The Absorption Spectrum and Absolute Absorption Cross Sections of Acetylperoxy Radicals, CH3C(O)O2 in the near IR

Acetylperoxy radicals (CH₃C(O)O₂) play an important role in the tropospheric chemistry. They are produced by the photooxidation of most emitted biogenic non-methane hydrocarbons. Recent studies show that the CH₃C(O)O₂ + HO₂ reaction, which is the most important tropospheric loss reaction of acetylperoxy radicals in regions that are dominated by biogenic emissions (low NO emissions), does not only lead to radical chain terminating products but can also regenerate OH. The competing secondary chemistry, e. g., the CH₃C(O)O₂ self-reaction, complicate kinetic measurements. The detection of acetylperoxy radicals in previous kinetic laboratory studies was mainly done in the UV region. However, the spectral overlap of different peroxy species in this region is prone to systematic errors in the quantitative detection. These complications can be avoided, if acetylperoxy radicals are detected by absorption in the near IR.

In our work, the near infrared CH₃C(O)O₂ spectrum was measured in the spectral ranges from 6094 cm^-1 to 6180 cm^-1 and 6420 cm^-1 to 6600 cm^-1. CH₃C(O)O₂ radicals were generated by pulsed photolysis of a acetaldehyde/Cl₂/O₂ mixture at 351 nm and were subsequently detected by time-resolved continuous-wave cavity ring-down spectroscopy (cw-CRDS). Experiments were done at 67 hPa in synthetic air and helium. The absorption cross sections of eight discrete absorption lines were determined relative to the absorption cross section of HO₂, which has previously been reported.