Predicted line intensity ratios for selected lines of interest for the study of dark clouds. The assumed kinetic temperature and Δv= are 10 K and 1 km s^-1, respectively. Solid/dashed lines correspond to the case of collisional excitation by ortho/para H₂. The results correspond to N(SO₂) = 3  ×  10¹³ and 3  ×  10¹⁴ cm^-2. The blue thick curve corresponds to N(SO₂) = 3  ×  10¹⁴ cm^-2 but using the collisional rates SO₂/He of Green (1995).