Photochemistry of Methylglyoxal in the Vapor Phase

The photolysis of methylglyoxal (CH 3 COCHO) in the presence of synthetic air was studied by laboratory experiments in a static reactor in order to determine its atmospheric lifetime. Quantum yields of the molecular photolysis products CO and HCHO were determined at 298 K as a function of wavelength (260 ≤ λ ≤ 440 nm) and pressure (30 ≤ P ≤ 900 Torr) using an optical resolution of 8.5 nm. The results can be distinguished with respect to both UV/VIS-absorption bands of methylglyoxal. For the short-wavelength band (260 ≤ λ ≤ 320 nm) photolysis quantum yields were found to be unity, independent of wavelength and pressure, consistent with a dissociation mechanism yielding peroxy radicals and CO according to CH 3 COCHO CH 3 COO 2 + HO 2 + CO (P1). For the long-wavelength band (380 ≤ λ ≤ 440 nm) two different processes were distinguished. The major process is photodissociation (P1) with quantum yields (φ D ) decreasing with increasing wavelength and pressure following the Stern−Volmer relationship: 1/φ D (λ) = 1/φ 0 (λ) + P / k ‘ D (λ) with φ 0 (λ) = (8.15 ± 0.5)(10 -9 ) [exp(7131 ± 267)] nm/λ and k ‘ D (λ) = (7.34 ± 0.1)(10 -9 Torr) [exp(8793 ± 300)] nm/λ. The minor process could be described by an H-atom transfer between electronically excited MG and ground-state MG, yielding the experimentally observed products [CH 3 COCHO]* + CH 3 COCHO CH 3 COO 2 + HCHO + CO + CH 3 COO 2 (R10). The atmospheric lifetime due to photolysis (τ phot ) was calculated using an atmospheric radiation model and the above expression, where φ 0 (λ) = 1 for λ < 380 nm, resulting in τ phot = (4.1 ± 0.7) h for a solar zenith angle of 50° at ground level. Therefore, photolysis can be identified as the most important degradation process of atmospheric methylglyoxal.