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NASA NTRS1995

Stratospheric ClO profiles from McMurdo Station, Antarctica, spring 1992

We describe ground-based measurements of ClO made at McMurdo Station, Antarctica, during September and October 1992. Vertical profiles were retrieved from molecular rotational emission spectra at 278 GHz. Peak mixing ratios of 1.6 +/- 0.3 ppbv were seen in mid-September at approximately 18 km altitude, suggestive of somewhat larger quantities than were measured at the same site and season in 1987. As the core of the polar vortex moved away from McMurdo by early October, the ClO mixing ratio at this altitude dropped to less than 0.2 ppbv, coincident with increasing temperature, ozone, and NO2. The diurnal variation of ClO was also observed. The lower stratospheric layer from 15 to 27 km was found to reach approximately midday abundance by 2 - 3 hours after sunrise. The column abundance in this layer began to decrease by the period 4 - 2 hours before sunset and had declined to approximately one quarter of its midday value by 2 - 0 hours before sunset. In contrast, the ClO column in the upper stratosphere, from 28 to 50 km, built up slowly until midday and remained large while sunlight persisted.

Emmons, L. K.↗
NASA NTRS1993

MLS CLO observations and arctic polar vortex temperatures

Analysis of Upper Altmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) observations in early January 1992 shows a clear relationship between predicted polar stratospheric cloud formation along the back trajectory and elevated ClO amounts. These findings are in good agreement with aircraft observations. The MLS observed variation of ClO amounts within the vortex also fits the pattern of ClO change as a result of air parcel solar exposure and nitric acid photolysis. Outside the polar vortex, the occasional highly elevated ClO appear statistically consistent with MLS measurement noise.

Schoeberl, M. R.↗
NASA NTRS1995

UARS MLS observations of lower stratospheric ClO in the 1992-93 and 1993-94 Arctic winter vortices

Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) measurements of lower stratospheric ClO during 1992-93 and 1993-94 Arctic winters are presented. Enhanced ClO in the 1992-93 winter was first observed in early December, and extensively during February when temperatures were continually low enough for polar stratospheric cloud (PSCs). Sporadic episodes of enhanced ClO were observed for most of the 1993-94 winter as minimum temperatures hovered near the PSC threshold, with largest ClO amounts occurring in early March after a sudden deep cooling in late February.

Waters, J. W.↗
NASA NTRS1996

Latitudinal Distribution of Upper Stratospheric ClO as Derived from Space Borne Microwave Spectroscopy

Latitudinal distributions of upper stratospheric ClO measured by MAS during the three ATLAS missions are presented for northern hemisphere (NH) spring equinox in 1992, southern hemisphere (SH) early fall in 1993, and NH fall in 1994. The MAS ClO results are shown along with correlative MLS observations. The results of both instruments consistently show the same latitudinal features. The ClO maximum in the NH spring occurs at mid latitudes, whereas the latitudinal ClO maximum in both the NH and SH fall occurs at high latitudes. The volume mixing ratio maxima were significantly higher in the fall (0.7-0.8 ppbv) than in spring (0.5-0.6 ppbv). Qualitatively, these results are consistent with calculations of several 2-D models.

Aellig, C. P.↗
NASA NTRS1995

UARS MLS Observations of Lower Stratospheric ClO in the 1992-93 and 1993-94 Arctic Winter Vortices

UARS (Upper Atmosphere Research Satellite) MLS (Microwave Limb Sounder) measurements of lower stratospheric ClO during the 1992-93 and 1993-94 Arctic winters are presented. Enhanced ClO in the 1992-93 winter was first observed in early December, and extensively during February when temperatures were continually low enough for PSCs. Sporadic episodes of enhanced ClO were observed for most of the 1993-94 winter as minimum temperatures hovered near the PSC threshold, with largest ClO amounts occurring in early March after a sudden deep cooling in late February.

Waters, J. W.↗
NASA NTRS1977

Measurements of the electronic transition moment of the A 2 Pi-X 2 Pi system of ClO

The electronic transition moment of the A 2 Pi-X 2 Pi band system of ClO has been determined from spectral emission measurements behind incident shock waves in Cl2-O2 mixtures. The temperature of the radiating gas was typically 3500 K. Narrow bandpass radiometers were used to obtain absolute measurements of the ClO emission from which electronic transition moments were derived by a synthetic spectrum analysis. Absolute measurements of the O2 Schumann-Runge system and the photo-dissociation cross section of ClO are also reported.

Cooper, D. M.↗
NASA NTRS1978

Infrared vibration-rotation spectra of the ClO radical using tunable diode laser spectroscopy

Tunable diode laser spectroscopy is used to measure the infrared vibration-rotation spectra of the ClO radical. The radical is generated in a flow system where a Cl2-He mixture passes through a microwave discharge to dissociate the Cl2. An O3-O2 mixture from an ozone generator is injected into the system downstream of the microwave discharge where O3 combines with Cl to form ClO. By adjusting the gas flow rates to yield an excess of Cl atoms, all the ozone is combined. ClO concentration is measured with UV absorption at 2577 and 2772 A and a deuterium lamp as a continuous source. Total cell pressure is 5.5 torr. The diode laser spectrometer is calibrated with ammonia lines as a reference where possible. The frequency of vibration-rotation lines is expressed as a function of rotational quantum number, fundamental vibrational frequency, and the rotational constants of the upper and lower vibrational states.

Rogowski, R. S.↗
NASA NTRS1980

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

The influence of short lived photochemically produced species on solar occultation measurements of ClO and NO was examined. Time varying altitude profiles of ClO and NO were calculated with a time dependent photochemical model to simulate the distribution of these species during a solar occultation measurement. These distributions were subsequently used to calculate simulated radiances for various tangent paths from which mixing ratios were inferred with a conventional technique that assumes spherical symmetry. These results show that neglecting the variation of ClO in the retrieval process produces less than a 10 percent error between the true and inverted profile for both sunrise and sunset above 18 km. For NO, errors are less than 10 percent for tangent altitudes above about 35 km for sunrise and sunset; at lower altitudes, the error increases, approaching 100 percent at altitudes near 25 km. the results also 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 of the ClO + NO2 + M reaction

The ClO + NO2 + M reaction has been studied with two techniques: Fourier transform infrared spectroscopy of the products, and flash photolysis-ultraviolet absorption to monitor the decay of ClO in excess NO2. The measured third-order rate constant is 1.5 x 10 to the -31st cm to the 6th/molecule-squared per sec at 298 K with M = N2, in good agreement with previous literature values, but the rate constant appears to decrease by up to a factor of three in the presence of increasing amounts of OClO. For the infrared studies a stoichiometric mixture of ClO and NO2 was prepared in a flow system by mixing NO with OClO; at least as much NO2 as ClONO2 was produced under a variety of experimental conditions. These two sets of results are incompatible with the assumption made in previous kinetic studies that ClONO2 is the only recombination product; other isomers such as OClONO or ClOONO are likely to be formed three to four times faster. These results imply that potential stratospheric ozone depletion due to chlorofluoromethanes may be even larger than previously thought.

Molina, M. J.↗
NASA NTRS1984

Kinetics of the reaction O + ClO yields Cl + O2

The bimolecular rate constant for the reaction O + ClO yields Cl + O2 has been measured over the temperature range 236-422 K in a discharge flow system using atomic oxygen resonance fluorescence at 130 nm to monitor the decay of O in an excess of ClO. The results are found to be independent of the method used to produce the ClO radical. At 296 K, the rate constant is given by (3.6 + or - 0.7) x 10 to the -11th cu cm/sec and the temperature dependence expressed in Arrhenius form by k2 = (5.0 + or - 1.0) x 10 to the -11th exp-(96 + or - 20)/T cu cm/sec. The results are compared with earlier values, and the implications for stratospheric chemistry are discussed briefly.

Leu, M.-T.↗
NASA NTRS1984

Kinetics of the reaction O + ClO yields Cl + O2

The kinetics of the reaction O + ClO yields Cl + O2 (k1) were studied by using resonance fluorescence to monitor the decay of O atoms which had been generated from the laser photolysis of ClO. A discharge flow system was used to generate the ClO and its concentration was measured directly by multipass absorption spectrometry. The results are k1(298) = (4.2 + or -0.8) x 10 to the -11th cu cm/s (including systematic errors) with E/R determined to lie in the range + or - 200 K. The literature data are reviewed and a composite value of k1(T) = 6.0 x 10 to the -11th exp-(100/T) is recommended for stratospheric modeling calculations. The atmospheric implications of the revised rate constant are discussed briefly.

Margitan, J. J.↗
NASA NTRS1988

ClO and O3 stratospheric profiles - Balloon microwave measurements

Stratospheric ClO and O3 vertical profiles were measured at 32-deg N in May 1985 and October 1986. The ClO profiles and diurnal variation are in general agreement with theory, but have somewhat less midday ClO near 34 km. Measured 35-45 km O3 is larger than theoretical, and no stratospheric O3 diurnal variation was observed, as expected. HNO3 was detected, and an H2O2 upper limit was obtained.

Waters, J. W.↗
NASA NTRS1989

Kinetics of O3 destruction by ClO and BrO within the Antarctic vortex - An analysis based on in situ ER-2 data

The kinetics of ozone destruction within the Antarctic polar vortex are studied via simultaneous in situ observations of ClO, BrO, O3, N2O, pressure, and temperature. It is found that the chlorine dimer mechanism rate, limited by the reaction ClO + ClO + M yields ClOOCl + M, contributes the most to the integrated rate of ozone destruction within the vortex on isentropic surfaces between altitudes of 14 and 18.3 km.

Anderson, J. G.↗
NASA NTRS1989

Kinetics and product studies of the reaction ClO + BrO using flash photolysis-ultraviolet absorption

The reaction between BrO and ClO was studied over the pressure range 50-700 torr and temperature range 220-400 K, using the flash photolysis-ultraviolet absorption method described by Watson et al. (1979). In order to investigate the mechanism of the BrO + ClO reaction, the product branch reactions Br + Cl2O yielding ClO + BrCl and Cl2O + h(nu) yielding products were examined. The rate constant for the overall reaction and the Arrhenius expression for the Br + Cl2O reaction are given, as well as the quantum yield for the production of atomic oxygen from the Cl2O photolysis.

Sander, Stanley P.↗
NASA NTRS1989

In situ observations of ClO in the Antarctic - ER-2 aircraft results from 54 deg S to 72 deg S latitude

The spatial and temporal distributions of ClO in the latitude belt from 54 to 72 deg S during the later stages of austral winter were obtained from measurements aboard the ER-2 aircraft flying at the altitude of about 18 km into the Antarctic polar vortex. Mixing ratios of about 10 parts per trillion by volume (pptv) were found in the vicinity of 55 deg S, increasing to 50 pptv at 60 deg S. A steep gradient of the ClO mixing ratio occurred in the vicinity of 65 deg S, where, at a fixed potential temperature, the value of the ClO mixing ratio increased by an order of magnitude within a very few degrees of latitude, defining the edge of the 'chemically perturbed region' (CPR). At the southern extension of the flight track, peak mixing ratios increased sharply from 800 pptv on August 23 to 1100 pptv in early September, and then increased more slowly to 1200 pptv on September 22. This corresponds to enhancements 500 times the typical mid-latitude values.

Brune, W. H.↗
NASA NTRS1991

In situ measurements of midlatitude ClO in winter

In situ measurements of ClO in the winter lower stratosphere are presented for six flights of the NASA ER-2 aircraft from 38 deg N to 61 deg N. Enhanced abundances, increasing in severity with date, were observed below 20 km, where HCl and ClONO2 dominate the inorganic chlorine budget. The greatest mixing ratios, over 150 pptv, were encountered on February 20 and 21, 1989, as the vortex experienced a major warming. Although the timing of these ClO enhancements and the evidence that vortex air can reach midlatitudes suggest that heterogeneous conversion of chlorine compounds within the vortex influences chemistry at midlatitudes, the enhacements observed early in the winter could have been caused by unknown chemistry occurring outside the vortex. In either case, photochemical loss of ozone due to catalytic reactions involving ClO at these mixing ratios may be responsible in part for the ozone decreases observed at high latitudes in the Northern Hemisphere.

Toohey, D. W.↗
NASA NTRS1993

Balloon-borne in situ measurements of ClO and ozone - Implications for heterogeneous chemistry and mid-latitude ozone loss

In situ measurements of chlorine oxide (ClO) obtained on 31 March 1991 with a new balloon-borne instrument are compared to results from a photochemical model which incorporates hydrolysis of N2O5 on sulfate aerosols. With the addition of this process, there is better agreement between calculation and measurement over most of the profile, except below 20 km where observed ClO is greater by as much as a factor of four. In a model which is constrained to reproduce the observed ClO below 20 km, ozone loss by catalytic cycles involving halogen oxides becomes larger than that from NO(x), which would dominate under gas-phase or standard heterogeneous conditions.

Avallone, L. M.↗
NASA NTRS1994

Measurements of the vertical profile, diurnal variation, and secular change of ClO in the stratosphere over Thule, Greenland, February-March, 1992

We report observations of stratospheric chlorine monoxide over the altitude range approx. 16 to 50 km at Thule, Greenland from Feb. 8 to Mar. 24, 1992. A new, more sensitive ground-based mm-wave spectrometer was employed for these measurements, similar in principle to that used earlier for the discovery of low altitude ClO in the Antarctic springtime. In this report, we discuss different aspects of vertical distribution, secular trends, and diurnal variation of ClO in the Arctic stratosphere, based on a preliminary analysis of our Thule data. We see no evidence for large (approx. 1.2-1.5 ppb) amounts of ClO in the lower stratosphere at any time during February or March, in agreement with UARS-MLS findings for this period, and in marked contrast to findings reported for the Arctic in January. We have some evidence for small enhancements (approx. 0.2-0.5 ppb) in the 18-30 km range in late February-early March, which might be associated with volcanic aerosol, rather than PSC, processing.

Dezafra, Robert L.↗