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operation on 8th April 2012. During this time of almost ten years, it orbited the
Earth every 100.6 min in a repeat interval of 35 days (corresponding to 501 orbits)
with a descending node at 10:00 a.m. local time.
In the limb geometry, the SCIAMACHY instrument scanned the atmosphere
from 0 km to about 93 km altitude with tangent height steps of 3.3 km, whereby
vertical pro
les of the scattered solar radiation between 214 and 2,386 nm (spectral
sampling between 0.11 and 0.74 nm) were measured (e.g., Gottwald and Bo-
vensmann
2011
). The advantage of this geometry is a high vertical resolution
compared to the nadir geometry and a near-global coverage within a few days that
cannot be achieved by the occultation geometry.
In order to retrieve the aerosol extinction coef
cients, the vertical radiation
pro
les at 470 and 750 nm (SCIAMACHY Level 1 data version 7.04) were used.
Each measurement vector is generated in a two-step approach. The
first step is a
tangent height normalisation (reference tangent height is 35 km), and the second
step is the so-called wavelength pairing, where the normalised limb radiance pro
le
of the longer wavelength, i.e. 750 nm, is divided by the one of the shorter wave-
length, i.e. 470 nm, yielding the measurement vector (Bourassa et al.
2007
). The
es
the Mie scattering signal compared to the Rayleigh scattering signal, which is
useful, as aerosols are Mie scatterers. Subsequently, the underlying inverse problem
is solved employing the optimal estimation method (Rodgers
2000
).
For the aerosol parametrisation, extinction coef
first step reduces the in
fl
uence of the ground albedo and the second step ampli
les of a climatological
model (ECSTRA, Fussen and Bingen
1999
) were used and the phase function was
calculated with a standard Mie code. To perform forward simulations, i.e., to cal-
culate the scattered solar radiation and weighting functions of various atmospheric
parameters in the spectral range of the measurements, the radiative transfer model
SCIATRAN developed at the University of Bremen (Rozanov et al.
2014
) was
used.
This work is based on aerosol extinction coef
cient pro
les in units of 1/km at
750 nm of the retrieval version V1.1 (Ernst et al.
2012
; Ernst
2013
). The data is on a
monthly-mean, 1-km-altitude, 5
cient pro
-longitude grid with a vertical
resolution of about 4 km, and horizontal resolution of about 240 km across the
fl
°
-latitude, and 5
°
flight direction, and about 400 km along
fl
flight direction.
3 Results and Discussion
Figure
1
shows a latitude-time cross section of the zonal and monthly mean aerosol
extinction coef
cients for 750 nm at about 30 km altitude. The enhanced aerosol
extinction coef
cients in the tropics and subtropics compared to the mid and high-
latitudes result from tropical upwelling of the residual mean meridional circulation.
In addition, Fig.
1
indicates that the tropical aerosol extinction varies with an almost
two-year period. Maximum extinction coef
cients are found around January 2003,
2005, 2007, 2009, and with a slight delay and a weaker extent in 2011. This
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