Observed SO₂/ SO ratio compared to the model prediction. The upper panel is adopted from Fig. 5 of Wakelam et al. (2004) (100 K, 10⁷ cm^-3), where different line types are from different grain component models. The lower panel is from our observed SO₂/ SO ratio by assuming (1) all substructures have similar density 10⁷ cm^-3 and (2) N_SO2/N_³⁴SO2 = 22 (Wilson & Rood 1994). The black solid line comes from the assumption that all substructures have a temperature close to 100 K, and the red dotted line comes from different temperatures derived in Sect. 4.1 (labelled in red).