Fig. 1. Fourier-transform infrared spectroscopy (FT-IR) spectrum of ambient air between 600 and 4000 cm–1 (reverse scale increases with wavelength; measured with 20 m long path gas cell). The IR lines of the strong absorbers CO2 and H2O stand out. Sections with absorption lines of environmentally relevant trace gases are marked by circled numbers: 1: CO; 2: NH3; 3: N2O; 4: CH4.