Peak wavelength vs. the FWHM of the absorption bands at (a) 7.24 μm, (b) 7.42 μm, (c) 7.67 μm, and (d) 7.02 μm in the JWST/MIRI-MRS spectra and the laboratory spectra of C₂H₅OH, CH₃CHO, and HCOO⁻ ices (labeled as NH₃:HCOOH mixtures). Values measured from laboratory spectra are denoted by multi-shape markers in different colors (see legend in each panel), and the number next to each marker indicates the temperature in Kelvin. The laboratory data are taken from J. Terwisscha van Scheltinga et al. (2018) for CH₃CHO and C₂H₅OH, M. G. Rachid et al. (2020) for CH₃COCH₃, and this work (Appendix D) for HCOO⁻. The legend convention for observational data points is the same as in Figure 3. In the laboratory spectra of some C₂H₅OH ice mixtures, the band around 7.24 μm is split into two, which are indicated by two data points linked with dotted lines. To avoid clutter, representative uncertainty bars in peak wavelength (0.0025 and 0.005 μm) and FWHM (0.0035, 0.007, and 0.01 μm) are indicated in the upper right corner of panel (a). The observational data points have Δλ_peak = 0.005 μm and ΔFWHM = 0.01 μm; the experimental data points have Δλ_peak = 0.0025 μm and ΔFWHM = 0.0035 μm for HCOO⁻ (panels (a), (b)) and CH₃COCH₃ ices (panel (d)), and Δλ_peak = 0.005 μm and ΔFWHM = 0.007 μm for C₂H₅OH (panel (a)), CH₃CHO (panels (b), (d)), and CH₄ ices (panel (c)). See Appendix C for the discussion of uncertainty estimation.