Thomas Owano

The Measurement of Aerosol Optical Properties Using Continuous Wave Cavity Ring-Down Techniques

Abstract Large uncertainties in the effects that aerosols have on climate require improved in situ measurements of extinction coefficient and single-scattering albedo. This paper describes the use of continuous wave cavity ring-down (CW-CRD) technology to address this problem. The innovations in this instrument are the use of CW-CRD to measure aerosol extinction coefficient, the simultaneous measurement of scattering coefficient, and its small size, suitable for a wide range of aircraft applications. The prototype instrument measures extinction and scattering coefficient at 690 nm and extinction coefficient at 1550 nm. The instrument itself is small (60 cm × 48 cm × 15 cm) and relatively insensitive to vibrations. The prototype instrument has been tested in the lab and used in the field. While improvements in performance are needed, the prototype has been shown to make accurate and sensitive measurements of extinction and scattering coefficients. Combining these two parameters, one can obtain the single-s...

Methyl radical measurement by cavity ring-down spectroscopy

Abstract Cavity ring-down spectroscopy has been used to measure the absorbance of methyl radicals near 216 nm. The methyl radicals are generated by a hot tungsten filament heated to 2300 K in a mixture of 0.5% CH 4 in H 2 slowly flowing through the reactor at 20 Torr total pressure. CH 3 absorbances are detected with a noise-equivalent sensitivity of two parts in 10 5 using a narrow pencil of UV light 0.5 mm in diameter, which allows measurement of spatial profiles of CH 3 for column densities of 3×10 12 radicals/cm 2 (3×10 12 radicals/cm 3 ×1 cm absorption pathlength).

Measurement of δ18O, δ17O, and 17O-excess in water by off-axis integrated cavity output spectroscopy and isotope ratio mass spectrometry.

Stable isotopes of water have long been used to improve understanding of the hydrological cycle, catchment hydrology, and polar climate. Recently, there has been increasing interest in measurement and use of the less-abundant (17)O isotope in addition to (2)H and (18)O. Off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and precise measurements δ(18)O, δ(17)O, and (17)O-excess in liquid water. OA-ICOS involves no sample conversion and has a small footprint, allowing measurements to be made by researchers collecting the samples. Repeated (514) high-throughput measurements of the international isotopic reference water standard Greenland Ice Sheet Precipitation (GISP) demonstrate the precision and accuracy of OA-ICOS: δ(18)OVSMOW-SLAP = -24.74 ± 0.07‰ (1σ) and δ(17)OVSMOW-SLAP = -13.12 ± 0.05‰ (1σ). For comparison, the International Atomic Energy Agency (IAEA) value for δ(18)OVSMOW-SLAP is -24.76 ± 0.09‰ (1σ) and an average of previously reported values for δ(17)OVSMOW-SLAP is -13.12 ± 0.06‰ (1σ). Multiple (26) high-precision measurements of GISP provide a (17)O-excessVSMOW-SLAP of 23 ± 10 per meg (1σ); an average of previously reported values for (17)O-excessVSMOW-SLAP is 22 ± 11 per meg (1σ). For all these OA-ICOS measurements, precision can be further enhanced by additional averaging. OA-ICOS measurements were compared with two independent isotope ratio mass spectrometry (IRMS) laboratories and shown to have comparable accuracy and precision as the current fluorination-IRMS techniques in δ(18)O, δ(17)O, and (17)O-excess. The ability to measure accurately δ(18)O, δ(17)O, and (17)O-excess in liquid water inexpensively and without sample conversion is expected to increase vastly the application of δ(17)O and (17)O-excess measurements for scientific understanding of the water cycle, atmospheric convection, and climate modeling among others.

Measurement of the methyl radical concentration profile in a hot‐filament reactor

The spatial profile of methyl radical concentration in a hot‐filament reactor has been measured using cavity ring‐down spectroscopy (CRDS) at a wavelength of 213.9 nm for which the CH3 absorption cross section has been shown to be nearly independent of temperature. Methyl radicals are generated with a 25 mm long tungsten filament heated to 2400 K in a slowly flowing mixture of 0.6% CH4 in H2 (20 Torr total pressure). CRDS is employed to measure CH3 absorbance as a function of a distance perpendicular to the axis of the filament. The CH3 absorbance profiles do not change when the direction of the process gas flow through the reactor is reversed, which indicates cylindrical symmetry of the CH3 distribution about the filament. Consequently, the radial CH3 concentration in the reactor is determined by Abel inversion of the CH3 absorbance profile. The CH3 concentration peaks ∼4 mm from the filament (1.04×1014 molecules/cm3). Methyl radicals decay rapidly as a function of a distance from the filament and disapp...

Boundary layer profiles in plasma chemical vapor deposition

A nonlinear optical spectroscopy based on degenerate four-wave mixing has made possible direct measurements of species temperature and concentration profiles through the boundary layer of a reactive plasma at atmospheric pressure. Spectra were obtained for CH and C2 radicals over a range of conditions including those for the plasma chemical vapor deposition of diamond films. Numerical simulations based on a one-dimensional stagnation-point flow model are in good agreement with the measurements. The CH mole fraction is shown to rise and fall as a function of distance from the substrate, which is compelling experimental evidence for the complex chemistry that is occurring in the plasma boundary layer.

Measurement of absolute CH3 concentration in a hot-filament reactor using cavity ring-down spectroscopy

Abstract Methyl radicals were generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (length, 20 mm) in a slowly flowing mixture of 0.5% CH 4 in H 2 . The UV absorbance of CH 3 was measured during deposition using a new line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements were carried out at 213.9 nm, a wavelength at which the CH 3 absorption cross-section has been shown by others to be independent of the temperature over a large range. We observed that the CH 3 absolute concentration varied strongly as a function of the position between the substrate and the filament, and its value was strongly influenced by the substrate temperature.

Cavity-enhanced quantum-cascade laser-based instrument for carbon monoxide measurements

An autonomous instrument based on off-axis integrated cavity output spectroscopy has been developed and successfully deployed for measurements of carbon monoxide in the troposphere and tropopause onboard a NASA DC-8 aircraft. The instrument (Carbon Monoxide Gas Analyzer) consists of a measurement cell comprised of two high-reflectivity mirrors, a continuous-wave quantum-cascade laser, gas sampling system, control and data-acquisition electronics, and data-analysis software. CO measurements were determined from high-resolution CO absorption line shapes obtained by tuning the laser wavelength over the R(7) transition of the fundamental vibration band near 2172.8 cm(-1). The instrument reports CO mixing ratio (mole fraction) at a 1-Hz rate based on measured absorption, gas temperature, and pressure using Beers Law. During several flights in May-June 2004 and January 2005 that reached altitudes of 41,000 ft (12.5 km), the instrument recorded CO values with a precision of 0.2 ppbv (1-s averaging time) and an accuracy limited by the reference CO gas cylinder (uncertainty < 1.0%). Despite moderate turbulence and measurements of particulate-laden airflows, the instrument operated consistently and did not require any maintenance, mirror cleaning, or optical realignment during the flights.

Deployment of a carbon isotope ratiometer for the monitoring of CO2 sequestration leakage.

In an effort to monitor leakage from underground CO(2) storage, a field-deployable analyzer capable of rapidly measuring the CO(2) mixing ratio and δ(13)C values (±0.05 ppm(v) ± 0.2‰, 60 s) was deployed to distinguish between biogenic and fossil CO(2) sources. The analyzer was interfaced with a multiport inlet unit to allow autonomous sampling from multiple locations. The instrument and inlet interface were deployed at the Zero Emissions Research and Technology (ZERT) site (Bozeman, Montana, July 14-22, 2009) during a controlled, subsurface release of CO(2) depleted in (13)C. A biogenic diurnal cycle was observed far from the release, and the associated Keeling plot suggested a CO(2) source (δ(13)C = -27.0 ± 0.5‰) consistent with local C(3) vegetation. Inlets near the leak showed large CO(2) mixing ratios (388/>40 000 ppm(v)) whose predominant source was the release CO(2) (inferred δ(13)C = -58.2 ± 0.7‰). Measurements 3 m from the source showed diurnal CO(2) cycles (382-2400 ppm(v)) influenced by leaked CO(2), possibly due to diel air mixing. Finally, the data from all of the sampling inlets was combined to spatially localize the leak position.

High growth rate diamond synthesis in a large area atmospheric pressure inductively coupled plasma

Abstract : Diamond synthesis is demonstrated in an atmospheric pressure inductively coupled argon-hydrogen-methane plasma. The plasma generated has a free stream active area of 20 sq cm and has an equilibrium electron temperature T sub e of approximately 4000 K. Growth rates are typically in the range of 25- 50 microns/hour and are found to depend on the processing conditions and position on the substrate. Individual crystallites are analyzed by micro-raman spectroscopy. Large (50 microns) size well-faceted crystallites are found to be under a state of compression, displaying shifts in the principal phonon mode as great as 3/cm from that of smaller polycrystalline diamond clusters or secondary nucleation sites. These shifts are shown to be expected within the framework of linear thermal expansion theory and the assumption of negligible stress relief at the crystallite-substrate interface. Keywords: Diamond synthesis, Thermal plasma, Raman spectroscopy.

Near-infrared cavity ringdown spectroscopy of water vapor in an atmospheric flame

We have used cavity ringdown spectroscopy CRDS to measure near-infrared overtone transitions of water in . atmospheric flames propane premixed jet and laminar methane-air flat flame burner . The strong signal output with a well-defined laser beam direction and the insensitivity to strong background emission present in hostile environments make CRDS ideal for the study of combustion environments. We have obtained spectra of water vapor from within a flame and extracted a profile of the rotational temperature and concentration of water vapor as a function of distance from the plane burner surface. q 1998 Elsevier Science B.V.