Upper panel: observed spectral line profiles of the H³⁵Cl and H³⁷Cl J = 1–0 transitions towards G291.272−00.714, G009.621+00.194, G029.954−00.016, and HGAL 285.264−00.049, along with their XCLASS fitted models (red curves). The dashed grey lines mark the systemic velocities, while the dotted grey lines mark the fitted continuum temperatures around the presented lines for each source. Bottom panel: the hyperfine structure splitting deconvolved column density profiles. The derived dN/dυ distributions with the corresponding 1σ errors are displayed by the solid blue curves and blue shaded regions, respectively.