Carleton J. Howard

Do Hydrofluorocarbons Destroy Stratospheric Ozone

Hydrofluorocarbons, many of which contain a CF3 group, are one of the major substitutes for the chlorofluorocarbons and halons that are being phased out because they contribute to stratospheric ozone depletion. It is critical to ensure that CF3 groups do not cause significant ozone depletion. The rate coefficients for the key reactions that determine the efficiency of the CF3 radical as a catalyst for ozone loss in the stratosphere have been measured and used in a model to calculate the possible depletion of ozone. From these results, it is concluded that the ozone depletion potentials related to the presence of the CF3 group in hydrofluorocarbons are negligibly small.

Gas phase ion chemistry of HNO3

The ion chemistry of HNO3 is studied at 298 °K. The attachment of thermal electrons to HNO3 to produce NO−2 is found to have a rate constant of (5±3) ×10−8 cm3/sec. HNO3 reacts rapidly (k∼10−9 cm3/sec) with a large variety of negative ions including Cl−, NO2−, O2−, and CO−3. HNO3 is also found to bond strongly to NO3−. The proton affinity of HNO3 is determined to be 176±7 kcal/mole. The present results indicate a strong similarity between protonated HNO3 and hydrated NO2+, suggesting that these ion species are identical. The implication of the present results for atomospheric ion chemistry is discussed.

Absolute rate constant determinations for the deactivation of O(1D) by time resolved decay of O(1D) →O(3P) emission

Absolute rate constants for the deactivation of O(1D) atoms by some atmospheric gases have been determined by observing the time resolved emission of O(1D) at 630 nm. O(1D) atoms were produced by the dissociation of ozone via repetitive laser pulses at 266 nm. Absolute rate constants for the relaxation of O(1D) (×10−10 cm3 molecule−1⋅s−1) by N2(0.30±0.01), O2(0.41±0.05), CO2(1.2±0.09), O3(2.4±0.1), H2(1.3±0.05), D2(1.3±0.05), CH4(1.3±0.3), HCl(1.4±0.3), NH3 (3.4±0.3), H2O(2.1±1.0), N2O(1.4±0.1), and Ne (<0.0013) are reported at 298 K. The results obtained are compared with previous relative and absolute measurements reported in the literature.

Temperature dependence of O(1D) rate constants for reactions with O2, N2, CO2, O3, and H2O

Absolute rate constants and their temperature dependencies for the deactivation of O(1D) by five important atmospheric gases are reported. O(1D) atoms are produced by photolysis of ozone at 266 nm and the atoms are directly observed in time resolved decay of the O(1D) →O(3P) radiation at 630 nm. Gases which only quench O(1D) (O2, N2, and CO2) are observed to have a small negative temperature dependence while O3 and H2O, which also have a reactive channel, display no temperature dependence. Arrhenius expressions for the reactions measured are (A in units of 10−10 cm3/molecule⋅ s, E in cal/mole) O2(T=104–354 K) 0.29 exp(134/RT), N2(T=104–354 K) 0.20 exp(214/RT), CO2(T=139–200 K) 1.2 and (T=200–354 K) 0.68 exp(233/RT). The rate constants for O3 and H2O are 2.4×10−10 and 2.3×10−10 cm3/molecule⋅ s over ranges of 103–393 K and 253–353 K, respectively. The results are compared with other energy dependent measurements and with the theories reported in the literature.

Emissions of volatile organic compounds from cut grass and clover are enhanced during the drying process

The release of volatile organic compounds (VOCs) by drying grass and clover leaves and stems was studied in the laboratory using proton-transfer chemical-ionization mass spectrometry, which enables the simultaneous, on-line monitoring of VOC concentrations. A burst of VOC emissions due to cutting the leaves and stems was followed by a second, more intense emission lasting for several hours when the vegetation was starting to dry out. In addition to (Z)-3-hexenal, (Z)-3-hexenol, and hexenyl acetate, that were emitted by the plant tissue in response to the wounding, enhanced emissions of methanol, acetaldehyde, acetone, butanone, and possibly formaldehyde were observed. These findings may have important implications for regional air quality in agricultural and urban areas.

Temperature dependence of O(1D) rate constants for reactions with N2O, H2, CH4, HCl, and NH3

Absolute rate constants are reported for the deactivation of O(1D) by five trace atmospheric constituents for the temperature range 200–350 K. O(1D) atoms produced by photolyzing ozone with a frequency quadrupled pulse from a Nd‐YAG laser are monitored directly by means of the O(1D) →O(3P) emission at 630 nm. All the gases studied here are thought to deactivate O(1D) primarily through reactive channels. Their rate constants exhibit no detectable temperature dependence. The rate constants are (in units of 10−10 cm3/molecule⋅s) N2O: 1.1,±0.2, H2: 0.99±0.3, HCl: 1.4±0.4, NH3: 2.5±0.5, CH4: 1.4±0.4. The present results and those of our previous studies are compared with rate constants obtained using the time‐resolved attenuation of resonance absorption technique.

Rate constants for the reactions of O2+, NO2+, NO+, H3O+, CO3−, NO2−, and halide ions with N2O5 at 300 K

The reactions of N2O5 with CO3−, NO2−, and the halide ions, F−, Cl−, Br−, and I−, have been found to be very fast (k∼10−9 cm3 s−1) at 300 K and to produce the NO3− ion. It is inferred from the thermochemistry of the halide reactions that the neutral products must be XNO2. The positive ions O2+ and NO+ react with N2O5 to produce NO2+, while H3O+ reacts with N2O5 to form NO2+ and H2NO3+. NO2+ was observed not to react readily with N2O5.

Rate constants for the reactions of OH with CH4 and fluorine, chlorine, and bromine substituted methanes at 296 K

The absolute rate constants for the reactions of OH radicals with CH4 and fifteen fluorine, chlorine, and bromine substituted methane molecules have been measured using a discharge flow system and laser magnetic resonance detection of OH. Measurements were made at 296 K and at pressures between 107 and 1300 Pa. The results indicate that the reaction mechanism involves the abstraction of a hydrogen atom and formation of H2O and a methyl type radical product. Completely halogenated methane molecules are found to be relatively unreactive. Hydrogen containing molecules react with rate constants ranging from about 0.2 to 160×10−15 cm3/molecule⋅sec. The reactivity increases with decreasing carbon–hydrogen bond energies. Rough estimates are made of the Arrhenius parameters for the reactions.

HO2 detected by laser magnetic resonance

Far‐infrared absorption spectra of HO2 in the gas phase have been detected at six wavelengths of a water vapor laser magnetic resonance spectrometer. The identification of HO2 as the absorbing molecule is based on a partial analysis of the spectra and on a variety of different chemical methods used to produce the radical. Approximate values of rotational constants and spin doublet separations are derived from the spectra.

Rate constants for the reactions of OH with ethane and some halogen substituted ethanes at 296 K

Absolute rate constants for the reactions of OH radicals with C2H6 and twelve fluorine, chlorine, and bromine substituted ethane compounds are reported. The measurements are made at 296 K and pressures ranging from 100 to 1000 Pa (0.7–7 torr) using a discharge‐flow system and laser magnetic resonance detection of OH. The results are similar to those of an earlier work on a series of methane compounds and indicate that the reaction mechanism is the abstraction of an H atom. Thus, completely halogenated molecules are relatively inert. The hydrogen containing molecules react with rate constants ranging from about 3 to 400×10−15 cm3 molecule−1⋅sec−1.