. While HCl is a 'safe' (i.e., non
ozone depleting) form of chlorine, HCl molecules are occasionally broken down -
by sunlight or reaction with other chemicals - into other forms such as the
ozone depleting ClO.
Prolonged low temperatures in
September 2006 increased by the longevity of 'active chlorine', leading to a
record area and depth of the ozone hole.
Observations from the Microwave Limb Sounder (MLS) on Aura (Figure 1) show that
the amount of HCl in the upper atmosphere (~55 km) is slowly decreasing,
reflecting the cuts in emissions of CFCs following international agreements. The
long lifetime of CFCs, however, means it will be many decades before the amount
of chlorine in the Earth's stratosphere will return to pre-industrial levels.
The 2006 Antarctic ozone hole was a 'double record breaker'. Column ozone (the
total amount of ozone overhead at a given point on Earth) reached record low
values, with low ozone seen over an area larger than that in any earlier year.
In addition to measuring HCl and ozone (O3) itself, the Microwave Limb Sounder
(MLS) instrument on Aura measures the abundances of other chemicals that play
key roles in the ozone hole process.
Each winter, air in the stratosphere over Antarctica is mostly trapped in a
large (continent sized) pool of rotating air (the 'polar vortex'). This
trapping, combined with the absence of sunlight, leads to significant cooling.
When temperatures fall below 195 K (-78°C) clouds form from a combination of
water (H-2O) and nitric acid (HNO3). Figure 2 shows maps of observations from
MLS at around 20 km altitude on two days in 2005. May 20 is early in the winter,
before these 'polar stratospheric clouds' (PSCs) have formed. By October 1,
2005, a significant reduction is seen in both HNO3 and H2O, implying that these
gases have been converted into PSC form. Reactions on the surfaces of the PSC
particles convert the bulk of the chlorine from HCl into other species,
including the ozone-depleting ClO. Figure 2 shows a notable decrease in HCl
observed between the two dates, with a corresponding increase in ClO. Finally, a
dramatic decrease in ozone is seen, in large part due to the chemical
interactions between ozone and ClO.
During the 2006 Antarctic winter, stratospheric temperatures remained cold for
longer than in previous years. This prolonged the duration of the PSCs and in
turn the period of chlorine 'activation', increasing the amount of chemical
ozone loss. For comparison, by the same October 1 date in 2005, the amount of
ClO was already on the decline, with significantly less than seen in 2006.
Monitoring the Decrease in Stratospheric Chlorine
- MLS measures Hydrogen Chloride
(HCI), the primary reservoir for stratospheric chlorine
- Decreasing upper stratospheric HCI reflects
cuts in Chlorofluorocarbon (CFC) emissions
- MLS continues the record of
HCI observations established by HALOE
- MLS global coverage gives
excellent precision for global HCI mean
- The -0.8% year HCI decrease can be seen
at ~seasonal resolution
- Offsets between instruments
are within expected accuracies
- MLS observes less
month-to-month variablity than HALOE
Observations of Hydrogen
Chloride (HCI) at ~53 km in the atmosphere. Purple points are data from the
UARS HALOE instrument, while red are from Aura MLS. Data from the ATMOS and
ACE instruments are shown in cyan and green respectively. In all cases the
error bars indicate the estimated accuracy of the measurements. The solid and
dashed lines indicate the expected abundances of HCl based on chlorine
emissions and models of atmospheric transport.
Feature Released 2.22.2007 NASA Goddard
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