Observations of Ch4 and N2o from Ground-based Fourier Transform Infrared Spectrometer at Addis Ababa: Retrieval, Source Identification and Characterization of Retrieval Errors.
| dc.contributor.advisor | Mengistu, Gizaw (Professor) | |
| dc.contributor.author | Yirdaw, Temesgen | |
| dc.date.accessioned | 2021-04-13T06:36:17Z | |
| dc.date.accessioned | 2023-11-09T11:26:32Z | |
| dc.date.available | 2021-04-13T06:36:17Z | |
| dc.date.available | 2023-11-09T11:26:32Z | |
| dc.date.issued | 2020-09-16 | |
| dc.description.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. | en_US |
| dc.identifier.uri | http://10.90.10.223:4000/handle/123456789/26099 | |
| dc.language.iso | en | en_US |
| dc.publisher | Addis Ababa University | en_US |
| dc.subject | Observations of Ch4 | en_US |
| dc.subject | N2o from Ground-Based | en_US |
| dc.subject | Fourier Transform | en_US |
| dc.subject | Infrared Spectrometer | en_US |
| dc.subject | Addis Ababa | en_US |
| dc.subject | Retrieval | en_US |
| dc.subject | Source Identification | en_US |
| dc.subject | Characterization | en_US |
| dc.subject | Retrieval Errors | en_US |
| dc.title | Observations of Ch4 and N2o from Ground-based Fourier Transform Infrared Spectrometer at Addis Ababa: Retrieval, Source Identification and Characterization of Retrieval Errors. | en_US |
| dc.type | Thesis | en_US |