Reference ATM model for the APEX site, considering 555 hPa and 273 K as physical parameters at the ground, and 0.5 (dotted black and red lines), 1.0 (solid black and red lines), and 2.0 mm (dashed black and red lines) of the PWVC. Note that the H₂O–H₂O (self wet) CIA is in practice negligible in these very dry conditions (only the curve for 2.0 mm PWVC is shown in a dashed green colour as the curves for 1.0 and 0.5 mm PWVC are below the bottom Y limit of the plot). The frequency ranges covered by the SEPIA and nFLASH receivers used in this work are plotted for reference, as are ALMA bands 5–9. The central frequencies of the six double side-band channels of the APEX WVR, on both sides of the 183.31 GHz H₂O line, are also plotted. The red and pink ClA curves are the ones under validation in this paper. Cyan (minor gases opacity), blue (O₂ lines opacity) and pink (dry CIA opacity) curves remain unchanged for all PWVC values.