Wavelength-dependent photolysis of methylglyoxal in the 290-440 nm region

Photodissociation of methylglyoxal (CH3COCHO; MGLY) in the 290-440 nm region has been investigated at 10 nm intervals by using dye laser photolysis in combination with cavity ring-down spectroscopy. Absorption cross sections of methylglyoxal were obtained at each wavelength studied. The HCO radical was a photodecomposition product. The formation yield of HCO, determined by monitoring its absorption at 613.8 nm, decreased with increasing MGLY pressure in the 1-10 Torr range. The HCO yields (extrapolated to zero MGLY pressure) were unity in the 320-360 nm region, consistent with the photodissociation mechanism CH3COCHO + hv --> CH3CO + HCO (P1). The HCO yields decreased at both the shorter (290-310 nm) and the longer (370-440 nm) wavelength end. The dependence of the HCO yields on nitrogen buffer gas pressure was also examined. The HCO radical yields were independent of nitrogen pressure (10-400 Torr) between 290 and 370 nm, but decreased with increasing nitrogen pressure in the 380-440 nm region. The coefficients for the dependence of the HCO radical yields on MGLY pressure and total pressure were given. Absorption cross sections and quantum yields thus obtained were used to calculate atmospheric photodissociation rate constants and lifetimes of MGLY as a function of zenith angle for cloudless conditions at sea level and at 760 Torr nitrogen pressure. Photolysis lifetimes of MGLY were on the order of 1.4-2.3 h for zenith angles in the 0-60 degrees range.