PHOTOCHEMISTRY OF METHYLGLYOXAL IN THE VAPOR-PHASE

The photolysis of methylglyoxal (CH3COCHO) in the presence of syntheti c air was studied by laboratory experiments in a static reactor in ord er to determine its atmospheric Lifetime. Quantum yields of the molecu lar photolysis products CO and HCHO were determined at 298 K as a func tion of wavelength (260 less than or equal to lambda less than or equa l to 440 nm) and pressure (30 less than or equal to P less than or equ al to 900 Torr) using an optical resolution of 8.5 nm. The results can be distinguished with respect to both UV/VIS-absorption bands of meth ylglyoxal. For the short-wavelength band (260 less than or equal to la mbda 320 nm) photolysis quantum yields were found to be unity, indepen dent of wavelength and pressure, consistent with a dissociation mechan ism yielding peroxy radicals and CO according to CH3COCHO --> (hv) --> (O)(M)(2,) CH3COO2 + HO2 + CO (P1). For the long-wavelength band (380 less than or equal to lambda less than or equal to 440 nn) two differe nt processes were distinguished. The major process is photodissociatio n (P1) with quantum yields (phi D) decreasing with increasing waveleng th and pressure following the Stern-Volmer relationship: 1/phi D(lambd a) = 1/phi o(lambda) + P/k'D(lambda) with phi o(lambda) = (8.15 +/- 0. 5)(10-9) [exp(7131 +/- 267)] nm/gamma and k'D(gamma) = (7.34 +/- 0.1)( 10-9 Torr) [exp(8793 +/- 300)] nm/lambda. The minor process could be d escribed by an I-I-atom transfer between electronically excited MG and ground-state MG, yielding the experimentally observed products [CH3CO CHO]similar to + CH3COCHO o2,M CH3COO2 + HCHO + CO + CH3COO2(R10). The atmospheric lifetime due to photolysis (tau phot) was calculated usin g an atmospheric radiation model and the above expression, where phi o (lambda) = 1 for /Z ( 380 nm, resulting in tau phot = (4.1 +/- 0.7) h for a solar zenith angle of 50 degrees at ground level. Therefore, pho tolysis can be identified as the most important degradation process of atmospheric methylglyoxal.