Intensity (upper panels) and Stokes Q/I (lower panels) profiles as a function of air wavelength, for the 0.8 Å–wide spectral ranges centered on the K I D₂ (left panels) and D₁ (right panels) lines. Calculations were carried out using the radiative transfer (RT) code introduced in Sect. 2, considering a two-term atomic model with HFS, accounting for partial frequency redistribution (PRD) effects, and in the absence of a magnetic field. For all the figures in this work, a line of sight (LOS) with μ = 0.2 is taken except where otherwise noted. The results of calculations considering the two most abundant isotopes of potassium (solid red curves) are compared to those in which only ³⁹K was considered (solid black curves). The atmospheric model C of Fontenla et al. (1993), hereafter FAL-C, was considered for both calculations. Throughout this work, the reference direction for positive Stokes Q is taken to be parallel to the limb.