Rotation measure as a function of frequency for 3C 273. The RM values at lower than ALMA 224– 242 GHz frequency from this study are taken from the near-core region in VLBA observations from the literature (see text for references). Apart from the ALMA observation, the uncertainty in RM is a range of reported values, not an uncertainty in a single measurement. Similarly, the frequency range of the observations over the which the RM was estimated is shown by a solid line.