Photolysis and OH-Initiated oxidation of glycolaldehyde under atmospheric conditions

The photolysis and OH-initiated oxidation of glycolaldehyde (HOCH2CHO), which are relevant atmospheric processes, have been investigated under different conditions using complementary methods in three different laboratories. The UV absorption cross sections of glycolaldehyde, determined in two of the laboratories are in excellent agreement. The photolysis of glycolaldehyde in air has been investigated in a quartz cell with sunlamps and in the EUPHORE chamber irradiated by sunlight. The mean photolysis rate measured under solar radiation was (1.1 +/- 0.3) x 10(-5) s(-1) corresponding to a mean effective photolysis quantum yield of (1.3 +/- 0.3). The major products detected were HCHO and CO, whereas CH3OH was also observed with an initial yield around 10%. Evidence for OH production was found in both experiments using either OH scavenger or OH tracer species. Photolysis of glycolaldehyde was used as the OH source to measure the reaction rate constants of OH with a series of dienes by the relative method and to identify and quantify the oxidation products of the CH-initiated oxidation of 2-propanol. The different experiments suggest that OH is produced by the primary channel: HOCH2CHO + hv -> OH + CH2CHO (1). The rate constant of the OH reaction with glycolaldehyde has been measured at 298 K using the relative method: k(glyc) = (1.2 +/- 0.3) x 10(-11) cm(3) molecule(-1) s(-1). The product study of the OH-initiated oxidation of glycolaldehyde in air has been performed using both a FEP bag and the EUPHORE chamber. HCHO was observed to be the major product with a primary yield of around 65%. Glyoxal (CHOCHO) was also observed in EUPHORE with a primary yield of (22 +/- 6)%. This yield corresponds to the branching ratio (approximate to 20%) of the H-atom abstraction channel from the CH2 group in the OH + HOCH2CHO reaction, the major channel (approximate to 80%) being the H-atom abstraction from the carbonyl group. The data obtained in this work, especially the first determination of the photolysis rate of glycolaldehyde, under atmospheric conditions, indicate that the OH reaction and photolysis can compete as tropospheric sinks for glycolaldehyde. Since glycolaldehyde is a significant oxidation product of isoprene whereas the photolysis of glycolaldehyde is a significant source of methanol, isoprene might contribute a few percent of the global budget of methanol.