Column densities derived from total-N(NH₃) vs. the evolutionary sequence of the six W33 source (top left), total fractional NH₃ abundance, N(total-NH₃)/N(H₂), vs. the evolutionary sequence (top right), ortho-to-para ratio of NH₃ vs. the evolutionary sequence (bottom left), and gas temperature derived from NH₃ vs. the evolutionary sequence (bottom right). For the chosen positions, see Table 1. We note that the ortho-to-para ratio of W33 Main may be affected by systematic errors which cannot be quantified (see Sect. 4.5).