Murray J. McEwan

Ion-Molecule Chemistry in Titan's Ionosphere

Abstract A summary is presented of the information available from laboratory studies of ion-molecule reactions that is relevant to the chemistry occurring in Titans ionosphere. Reaction information from the literature has been collated and many new reactions have been measured, including some ion-atom reactions. The sequences of ion-neutral reactions can lead to a rapid increase in ion size. How this increase may lead to aerosol production at the base of the ionosphere is briefly discussed. Laboratory observations of extremely rapid termolecular ion-neutral association reactions indicate that these association reactions are viable contribution to the ion chemistry at the base of Titans ionosphere.

Application of selected ion flow tube mass spectrometry to real-time atmospheric monitoring.

Data are presented for real-time atmospheric monitoring of volatile organic chemicals (VOCs) in air using selected ion flow tube mass spectrometry (SIFT-MS) technology. These measurements were made by one of the new generation of SIFT-MS instruments. Results are shown for five VOCs that were continually monitored from a stationary sampling point over a 4-day period: ethene, ethanol, 1,3-butadiene, benzene and toluene. All analytes except ethene in the study have at least two simultaneous and independent measures of concentration. These results demonstrate the great advances in SIFT-MS that have been made in recent years. 1,3-Butadiene is measured at a concentration of 9 pptv with a precision of 44%. For a 1-s integration time, a detection limit of 50 pptv is achieved. Instrument sensitivities are reported for all five analytes.

ION-MOLECULE REACTIONS RELEVANT TO TITAN'S IONOSPHERE

Abstract Twenty four new ion-molecule reactions are presented for inclusion in the modeling of the ionosphere of Saturns satellite Titan. Sixteen reactions were re-examined to reduce uncertainties in the previous literature results. In this study we have examined the reactions of N + and N 2 + with CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , HCN, CH 2 CHCN and HC 3 N; the reaction of N + with CH 3 CN; the reactions of C 3 H 5 + with CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , H 2 , HCN, HC 3 N and CH 2 CHCN; the reactions of C 2 N 2 + with C 2 H 2 ; C 2 H 2 + and C 2 N 2 ; C 2 H 4 with C 2 H 3 + , C 2 H 4 , CHCCNH + , and HC 5 N + ; HCNH + with C 2 H 6 ; C 3 H 6 + with C 3 H 6 ; HCN with C 2 H 6 + , C 3 H 6 + , cC 3 H 6 + ,C 2 N 2 + and NO + ; N 2 with C 2 H 2 + and C 2 H 5 + ; C 2 H 4 + and HC 3 N. The ions selected for this study were derived either from nitrogen, appropriate hydrocarbons or nitrites. The reactant neutrals were selected on the basis of their known presence in Titans atmosphere. The reaction products are consistent with the expected increase in ion size through ion molecule reaction processing. Data are also presented for the reactions of 23 ions with molecular nitrogen. Almost all of these ions are unreactive with N 2 .

Gas-phase reactions of some positive ions with atomic and molecular nitrogen

The reactions of the cations CN+, HCN+, HCNH+, HC3N+, HC3NH+, H3+, H2O+, H3O+, N2+, CO+, HCO+, O2+, CO2+, HCO2+, and C2H2+ with atomic and molecular nitrogen have been characterized using a selected ion flow tube (SIFT) operating at room temperature. Rate coefficient and branching ratio data are reported for all ion–neutral reactions studied. Constraints arising from spin conservation considerations are found to be unimportant in cation-N atom processes.

Emission of nitrogen oxides and ammonia from varying rates of applied synthetic urine and correlations with soil chemistry

Synthetic urine was applied at 5 rates, from 0 to 1000 kg synthetic urine-N/ha, to a pasture soil under controlled laboratory conditions. Gaseous emissions of NOx and NH3 were monitored for up to 21 days following application using selected ion flow tube mass spectrometry with N2O measured using electron-capture gas chromatography. During this period soil replicates were destructively sampled to measure changes in soil pH and inorganic-N concentrations. Comparisons were made between measured soil variables, calculated soil concentrations of NH3(g), HNO2, and the measured gas fluxes. At N rates up to 500 kg N/ha, inorganic-N concentrations increased as nitrification progressed over time. With the exception of the 1000 kg N/ha treatment, NO production followed the pattern of increasing nitrification, reaching a maximum of 905 ng NO-N/cm2.h in the 500 kg N/ha treatment 14 days after synthetic urine application. At this time the NO flux was associated best with soil pH, NH4+, and NO2– levels. Over 21 days the maximum cumulative loss as NO-N and N2O-N occurred under the 100 kg N/ha urine treatment, with 6.6 and 6.4% of N applied lost as gas, respectively. NO2 gas fluxes paralleled the NO emissions but were an order of magnitude smaller. Nitrification was inhibited in the 1000 kg N/ha treatment due to the sustained high ammoniacal-N and pH conditions present. These conditions prolonged the NH3 volatilisation from this treatment. NH3 volatilisation, as determined by selected ion flow tube-mass spectrometry, was linearly related to calculated soil NH3 gas concentrations up to 500 kg N/ha on Day 1.

The relationship between recombination, chemical activation and unimolecular dissociation rate coefficients

A new solution to the master equation relating the rate coefficients for unimolecular, recombination (association) and chemical activation reactions, incorporating weak collision effects, is presented. The solution establishes conditions for the validity of the commonly used procedure of relating the recombination rate coefficient, throughout the falloff regime, to the reverse single‐channel unimolecular rate coefficient via the equilibrium constant. In addition, a relationship between the rate coefficient for stabilization in a chemical activation reaction and the reverse multichannel unimolecular dissociation rate coefficient is derived. This result, in conjunction with recently developed methods for fully incorporating angular momentum conservation into the solution of the master equation for unimolecular dissociation, enables both angular momentum and weak collision effects to be accurately incorporated into the solution of the master equation for chemical activation reactions in the falloff regime. A...

Quantitative analysis of trace gases of breath during exercise using the new SIFT–MS technique

Abstract We show how the concentration of the breath gases ammonia, acetone, and isoprene vary with time during exercise using the new selected ion flow tube mass spectrometry (SIFT–MS) technique. The expired breath concentrations of ammonia, acetone and isoprene were observed within the range of 50–500, 100–1400 and 5–400 ppb, respectively. Increasing acetone levels were observed for most subjects during the exercise period. However, isoprene levels decreased with time during exercise. Older subjects showed higher levels of isoprene compared with younger subjects. The ammonia time profile with exercise showed both decreasing and increasing patterns for different subjects.

The reactivity of HOC+ and the proton affinity of CO at O

Abstract Two ion/molecule reactions were observed to produce the isoformyl ion, HOC+, as a product in conjunction with the structural isomer, HCO+. These two reactions are C+ + H2O and CO+ + H2. The reaction mainly used in this study to produce an HOC+/HCO+ mixture in the flow tube was CO+ + H2 and the branching ratio for the production of HOC+ was 0.48. A technique based on the very different proton affinity of CO at O compared with CO at C, was used to distinguish between the isomers. The rates of proton transfer from HOC+ to O2, H2, Kr, Xe, NO, CO, CO2, CH4, and N2O were measured at room temperature. In addition, the reaction with H2 was also observed to isomerize HOC+ into HCO+. The proton affinity of CO at O that best fits these results is 427 kJ mol−1 which yields a value of ΔHf[HOC+] = 990 kJ mol−1.

Fluorescence of aliphatic amines

Abstract Fluorescence of several tertiary aliphatic amines has been excited in the gas phase by absorption of 206.2 nm radiation, i.e. in the band corresponding to the A - X transition of NH3. No fluorescence was obtained from molecules having hydrogen or deuterium bonded directly to the nitrogen atom. The experiments support the view that quantum-mechanical tunnelling is the major factor governing the rates of predissociation of the excited states of NH3, ND3, and simple amines.

Unimolecular decomposition of a polyatomic ion in a variable-temperature selected-ion-flow-drift tube: experiment and theoretical interpretation

Abstract The unimolecular decomposition of CH 3 CH 2 OH + 2 , protonated ethanol ions, has been studied in a variable-temperature selected-ion-flow-drift tube, and unimolecular rate coefficients, k uni , have been determined as a function of the mean centre-of-mass collision energy of the ions with the helium carrier gas atoms, E c . The only product ions observed are H 3 O + and C 2 H + 5 . Quantitative modelling of the decomposition has been carried out making the following assumptions: (1) the induction period for attaining steady-state distributions is brief compared to the timescale of the experiment; (2) the translational energy distribution of the primary CH 3 CH 2 OH + 2 ions is Maxwellian with a mean energy E c and that a thermal probability distribution function is appropriate for vibrational energy and angular momentum transfer in collisions between the primary ion and the helium atoms. The experimental results indicate that assumption (1) is valid but that assumption (2) is not, because the values of k uni measured at two different carrier gas temperatures of 300 and 500 K but at the same E c are not the same. In principle, it should be possible to deduce thermal rate coefficients for collisional decomposition at higher temperatures T than are accessible experimentally by extrapolating to zero E -field plots of the experimental rate coefficients against the contribution to E c due to the drift field obtained for fixed values of E c (assuming E c = 1.5 kT ). It is shown that the form of the extrapolation greatly influences the value of k uni so derived, and it is apparent that more experimental data and careful modelling are required before meaningful k uni versus T data can be deduced.