Goals
This study will measure the action spectra (or the product of the absorption cross section and the photodissociation quantum yield) of hydrogen peroxide (H2O2) and organic peroxides (ROOH) (e.g., methyl hydroperoxide, CH3OOH; ethyl hydroperoxide, CH3CH2OOH; hydroxylmethyl hydroperoxide, HOCH2OOH; 1-hydroxyethyl hydroperoxide, CH3CH(OH)OOH; tert-butyl hydroperoxide, (CH3)3OOH) as a function of wavelength and temperature. Peroxides are to be synthesized and purified by known techniques. Action spectra will be obtained for each peroxide by photolyzing the peroxide and then monitoring the OH production via laser induced fluorescence (LIF). Accurate UV cross sections of these important urban and global atmospheric trace constituents are required for accurate modeling of the photochemistry.
Background
Hydrogen peroxide and organic peroxides (i.e. methyl hydroperoxide, ethyl hydroperoxide, hydroxymethyl hydroperoxide, 1-hydroxyethyl hydroperoxide, tert-butyl hydroperoxide) are important trace constituents of the urban and global atmosphere. They are principally formed via association reactions between peroxy radicals:
HO2 + HO2 H2O2 + O2
RO2 + HO2 ROOH + O2
and therefore act as sinks and temporary reservoirs for HOx and ROx species. Other formation pathways include reaction with formaldehyde and ozonolysis of alkenes. Hydrogen peroxide, and to a lesser extent organic peroxides, are also the dominant oxidants of species such as SO2 found in clouds, fogs, or rain in the atmosphere. In addition, peroxides may contribute to forest damage due to their phytotoxic properties. Transport of these reservoir species can have large regional and potentially global effects.
In addition to wet and dry deposition, peroxides are removed from the atmosphere by reaction with OH radicals or by photolysis.
ROOH + OH products
ROOH + hv products
Atmospheric lifetimes, which are determined in part by these processes, will influence the role of these species in the transport of active urban photochemistry to more remote environments.
The photolysis rates of the peroxides are determined by the magnitude of the absorption features at wavelengths longer than 305 nm (the so-called ozone "cut-off".) Accurate UV absorption cross sections are required to assess the importance of photochemical destruction of atmospheric hydrogen peroxide and organic peroxides. Such data is also useful in laboratory studies, where concentrations of these species are often measured by UV absorption.
Method of Approach and Facilities
Peroxide photolysis. A Nd:YAG-pumped Type II BBO optical parametric oscillator (OPO) developed in Caltech Prof. Geoff Blake's research group at Caltech will be used to generate tunable laser light from 250 to 1000 nm. Operated at 100 Hz pulse repetition rate, 1/2 Watt of narrow-band (0.7 - 1.5 cm-1) fundamental (410 - 2500 nm) radiation can be produced from the OPO with a single optical set. Because frequency doubling efficiencies exceed 40%, the entire spectrum from 250 to 345 nm can be scanned quickly. This provides a means of determining the cross sections as a function of wavelength relative to the cross section at shorter wavelengths where the absolute cross sections of these compounds are easier to measure. The photolysis will occur by directing the laser across a slow flow (100 cm/s) of ROOH (mixed in N2) in a cooled flow tube. To minimize wall effects, the peroxides will be added through an injector into the middle of this large diameter flow tube. Photolysis will occur approximately 10 cm (0.1 sec) before the LIF axis providing sufficient time to thermalize the nascent OH distribution via collisions with the N2 bath gas.
OH detection. A laser induced fluorescence (LIF) detection method similar to that used for the ER2 HOx instrument (previously built by the P.I.) is employed. The LIF optical system used in the HOx instrument has demonstrated the ability to measure concentrations of OH below 103 molecules cm-3 at low pressure (<5 torr) in the laboratory. Approximately 3 mW of UV light at 282 nm is used in a 32-pass White cell to obtain this sensitivity. For these studies, however, the photolysis of the peroxides by the LIF laser will limit our ability to observe the OH produced by the OPO laser. Conservatively, if 1 mW of laser light is used without a multipass cell, the OH sensitivity will still have a limit of detection of below 105 OH molecule cm-3. Because of this sensitivity, only a small fraction of the ROOHs will need to be photolysed thereby minimizing secondary chemistry.
HO2 detection. HO2 will be detected by chemical titration of the HO2 to OH with nitric oxide. The relative yield of OH and HO2 can thus be measured as a function of wavelength.
Determination of the cross sections. With UV optical power in excess of 200 mW from the photolysis laser, and requiring that the signal from the tunable photolysis laser generate at least 1/10 of the OH produced by the LIF laser, we will be able to observe cross sections 103 times smaller than the cross section at 282 nm. For most peroxides, the cross section at 282 nm is approximately 10-20 cm2 molecule-1. Thus, cross sections as small as 10-23 can, in principle, be observed. This sensitivity is one order of magnitude better than the earlier studies. The OH and HO2 yields will be measured as the photolysis laser is tuned rapidly from 240 to 350 nm. By simply normalizing the OH and HO2 signals by the intensity (photons/sec) of the laser light, the product of the absorption cross section and the photodissociation quantum yield can be determined relative to that at 250 nm. At 250 nm the cross sections are large and can be measured by standard absorption spectroscopy techniques. Although precise knowledge of the absolute concentration of the ROOH concentration is not required to determine these relative cross sections - careful study of the purity of the peroxides is very important due to potential spectral interferences.
Summary of Progress and Accomplishments
During the first six months of this effort, we have been constructing both the optical apparatus and the gas synthesis / characterization capability.
For the optical system, laser outputs from both the 532 nm-pumped Type II BBO optical parametric oscillator (pump laser) and the UV OH probe laser have been optically aligned and optimized for power. We have characterized the output of the signal and idler beams of the OPO by separating the infrared from YAG pump beam with a prism and directing the radiation into a wavemeter for calibrations. These calibrations are used to automate of the OPO crystal mount drives. The OPO output beam is focused and co-aligned with the UV beam into the photolysis cell.
We have worked to reduce the laser scatter in the photolysis chamber. The flat windows where the laser beams enter the gas cell have been replaced with purged Brewster angle windows. A small N2 purge helped to decrease the dead volume in the side arms, while the Brewster angled windows reduced scatted light to the detector.
A gas handling system has been designed to deliver a diluted pexoxide mix to the photolysis cell. Gaseous peroxide is introduced into a flowing system by passing a calibrated N2 flow through the selected peroxide sample, which was contained in a Quartz reservoir placed in an ice bath. A gas handling system has been built to couple the reservoir to a Fourier Transform Infrared spectrometer (FTIR) for in-line peroxide quantification. A jacketed flow cell was designed to minimize the decomposition on the wall surfaces. Pre-cooled liquid (in this case ice water) is circulated through the entire length of the jacket. The residence time in the flow cell photolysis region was adjusted by varying the total flow rates and the photolysis cell pressure.
Several preliminary experiments have been performed to test the OH detection system and to optimize the flow conditions for the conversion of HO2 to OH with NO addition. A data acquisition system including a pulse gated photon counter has been assembled and characterized. OH generated by passing water vapor over a hot filament has been built and used to test the OH detection system.
Status of Original Plan
This new project is just underway and is following along the outline described in the original plan.
Future Plans
Handling of QA/QC
Calibration and testing of the laser and OH detection systems has simultaneously been carried out with the equipment development. An FTIR has been incorporated for on-line peroxide quantification.
Key Personnel
Staff Scientist:
Coleen M. Roehl
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