The ratio of integrated line brightness of 3-mm to 7-mm lines of HNCO, HOCO⁺, HC₃N, SiO and ¹³CS expressed as ln (∫T_{B, 3 mm}dv/∫T_{B, 7 mm}dv), averaged over latitude, to show the variation with longitude. Note the variation in line ratio across the CMZ, most clearly shown in HNCO, HC₃N and SiO.