Infrastructure

The EM27/SUN is a portable ground-based Fourier Transform InfraRed (FTIR) spectrometer developed by KIT in cooperation with Bruker Optics GmbH to measure direct solar radiation in the near-infrared spectral range (4000–12000 cm⁻¹) with a resolution of 0.5 cm⁻¹. It is designed to observe total column abundances of key atmospheric gases such as CO₂, CH₄, CO, and H₂O. The compact system consists of a spectrometer and an integrated solar tracker equipped with a camera and image-processing software, allowing highly accurate solar pointing better than 0.01°. With a total weight of about 27 kg, the instrument is easily transportable and well suited for field deployments. Each measurement is completed in approximately 58 seconds using double-sided interferograms. Gas columns are retrieved in units of molecules per cm². To reduce sensitivity to surface pressure and other systematic effects, column-averaged dry-air mole fractions (Xg) are calculated by normalizing the gas columns with the dry-air column derived from O₂ measurements. Since the atmospheric O₂ mixing ratio is constant, this approach yields robust and comparable greenhouse gas observations.