Photolysis of glyoxal in air

The photolysis of glyoxal in synthetic air was investigated in a quartz cell at 298 K using three types of UV sources (TL/12 lamps (275–380 nm), TL/03 lamps (390–470 nm) and mercury lamps (254 nm)) and products were identified and quantitatively analyzed using long-path FTIR spectroscopy. For all light sources, the observed products were CO, HCHO and HCOOH. Absolute quantum yields were determined using Cl2 and Br2 as actinometers. Photolysis in the first absorption band of glyoxal, using TL/12 lamps, provided an overall quantum yield of ΦT = 0.97 ± 0.05, independent of total pressure ranging from 100 to 700 Torr air. The absolute quantum yields obtained with the TL/03 lamps, covering the second absorption band of glyoxal, showed dependency on total pressure, ranging from ΦT = 0.12 at 100 Torr to ΦT = 0.042 at 700 Torr, which can be expressed as a Stern–Volmer-type equation 1/ΦT = (6.80 + 251.8) × 10−4 × P (Torr).By combining the product yields with literature data, we deduced the detailed picture of glyoxal photolysis, including the dependency of the quantum yield of each particular channel: CHOCHO + hν → 2HCO (Φ1); CHOCHO + hν → H2 + 2CO (Φ2); CHOCHO + hν → H2CO + CO (Φ3) on the applied wavelength. The product quantum yields indicate that dissociation into two HCO radicals is the most important pathway under atmospheric conditions. The mean photolysis rate was measured under solar radiation in the EUPHORE outdoor chamber to be Jobs = 1.04 ± 0.10 × 10−4 s−1, corresponding to a mean effective quantum yield ϕeff = 0.035 ± 0.007. Although glyoxal has a very low effective quantum yield, photolysis remains an important removal path in the atmosphere.