Degradación en fase gaseosa de compuestos orgánicos oxigenados y/o pesticidas

Abstract

Oxygenated volatile organic compounds (OVOC´s) are currently being investigated as an environmental friendly alternative to replace solvents, pesticides and fuel additives. Such compounds are not only emitted by industrial activities, it is known that a large proportion of OVOC's is released also into the atmosphere from biogenic sources (BVOCs) like vegetation, biomass burning, oceans and volcanic emissions. Once in the atmosphere, these compounds can contribute to the formation of secondary organic aerosols (SOA), acid rain production, along with other environmental problems associated with the photochemical air pollution and persistent organic pollutant (POPs) formation. Consequently, detailed knowledge of their chemical behavior in the atmosphere is required to assess possible implications of their widespread application. In this aspect, the electrophilic addition reactions of atmospherically relevant species (OH radicals, Cl atoms and O3 molecules) over selected OVOC's were studied. For these purposes, a series of long chain unsaturated alcohols emitted on a large scale by plants, with potential use as biopesticides was selected. On the other hand, the product distribution of the OH-initiated degradation of a man-made unsaturated ester were studied from the experimental and theoretical point of view regarding the effect of different kind of atmospheres (presence and absence of NOx). The kinetics of the reactions and products studies were obtained by different experimental systems:  80 L collapsible Teflon reaction chamber with solid phase microextraction (SPME) and GC-FID/MS.  405 L multiple pass Pyrex reaction chamber with in situ FTIR detection.  1080 L multiple pass quartz-glass reaction chamber with in situ FTIR detection. The following rate coefficients of the reactions of a series of biogenic alcohols at room temperature and atmospheric pressure were obtained (in cm3 molecule-1 s-1): BVOC kOH × 1010 cm3 molecule-1 kCl × 1010 cm3 molecule-1 kO3 × 1017 cm3 molecule-1 (E)-2-hexen-1-ol (1.010.32) (3.490.82) (5.98 ± 0.73) (E)-3-hexen-1-ol (1.200.24) (3.420.79) (5.83 ± 0.86) (Z)-3-hexen-1-ol (1.400.33) (2.940.72) (6.04 ± 0.95) (Z)-3-hepten-1-ol (1.28±0.23) (3.800.86) (Z)-3-Octen-1-ol (1.49±0.35) (4.130.68) In addition, a product study on the reactions of OH radicals with (E)-3-hexen-1-ol and (Z)-3-hepten-1-ol in the absence of NOx were determined. Molar formation yields of (377) % for propanal and (333) % for butanal where obtained as main degradation products of (E)-3-hexen-1-ol and (Z)-3-hepten-1-ol respectively. Based on the experimental data obtained, possible reactions mechanisms were postulated and their atmospheric implications assessed. Furthermore, the reactivity of different kind of electrophiles toward the OVOCs studied was correlated with their ionization potentials to calculate rate coefficients of reactions that have not been measured yet from its experimental difficulties. On the other hand, the product distribution of the gas-phase reaction of OH radicals with methyl methacrylate (MMA) in the absence of NOx was also studied at 298 K and 1 atm of air. In the absence of NOx, methyl pyruvate was identified with a yield of 76 ± 13%. In addition, a detailed quantum chemical study of the degradation of MMA was performed by density functional theory (DFT) methods. This study allow us to propose that reaction between peroxy radical (RO2•) and hydroxyl radical (OH) became relevant at NOx- free environments. The results obtained in this work allowed us to establish the atmospheric impact of these reactions and detailed photooxidation mechanisms for the VOCs studied required to develop realistic atmospheric models to help environmental policy decisions.