R. Anthony Cox

Gas-phase ultraviolet absorption cross-sections and atmospheric lifetimes of several C2C5 alkyl nitrates

Abstract Gas-phase ultraviolet absorption cross-sections of ethyl nitrate, 1-propyl nitrate, 2-propyl nitrate, 2-methyl, 1-propyl nitrate, 1-butyl nitrate and 1-pentyl nitrate have been measured over the wavelength range 220–340 nm using a dual-beam, diode array spectrometer. Each alkyl nitrate spectrum appears to be the sum of at least two Gaussian-shaped absorptions with an intense π−π ∗ band extending from 190–240 nm having a shoulder between 250–340 nm due to a π−π ∗ system. The absorption cross-sections recorded for ethyl nitrate, 1-propyl nitrate, 2-propyl nitrate and 1-butyl nitrate are within 10% of previous data: those of 2-methyl, 1-propyl nitrate and 1-pentyl nitrate have been measured for the first time. For ethyl nitrate, absorption cross-sections between 280–340 nm in the tail of the near-ultraviolet band declined with decreasing temperature from 298-233 K. At two-dimensional numerical model of tropospheric chemistry was used to calculate atmospheric lifetimes with respect to photodissociation and OH radical reaction that are markedly dependent on season, latitude and altitude. Relatively long, surface level atmospheric lifetimes of several days to weeks confirm that the C2C5 alkyl nitrates may act as temporary reservoirs for NOx and suggest that they may also constitute a significant fraction of total reactive odd-nitrogen, NOy, particularly during winter at northern hemisphere high latitudes.

UV–VIS absorption cross-sections and atmospheric lifetimes of CH2Br2, CH2I2 and CH2BrI

The UV–VIS absorption spectra of CH2Br2, CH2I2 and CH2BrI have been measured over the wavelength range 215–390 nm using a dual-beam diode array spectrometer. The spectra consist of broad continuous absorption bands. CH2Br2 exhibits its maximum cross-section of σ=2.71(±0.16)×10-18 cm2 molecule-1 at λ=219 nm. The magnitude of the peak cross-sections for the iodine-containing molecules above λ=210 nm are σ=1.62(±0.10)×10-18 cm2 molecule-1 at λ=248 nm and σ=3.78(±0.23)×10-18 cm2 molecule-1 at λ=288 nm for CH2I2, and σ=5.67(±0.34)×10-18 cm2 molecule-1 at λ=215 nm and σ=2.34(±0.14)×10-18 cm2 molecule-1 at λ=267 nm for CH2BrI. The temperature dependence of the absorption cross-sections was investigated over the temperature range 348–250 K. A decline in the cross-sections with decreasing temperature was observed in the tail of the spectra. At the peaks the opposite effect was observed. All three gases have been found in the atmosphere and the atmospheric photolysis rates of CH2Br2, CH2I2 and CH2BrI were calculated as a function of altitude and solar zenith angle using the measured cross-sections. Model calculations show that, during sunlit hours, CH2I2 and CH2BrI will be photolysed within minutes and hours, respectively. The photolysis of CH2Br2 is much slower and reaction with the OH radical is the dominant atmospheric loss process.

UV absorption cross-sections and atmosphericphotolysis rates of CF3I, CH3I,C2H5I andCH2ICl

The absorption cross-sections of trifluoromethyl iodide (CF 3 I), methyl iodide (CH 3 I), ethyl iodide (C 2 H 5 I) and chloroiodomethane (CH 2 ICl) have been determined over the wavelength range 235–400 nm, with a spectral resolution of 0.6 nm (FWHM), using a diode array spectrometer. The spectra consist of a broad continuous absorption band with maximum cross-sections of (6.0±0.1)×10 -19 cm 2 molecule -1 for CF 3 I at 267 nm, (1.09±0.02)×10 -18 cm 2 molecule -1 for CH 3 I at 258 nm, (1.18±0.04)×10 -18 cm 2 molecule -1 for C 2 H 5 I at 258 nm and (1.21±0.07)×10 -18 for CH 2 ICl at 270 nm. The temperature dependence of the cross-section was investigated over the range 333–243 K. A decline in the cross-section with decreasing temperature at wavelengths longer than ca. 280 nm was observed in all cases, the decrease being most pronounced in the long wavelength tail of the absorption band. At wavelengths shorter than ca. 270 nm the cross-section increased with decreasing temperatures; the effect, however, was significantly smaller (5–10%), being most pronounced around the band maximum. The temperature dependence was parametrised in order to calculate the atmospheric photolysis rate as a function of altitude, latitude and season. Model calculations show that during sunlit hours the iodides will be rapidly photolysed with tropospheric photodissociation lifetimes of ca. 1 day for CF 3 I, several days for CH 3 I and C 2 H 5 I and several hours for CH 2 ICl.

Reduction of NO2 to nitrous acid on illuminated titanium dioxide aerosol surfaces: implications for photocatalysis and atmospheric chemistry

TiO2, a component of atmospheric mineral aerosol, catalyses the reduction of NO2 to nitrous acid (HONO) when present as an aerosol and illuminated with near UV light under conditions pertinent to the troposphere.

Studies of single aerosol particles containing malonic acid, glutaric acid, and their mixtures with sodium chloride. II. Liquid-state vapor pressures of the acids

The vapor pressures of two dicarboxylic acids, malonic acid and glutaric acid, are determined by the measurement of the evaporation rate of the dicarboxylic acids from single levitated particles. Two laboratory methods were used to isolate single particles, an electrodynamic balance and optical tweezers (glutaric acid only). The declining sizes of individual aerosol particles over time were followed using elastic Mie scattering or cavity enhanced Raman scattering. Experiments were conducted over the temperature range of 280-304 K and a range of relative humidities. The subcooled liquid vapor pressures of malonic and glutaric acid at 298.15 K were found to be 6.7(-1.2)(+2.6) x 10(-4) and 11.2(-4.7)(+9.6) x 10(-4) Pa, respectively, and the standard enthalpies of vaporization were respectively 141.9 ± 19.9 and 100.8 ± 23.9 kJ mol(-1). The vapor pressures of both glutaric acid and malonic acid in single particles composed of mixed inorganic/organic composition were found to be independent of salt concentration within the uncertainty of the measurements. Results are compared with previous laboratory determinations and theoretical predictions.

Reactive uptake of N2O5 by aerosols containing dicarboxylic acids. Effect of particle phase, composition, and nitrate content.

Reactive uptake coefficients for loss of N(2)O(5) to micron-size aerosols containing oxalic malonic, succinic, and glutaric acids, and mixtures with ammonium hydrogen sulfate and ammonium sulfate, are presented. The uptake measurements were made using two different systems: atmospheric pressure laminar flow tube reactor (Cambridge) and the Large Indoor Aerosol Chamber at Forschungszentrum Juelich. Generally good agreement is observed for the data recorded using the two techniques. Measured uptake coefficients lie in the range 5 x 10(-4)-3 x 10(-2), dependent on relative humidity, on particle phase, and on particle composition. Uptake to solid particles is generally slow, with observed uptake coefficients less than 1 x 10(-3), while uptake to liquid particles is around an order of magnitude more efficient. These results are rationalized using a numerical model employing explicit treatment of both transport and chemistry. Our results indicate a modest effect of the dicarboxylic acids on uptake and confirm the strong effect of particle phase, liquid water content, and particulate nitrate concentrations.

Modelling the response of a tungsten oxide semiconductor as a gas sensor for the measurement of ozone

The behaviour of gas-sensitive resistors based on WO3 towards small concentrations of ozone in air can be understood with a simple model involving the reaction of ozone with surface oxygen vacancies. This model has been validated by comparison with experimental results for the effects of varying oxygen partial pressure on the ozone response. A complete description of the behaviour of devices constructed by printing WO3 as porous layers onto an impermeable substrate requires consideration of the effects of the microstructure of such a device upon its response. A very simple series-parallel equivalent circuit model captures the effects and allows a simple interpretation of the sensor behaviour, including the quadratic limiting steady state resistance response to ozone and the effects of variation of device thickness. An important fact that allows WO3 to be used at rather high temperatures as an effective ozone sensor is that ozone does not decompose at any discernible rate on the oxide surface. Saturation of the oxide surface at ambient temperature with water vapour inhibits the ozone response when the sensor is subsequently heated. The effect can be removed by heating at sufficiently high temperature. Water vapour also gives a high-temperature sensor response, but appears to act at sites different to those that mediate the response to ozone.

Uptake of gaseous hydrogen peroxide by submicrometer titanium dioxide aerosol as a function of relative humidity.

Hydrogen peroxide (H(2)O(2)) is an important atmospheric oxidant that can serve as a sensitive indicator for HO(x) (OH + HO(2)) chemistry. We report the first direct experimental determination of the uptake coefficient for the heterogeneous reaction of gas-phase hydrogen peroxide (H(2)O(2)) with titanium dioxide (TiO(2)), an important component of atmospheric mineral dust aerosol particles. The kinetics of H(2)O(2) uptake on TiO(2) surfaces were investigated using an entrained aerosol flow tube (AFT) coupled with a chemical ionization mass spectrometer (CIMS). Uptake coefficients (gamma(H(2)O(2))) were measured as a function of relative humidity (RH) and ranged from 1.53 x 10(-3) at 15% RH to 5.04 x 10(-4) at 70% RH. The observed negative correlation of RH with gamma(H(2)O(2)) suggests that gaseous water competes with gaseous H(2)O(2) for adsorption sites on the TiO(2) surface. These results imply that water vapor plays a major role in the heterogeneous loss of H(2)O(2) to submicrometer TiO(2) aerosol. The results are compared with related experimental observations and assessed in terms of their potential impact on atmospheric modeling studies of mineral dust and its effect on the heterogeneous chemistry in the atmosphere.

Mechanism of atmospheric oxidation of 1,1,1,2-tetrafluoroethane (HFC 134a)

The chlorine-initiated photooxidation of hydrofluorocarbon 134a (CF3CH2F) has been studied in the temperature range 235–318 K and at 1 atm total pressure using UV absorption. Trifluoroacetyl fluoride [CF3C(O)F] and formyl fluoride [HC(O)F] were observed as the major products. IR analysis of the reaction mixture also showed carbonyl fluoride [C(O)F2] as a product. By measurement of the yields of [HC(O)F] from the photooxidation as a function of [O2] and temperature, the rate of the unimolecular decomposition of the oxy radical, CF3CHFO, reaction (5), was determined relative to its reaction with O2, reaction (4): CF3CHFO + O2→ CF3C(O)F + HO2(4), CF3CHFO → CF3+ HC(O)F (5) The results were treated using both an arithmetic derivation and numerical integration with a detailed reaction scheme. Inclusion of other recently published kinetic data leads of the following recommended rate expression for reaction (5) at 1 atm k5= 7.4 × 1011 exp[(–4720 ± 220)/T] s–1 The errors are 1σ.The observation of enhanced product yields in the present work is attributed to the reaction of the CF3O radical with HFC 134a leading to further peroxy radical formation. The results have been incorporated into a 2D atmospheric model to assess the environmental implications of HFC 134a release in the troposphere.

The interaction of HCl with water‐ice at tropospheric temperatures

The uptake of HCl on frozen film ice surfaces was studied as a function of temperature and HCl partial pressure (PHCl) in a flow reactor. The temperatures ranged from 205 to 230 K, and PHCl was (0.4–2.0) × 10−6 Torr. The initial uptake coefficients, γ, and the HCl surface coverage are reported. It was observed that the coverage varied from 3.0 × 1014 molecules/cm² at 205 K to 1.0 × 1014 molecules/cm² at 230 K. At 205 K the coverage showed no dependence on PHCl; the average surface coverage was ∼ (2.0±0.7) ×1014 molecules/cm² at saturation. This value is consistent with other studies on similarly prepared ice films. It was found that γ ∼0.1 at 205 K, but decreased substantially when the temperature increased; however, γ was insensitive to PHCl.