Fourier Transform Infrared Spectroscopy (FTIR)

Main Scientific Goals

Earth observations are fundamental for investigating the processes driving climate change and, thus, for supporting decisions on climate change mitigation strategies. Atmospheric remote sounding from space and ground are essential components of this observational strategy. In this context, the Fourier transform infrared spectroscopy (FTIR) programme at the IARC was established with the main goals of long-term monitoring of atmospheric gas composition (e.g. ozone and ozone-related species, greenhouse gases, air quality gases,…) and the validation of satellite remote sensing measurements and climate models.

The FTIR programme at the IARC is the result of the close and long lasting collaboration of more than a decade between the IARC-AEMET and the IMK-ASF-KIT (Institute of Meteorology and Climate Research- Atmospheric Trace Gases and Remote Sensing, Karlsruhe Institute of Technology, Germany). The IMK-ASF has operated high-resolution ground-based FTIR systems for almost two decades and they are leading contributors in developing FTIR inversion algorithms and quality control of FTIR solar measurements. As a result of this collaboration, the FTIR experiment at IZO has contributed to the prestigious international networks NDACC and TCCON since 1999 and 2007, respectively. 

Measurement Programme

Ground-based high-resolution FTIR Spectrometry

In Fourier transform (FT) spectrometry, the source radiation (typically the Sun for atmospheric ground-based measurements) is modulated by an interferometer and all optical frequencies are recorded simultaneously in the measured interferogram. Then, a mathematical FT is used to retrieve the atmospheric absorption spectrum from the interferogram. The application of the FT measurement technique in the IR spectral region has a well-known capability for atmospheric gas monitoring, and therefore for climate research, since most atmospheric molecules interact with solar electromagnetic radiation in this spectral region.
A ground-based high-resolution FTIR experiment (HR FTIR in the following) for atmospheric composition monitoring has two main components (Figure 1): a precise solar tracker that captures the direct solar light beam and couples it into a high-resolution Michelson interferometer (IFS). IARC’s FTIR activities started in 1999 with a Bruker IFS 120M spectrometer, which was replaced by a Bruker IFS 120/5HR spectrometer in 2005 that has been operating until present.

Figure 1: The ground-based FTIR experiment at the IARC (scientific container, upper panel, hosting the Bruker IFS 120/5HR spectrometer, lower panel).

The IZO FTIR instrument records direct solar spectra in the middle- and near-infrared spectral region (MIR and NIR, respectively), using a set of different apertures, narrow-bandpass filters and detectors. Within the NDACC activities, solar measurements are acquired between 740 and 4250 cm-1 with a spectral resolution of 0.005 cm-1, while for TCCON retrievals the NIR spectra are taken between 3800 and 9000 cm-1 at a spectral resolution of 0.02 cm-1.
By evaluating the spectral signatures of vibrational-rotational transitions contained in these measured solar absorption infrared spectra, the FTIR technique allows total column amounts and low-resolution vertical profiles of different atmospheric trace gases to be retrieved with high precision. To do so, refined FTS retrieval strategies and remote sensing inversion principles are used. Routinely, the IARC HR FTIR have contributed to NDACC with C2H6, ClONO2, CO, CH4, COF2, HCl, HCN, HF, H2CO, HNO3, N2O, NO2, NO, O3 and OCS observations (total column amounts and VMR vertical profiles) since 1999, while total column-averaged abundances of CO2, N2O, CH4, HF, CO, H2O and HDO are measured within TCCON since 2007. An example of the IARC FTIR time series is displayed in Figure 2.

Ground-based low-resolution FTIR Spectrometry

In 2018 a portable and low-resolution FTIR spectrometer (LR FTIR in the following), the Bruker EM27/SUN, was acquired by the IARC within the Spanish infrastructure project (AEDM15-BE-3319) and installed at IZO in May 2018 (Figure 3). This instrument operates within the recently formed COCCON (Collaborative Carbon Column Observing Network).

Figure 3: Installation of the EM27/SUN FTIR at the IARC in May 2018