The atmospheric oxidation of aromatic compounds plays an important role in the generation of pollutants in urban atmospheres. Reaction rate constants are well established only for the initial oxidation step, which is predominantly reaction with the hydroxyl (OH) radical; in contrast, the rates for most of the subsequent steps have not been directly characterized in the laboratory. Reliable measurements of such rate constants will increase current understanding of the mechanism of formation of ozone and other pollutants in urban atmospheres, and will enable improved model predictions of photochemical smog production.
The formation of reaction intermediates will be investigated by means of a steady state turbulent flow reactor operating at about 100 Torr total pressure, and fitted with a chemical ionization mass spectrometer (CIMS) for the detection of reactants and products. The intermediates of interest are generated in the flow tube by mixing the parent aromatic compound, present at small concentrations in an inert carrier gas, with a radical such as OH generated with a microwave discharge. The CIMS detector consists of a chamber operated at pressures below those of the flow tube -- typically at 15 Torr, in which a portion of the flow tube effluents are mixed with a reactant ion such as SF6- or O2+, generated with a corona discharge or a radioactive polonium source. The product ions are extracted into a differentially pumped vacuum chamber containing a quadrupole mass analyzer; the ions are collimated and transported by means of an ion guide towards the entrance of the mass analyzer.
Our initial exploratory work consisted of developing the CIMS technique for the detection of organic radicals. We succeeded in monitoring the formation of the methyl-hydroxycyclohexadienyl radical (the toluene-OH adduct). We have generated this radical in the flow tube by mixing toluene with OH radicals produced in several methods. A microwave discharge of helium or argon containing a trace amount of H2 or F2 produces H-atoms or F-atoms, and the subsequent reaction of F + H2O, H + NO2 or H + O3 yields OH radicals at concentrations in the range from 1011 to 1012 molecule cm-3. To monitor the toluene-OH adduct, we found the most efficient positive ionization reagents to be O2+ and NO+.
We have also succeeded in monitoring the subsequent formation of aromatic peroxy radicals and nitrates using instead negative ion CIMS, with O2- as the ionization reagent. These compounds are produced by mixing the toluene-OH adduct in the flow tube with O2 and/or NO2, introduced through the moveable injector. We have been able to identify these intermediate species based solely on their mass, taking advantage of the fact that little if any fragmentation of the product ions takes place with the CIMS approach. In contrast, conventional electron impact ionization induces significant fragmentation of the parent ions, and hence with such an approach it would be difficult, if not impossible to differentiate species produced in the ionization process from those generated by neutral gas phase reactions. Clearly, the structure of the intermediates in question cannot be established unambiguously from the mass alone; however, it can be inferred by resorting to the results of theoretical investigations.
Our results indicate that the OH-toluene adduct can add two oxygen molecules. The most likely explanation is that addition of the first oxygen molecule generates a peroxy radical that isomerizes to produce a bicyclic radical intermediate, which in turn adds a second oxygen molecule to produce a bicyclic peroxy radical. Other species that we have identified as reaction products include cresols and nitrates.
The original plan is proceeding as expected.
Luisa T. Molina, Christophe Guimbaud