Relative abundance of CH₃ OH ice in the CMZ sources, shown with respect to H₂O ice (top panel) and to the CO₂ shoulder component (bottom panel), as a function of reduced extinction $A_{K,0}^{\ast }$. The fractional abundance is defined as the column density of CH₃OH ice divided by the sum of N(solid CH₃OH) and either H₂O or CO₂ shoulder column densities, respectively. Circles show our measurements, and arrows mark 3σ upper limits. Only sources with significant detections (>2.5σ) of 3 μm H₂O ice are included. In the top panel, the thick red line shows the weighted mean abundance in four adaptive intervals, while the thick blue line shows the abundance measured in the envelope of SMM 4, a low-mass YSO in the Serpens cloud (Pontoppidan et al. 2004). The approximate length scale shown at the top corresponds to the projected distance from the centre of SMM 4.