Bond energies of the molecules H2O, SO2, H2O2, and HCl to various atmospheric negative ions

Thermochemical data for some negative cluster ions of importance in atmospheric ion chemistry have been determined from equilibrium constant measurements in a flowing afterglow apparatus. The ions studied have the structure X−⋅M with X−=Cl−, NO−2, NO−3, and HSO−4 and M=H2O, SO2, H2O2, and HCl. The bond strengths of these cluster ions were found to be in the range ΔH=50 to 102 kJ/mol. ΔH increases as a function of the ligand molecules in the following order (H2O)<(SO2)≲(H2O2)≲(HCl). The equilibrium constants for the association of H2O2 and HCl to negative ions are three to four orders of magnitude higher than those for H2O association at temperatures from 250 to 300 K. This implies that the stratospheric trace gases H2O2 and HCl can cluster to negative ions in the presence of the much higher abundance of water vapor in the stratospheric environment. Negative cluster ions containing H2O2 and HCl molecules may therefore exist in measurable quantities in the stratosphere.