Left: N(ArH⁺) vs. N(H). Right: X(ArH⁺) = N(ArH⁺)∕N(Hi) vs. N(H). The data points include column density values derived towards the MC and LOS features from our sample of sources indicated by filled grey and black triangles, as well as those from Schilke et al. (2014) marked by unfilled red and blue diamonds, respectively. The solid black line displays the power-law relation to the combined data-set, , with the grey shaded region representing the 1σ-level of the weighted fit. The dashed cyan and dashed-dotted pink curves represent the fits to only the LOS and only the MC components, respectively.