Panela: relation between the abundances of C₂H and N₂ H⁺ of CO-selected cores and the H₂ column densities of their host PGCCs (N^d(H₂)). The abundances of C₂H and N₂ H⁺ of our sourceslocate in two different regions separated by the green line. The dashed red line shows the result of linear least-square fitting on data represented by red circles. Panel b: blue line shows number density distribution of log(N(N₂H⁺)/N(C₂H)) for our detected PGCC sources. The value of this parameter for typical sources are also shown, including well known starless cores TMC-1 (Hirota et al. 2004; Liszt et al. 2018), L1498 and CB246 (Padovani et al. 2009), massive clumps associated with infrared dark clouds I18151-1208 MM3 (abbreviated as I18151MM3) and G019.27+0.07 MM1 (abbreviated as G019MM1; Sakai et al. 2008, 2010), as well as the infrared dark cloud G028.23-00.19 (Sanhueza et al. 2013).