Spectroscopic Characterization of HSO and HOSO Intermediates Involved in SO Geoengineering.

Sulfur-containing radicals HSO and HOSO are key intermediates involved in stratospheric sulfur geoengineering by SO injection. The spectroscopic characterization and photochemistry of both radicals are crucial to understanding the chemical impact of SO chemistry in the stratosphere. On the basis of the efficient generation of HOSO by flash pyrolysis of gaseous sulfinic acid, CHFS(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 (XA) to the CA/DA and AA/BA states, respectively. Upon a 266 nm laser irradiation, the broad absorption band in the range 320-500 nm for HSO appears, and it corresponds to the combination of three excitations from the XA state to the first (AA), second (BA), and third (CA) excited states. Assignment of the UV-vis spectra is consistent with the photochemistry of HOSO and HSO as observed by matrix-isolation IR spectroscopy and also by the agreement with high-level calculations.