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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The sunrise and sunset variation of ClO in the lower stratosphere

The abundances of ClO have been measured, in situ, in the lower stratosphere during sunrise and sunset. Measurements were made with an instrument mounted on the NASA ER-2 aircraft, which was flown at an altitude of 20 km and latitudes between 35 and 47 deg N during a morning and an evening flight. The abundances of ClO were observed over a dynamic range of 20 from a detection threshold of 1 part per trillion volume (pptv). These data confirm the sunrise variation of the photolysis of chlorine nitrate that is predicted by a zero-dimensional photochemical model. They also suggest that the absolute photolysis and termolecular formation of chlorine nitrate occur at rates consistent with nominal ClONO2 and NO2 concentrations.

Brune, W. H.↗
NASA NTRS1991

Measurements of ClO and O3 from 21 deg N to 61 deg N in the lower stratosphere during February 1988 - Implications for heterogeneous chemistry

The decline in stratospheric ozone at northern midlatitudes in wintertime may be caused by chlorine photochemistry that has been enhanced by heterogeneous reactions. The possibility that the heterogeneous reaction of N2O5 on sulfate aerosols is the cause of this decadal ozone decline is examined by comparing ClO and O3 measurements made in the lower stratosphere during February, 1988, with results from a 2D model. At midlatitudes, the abundances, latitudinal, and seasonal gradients of the observed ClO are similar to the results of a model with heterogeneous chemistry, but are in strong disagreement with the results from the model with only gas-phase chemistry. At low latitudes, agreement is best with the results of the model with only gas-phase chemistry. Limited observations indicate that the amount of reactive chlorine is being enhanced, and that heterogeneous chemistry is a likely cause.

King, J. C.↗
NASA NTRS1993

In situ measurements of ClO at mid-latitudes: Is there an effect from Mt. Pinatubo?

Observations of ClO from 20 to 60 deg N made before and after the eruption of Mt. Pinatubo are compared for changes which may result from increased sulfate aerosol surface area. Using ozone as a vertical coordinate and examining data at similar latitudes and seasons, elevated abundances of ClO are found at low latitudes (20 to 30 deg N), an effect which decreases with increasing latitude. For the flights compared, there appear to be no differences, within the uncertainty of the measurements, at latitudes poleward of 40 deg N. These results are consistent with the idea that the hydrolysis of N2O5 on sulfate aerosols becomes saturated at moderate aerosol loadings.

Avallone, L. M.↗
NASA NTRS1993

Balloon-borne measurements of ClO, NO, and O3 in a volcanic cloud: An analysis of heterogeneous chemistry between 20 and 30 km

Balloon profiles of chlorine monoxide (ClO), nitric oxide (NO), and ozone (O3) were measured on March 11, 1992 from 100 to 10 mb over Greenland (67.0 deg N, 50.6 deg W). Measurements from SAGE II indicate that the aerosol surface area in the region was enhanced by sulfur from the eruption of Mt. Pinatubo, reaching 50 times background near 20 km. Concentrations of ClO were enhanced and concentrations of NO were suppressed relative to low aerosol conditions consistent with the effects of hydrolysis of N2O5 on the surface of sulfuric acid aerosols. The data are consistent with a value of 2 x 10(exp -4) for the reaction probability of the heterogeneous hydrolysis of ClONO2, indicating a minor role for this reaction at as temperature of 220 K. At these temperatures, we find no evidence for the catastrophic loss of ozone predicted to occur under conditions of enhanced aerosol surface area.

Dessler, A. E.↗
NASA NTRS1994

Pressure broadening of ClO by N2 and O2 near 204 and 649 GHz and new frequency measurements between 632 and 725 GHz

The N2 and O2 pressure broadening of the ClO transitions near 204 and 649 GHz have been measured between 200 and 300 K. Oxygen broadening has been measured for the transitions near 278 GHz. The accuracy of the derived air broadening is comparable to that for the air broadening of stable species and is estimated to be within approximately 3% over the entire temperature range. These transitions are currently being used for satellite, balloon, and ground based monitoring of atmospheric ClO, respectively. Some new frequency measurements are reported in the 632 - 725 GHz range. These are in good agreement with previous measurements and predictions.

Oh, J. J.↗
NASA NTRS1996

Stratospheric Chlorine partitioning: Constraints from Shuttle-borne Measurements of [HCl], [ClNO3] and [ClO]

Measured stratospheric mixing ratios of HCl, ClNO3, and ClO from ATMOS and MAS are poorly reproduced by models using recommended kinetic parameters. This discrepancy is not resolved by new rates for the reactions Cl+CH4 and OH+HCl derived from weighted fits to laboratory measurements. A deficit in modeled [HCl] and corresponding overprediction of [ClNO3] and [ClO], which increases with altitude, suggests that production of HCl between 20 and 50 km is much faster than predicted from recommended rates.

Michelsen, H. A.↗
NASA NTRS1993

In Situ Measurements of CLO at Mid-Latitudes: Is there an Effect from Mt. Pinatubo?

Observations of ClO from 20 to 60 deg N made before and after the eruption of Mt. Pinatubo are compared for changes which may result from increased sulfate aerosol surface area. Using ozone as a vertical coordinate and examining data at similar latitudes and seasons, elevated abundances of ClO are found at low latitudes (20 to 30 deg N), an effect which decreases with increasing latitude. For the flights compared, there appear to be no differences, within the uncertainty of the measurements, at latitudes poleward of 40 deg N. These results are consistent with the idea that the hydrolysis of N2O5 on sulfate aerosols becomes saturated at moderate aerosol loadings.

Avallone, L. M.↗
NASA NTRS1993

Balloon-Borne Measurements of CLO, NO and O3 in a Volcanic Cloud: An Analysis of Heterogeneous Chemistry between 20 and 30 KM

Balloon profiles of chlorine monoxide (ClO), nitric oxide (NO), and ozone (O3) were measured on March 11, 1992 from 100 to 10 mb over Greenland (67.0 deg N, 50.6 deg W). Measurements from SAGE II indicate that the aerosol surface area in the region was enhanced by sulfur from the eruption of Mt. Pinatubo, reaching 50 times background near 20 km. Concentrations of ClO were enhanced and concentrations of NO were suppressed relative to low aerosol conditions consistent with the effects of hydrolysis of N2O5 on the surface of sulfuric acid aerosols. The data are consistent with a value of 2 x 10(exp -4) for the reaction probability of the heterogeneous hydrolysis of ClONO2, indicating a minor role for this reaction at a temperature of 220 K. At these temperatures, we find no evidence for the catastrophic loss of ozone predicted to occur under conditions of enhanced aerosol surface area.

Dessler, A. E.↗
NASA NTRS1996

Polar Vortex Conditions During The 1995-96 Arctic Winter: MLS ClO and HNO3

Microwave Limb Sounder (MLS) measurements of lower stratospheric ClO and HNO3 during the 1995-96 Arctic winter are presented. The 1995-96 Arctic winter was both colder and more persistently cold than usual, leading to an enhancement in lower stratospheric ClO of greater magnitude, vertical extent, and duration than previously observed in the Arctic. Vortex concentrations of HNO3 in mid-December were large due to diabatic descent. Trajectory calculations indicate that localized severe depletions of gas-phase HNO3 in mid-February and early March did not arise from entrainment of midlatitude air into the vortex and were therefore probably related to polar stratospheric cloud (PSC) formation. A strong correlation between temperature and gas-phase HNO3 was evident, consistent with recurring PSC condensation and evaporation cycles.

Santee, M. L.↗
NASA NTRS1977

MCSCF + Cl wavefunctions and properties of the X2-Pi and A2-Pi states of ClO

A multiconfiguration, self-consistent field plus configuration-interaction calculation has been performed on the X2-Pi and A2-Pi electronic states of ClO. The values of certain molecular properties computed from the wavefunctions agree well with those from experiment. The sum of the squares of the computed electronic transition moments between the X2-Pi and A2-Pi states at 3.07 bohr is 1.75 a.u., which corresponds to the experimental result of 1.58 plus or minus 0.20 a.u. The computed ground state dipole moment at the experimental equilibrium separation is 1.232 D, which compares well with two experimental results of 1.18 plus or minus 0.12 and 1.239 plus or minus 0.010 D. The value of the computed dissociation energy is 2.75 eV, which agrees well with the experimental value of 2.803 plus or minus 0.001 eV.

Arnold, J. O.↗
NASA NTRS1978

Rate constant for the reaction ClO + NO yields Cl + NO2

The rate constant for the reaction ClO + NO yields Cl + NO2 has been determined over the temperature range 226.7-415.4 K in a discharge flow system using a mass spectrometer as a detector. The results, expressed in the Arrhenius form, are compared with previous measurements.

Leu, M. T.↗
NASA NTRS1979

Remote measurement of ClO in the stratosphere

ClO has been detected in the stratosphere from observations of the solar spectrum in the infrared, in a small spectral interval near 12 micrometers. The observations were made with a balloon-borne laser heterodyne radiometer, launched from Palestine, Texas on September 20. By comparing high sun spectra with a number of sequential spectra taken during sunset, an altitude profile has been calculated in the 29-38 km altitude range. The results show a peak mixing ratio in excess of one ppb above 34 km, and a rapid decrease in mixing ratio with decreasing altitude below 34 km.

Menzies, R. T.↗
NASA NTRS1980

The influence of NO and ClO variations at twilight on the interpretation of solar occultation measurements

Measurement of short-lived photochemically-produced species in the stratosphere by solar occultation is difficult because the rapid variation of such species near the terminator introduces ambiguities in interpreting the measured absorption in terms of meaningful atmospheric abundances. These variations produce tangent path concentrations that are asymmetric relative to the tangent point, as opposed to the symmetrical distribution usually assumed in most inversion algorithms. Neglect of this asymmetry may yield an inverted profile that deviates significantly from the true sunset/sunrise profile. In the present paper, the influence of this effect on solar occultation measurements of ClO and NO is examined. The results show that average inhomogeneity factors, which measure the concentration variation along the tangent path and which can be calculated from a photochemical model, can indicate which species require more careful data analysis.

Boughner, R.↗
NASA NTRS1980

Kinetics study of the Cl/2P/ + Cl2O yields Cl2 + ClO reaction at 298 K

The kinetics of the Cl + Cl2O reaction, a possible source of ClO(2 Pi) radicals for atmospheric photochemical studies, are investigated at 298 K. The discharge flow/mass spectrometry and discharge flow/resonance fluorescence techniques were used to monitor the decay of C12O in the presence of excess concentrations of atomic chlorine and chlorine monoxide, respectively. The pseudo-first order rate constants obtained from both experiments are found to be in excellent agreement, averaging 9.8 + or - 0.8 x 10 to the -11th cu cm/molecule per sec. Results are consistent with the lower limit obtained by Edgecombe et al. (1957) but differ by a factor of 150 from those of Basco and Dogra (1971). The present value is also noted to be consistent with a lower value for the rate constant of the reaction of oxygen atoms with Cl2O.

Ray, G. W.↗
NASA NTRS1981

Pressure broadening of ClO by nitrogen

The temperature and pressure dependence of linewidths for the J = 11/2 - 9/2 and J = 15/2 - 13/2 transitions of ClO are determined by microwave spectroscopy. It is found that the widths for both transitions are the same within experimental error. The width parameter for nitrogen broadening is shown to be 3.35 + or - 0.17 MHz/torr at 317 K and 4.44 + or - 0.23 MHz/torr at 218 K.

Pickett, H. M.↗
NASA NTRS1983

Chlorine nitrate - The sole product of the ClO + NO2 + M recombination

A laser operating at 266 nm is used to photolyze the product of the ClO + NO2 + M reaction, and the yield of Cl atoms, measured by resonance fluorescence, is found to be identical with that from equivalent concentrations of ClONO2. This is interpreted as a strong indication that no other isomers are formed in the recombination reaction. Evidence for isomers is surveyed, and the apparent forward-and-reverse-rate constant discrepancy is explained as a probable uncertainty in the thermochemistry.

Margitan, J. J.↗
NASA NTRS1985

A critical analysis of CLO and O3 in the mid-latitude stratosphere

The Caltech one-dimensional photochemical model is used to analyze important questions concerning the upper stratospheric O3 and the key HO(x), NO(x), and ClO(x) free radicals. The model is described and first order effects of the inclusion of diffuse radiation in a spherical rather than plane parallel atmosphere at solar zenith angles close to 90 deg are assessed. A comparison is made between photochemical theory and observations for upper stratospheric O3, where local photochemistry rather than dynamics should control the O3 abundance. The photochemical equilibrium relation for odd oxygen is studied, including the important radicals involves. It is concluded that a significant model ozone deficit exists in the upper stratosphere. Possible causes for such a discrepancy are investigated in light of current model and experimental uncertainties. In particular, the observational ratio of atomic oxygen to zone appears to disagree with the model results.

Froidevaux, L.↗