Atmospheric Chemistry of 2-Ethyl Hexanal: Photochemistry and Oxidation in presence of NO2

JUAN C. FRAIRE; FABIO E. MALANCA *; GUSTAVO A. ARGÜELLO

Mendoza

Congreso; 21st IAPS Winter Conference; 2011

IAPS

Aldehydes are important atmospheric constituents as they are emited by a variety of sources (natural and antropogenic). They arise from incomplete fossil fuel combustion, are emitted by vegetation and are produced during biomass burning as well as atmospheric oxidation of volatile organic compounds (VOCs). They are important precursors of radicals and peroxyacyl nitrates (RC(O)OONO2). The atmospheric degradation of branched-chain aldehydes is controlled by photolysis and by reaction with OH radicals. In this work we present a kinetic and photochemical study of 2-ethyl hexanal. It is a volatile liquid used as solvent  and the main product of the atmospheric degradation of 2-ethyl hexanol (which is used in the manufacture of plastics, as an additive for lubricants, surfactants, etc.). Previous studies of the oxidation of this aldehyde proposes it as a new reagent to obtain 2-ethyl hexanoic acid. According to these industrial scale use, an increase of it concentration in the atmosphere is expected, and therefore it is important to know its atmospheric behavior. The rate constant (k) for reaction of 2-Ethal with Cl atoms was determined through a relative rate method with two reference compounds (i-pentane and cyclohexane). The mean value obtained is in agreement with those expected for other long chain aldehydes. The oxidation mechanism in the presence of NO2 was determined by using FTIR spectroscopy for the product analysis. The acyl radical forming channel was the most important in the oxidation mechanism leading to the formation of CO2, CO, peroxyacetyl, peroxypropionyl, and ethyl nitrate. The mechanism proposed involves a series of aldehydes: formaldehyde, acetaldehyde and propyonaldehyde, which react to form the nitrates and peroxynitrates. In order to determine the quantum yields of the molecule, the UV absorption cross section was recorded previously over the range 200-350 nm at 298 K. The 254 nm photolysis mechanism in 700 torr of O2 was determined through FTIR analysis of the products. Two primary photodissociation channels occur: the formation of radicals and the dissociation in closed shell molecules. The total quantum yields (0.51 ± 0.09) agrees with those reported in bibliography for similar non-linear carbonated chain aldehydes. The formation of CO, heptane, 3-heptanol and 3-heptanone, as products of reactions are in accordance with the proposed mechanism.