Cm6 Molecule Research Articles

Gas-phase kinetics of ground-state platinum with O2,NO, N2Oand CH4

The gas-phase reactivity of ground-state platinum with O2, NO, N2O and CH4 is reported. Platinum atoms were produced by the photodissociation of [Pt(CH3)3(C5H4CH3)] and detected by laser-induced fluorescence. The reaction rates of platinum with all the reactants are pressure dependent indicating adduct formation; however, the reaction with N2O has a bimolecular component. The room-temperature limiting low-pressure third-order rate constants in argon buffer are (2.3±0.2)×10-31 molecule-2 cm6 s-1, (4.3±0.4)×10-31 molecule-2 cm6 s-1, (3.7±0.5)×10-31 molecule-2 cm6 s-1 and (2.1±0.9)×10-28 molecule-2 cm6 s-1 for O2, NO, N2O and CH4, respectively, where the uncertainties are ±2σ. The limiting high-pressure second-order rate constants are (2.5±0.5)×10-12 molecule-1 cm3 s-1, (2.3±0.8)×10-11 molecule-1 cm3 s-1, (2.3±0.3)×10-12 molecule-1 cm3 s-1 and (6.3±0.3)×10-12 molecule-1 cm3 s-1 for O2, NO, N2O and CH4, respectively. The second-order rate constant for the abstraction channel for the reaction with N2O at 296 K is approximately 1×10-13 molecule-1 cm3 s-1.

Temperature dependent study of the gas-phase kinetics of Zr(a3F2) and Hf(a3F2)

The second-order rate constants of gas-phase Zr(a3F2) and Hf(a3F2) with O2, N2O, CO2, NO, H2O, SO2 and SF6 as a function of temperature are reported. For Zr(a3F2), the bimolecular rate constants (in molecule-1 cm3 s-1) are described in Arrhenius form by k(O2)=(2.6±0.5)×10-10 exp(-6.2±0.7 kJ mol-1/RT), k(N2O)=(1.6±0.3)×10-10 exp(-3.3±0.6 kJ mol-1/RT), k(CO2)=(1.4±0.2)×10-10 exp(-5.1±0.4 kJ mol-1/RT), k(NO)=(1.6±0.3)×10-10 exp(-1.9±0.5 kJ mol-1/RT) and k(SF6)=(3.4±1.3)×10-10 exp(-28±2 kJ mol-1/RT), where the uncertainties are ±2σ. The rate constants for Zr reacting with H2O and SO2 were temperature insensitive with room-temperature rate constants of 9.1×10-11 and 3.5×10-10 molecule-1 cm3 s-1, respectively. For Hf(a3F2), the bimolecular rate constants are described in Arrhenius form by k(O2)=(1.2±0.1)×10-10 exp(-2.8±0.3 kJ mol-1/RT), k(N2O)=(1.5±0.2)×10-10 exp(-11.3±0.5 kJ mol-1/RT), k(CO2)=(1.4±0.2)×10-10 exp(-17.5±1.2 kJ mol-1/RT), k(H2O)=(2.1±0.3)×10-11 exp(-3.4±0.6 kJ mol-1/RT), k(SF6)=(7±5)×10-10 exp(-41±3 kJ mol-1/RT). The rate constants for Hf reacting with NO and SO2 were temperature insensitive with room-temperature rate constants of 1.0×10-10 and 3.0×10-10 molecule-1 cm3 s-1, respectively. The disappearance rates for all the reactants are independent of total pressure, indicating bimolecular abstraction processes.

Ground-based FTIR Measurements of Stratospheric Trace Species from Aberdeen During Winter and Spring 1993/94 and 1994/95 and Comparison with a 3D Model

Vertical column abundances of HCl, ClONO2, HF and HNO3 have been obtained from infrared solar absorption measurements made at Aberdeen, UK (57°N, 2°W) during the periods January 13 1994 - May 8 1994 and November 23 1994 - April 19 1995. The measurements reveal the partitioning of inorganic chlorine (Cly) inside and outside the polar vortex during these two winter and spring periods. Stratospheric temperatures within the northern polar vortex during 1993/94 were not cold throughout January and most of February. The measurements reported here suggest that following a brief period of chlorine activation in late February and early March, the active chlorine within the vortex recovered rapidly to form ClONO2 resulting in in-vortex ClONO2 columns of 7 × 1015 molecules cm-2. In contrast, measurements during January 1995 suggest extensive invortex activation with in-vortex HCl + ClONO2 as low as 3.6×1015 molecules cm-2. High day-to-day variability in the ClONO2 columns observed during February is evidence for the transport of ClONO2 rich air from high to mid latitudes during the late winter. The implications for mid latitude O3 loss are discussed. A preliminary comparison of the HCl, ClONO2, and HNO3 column data from winter 94/95 with a three-dimensional chemical transport model shows that the model generally reproduces well the day-to-day variability and absolute magnitude of the observed columns, especially for HNO3 outside of the vortex.

Measurement of the NO + O3 Reaction Rate at Atmospheric Pressure Using Realistic Mixing Ratios

Accurate values for the rate and temperature dependence of the reaction NO + O3 → NO2 + O2 are important in the chemical modelling of photochemical processes in the atmosphere. Previous measurements have been made at low total pressures and/or with very large mixing ratios relative to those observed in the atmosphere. In this study the reaction rate has been measured using a novel approach under tropospheric conditions of temperature and pressure, and at tens of ppb (mixing ratios of 1 in 108) between 263 and 328 K. The resultant Arrhenius expression (k=Ae-Ea/RT) gives a larger activation energy (Ea/R=1670 ± 100) than the recommended literature value (Ea/R=1400 ± 200), and a larger pre-exponential factor (A=5.1 ± 1.6 × 10-12 cf. recommended A=2.0 × 10-12), but the second-order rate constant at 298 K (1.90 × 10-14 molecules cm-3 s-1 ± 10%) is similar to the recommended value. The results confirm a lack of pressure dependence of the reaction, but were made over too small a range in temperature to address the issue of curvature of the simple Arrhenius expression.

Pressure and Temperature Dependence of the Gas-Phase Reaction of SO3 with H2O and the Heterogeneous Reaction of SO3 with H2O/H2SO4 Surfaces

The gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O−H2SO4 surfaces have been studied in a fast flow reactor coupled to a chemical ionization mass spectrometer (CIMS) for species detection. The gas-phase reaction was studied under turbulent flow conditions over the pressure range from 100 to 760 Torr N2 and the temperature range from 283 to 370 K. The loss rate of SO3 was measured under pseudo-first-order conditions; it exhibits a second-order dependence on water vapor concentration and has a strong negative temperature dependence. The first-order rate coefficient for the SO3 loss by gas-phase reaction shows no significant pressure dependence and can be expressed as kI(s-1) = 3.90 × 10-41 exp(6830.6/T)[H2O]2 where [H2O] is in units of molecule cm-3 and T is in Kelvin. The overall uncertainty of our experimentally determined rate coefficients is estimated to be ±20%. At sufficiently low SO3 concentrations (<1012 molecule cm-3) the rate coefficient is independent of the initial SO3 level, as expected for a gas-phase reaction mechanism involving one SO3 and two H2O molecules. However, at higher concentrations and lower temperatures, increased rate coefficients were observed, indicating a fast heterogeneous reaction after the onset of binary homogeneous nucleation of acid hydrate clusters leading to particle formation, which was verified by light-scattering experiments. The heterogeneous loss of SO3 to the reactor walls has also been investigated under low pressure (1.1−12.5 Torr) laminar flow conditions. The loss rate is highly dependent on the humidity of the surface. In the presence of excess water the reactive sticking coefficient approaches unity and the wall loss rate is gas diffusion limited; under dry conditions it approaches zero, as expected. The atmospheric implications of the homogeneous and heterogeneous SO3−water reaction are discussed.

Determination of the Product Branching Fraction for HNF in the Reaction of HN3 with F

The reaction of F atoms with HN3 forms the products HF (+N3) or HNF (+N2). The product branching fraction for this reaction has been investigated in a room-temperature flow reactor using laser-induced fluorescence to monitor the concentration of HNF. A microwave discharge applied to a dilute flow of CF4 in argon served as the F atom source. Using reactant concentrations of (0.7−3.0) × 1012 and (1.7−20.0) × 1012 molecules cm-3 for CF4 and HN3, respectively, the rate constant for formation of HNF from HN3 + F was determined to be (6.3 ± 3.5) × 10-12 cm3 molecule-1 s-1. The average product branching fraction for formation of HNF was determined to be . The quenching rate constant of electronically excited HNF by Ar was determined to be (3.0 ± 0.3) × 10-11 cm3 molecule-1 s-l. In addition, secondary reactions between HNF and HN3, F, and H2S were examined. No reaction was observed to occur between HNF and HN3. The reaction of HNF + F was observed to occur with an estimated rate constant of 2.0 × 10-10 cm3 molecu...

Experimental and Theoretical Studies of the Reaction of Al Atoms with OCS and CS2

The temperature (295−382 K) and pressure (5−50 Torr) dependences of the rate constants of the Al + OCS and Al + CS2 reactions have been studied by using time-resolved laser-induced fluorescence spectroscopy. The temperature dependence of the Al + OCS reaction is characterized by the rate expression = (4.2 ± 2.1) × 10-11 exp[5.3 ± 1.4 kJ mol-1/RT] cm3 molecule-1 s-1. The rate expression for the former reaction was found to be independent of total pressure up to 50 Torr. The pressure dependent rate expression for the reaction of Al + CS2 obeys = (1.2 ± 0.3) × 10-28 cm6 molecule-2 s-1. Ab initio correlated calculations have been performed to determine the relative stability of the Al−CS2 and Al−OCS collision complexes as well as the relative energetics of reactants and products.

Kinetics and Mechanism of the Gas Phase Reaction of Atomic Chlorine with CH2ICl at 206−432 K

The title reaction was studied using two different experimental techniques: laser flash photolysis with resonance fluorescence detection of Cl atoms and continuous photolysis with FTIR detection of end products. Over the temperature range 206−432 K the rate constant for reaction of Cl atoms with CH2ICl is given (to within ±15%) by the Arrhenius expression k1 = 4.4 × 10-11 exp(195/T) cm3 molecule-1 s-1, which gives k1 = 8.5 × 10-11 cm3 molecule-1 s-1 at 298 K. Variation of the total pressure of N2 diluent over the range 5−700 Torr at 295 K had no discernible (<10%) effect on the rate of reaction. At 295 K in 100−700 Torr of N2 the reaction proceeds via iodine transfer to give CH2Cl radicals. As part of this work the rate constant k(CH2Cl+O2+M) was measured at 295 K in the presence of 1−800 Torr of N2 diluent. The results were well described by the Troe expression with a broadening factor Fc of 0.6 and limiting low- and high-pressure rate constants of k0 = (1.8 ± 0.1) × 10-30 cm6 molecule-2 s-1 and k∞ = (3...

Visualisation of a supersonic free-jet expansion using laser-induced fluorescence spectroscopy: Application to the measurement of rate constants at ultralow temperatures

Σ+, v′=0) by air at 26±4 K. A value of 2.56±0.40×10-10 molecule-1 cm3 s-1 was obtained, a factor of more than four higher than at room temperature, and consistent with attractive forces dominating the quenching of OH(A2Σ+). A d etailed computational fluid dynamics (CFD) simulation of the supersonic free-jet expansion was performed, providing a two-dimensional visualisation of temperature and density variations throughout the expansion. The CFD calculations reproduced the salient features of the experimental temperature and density profiles along the centreline. Comparison between experiment and computation has allowed validation of CFD codes.

Solar absorption measurements of stratospheric OH in the UV with a Fourier-transform spectrometer

We performed solar absorption measurements of OH in the UV using a Fourier-transform spectrometer (FTS). The experiments were carried out in the high Arctic at Ny- Alesund (79 degrees N, 12 degrees E) during the summer of 1996. We accomplished the analysis in two ways: (1) by studying single solar-absorption spectra recorded in the middle of the solar disk and (2) by utilizing the Doppler shift of two spectra, recorded on the east and west sides of the solar disk. The results of both analysis methods agree and give total columns of approximately 6 x 1013 molecules cm-2 for solar zenith angles of 60 degrees . To find out the main noise contribution in the spectra, we compared the measured and calculated signal-to-noise ratios (SNR 's). During clear-sky conditions the photon noise determines the total SNR. However, because a FTS is extremely sensitive to source fluctuations, conditions that were already slightly cloudy increased the scintillation noise, preventing OH analysis. The noise contribution caused by the instrumental sampling process itself was found to be negligible; even through two sampling positions had to be interpolated between the laser zero crossings.

Kinetics and Thermochemistry of SiH3 + NO and SiD3 + NO Reactions: Pressure Falloff and SiH3−NO Bond Energy

The reactions of silyl and silyl-d3 radicals with nitric oxide, SiH3 + NO → SiH3NO (1) and SiD3 + NO → SiD3NO (2), have been studied using pulsed excimer laser photolysis coupled with time-resolved photoionization mass spectrometry over the temperature range 301−817 K. The rate constants were measured in the ranges 0.75 × 1016 ≤ [He] ≤ 32.0 × 1016 molecules cm-3 and 301 ≤ T ≤ 611 K. Both reactions are in the pressure falloff regions considerably far from the true low-pressure limit. RRKM fittings using the Troe factorization approach yielded the low-pressure limit and high-pressure limit rate constants: k1,tri = (3.12 ± 0.12) × 10-29(T/298)-2.96±0.09 cm6 molecule-2 s-1, k2,tri = (9.02 ± 0.40) × 10-29(T/298)-3.36±0.11 cm6 molecule-2 s-1, k1,inf = (3.9 ± 1.1) × 10-11(T/298)-1 cm3 molecule-1 s-1, and k2,inf = (5.0 ± 1.3) × 10-11(T/298)-1 cm3 molecule-1 s-1. Equilibrium constants were measured in the temperature range 711−817 K. The standard enthalpies of reactions 1 and 2 were determined using the third law...

Atmospheric Chemistry of HFC-236cb: Fate of the Alkoxy Radical CF3CF2CFHO

An FTIR/environmental chamber technique was used to study the fate of the alkoxy radical CF3CF2CFHO formed in the atmospheric degradation of HFC-236cb (CF3CF2CFH2). Experiments were performed over the temperature range 228−296 K at 7.8−1000 Torr total pressure. Two reaction pathways are possible for CF3CF2CFHO radicals: reaction with oxygen, CF3CF2CFHO + O2 → CF3CF2C(O)F + HO2 (kO2) and decomposition via C−C bond scission, CF3CF2CFHO → CF3CF2 + HC(O)F (kd). CF3CF2CFHO radicals were produced by two reactions: the CF3CF2CFHO2 self-reaction and the CF3CF2CFHO2 + NO reaction. In the absence of NO at 800 Torr total pressure the rate constant ratio kd/kO2 was determined to be ( ) × 1025 exp(−(3560 ± 295)/T) molecules cm-3. The pressure dependence of kd/kO2 was studied at 238 K and was well described by a Troe type expression using kd,0/kO2 = 30.8 ± 6.9 and kd,∞/kO2 = (2.31 ± 0.12) × 1019 molecules cm-3 where kd,0 and kd,∞ are the second- and first-order rate constants for decomposition in the low- and high-pressure limits, respectively. CF3CF2CFHO radicals formed in the CF3CF2CFHO2 + NO reaction undergo more C−C bond scission than those generated in the CF3CF2CFHO2 self-reaction. This is consistent with a significant fraction ( %) of the alkoxy radicals being formed with sufficient internal energy to undergo prompt decomposition. Overall, we calculate that less than 1% of the CF3CF2CFH2 (HFC-236cb) released to the atmosphere degrades to form CF3CF2C(O)F while >99% gives CF3CF2 radicals and HC(O)F.

Products of the Gas-Phase Reactions of NO3 Radicals with Furan and Tetramethylfuran

Furan and substituted furans were found to be atmospheric reaction products of the OH radical-initiated degradation of conjugated dienes, such as 1,3-butadiene, cis-1,3-pentadiene, and isoprene. They are also products of combustion processes. The purpose of this work is to identify the primary formed stable reaction products of the reaction of NO3 radicals toward furan and tetramethylfuran. Experiments were performed under flow conditions in the pressure range 5.5 < P (mbar) < 100 at 298 K. GC−MS/FID, direct MS, and long-path FT-IR served as detection techniques. cis-Butenedial and 3H-furan-2-one were found as main products of the reaction of NO3 radicals with furan in N2 with averaged formation yields of 0.77 and 0.19, respectively, independent of total pressure in the tube. In experiments with a maximum O2 concentration of 2.2 × 1018 molecule cm-3, an increase of the 3H-furan-2-one yield was observed up to 0.38. There were no indications for the formation of nitrates or peroxynitrates. In the reaction o...

Kinetics of Ground-State Cd Reactions with Cl2, O2, and HCl over Wide Temperature Ranges

Reactions of ground-state Cd(1S) have been investigated in the 10−80 mbar pressure range using the high-temperature fast-flow reactor (HTFFR) technique. The Cd + Cl2 → CdCl + Cl reaction yielded k(466−875 K) = 9.0 × 10-10 exp (−5959K/T) cm3 molecule-1 s-1, with a 2σ precision of about ±15% and corresponding accuracy of ±27%. For Cd + O2 and Cd + HCl reactions the upper limits are k(300 K) < 1 × 10-15 cm3 molecule-1 s-1, or < 5 × 10-34 cm6 molecule-2 s-1. Orbital symmetry rules are used to deduce the reaction products and, along with electron configuration arguments, help explain the slowness of the latter two reactions. The importance of these results for the speciation of Cd, Hg, and Zn in waste incineration is discussed, and the consequences of the present findings for transition metal reactions are mentioned.

Highly sensitive optically heterodyned, Raman-induced Kerr-effect spectrometer using pulsed lasers

A highly sensitive, optically heterodyned Raman-induced Kerr-effect (OHD-RIKE) spectrometer designed for the spectroscopy of radicals and molecular ions in discharges is presented. A linearly polarized, pulsed dye laser beam (probe) is crossed in the sample with the circularly polarized second harmonic of a pulsed Nd:YAG laser beam (pump). The nonlinear interaction between these beams and the sample induces a birefringence that creates a polarization component of the probe laser orthogonal to its original polarization. The magnitude of the birefringence peaks at Raman resonances as the probe laser is tuned. The combination of detection of the birefringence by the use of polarizers that have an extinction coefficient of the order of 108 with a pulsed probe laser with sufficient power to maximize the signal relative to the shot noise in the heterodyne field provides the basis for the high sensitivity. By careful adjustment of the polarization of the pump laser, we can decrease nonresonant contributions to the birefringence and further increase the signal-to-noise ratio. By subtracting a fraction of the magnitude of a reference signal from the OHD-RIKE signal, we further enhance the sensitivity by reducing pulse-to-pulse fluctuations of the pulsed probe laser to near (10 times) the shot-noise limit of the local oscillator intensity. The spectrometer is tested on the CO2 molecule in the Fermi-resonance region. Using a high-power (750-mJ) single-mode YAG laser as both the Raman pump laser and the pump for the dye laser that is used as the probe laser allows us to operate near Raman gain saturation, as observed in other coherent Raman spectroscopies. Preliminary studies of saturation for the OHD-RIKE in CO2 are presented. We present both broadband and narrow-band spectra of CO2. For broadband (∼10- GHz) studies we achieve sensitivities of 6×1013 molecules cm-3 for a signal-to-noise ratio of ∼1. In narrow-band spectra (∼400 MHz) we observe increased backgrounds owing to problems that arise because of the high intensities and the extremely high polarization extinction coefficients. These higher backgrounds prevent attainment of the improved signal-to-noise ratio expected from the larger Raman gains that are presumed to result from narrow-band operation. Possible solutions to the background problems are discussed.

Association and isotopic exchange reactions of CH(CD)[X 2Π]+CO

The reaction rates for CH12 and CD12 with normal isotopic abundance CO and CO13 have been studied at 293 K for pressures between 12.5 and 500 Torr and at 100 Torr for temperatures be-tween 293 and 650 K. The pressure and temperature dependence of the addition reaction of CH with CO have been measured. The addition rate coefficient can be fit to the expression 7.2±0.3×10−12(T/293)−2.4±0.2 cm3 molecule−1 s−1 at 100 Torr total pressure (He buffer). A fit of the pressure dependence to a Troe expression with Fc=0.6 yields a low-pressure rate constant (k0) of 2.4±0.3×10−30 cm6 molecule−2 s−1. The rate for carbon atom exchange has been measured by comparison of the C13 labeled and unlabeled reaction rates. The isotopic exchange reaction is 1.0±0.2×10−12 cm3 molecule−1 s−1 at 20 Torr. The deuterium isotope effect on the exchange rate is large, with an inverse kinetic isotope effect (kH/kD)=0.28±0.08 at 20 Torr. This inverse isotope effect reflects the competition between collisional stabilization and isomerization, and is a convolution of isotope effects for the isomerization, unimolecular dissociation, and stabiliza-tion rates. The experimental results are consistent with a mechanism for exchange that in-volves isomerization of an HCCO adduct via an oxiryl intermediate, and indicate that insertion into the C–O bond is not important in this reaction.

Unimolecular Decomposition of the FCO Radical

The kinetics of the unimolecular decomposition of the FCO radical has been studied experimentally in a heated tubular flow reactor coupled to a photoionization mass spectrometer. Rate constants for the decomposition were determined in time-resolved experiments as a function of temperature (951−1051 K) and bath gas density ((6−12) × 1016 molecules cm-3) in two bath gasesHe and N2. The rate constants are in the low-pressure limit under the conditions of the experiments and can be described by the following expressions: k°1(He) = 10-8.74±0.28 exp(−(14169 ± 653 K)/T) cm3 molecule-1 s-1 (from the fitting of experimental data) and k°1(N2) = 10-8.71 exp(−14154 K)/T) cm3 molecule-1 s-1 (evaluated from comparison of data obtained in nitrogen and helium bath gases). The thermochemistry of the reaction was analyzed on the basis of the data available in the literature and those obtained in the current study. An upper limit estimate was obtained for the rate constant of the reaction of FCO radicals with O2 at the temperature of 800 K: k(FCO+O2→products) ≤ 4 × 10-16 cm3 molecule-1 s-1.

ANALYSIS OF THE PLASMA-COMA OF COMET P/HALLEY BY IMAGE PROCESSING TECHNIQUES OF BOCHUM'S PHOTOPLATES

ABSTRACT Photographic and photoelectric observations of comet P/Halley's ion gas coma from CO+ at 4250A were part of the Bochum Halley Monitoring Program, conducted from 1986 February 17, to April 17 at the European Southern Observatory on La Silla (Chile). In this spectral range it is possible to watch the continuous formation, motion and expansion of plasma structures. To observe the morphology of these structures 32 CO^+ photos (glass plates) from P/Halley's comet have been analysed. They have a field of view of 28 degrees 6 X 28 degrees 6 and were obtained from 1986 March 29, to April 17 with exposure times between 20 and 120 minutes. All photos were digitized with a PDS 2020 GM (Photometric Data System) microdensitometer at the Astronomisches Institut der Westfalischen Wilhelms-Universitat in Munster (one pixel = 25 microns X 25 microns approximately 46 arcsec 88 X 46 arcsec 88). After digitization the data were reduced to relative intensities, and the part with proper calibrations were also converted to absolute intensities, expressed in terms of column densities using the image data systems MIDAS (Munich Image Data Analysis System; ESO - Image Processing Group, 1988) and IHAP (Image Handling And Processing; Middleburg, 1983). With the help of the Stellingwerf-Theta-Minimum-Method (Stellingwerf, 1978) a period of (2.22 ± 0.09) days results from analysis of structures in the plasma-coma by subtracting subsequent images. The idea behind subtracting subsequent images is that rotation effects are only 10% phenomena on gas distribution. Difference images are than used to supress the static component of the gas cloud. The CO+ column density data (in molecules cm-2) were compared with the data of CN column density from Schulz (1990) in all common days. The results show that the relations between CO+ and CN in average column density values (NCO^+ /NCN) are 11.6 for a circular slit with average diameter (Phi) of 6 arcminute 1 which corresponds to a distance from the nucleus (rho) equal to 6.3 X 104 km; 20.0 for Phi = 7 arcminute 1 and rho = 7.3 X 104 km; 8.1 for Phi = 8 arcminute 5 and rho = 8.7 X 104 km; 35.6 for Phi = 11 arcminute 9 and rho = 1.2 X 105 km; and 31.3 for Phi = 16 arcminute 7 and rho = 1.7 X 105 km. These values are in perfect agreement with the data from Vivekananda et al. (1990) for short distances (rho from 3.9 X 103 to 1.2 X 104 km) and small slit diameters (Phi from 0.4 to 1.2 arcminutes). With the use of slits with large diameters it is possible to get some information about the outer coma of the comet (in this work, from 60000 until 170000 km far away from the nucleus). At these distances (about 105 km) the CO+ column density changes only due to the geometrical dilution, because the CO+ parent molecules are already photoionized or photodissociated (Voelzke, 1989), which means that through different processes the neutral gas that flows to about 105 km from the nucleus is already transformed into ionized gas.

Estimation of hydroxyl radical concentrations in the marine atmospheric boundary layer using a reactive atmospheric tracer

Hydroxyl radical (OH) concentrations in the atmospheric boundary layer over a number of remote ocean locations are calculated from the measured diurnal variation in atmospheric dimethylsulfide (DMS). By using averaged DMS data sets from extended periods, the calculation yields OH concentrations averaged over periods from several days to weeks. These average OH concentrations range from 7×105 to 2.9×106 molecules cm-3, corresponding to midday maxima of 3 to 12×106 molecules cm-3. The lowest values correspond to studies with the lowest light intensity (Antarctic summer and South Atlantic winter), and the highest values to regions with probable anthropogenic influence. In addition to the long term averages, daily average OH levels can be calculated for most days in a two week period from a cruise in the tropical eastern Pacific. These calculations are in good argeement with global average OH levels derived from other tracers, and are consistent with model OH calculations when allowance is made for variation in ambient ozone levels between the studies. Estimates of gas exchange made from the diurnal variation of DMS suggest that either the gas exchange coefficient of DMS or the boundary layer mixing depth may have been overestimated in past analyses.

Measurements of stratospheric chlorine monoxide (ClO) from groudbased FTIR observations

Infrared absorption features due to ClO in the lower stratosphere have been identified from groundbased solar absorption spectra taken from Aberdeen, U.K. (57° N, 2° W) on 20 January 1995. A vertical column abundance of 3.42 (±0.47)×1015 molec cm-2 has been derived from 13 independent absorption features in the P and R branches of the (0–1) vibration-rotation band of 35ClO, spanning the spectral region 817–855 cm-1. The observed absorption features are consistent with very high levels of ClO (approximately 2.6 parts per billion by volume (ppbv)) in the altitude range 16–22 km. A comparison of this profile with a 3D chemical transport model profile indicates the observation was made inside the polar vortex and shows good qualitative agreement but the model underestimates the concentrations of ClO. Simultaneous measurements of other species were made including HCl, HF and ClONO2. These columns yield a value for HCl+ClONO2+ClO of 7.02±0.65×1015 molec cm-2. This is lower than the total inorganic chlorine (ClO y ) column of 10.7±1.6×1015 molec cm-2 estimated from mean measured (HCl+ClONO2)/HF ratios together with in-vortex HF measurements. The discrepancy is probably due to significant amounts of the ClO dimer (Cl2O2) in the lower part of the stratosphere. The measurements of highly elevated levels of ClO are used to estimate O3 loss rates at the 400, 475 and 550 K levels making assumptions about the probable distribution of ClO and Cl2O2. These are compared with loss rates derived from ozone sonde data.