Spectroscopic Characterization of HSO2• and HOSO• Intermediates Involved in SO2 Geoengineering.

Sulfur-containing radicals HSO2• and HOSO• are key intermediates involved in stratospheric sulfur geoengineering by SO2 injection. The spectroscopic characterization and photochemistry of both radicals are crucial to understanding the chemical impact of SO2 chemistry in the stratosphere. On the basis of the efficient generation of HOSO• by flash pyrolysis of gaseous sulfinic acid, CHF2S(O)OH, a strong absorption is observed at 270 nm along with a shoulder up to 350 nm for HOSO• isolated in low-temperature noble gas matrixes (Ar and Ne). These mainly arise from the excitations from the ground state (X2A) to the C2A/D2A and A2A/B2A states, respectively. Upon a 266 nm laser irradiation, the broad absorption band in the range 320-500 nm for HSO2• appears, and it corresponds to the combination of three excitations from the X2A state to the first (A2A), second (B2A), and third (C2A) excited states. Assignment of the UV-vis spectra is consistent with the photochemistry of HOSO• and HSO2• as observed by matrix-isolation IR spectroscopy and also by the agreement with high-level ab initio calculations.