RESUMO
The photolysis and OH-initiated oxidation of glycolaldehyde (HOCH(2)CHO), 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 CH(3)OH 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 OH-initiated oxidation of 2-propanol. The different experiments suggest that OH is produced by the primary channel: HOCH(2)CHO + hnu --> OH + CH(2)CHO (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 ( approximately 20%) of the H-atom abstraction channel from the CH(2) group in the OH + HOCH(2)CHO reaction, the major channel ( approximately 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.
RESUMO
The OH-initiated oxidation of two VOCs directly emitted to the atmosphere through their use as industrial solvents, hexylene glycol (HG, (CH3)2C(OH)CH2CH(OH)CH3) and diacetone alcohol (DA, (CH3)2C(OH)CH2C(O)CH3), has been studied in two photoreactors: a 140 L Teflon bag irradiated by lamps at CNRS-Orleans and the 200 m3 European photoreactor, EUPHORE, irradiated by sunlight. The rate constants for the reactions of HG and DA with OH radicals have been determined at (298 +/- 3) K using a relative rate method: k(HG) = (1.5 +/- 0.4) x 10(-11) and k(DA) = (3.6 +/- 0.6) x 10(-12) cm(3) molecule(-1) s(-1) and have been found in good agreement with estimations from structure-reactivity relationships. The study at Orleans and EUPHORE of the OH-initiated oxidation of hexylene glycol showed the formation of diacetone alcohol, acetone, and PAN as the principal products. The branching ratio of the H-atom abstraction from the > CH- group of HG has been estimated to be (47 +/- 4)% corresponding to the measured formation yield of DA. The formation yields of acetone and PAN lead to the determination of a lower limit of (33 +/- 7)% for the branching ratio of the H-atom abstraction of the -CH2- group of HG. For diacetone alcohol, studies at EUPHORE have shown negligible photolysis under atmospheric conditions (J < 5 x 10(-6) s(-1)) and the formation of acetone, PAN, HCHO, and CO in the OH-initiated oxidation experiments. The molar yield of acetone, close to 100%, corresponds to the branching ratio of the H-atom abstraction from the -CH2- group of DA. The present study has allowed the identification of the nature and the fate of the oxy radicals as intermediates in the oxidation mechanism of both HG and DA. The atmospheric implication of these results, especially the ozone formation potential of HG and DA, is discussed.