Top: 3 mm A-array (black line) and B-array (thick blue line; Aalto et al. 2012a) spectra of Mrk 231 towards the central beam. Bottom: 2 mm D-array spectrum. The frequency of the HCN 2–1 v₂ = 1 line is indicated in the plot, but this line is not significantly detected. Both figures: the most prominent outflow bumps of HCN and HCO⁺ emission are also indicated. The frequency axes of the two spectra are corrected for the redshift of the source (z = 0.042170). The beam sizes can be found in Table 1.