Most studies on SO2 capture using ionic liquids (ILs) have been carried out under mild conditions (e.g., about 1 bar of SO2 and 20-40 °C). However, these ILs only exhibit a fraction of their original SO2 uptake under harsher conditions, i.e., much lower SO2 partial pressure and evidently higher temperature (e.g., 0.2 mbar of SO2 and ≥110 °C). In this work, we screen 127 reported ILs for multi-molar SO2 absorption under the harsher conditions, covering nearly all the chemical space explored of the SO2 capturing IL. A unique cooperative anion-SO2 interaction mode with two forms (the insertion adduct and the ringed adduct) has been identified for branched anions, where SO2 acts as both acid and base to attack different reactive sites on anions. The simulated Langmuir isotherms further help screen out potential sorbents (amino-based branched singly charged anions with an Al atom as the center and carboxyl-based divalent anions) with both high overall capacity and high per-cycle absorption capacity. The energies of the reacting orbitals of the anions are linearly correlated to their SO2 binding energies. This work provides new insights for the design of task-specific ILs under harsh conditions.