Observed spectrum of SO₂ in the W band. The integration time per W sub-band was 4.4 h. Three upper panels: W band with different intensity scales. Middle panels: 8_1, 7−8_0, 8 transition of SO₂, ³⁴SO₂, ³³SO₂, SO₂ ν₂ and 2ν₂, and ³⁴SO₂ ν₂. The strongest hyperfine components of ³³SO₂ are clearly detected. Data for all lines observed during this experiment are given in Table C.1. Bottom left panel: difference, Δν(kHz), between the observed frequencies and those found in the CDMS (Müller et al. 2005), JPL (Pickett et al. 1998), and MADEX (Cernicharo 2012) catalogues. The colours correspond to different degrees of accuracy: |Δν(kHz)| ≤ 2 (red; 75 lines), 2 <  |Δν(kHz)| ≤ 4 (green; 53 lines), 4< |Δν(kHz)| ≤6 (blue; 17 lines), and 6 <  |Δν(kHz)| ≤ 12 (purple; 16 lines). Bottom right panels: uncertainty (in kHz) assigned by the Gaussian fitting algorithm of the CLASS/GILDAS program to all observed lines in the W band as a function of the S/N of the peak intensity.