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Introduction
Retrieval Data
References Links
Contact
Introduction:
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Fig. 1: IO slant columns on the Southern Hemisphere averaged
over 3 months (Sep-Nov 2005). Highest values are found in the Weddell sea and the
shelf ice areas.
Halogens (chlorine, bromine, iodine,…)
play an important role in our atmosphere in different respects. Well known is
the effect of chlorine species in the stratosphere, where they are responsible
for the formation of the ozone hole. But also in the lower layers, in the
troposphere, halogens lead to ozone destruction. Especially in the polar
springtime periods, bromine species have been identified as a cause for local
ozone depletion events, where nearly all ground-level ozone can be destroyed
within short time. In addition to chlorine and bromine, also iodine influences
atmospheric chemistry. It is very effective in ozone destruction and is
additionally involved in the formation of fine particles in cases of high
concentrations of iodine oxides. It is possible to detect iodine monoxide (IO)
by means of the DOAS (Differential Optical Absorption Spectroscopy) technique.
This has been done before using ground-based and balloon-borne instruments.
Here, measurements of IO from the SCIAMACHY satellite instrument are discussed.
The Antarctic is an interesting place
considering halogen chemistry. Here, the appearance of extended bromine oxide
(BrO) clouds has been observed from satellite, and occasions with extremely low
ozone concentrations close to the ground have been reported. Many open questions are
still connected to the subject of halogen chemistry, one of them being the
importance of iodine for ozone depletion and particle formation. In addition, the influence of iodine species on triggering and enhancement of
bromine release is not yet fully understood. Certain chemical reactions in the liquid phase, for example on
aerosol surfaces, can amplify the activation of gaseous bromine, and also cross
reactions between IO and BrO can lead to additional release of atomic bromine
and iodine, thereby increasing the chain lengths of the catalytic cycles.
Although the IO absorption signal is very small, it can be
retrieved from satellite measurements of backscattered solar light using the
DOAS method. As an example, the observed IO distribution for September to
November 2005 is shown in Fig. 1. More on the retrieval and
the detection limit can be found below.
The seasonal cycle extracted from the
satellite data for Halley Station (including all measurements within a square of
500 km side length enclosing Halley Station) is shown in Fig. 2. A maximum of IO
slant columns with values around 7-8x1012molec/cm2
is observed in Antarctic springtime (around October), still positive but lower
values during the summer period and a second slightly less pronounced maximum in
autumn. During the winter, hardly any measurements from satellite are available
for Halley Station due to darkness, but in the dataset the IO amounts decrease
towards winter. High IO values are not expected for the winter season as
sunlight would be needed for the photolysis of the precursor substances. The
variation in IO columns is a product of the seasonal cycle of the precursor
substances and available sunlight necessary for the photolysis. This seasonal
cycle is repeated in each of the three analysed years from 2004 to 2006.
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Fig 2: Seasonal variation of IO. The
figure shows SCIAMACHY IO slant columns within 500 km of Halley Station for the
three years from 2004-2006; daily values in blue and a two week average in
black. This seasonal cycle matches well with ground based long path DOAS
results, cp. (Saiz-Lopez et al., 2007).
Retrieval and Detection Limits:
IO has very strong differential
absorption structures in the blue wavelength region as can be seen in
Fig. 3
below. The maximum absorption cross section, depending on the resolution of the
spectrometer, lies around
smax~2.8x10-17cm2/molec.
Using a fitting window for the DOAS retrieval of 416–430 nm, two of the
absorption peaks are included.
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Fig. 3: Absorption cross section spectrum of IO (Spietz
et al., 2005) with an original FWHM of 0.07 nm, convoluted with the
SCIAMACHY slit function. Eight excitation bands (six of them visible in the
figure) from the ground state to the first excited state with different
vibrational levels (A23/2ß
X23/2) can be
identified. |
The detection limit of the instrument
determines in which cases the IO can be retrieved. Apart from possible
systematic effects or errors, this detection limit depends on the
signal-to-noise ratio (S/N) of the satellite measurement. The S/N again depends
on atmospheric and measurement related parameters such as integration time, the
amount of spatial or temporal averaging, and ground spectral reflectance. This
is again also dependant on the specific wavelength region under consideration.
For the case of IO, the best detection can be expected over bright surfaces,
such as the Antarctic for example.
For IO
slant columns and a surface spectral reflectance of 90%, the detection limit is given by SClim=
7x1012molec/cm2
for a single measurement. For a surface spectral
reflectance of 5% instead of 90%, the IO slant column detection limit for a
single measurement corresponds to 2x1013molec/cm2.
For 90% surface spectral reflectance and the spatially averaged ground scene of
60x120 km2,
used in this work, the limit is reduced to 3x1012molec/cm2.
In the monthly or even longer time average, the overall detection limit can
further decrease.
In the retrieval of IO by the DOAS
technique, other effects and absorptions have to be taken into account. The
current fit includes several effects such as the absorption of NO2, O3,
the Ring effect, and a quadratic polynomial accounting for the broad band
structures in the spectrum (resulting from instrumental characteristics and
elastic scattering). The results of the fitting procedure are the slant columns
of the respective trace gas, which can be converted into vertical columns
using appropriate airmass factors. As little is known on the vertical profile of
IO, so far only slant columns are shown.
For more details on the retrieval and the
results see the paper of
Schönhardt et al., 2007.
Data:
If you are interested in
more SCIAMACHY IO data, please contact Anja
Schönhardt.
References:
-
Schönhardt, A., Richter, A., Wittrock, F., Kirk, H., Oetjen, H., Roscoe, H.
K. and Burrows, J. P.,
Observations of iodine monoxide (IO) columns from satellite, Atmos.
Chem. Phys., 8, 637-653, 2008
-
First observations of iodine oxide columns from satellite, A. Schönhardt et al., EGU meeting, April 2007
-
Saiz-Lopez, A., Mahajan, A. S., Salmon, R. A., Bauguitte,
S. J.-B., Jones, A. E., Roscoe, H. K., and J. M. C. Plane: Boundary layer
halogens in coastal Antarctica, Science, 317, 348,
DOI:10.1126/science.1141408, 2007.
-
Spietz, P., Gomez Martin, J. C., and Burrows, J. P.:
Spectroscopic studies of the I2/O3 photochemistry, Part 2: Improved Spectra of Iodine
Oxides and Analysis of the IO Absorption Spectrum, J. Photochem. Photobiol. A,
176, 50-67, doi:10.1016/j.jphotochem.2005.08.023, 2005.
Links:
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More information on SCIAMACHY can be found
here.
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Many SCIAMACHY related links can be found on the
German SCIAMACHY page.
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More on SCIAMACHY BrO measurements can be found
here.
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For information on the satellite and other ENVISAT instruments check
the ESA ENVISAT page
Contact:
If you are interested in more information or SCIAMACHY IO data, please contact
Anja Schönhardt.
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