Remote sensing of tropospheric methane and isotopes of atmospheric carbon dioxide using Fourier Transform Spectrometry

Abstract

Due to the warming effect of atmospheric trace gases, such as CH4 and CO2 , it is important to monitor their variations and understand their sources and sinks. While an increasing trend in atmospheric concentrations of CH4 and CO2 is confirmed by many measurements, the sources and sinks are not completely understood. As one of the methods determining sinks and sources of traces gases, inverse modeling, its accuracy depends on assimilated observation and the simulation ability of atmospheric motion. For CH4 measurements, a method has been developed to determine tropospheric column-averaged mole fractions of CH4 from total columns of CH4 and N2O measured by solar absorption Fourier transform spectrometer (FTS). The motivation of this work is that tropospheric CH4 is more directly related to its sources and sinks compared to the total columns, which contain variabilities of stratospheric CH4 mostly originating from dynamic processes. Another reason is that the chemical transport model, in its current state, represents the troposphere better than the stratosphere. The method is applied to most of the FTS sites within the Total Carbon Column Observing Network. The measured tropospheric and stratospheric column-average CH 4 are used to assess performances of three European CH4 models in the troposphere and stratosphere separately. In addition, the isentropic mixing processes and evolutions of the polar vortex in the models are evaluated using equivalent length. It is found the southern surf zone is not developed to a real extent. Together with the southern surf zone, a region with both vertically and horizontally uniform CH4 occur between 450 and 850 K ( 18 and 30 km) in surf zone latitudes, which is absent in the models. The modeled polar vortex breaks too fast compared to the measurements. Measuring different isotopes of one species is another approach to improve knowledge on the sources and sinks of traces gases. This is explored for CO2 using ground-based solar FTS spectra, specifically the ratio 13 C/ 12 C in atmospheric CO2 . Retrieval from the FTS spectra is almost entirely determined by assumed molecular spectroscopy. The existing databases can not fulfill requirements for CO2 isotopes retrieval. An algorithm that inverts molecular spectroscopy from the ground-based spectra is developed. This algorithm is applied to H2O, CO2 and solar lines indicating its feasibility. While the inverted spectroscopy improves the accuracy of the column and profile retrieval of CO2 and H2O, the retrieved ratio 13C/12C is still beyond the required accuracy at a FTS site Bialystok.