Goals

• To identify and monitor in the laboratory various intermediate species formed in the atmospheric oxidation of organic compounds, in order to improve the current understanding of the mechanism of their atmospheric degradation process.
  
• To measure directly rate constants for various elementary reaction steps of atmospheric importance involving such intermediate species.

Background

The atmospheric oxidation of organic compounds plays an important role in the generation of pollutants in urban atmospheres. Except for the smallest hydrocarbons, 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.

Method of Approach and Facilities

The formation of reaction intermediates is being 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 organic 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 F- 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 electrostatic focussing electrodes towards the entrance of the mass analyzer.

Summary of Progress and Accomplishments

We have continued to develop further improvements to the CIMS technique in order to increase its sensitivity to enable chemical kinetics studies of organic free radicals. In our first set of experiments we monitored the formation of the methyl-hydroxycyclohexadienyl radical (the toluene-OH adduct). We 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 been able to monitor the subsequent formation of aromatic peroxy radicals and nitrates using instead negative ion CIMS, with SF6- as the ionization reagent . These compounds were produced by mixing the toluene-OH adduct in the flow tube with O2 and/or NO2, introduced through the moveable injector.

The mass-spectrometric results indicate that the OH-toluene adduct can add two oxygen molecules. This is what is expected from theoretical calculations reported in the literature: addition of the first oxygen molecule generates a peroxy radical, which then isomerizes to produce a bicyclic radical intermediate, which in turn adds a second oxygen molecule to produce a bicyclic peroxy radical. We have also identified as reaction products species such as cresols and nitrates.

To further elucidate the chemistry and the ionization processes of the aromatic peroxy radicals, we have investigated other organic peroxy radicals. We have found that the reagent ion F- provides excellent sensitivity to various organic peroxy radicals. More specifically, we are able to detect hydrogen peroxy radicals and methyl peroxy radicals with high sensitivity. Furthermore, we have determined the rate constant for the reaction of the methyl peroxy radical with nitric oxide by monitoring decays of the peroxy radical; the result agrees very well with literature values.

Status of Original Plan

The original plan is proceeding as expected.

Future Plans

• Elementary reaction rate constants of various other organic peroxy radicals will be investigated by measuring the decay of the mass spectrometric signal as a function of reaction time.
  
• Reaction mechanisms involving additional intermediate species in the atmospheric oxidation of volatile organic compounds will be investigated.

Handling of QA/QC

The guidelines described in the Quality Assurance and Quality Control Plan will be followed in carrying out the laboratory research, as needed.

Key Personnel

Graduate Student:

Keith Broekhuizen, MIT

Research Staff:

Luisa T. Molina, MIT