Intensity formation breakdown figure for the Ca II K and Mg II k spectral lines corresponding to spicule 1 from the model atmospheres at the location indicated by the vertical marker in Fig. 7. Panel a: formation of the Ca II K spectral line at t = 140 s (upflowing phase). Panel b: formation of the Ca II K spectral line at t = 370 s (downflowing phase) with a strong asymmetry toward the redward side of the line center. Panel c: formation of the Mg II k line for the upflowing phase. Panel d: formation of a single peaked Mg II k spectral line for the downflowing phase. Each sub-panel shows (dark corresponds to low values and orange corresponds to high values) the quantity indicated at the top left corner as a function of geometric height z and frequency from the line center in km s⁻¹. The τ_ν = 1 curve (white) and vertical velocity (yellow, positive is downflow) are overplotted in each sub-panel. The solid gray line (indicating zero Doppler shift) is shown in the first sub-panels for reference. In addition, the first sub-panels show the ionization fraction (dotted cyan line) of Ca II (panels a and b) and Mg II (panels c and d) as a function of z. The upper right sub-panels also contain the plasma temperature (dashed green) as a function of z in logarithmic scale, specified along the top. The lower right panel also shows the specific intensity profiles (cyan) as radiation temperature with units specified on the right-hand side.