Observations of Ch4 and N2o from Ground-based Fourier Transform Infrared Spectrometer at Addis Ababa: Retrieval, Source Identification and Characterization of Retrieval Errors.
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Date
2020-09-16
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Addis Ababa University
Abstract
A ground-based high spectral resolution Fourier-transform infrared (FTIR) solar absorption
spectrometer has been operational at the Addis Ababa University (9.01◦ N, 38.76◦ E, 2443
m a.s.l) since May 2009 to obtain information on the total column abundances and vertical
distribution of various constituents in the atmosphere. Retrieval strategy and results
on information content and corresponding full error budget evaluation of methane and nitrous
oxide are presented. Optimal estimation retrieval technique is used to analyses the
observed spectra and provide regular vertical profiles, total and partial column measurements
of trace gases. The volume mixing ratio profiles of CH4 and N2O are simultaneously
retrieved from ground-based FTIR solar absorption spectra measured between May
2009 and March 2013. The ground-based FTIR CH4 and N2O at Addis Ababa, Ethiopia
were compared with Michelson Interferometer for Passive Atmospheric Sounding ( MIPAS)
(version V5R_CH4_220, V5R_CH4_224, V5R_N2O_220 and V5R_N2O_224), the
Microwave Limb Sounder on board of the Aura satellite (Aura/MLS) (MLS v3.3 of N2O
and CH4 derived from MLS v3.3 products of CO, N2O and H2O) and the Atmospheric Infrared
Sounder (AIRS) (CH4) to validate the instrument performance. Relative differences
range in vertical profiles of CH4 and N2O from the measurements between MIPAS (version
V5R_CH4_220, V5R_CH4_224, V5R_N2O_220 and V5R_N2O_224) and FTIR in
altitude ranges of 15-27 km are -15 % to -3.8 %, -4.8 % to 4.6 %, 2 % to 10 %, -7 % to 15
% respectively. Moreover, the mean differences in partial column of CH4 and N2O within
altitude ranges of 15 to 27 km are -11 % , -5.5 %, 2.39 % and 0.5 % respectively. Relative
differences in vertical profiles of CH4 and N2O from the measurements between MLS v3.3
and FTIR in altitude ranges of 17-27 km are less than 9 % and 15 % respectively. Relative
differences range in vertical profiles of CH4 from the measurements between AIRS and
FTIR in altitude ranges of 11-27 km is -4.8 % to 9 %. The bias obtained between FTIR
and the new data version of MIPAS_CH4_224 has been reduced in the altitude range of
15-27 km when the interfering gas H2O is fitted jointly with the target gas, CH4. Therefore,
the ground-based FTIR spectrometer can be recommended for CH4 and N2O satellite data
(MIPAS, MLS and AIRS) validation.
Uncertainties of tropical methane concentrations, retrieved from spectra recorded by the
MIPAS, MIPAS_CH4_220 are large at upper troposphere and lower stratosphere. The relation
of these uncertainties with water vapour variability is explored and the uncertainties
have been reduced in MIPAS_CH4_224. Coincident measurements of CH4 by MIPAS,
ground based FTIR and EOS MLS CH4 are used to estimate the standard uncertainty of MIPAS_CH4_220, MIPAS_CH4_224 and natural variability of H2O. Different methods such
as bias evaluation, differential method and correlation coefficient are employed to explore
the latitudinal variations of standard uncertainty of MIPAS_CH4_220 and natural variability
of water vapour as well as its reduction in MIPAS_CH4_224. The averaged bias between
MIPAS_CH4_220 and ground-based FTIR measurements in the altitude range 15-22 km
are 12.3 %, 8.9 % and -1.2 % for the tropics, mid-latitudes and high latitudes, respectively.
Whereas the averaged bias for MIPAS_CH4_224 is 3.4 %, -2.8 % and -2.4 %. The average
estimated uncertainties of MIPAS_CH4_220 methane are 5.9 %, 4.8 % and 4.7 % at altitude
ranges of 15 to 27 km in the tropics, mid-latitudes and high latitudes, respectively. On the
other hand, the average estimated uncertainties of MIPAS_CH4_224 methane were obtained
2.4 %, 1.4 % and 5.1 %. Moreover, the correlation coefficient between MIPAS_CH4_220
and MIPAS_N2O_220 are found to be 0.30, 0.98 and 0.96 in the lower stratosphere of the
tropics, mid and high latitudes respectively. However, the correlation coefficient between
MIPAS_CH4_224 and MIPAS_N2O_224 are 0.62, 0.80 and 0.66. The correlation coefficient
between the uncertainty of MIPAS_CH4_220, MIPAS_CH4_224 and the variability of
water vapour in the lower stratosphere are 0.70 and 0.51 on monthly temporal scales. It was
found that the relation between the uncertainty in methane and the variability of water vapor
has been reduced in the new data version. Therefore, the natural variability of water vapour
was a cause for high uncertainties in the MIPAS_CH4_220 measurements over tropics.
In summary, this study has derived column abundances and profiles of CH4 and N2O
from solar absorption measurements taken by FTIR at tropical high altitude site of Addis
Ababa, Ethiopia for a period that covers May 2009 to March 2013. The quality of the data
is assessed through comparison with data from MIPAS, MLS and AIRS sensors onboard
satellites. Moreover, the different retrieval errors and their sources are fully characterized to
allow the use of the data in further scientific studies as it represents unique environment of
tropical Africa, a region poorly investigated in the past.
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Keywords
Observations of Ch4, N2o from Ground-Based, Fourier Transform, Infrared Spectrometer, Addis Ababa, Retrieval, Source Identification, Characterization, Retrieval Errors