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At least 271 records · Page 15
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NASA NTRS1994

Ultraviolet absorption cross-sections of some carbonyl compounds and their temperature dependence

Ultraviolet absorption cross-section of phosgene (CCl2O), trichloroacetylchloride (CCl3-CClO) and trichloroacetaldehyde (CCl3-CHO) have been measured between 170 and 320 nm for temperature ranging from 210 to 295 K with classical double beam equipment. These data are compared with other available determinations performed at room temperature. Photodissociation coefficients are estimated and their temperature dependence is discussed. Impact of the photodissociation on the total atmospheric destruction of these compounds is illustrated.

Gillotay, D.↗
NASA NTRS1994

Southern hemisphere ground based measurements of Carbonyl Fluoride (COF2) and Hydrogen Fluoride (HF): Partitioning between Fluoride reservoir species

We report infrared ground based total column measurements of the stratospheric fluorine reservoir gases COF2 and HF above Lauder, New Zealand (45 deg S, 167.8 deg E) obtained between April 1993 and January 1994. The average retrieved COF2 and HF total columns are 2.81(+/- 0.56) x 10(exp 14) and 9.91(+/- 1.09) x 10(exp 14) molecules/sq cm respectively. The daily average COF2 and HF columns are correlated; this correlation is likely the result of dynamics. The average HF/COF2 column ratio on days with measurements of both HF and COF2 is 3.63 (+/- 0.55). Comparison of this ratio with model calculations implies that the quantum yield for COF2 photolysis is near unity. Our measured COF2 columns are higher than all previously reported values, but inconsistencies among the earlier measurements and uncertainty in the latitudinal gradient of the COF2 column preclude an accurate determination of the long-term COF2 trend.

Reisinger, Andreas R.↗
NASA NTRS1994

Increase of carbonyl fluoride (COF2) in the stratosphere and its contribution to the 1992 budget of inorganic fluorine in the upper stratosphere

Volume mixing ratio profiles of COF2 have been derived through most of the stratosphere between 30 deg N and 54 deg S from series of 0.01-/cm resolution infrared solar spectra recorded in the occultation mode by the atmospheric trace molecule spectroscopy (ATMOS) instrument during the ATLAS 1 space shuttle mission of March-April 1992. When compared with similar results obtained from the ATMOS/Spacelab 3 mission of April-May 1985, the cumulative increase in the burden of COF2 in the middle and upper stratosphere was found to be 67% for that 7-year time interval. By combining a subset of these COF2 results with upper stratospheric concentrations of HF also derived from the ATMOS observations, it was further found that the budget of inorganic fluorine above 35 km altitude increased by (60 +/- 10)% over the 1985-1992 time interval, which corresponds to an average exponential rate of increase of (6.7 +/- 1/1)%/yr, or a linear rate of increase reference to 1985 of (8.5 +/- 1.3)%/yr at the 1 sigma confidence level. The total inorganic F atom volume mixing ratio found in the upper stratosphere for 1985 and 1992 and the increase during this perid mirror the rise in man-made fluorine-bearing compounds at the ground during the early to mid 1980s. This demonstrates the negligible impact of natural sources of fluorine, in particular volcanic activity, on the observed change in F in the upper stratosphere. Implications of the present findings and comparison with model results are discussed.

Zander, R.↗
NASA NTRS1995

Photoproduction of carbonyl sulfide in south Pacific Ocean waters as a function of irradiation wavelength

Carbonly sulfide (OCS) photoproduction rates were measured at selected wavelengths of ultraviolet light between 297 and 405 nm in sea water samples from the southern Pacific Ocean. Near-surface and column production rate spectra for natural sunlit waters were calculated using sea-surface sunlight data measured near the austral summer solstice. These plots show that photoproduction rates are at a maximum at 313 nm in tropical waters and at 336 nm in Antarctic waters. Tropical surface and column rates were found to be 68 pM/day and 360 nmol/sq m/day, respectively, and Antarctic surface and column rates were found to be 101 pM/day and 620 nmol/sq m/day, respectively. A high degree of variability was observed between photoproduction rates from different ocean regions, with coastal rates being the highest, suggesting that natural environmental variability is an important factor. Photoproduction rates at 297 nm were found to be constant at individual locations with increasing irradiation time. Relative photoproduction rates from this work are compared to previously measured rates from coastal sea water.

Weiss, Peter S.↗
NASA NTRS1994

Increase of carbonyl fluoride (COF2) in the stratosphere and its contribution to the 1992 budget of inorganic fluorine in the upper stratosphere

Volume mixing ratio profiles of COF2 have been derived through most of the stratosphere between 30 deg N and 54 deg S from series of 0.01-cm(exp -1) resolution infrared solar spectra recorded in the occultation mode by the atmospheric trace molecule spectroscopy (ATMOS) instrument during the ATLAS 1 space shuttle mission of March-April 1992. When compared with similar results obtained from the ATMOS/Spacelab 3 mission of April-May 1985, the cumulative increase in the burden of COF2 in the middle and upper stratosphere was found to be 67% for that 7-year time interval. By combining a subset of these COF2 results with upper stratospheric concentrations of HF also derived from the ATMOS observations, it was further found that the budget of inorganic fluorine above 35 km altitude increased by (60 +/- 10) % over the 1985-1992 time interval, which corresponds to an average exponential rate of increase of(6.7 +/- 1.1) % /yr , or a linear rate of increase referenced to 1985 of (8.5 +/- 1.3) % /yr at the 1(sigma) confidence level. The total inorganic F atom volume mixing ratio found in the upper stratosphere for 1985 and 1992 and the increase during this period mirror the rise in man-made fluorine-bearing compounds at the ground during the early to mid 1980s. This demonstrates the negligible impact of natural sources of fluorine, in particular volcanic activity, on the observed change of F in the upper stratosphere. Implications of the present findings and comparison with model results are discussed.

Zander, R.↗
NASA NTRS2003

Effects of carbonyl bond, metal cluster dissociation, and evaporation rates on predictions of nanotube production in high-pressure carbon monoxide

The high-pressure carbon monoxide (HiPco) process for producing single-wall carbon nanotubes (SWNTs) uses iron pentacarbonyl as the source of iron for catalyzing the Boudouard reaction. Attempts using nickel tetracarbonyl led to no production of SWNTs. This paper discusses simulations at a constant condition of 1300 K and 30 atm in which the chemical rate equations are solved for different reaction schemes. A lumped cluster model is developed to limit the number of species in the models, yet it includes fairly large clusters. Reaction rate coefficients in these schemes are based on bond energies of iron and nickel species and on estimates of chemical rates for formation of SWNTs. SWNT growth is measured by the conformation of CO2. It is shown that the production of CO2 is significantly greater for FeCO because of its lower bond energy as compared with that of NiCO. It is also shown that the dissociation and evaporation rates of atoms from small metal clusters have a significant effect on CO2 production. A high rate of evaporation leads to a smaller number of metal clusters available to catalyze the Boudouard reaction. This suggests that if CO reacts with metal clusters and removes atoms from them by forming MeCO, this has the effect of enhancing the evaporation rate and reducing SWNT production. The study also investigates some other reactions in the model that have a less dramatic influence.

Models, Chemical↗
NASA NTRS2002

Effects of Carbonyl Bond and Metal Cluster Dissociation and Evaporation Rates on Predictions of Nanotube Production in HiPco

The high-pressure carbon monoxide (HiPco) process for producing single-wall carbon nanotubes (SWNT) uses iron pentacarbonyl as the source of iron for catalyzing the Boudouard reaction. Attempts using nickel tetracarbonyl led to no production of SWNTs. This paper discusses simulations at a constant condition of 1300 K and 30 atm in which the chemical rate equations are solved for different reaction schemes. A lumped cluster model is developed to limit the number of species in the models, yet it includes fairly large clusters. Reaction rate coefficients in these schemes are based on bond energies of iron and nickel species and on estimates of chemical rates for formation of SWNTs. SWNT growth is measured by the co-formation of CO2. It is shown that the production of CO2 is significantly greater for FeCO due to its lower bond energy as compared with that ofNiCO. It is also shown that the dissociation and evaporation rates of atoms from small metal clusters have a significant effect on CO2 production. A high rate of evaporation leads to a smaller number of metal clusters available to catalyze the Boudouard reaction. This suggests that if CO reacts with metal clusters and removes atoms from them by forming MeCO, this has the effect of enhancing the evaporation rate and reducing SWNT production. The study also investigates some other reactions in the model that have a less dramatic influence.

Scott, Carl D.↗