Freezing halide ion solutions and the release of interhalogens to the atmosphere

The effect of freezing on a variety of acidified and neutral, nitrite ion and halide-containing mixtures has been investigated using UV/vis spectroscopy. Several trihalide ions were formed and monitored, including I(2)Cl(-), I(2)Br(-), ICl(2)(-) and IBr(2)(-). A mechanism to explain the observations is given in terms of steps involving INO and the nitroacidium ion, [H(2)ONO](+). The transformation of sea salt components to specific trihalide ions by freezing represents a potentially important process in a polar atmospheric context. This is because the dichloro- and dibromo-trihalide ions can release chlorine- and bromine-containing gases, which are key intermediates in ozone destruction.     

Protonated nitrosamide and its potential role in the release of HONO from snow and ice in the dark

A joint Fourier transform reflection absorption infrared spectroscopy/thermal programmed desorption (RAIRS/TPD) study has provided good evidence for the existence of protonated nitrosamide (NH?NO(+)) on surfaces at cold temperatures. This species has long been proposed to exist in studies of the DeNO(x) process and the decomposition of ammonium nitrite. In the context of the current experiments, performed at low-temperatures in the absence and presence of water-ice, the results provided a firm mechanistic basis for understanding the release of HONO from snowpack in a "dark" mechanism and also under alkaline surface conditions.     

Eine Ab?nderung des Orsat'schen Apparates behufs Bestimmung des Wasserstoffes in Kamingasen

Ohne Zusammenfassung     

Release of nitric oxide and iodine to the atmosphere from the freezing of sea-salt aerosol components

The known room-temperature, solution-phase reaction between nitrite ions and iodide ions, which occurs in acidic conditions (pH < 5.5), is shown to be accelerated when neutral aqueous solutions are frozen. The reaction is proposed to occur in liquid "micropockets" within the ice structure at temperatures between the freezing point and the eutectic temperature. The products, nitric oxide and molecular iodine, are known to play significant roles in atmospheric compositional change, and therefore, the results obtained here, which are not dependent on acidification, may impact on observed snowpack chemistry. Investigation of the effect of oxygen on the chemical processing indicates that a chain reaction mechanism is operative.     

BACTERIAL PHOTOPROTECTION THROUGH EXTRACELLULAR CADMIUM SULFIDE CRYSTALLITES

Ultraviolet light and the heavy metal, cadmium, both have toxic effects on many microorganisms. In this communication we describe a method by which the bacterium Klebsiella aerogenes surmounts both problems using one biological process: the synthesis of cadmium sulfide (CdS) crystallites. These semiconductor particles absorb radiation in the UV spectral region and therefore, when K. aerogenes produces extracellular CdS material in response to environments containing cadmium ions, a photoprotective layer is formed. The effect of UVA radiation on cultures of Klebsiella aerogenes was monitored using electron microscopy, energy-dispersive X-ray analysis and electronic spectroscopy. The results show that at wavelengths 320 nm Λ 400 nm, a photoprotection period of between 4 and 6 h is induced, which eventually fails due to photodegradation of the semiconductor layer to metallic cadmium and elemental sulfur.     

Bacterial Cadmium Sulfide Semiconductor Particles: An Assessment of their Photoactivity by EPR Spectroscopy

Abstract— The bacterium Klebsiella aerogenes produces extracellular particles of cadmium sulfide in the presence of cadmium ions. The photoactivity of these particles has been studied using electron paramagnetic resonance (EPR) spectroscopy. Bacterial samples containing these semiconductor particles were irradiated with visible light in the presence of a spin trap, either phenyl-tert-butylnitrone (PBN) or 5,5-dimethyl-l-pyrroline-N-oxid(eD MPO). The results obtained reveal that a number of radicals, both oxygen and carbon based, are generated. Bacterial samples grown in the absence of cadmium ions exhibit weak, irradiation-independent EPR signals. These bacterially produced radicals are quenched when CdS particles are present. The observation of light-induced radicals provides evidence that the bacterial CdS particles are photoactive, behaving in a similar manner to inorganic CdS particles, and therefore could be used to mediate photocatalytic reactions.     

Cold-surface photochemistry of primary and tertiary alkyl nitrites

Reflection-absorption infrared spectroscopy (RAIRS) is used to explore the photochemistry of primary and tertiary alkyl nitrites deposited on a gold surface. The primary alkyl nitrites examined for this study were n-butyl, isobutyl, and isopentyl nitrite. These compounds showed qualitatively similar spectra to those observed in previous condensed-phase measurements. The photolysis of the primary nitrites involved the initial formation of an alkoxy radical and NO, followed by production of nitroxyl (HNO) and an aldehydic species. In addition, the formation of nitrous oxide, identified from its distinctive transition near 2230 cm(-1), was observed to form from the self-reaction of nitroxyl. The reaction rates for cis and trans conformer decay, as tracked through their intense N═O stretching modes, were found to be significantly different, potentially due to a structural bias that favors HNO formation for the initial trans conformer photoproducts over recombination. Tert-butyl nitrite demonstrates only the trans conformer in the RAIRS spectra prior to photolysis; however, recombination of the initial NO and RO(?) photoproducts was observed to produce the cis conformer in the photolyzed samples. The primary photoproducts from tert-butyl nitrite can also react to form acetone and nitrosomethane, but the absence of HNO prohibits the formation of N(2)O that was observed for the primary alkyl nitrites. Additionally, the RAIRS spectrum of isobutyl nitrite co-deposited with water was measured to examine the photolysis of this species on a water-ice surface. No change in the identity of the photoproducts was observed in this experiment, and minimal frequency shifting (1-3 cm(-1)) of the vibrational modes occurred. In addition to being a known atmospheric source of NO and various aldehydes, our results point to cold surface processing of alkyl nitrites as a potential environmental source of nitrous oxide.     

Cirrus cloud mimics in the laboratory: an infrared spectroscopy study of thin films of mixed ice of water with organic acids and ammonia

The structures of formic and acetic acids deposited on a thin gold substrate held in vacuum at low temperatures and their related water-ice promoted chemistry have been investigated. The condensed water/guest films were taken to act as cirrus cloud "mimics." Such laboratory representations provide a necessary prelude to understanding how low temperature surfaces can affect chemical composition changes in the upper atmosphere. The systems were characterized by reflection-absorption infrared spectroscopy and temperature-programmed desorption spectrometry. The interaction behavior of the binary acid ices was compared to that observed when ternary mixtures of water, formic acid, and ammonia were deposited. Differences in the chemistry were observed depending on deposition method: layering or mixing. The more atmospherically relevant codeposition approach showed that at low temperatures, amorphous formic acid can be ionized to its monodentate form by water ice within the bulk rather than on the surface. In contrast, the introduction of ammonia leads to full bidentate ionization on the ice surface. The thermal desorption profiles of codeposited films of water, ammonia, and formic acid indicate that desorption occurs in three stages. The first is a slow release of ammonia between 120 and 160 K, then the main water desorption event occurs with a maximum rate close to 180 K, followed by a final release of ammonia and formic acid at about 230 K originating from nonhydrous ammonium formate on the surface. The behavior of acetic acid is similar to formic acid but shows lesser propensity to ionize in bulk water ice.