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.