Map of the 339 GHz continuum emission of PKS 1830−211 showing the two resolved lensed images of the blazar (left). Overview of the ArH⁺ absorption spectrum toward PKS 1830−211 SW (bottom right) and NE (top right) from the current observations. Expected positions of the main absorption feature are indicated for ³⁶ArH⁺, ³⁸ArH⁺, and ⁴⁰ArH⁺; the last is not detected. The noise increases toward lower frequencies because of the proximity of the atmospheric water line near 325.2 GHz.