W49N. (Upper) WBS spectra of 1₀–0₀ ortho-NH₃ and 2₁–1₁ para-NH₃ normalised to SSB continuum. (Lower) The optical depth ratios are shown for absorptions larger than 3σ as a function of LSR velocity. The column density ratios (OPR), estimated with RADEX, are given by the right hand y-axis. The horisontal dashed line marks an ortho-to-para optical depth ratio of 4.2 corresponding to a column density ratio of unity.