Andrew B. Horn

Heterogeneous chemistry of toluene, kerosene and diesel soots.

Soot samples as potential mimics of atmospheric aerosols have been produced from the combustion of toluene, kerosene and diesel in order to compare the nature of soot produced from a simpler material, toluene, with soots from the fuels kerosene and diesel. Characterisation of the soots using elemental analysis, infrared spectroscopy, solvent extraction, thermal desorption and electron microscopy techniques before and after reaction with ozone allows assessment of the reactivity of soots from these different fuels. Despite the production of toluene and kerosene soots from identical combustion conditions, strong differences in structure and reactivity are observed in terms of their reaction with ozone. However, toluene soot is a much better mimic of diesel soot. It is proposed that the differing reactivities of the soots is related to the nature of the organic carbon and structure of the elemental carbon which vary with soots from the different fuels.

Ageing of alkanethiol self-assembled monolayers

Self-assembled monolayers of three straight-chain alkanethiols [CH3(CH2)nSH, n= 11, 15, 17] have been formed on a gold substrate from cyclohexane solution and characterised using IR spectroscopy. Over a period of ca. six months, the relative intensities of the C–H stretching bands in the IR spectra were observed to change. These changes are explained by a tilting of the molecules from their initial position away from the surface normal upon oxidation of the thiolate root. This temporal dependence of the tilt angle has implications for the stability of self-assembled monolayers.

Adsorption and ionization of HCl on an ice surface

In this article, we describe a series of experiments investigating the interaction of the important stratospheric reservoir species HCl with the surface of a thin ice film. Reflection absorption infrared spectroscopy and thermal desorption spectroscopy have been used to identify the nature of the ionic hydrates formed under a variety of pressure, temperature, and exposure regimes.

Ionisation and solvation of stratospherically relevant molecules on ice films

Heterogeneous reactions of stratospheric reservoir species such as HCl, N2O5 and ClONO2 on type I (nitric acid hydrates) and type II (water-ice) polar stratospheric cloud (PSC) particles are believed to play an important role in the extremely large losses of stratospheric ozone observed during the Antarctic spring. Laboratory studies of such processes, using thin ice films as PSC particle surface mimics, can provide mechanistic information about individual elementary steps and overall reaction schemes. IR and mass spectrometry have been used to identify reaction products and intermediates and it has been shown that the primary step in all the studied reaction schemes involves the formation of ionic surface species. Furthermore, the nature and stability of the ions are found to be inextricably linked with the amount of ‘free’ water available to solvate them. The following questions are also addressed: Are adsorbed ions (e.g. nitrate, chloride and oxonium) reactive on ice surfaces? If so, are the products formed the same as those suggested when their corresponding molecular parents are involved? The answers, which have important implications for understanding stratospheric heterogeneous chemistry, are discussed in terms of ionic reaction mechanisms and novel surface intermediates.

Infrared spectroscopic studies of the heterogeneous reaction of ozone with dry maleic and fumaric acid aerosol particles

Dicarboxylic acids, either directly emitted or formed in chemical processes, are found to be a significant component of tropospheric aerosols. To assess any potential chemical transformation of short unsaturated dicarboxylic acids in tropospheric heterogeneous chemistry, maleic and fumaric acid were selected as surrogates in this study. A novel aerosol flow tube apparatus is employed to perform kinetic studies of the oxidation of these organic compounds by gas-phase ozone. The system consists of a particle generation system, a vertically oriented glass flow tube and an infrared observation White cell with a Fourier transform infrared (FTIR) spectrometer for the detection system. A flow of single component organic aerosols with mean diameters ranging between 0.7 and 1.1 microm is introduced in a flow tube, in which the particles are subsequently exposed to a known concentration of ozone for a controlled period of time. A band assignment of infrared vibrational frequencies for dry maleic and fumaric acid aerosol spectra is presented. These studies are complemented with off-line analysis on the reaction products. The reaction exhibited pseudo-first-order kinetics on gas product formation, and the pseudo-first-order rate coefficients displayed a Langmuir-Hinshelwood dependence on gas-phase ozone concentration for both materials. By assuming a Langmuir-Hinshelwood behaviour, the following parameters were obtained: for the reaction of maleic acid aerosols, K(O3) = (3.3 + 0.5) x 10(-16) cm3 molecule(-1) and k(I)(max) = (0.038 + 0.004) s(-1); for the reaction of fumaric acid aerosols, K(O3) = (1.6 + 0.5) x 10(-16) cm3 molecule(-1) and k(I)(max) = (0.048 + 0.007) s(-1), where K(O3) is a parameter that describes the partitioning of ozone to the particle surface and k is the maximum pseudo-first-order coefficient at high ozone concentrations. Apparent reactive uptake coefficients were estimated from the pseudo-first-order rate coefficient and a trend of decreasing uptake coefficients with increasing ozone concentrations was observed, in good agreement with literature values.

Mechanisms for the heterogeneous hydrolysis of hydrogen chloride, chlorine nitrate and dinitrogen pentoxide on water-rich atmospheric particle surfaces

The heterogeneous interaction of the stratospheric reservoir species HCl, ClONO 2 , and N 2 O 5 with water-rich polar stratospheric particle mimics is characterized by the formation of solvated ionic products. Simple semiempirical calculations have been used to explain the nature of the species observed in infrared spectroscopic measurements and to elucidate the mechanism by which they are formed. The initial stages of the interaction appear to involve an S N 2-type nucleophilic attack by the oxygen atom of the surface water molecule upon the most accessible electrophilic site of the adsorbing reactant. Mechanistic schemes involving protonated acid intermediates and their subsequent decomposition or hydrolysis can be used to accurately predict and explain the stable reaction products observed spectroscopically under stratospheric conditions.

ATR-IR spectroscopic studies of the formation of sulfuric acid and sulfuric acid monohydrate films

The heterogeneous reaction of H2O with SO3 on a Ge surface was studied using infrared absorption spectroscopy in an internal reflection geometry. A concentrated sulfuric acid film was formed and subsequently diluted with H2O to produce a stable hydrate film at low temperature (190 K). Such films are important as potential substrates for spectroscopic probes of the surface chemistry of atmospheric sulfate aerosol particles. Co-deposition of H2O and SO3 at 250 K results in the formation of a thin film of liquid sulfuric acid which, on cooling to 190 K, crystallises to produce a stable solid molecular H2SO4 film. This is followed in detail through absorption band shifts and splitting in the IR spectra. Exposure to water vapour at 190 K dilutes this film to a simple hydrate which shows IR absorption bands which can be assigned to H3O+ and HSO4- ions, indicating the formation of sulfuric acid monohydrate. This is in agreement with predictions from the sulfuric acid phase diagram. It is proposed that further dilution using higher water partial pressures will enable stable hydrates of sulfuric acid to be produced in a reproducible and well characterised manner for reaction studies.

Kinetic studies of the heterogeneous oxidation of maleic and fumaric acid aerosols by ozone under conditions of high relative humidity

In this paper, a kinetic study of the oxidation of maleic and fumaric acid organic particles by gas-phase ozone at relative humidities ranging from 90 to 93% is reported. A flow of single component aqueous particles with average size diameters in the range 2.6-2.9 µm were exposed to a known concentration of ozone for a controlled period of time in an aerosol flow tube in which products were monitored by infrared spectroscopy. The results obtained are consistent with a Langmuir-Hinshelwood type mechanism for the heterogeneous oxidation of maleic/fumaric acid aerosol particles by gas-phase ozone, for which the following parameters were found: for the reaction of maleic acid aerosols, K(O(3)) = (9 ± 4) × 10(-15) cm(3) molecule(-1) and k = (0.21 ± 0.01) s(-1); for the reaction of fumaric acid aerosols, K(O(3)) = (5 ± 2) × 10(-15) cm(3) molecule(-1) and k = (0.19 ± 0.01) s(-1). From the pseudo-first-order coefficients, apparent uptake coefficient values were calculated for which a decreasing trend with increasing ozone concentrations was observed. Comparison with previous measurements of the same system under dry conditions reveals a direct effect of the presence of water on the mechanism of these reactions, in which the water is seen to increase the formation of CO(2) and formic acid (HCO(2)H) through increased levels of hydroxyacetyl hydroperoxide intermediate.

Infrared spectroscopic studies of the low temperature interconversion of sulfuric acid hydrates

IR spectra of single phase sulfuric acid hydrates have been measured and the conversion between sulfuric acid monohydrate (H3O+HSO4−, SAM) and sulfuric acid tetrahydrate ([H5O2+]2SO42−, SAT) as a function of temperature and water partial pressure pH2O has been followed using the principal absorption bands of the four main ionic species present, H3O+, H5O2+, HSO4− and SO42−. Temperature–pressure variation studies of the phase transition show that intermediate structures are formed on hydration of SAM that are not formed in the dehydration of SAT. Spectra taken at regular time intervals during the conversion process have been used to monitor these intermediates which can be attributed to changes in the local coordination geometry of the sulfate ion as a function of the amount of available water. The sulfate ion core in SAT is nearly tetrahedral and principally shows a strong asymmetric S–O stretching fundamental at ca. 1070 cm−1 in its mid-IR spectrum. The SO4 core of the bisulfate ion in SAM has pseudo-C3v local symmetry, with 3 IR-active modes (2A1 + E) which are observed to change markedly upon hydration. Slow hydration at 180 K results in the melting of SAM, with subsequent SAT crystallisation from this melt. At reduced temperatures (175 K), instead of melting, a sulfate ion is held in a solid matrix and successively coordinates to a second H3O+ ion in a structure with local C3v symmetry. This change in coordination allows different vibrational modes to become IR active. The IR absorption bands in each of these configurations can be assigned by comparison with the vibrational modes of metal sulfates for which structures and spectra are known. The ultimate effect of hydration is to deprotonate the bisulfate core forming sulfate and hydrated protons, i.e. the formation of SAT. Isothermal dehydration of SAT in acuo shows a simpler trend, through the loss of excess water from a SAT film until the overall stoichiometry reaches that of SAM: a smooth, direct conversion into SAM is observed. The rate of this process compared to the rate of hydration suggests that a barrier to SAT decomposition exists.

Reaction and diffusion in heterogeneous atmosphericchemistry studied by attenuated total internal reflection IRspectroscopy

The adsorption of gaseous HCl onto the surface of water ice films in the range 90–160 K is known to result in the formation of a layer composed of hydrated protons and chloride ions, with the general formula (H 2 O) n H 3 O Cl - . Using a novel attenuated total internal reflection (ATR)-based IR spectroscopic probe in which ice films are condensed upon the surface of a variable temperature internal reflection element in vacuo, the diffusion of material from this surface layer into the bulk of the ice film has been studied. The validity of the ATR-IR method for the study of these systems has been verified by monitoring the growth of ice films as a function of exposure time and comparing the resulting absorbance vs. time curves with simple models. For ice films with thicknesses greater than the effective depth of penetration of the probe beam, spectra recorded immediately after exposure to HCl do not show features attributable to ionised HCl hydrates. However, as material from the interfacial layer penetrates into the bulk of the film, spectra recorded as a function of time can be used to measure the rate of diffusion of HCl into the film from the ice/vacuum interface. By comparing the absorbance due to the H 3 O + ion vs. time curves with theoretical predictions, the Ficks law diffusion coefficient of (H 2 O) n H 3 O Cl - in ice is estimated to be ca. 10 -15 m2 s -1 at 150 K, in good agreement with values estimated from indirect techniques.