Stratospheric Aerosol Climatology from Sage 11 and Observation of its Deriver, Carbonyl Sulfide, over Equatorial Africa
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Date
2010-07
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Addis Ababa University
Abstract
The issues of climate change and its causes have become agenda of di erent governmental
and non-governmental discussions. The major atmospheric components responsible for
climate change are particulate matters (aerosols) and trace gases (mainly anthropogenic
green house gases). The stratospheric aerosol climatology over Ethiopia from SAGE II
data and carbonyl sul de (OCS) measurement from FTIR spectrometer over Addis Ababa
are discussed in this thesis. The vertical pro les of aerosol extinction over Ethiopia were
analyzed and the result shows seasonal variation of stratospheric aerosol distribution over
Ethiopia. The total column stratospheric aerosol also shows seasonal variation with maximum
value attained during Spring and minimum during Summer. The annual mean
value of total column AOD is observed to be steadily increasing from 1984 to 2005, which
revealed the increasing trend of stratospheric aerosols of both ne and course mode particles.
Results from the monthly mean stratospheric aerosol size distribution show that
the number density is maximum for months of maximum AOD values as expected however
the observed trend is not signi cant. The Bruker IFS120M was installed at tropical
site located in Addis Ababa, Capital of Ethiopia (9o10 ; 38o760 ; 2443 m.a.s.l.) and has
been continuously operating since May, 2009. Nearly regular cell (HBr cell) measurements
were taken using laboratory source to monitor the alignment of the instrument
and its stability over time. The analysis of cell measurement using LINEFIT (LINEFIT12)
software has indicated the performance of the instrument is very good. The data
measured between May, 2009 to April, 2010 were analyzed and interpreted. The latest
version of PROFFIT (PROFFIT95) was used to retrieve vertical pro les and total
column values of carbonyl sul de from pressure dependent broadened spectra with the
help of Tikhonov-Philips regularization method. The OCS VMR observed at Addis site
reaches up to 0.6 parts per billion by volume (ppbv) and the total column amount reaches
up to 8 1019 molecules=cm2. The total column and vertical pro le observed in our
xi
site is greater than the same quantity reported at Izana, Spain. The tropospheric and
stratospheric mass of OCS is also estimated in gram and we found it to be 4.6056 Tg and
0.3427 Tg respectively which sum up to yield of 4.9484 Tg of total mass. The atmopheric
life time of OCS in our region is estimated to be 4.1236 years. Because of its longer
life time in the atmosphere (as compared to other sulfur containing compounds), it is
transported to the stratosphere through the tropical tropopause by di erent mechanisms
and latter converted to form stratospheric background sulfate aerosol layer, which in turn
determines the Earth's radiation budget. Random and systematic Error contribution
to retrieval were analyzed and the result shows that the total error contribution due to
atmospheric and auxiliary parameters is less than 10%
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Keywords
Carbonyl Sulfide, over Equatorial Africa