Raman spectroscopy of solutions and interfaces containing nitrogen dioxide, water, and 1,4 dioxane: evidence for repulsion of surface water by NO2 gas.

The interaction of water, 1,4 dioxane, and gaseous nitrogen dioxide, has been studied as a function of distance measured through the liquid-vapour interface by Raman spectroscopy with a narrow (<0.1 mm) laser beam directed parallel to the interface. The Raman spectra show that water is present at the surface of a dioxane-water mixture when gaseous NO2 is absent, but is virtually absent from the surface of a dioxane-water mixture when gaseous NO2 is present. This is consistent with recent theoretical calculations that show NO2 to be mildly hydrophobic.

Atmospheric reactions on electrically charged surfaces.

It is proposed that tropospheric NO2 at concentrations in the parts-per-billion range can be efficiently converted to HONO in a dust storm, by a process that is initiated by electron capture by NO2 from a negatively-charged dust particle. The electron capture is visualized as a harpoon-type process that does not require the NO2 to be adsorbed on the particle. The resulting electronically excited [NO2(-)]* ion reacts with water to form an HONO molecule plus an OH(-)·(H2O)n cluster ion. It is suggested that analogous processes can occur on other atmospheric aerosol particles with both positive and negative charges, with other molecules of high electron affinity such as SO2, and also, because the earth's surface is effectively the negative plate of a planet-sized capacitor, at the surfaces of terrestrial solids, lakes and oceans.

Nitrogen dioxide at the air-water interface: trapping, absorption, and solvation in the bulk and at the surface.

The interaction of NO(2) with water surfaces in the troposphere is of major interest in atmospheric chemistry. We examined an initial step in this process, the uptake of NO(2) by water through the use of molecular dynamics simulations. An NO(2)-H(2)O intermolecular potential was obtained by fitting to high-level ab initio calculations. We determined the binding of NO(2)-H(2)O to be about two times stronger than that previously calculated. From scattering simulations of an NO(2) molecule interacting with a water slab we observed that the majority of the scattering events resulted in outcomes in which the NO(2) molecule became trapped at the surface or in the interior of the water slab. Typical surface-trapped/adsorbed and bulk-solvated/absorbed trajectories were analyzed to obtain radial distribution functions and the orientational propensity of NO(2) with respect to the water surface. We observed an affinity of the nitrogen atom for the oxygen in water, rather than hydrogen-bonding which was rare. The water solvation shell was less tight for the bulk-absorbed NO(2) than for the surface-adsorbed NO(2). Adsorbed NO(2) demonstrated a marked orientational preference, with the oxygens pointing into the vacuum. Such behavior is expected for a mildly hydrophobic and surfactant molecule like NO(2). Estimates based on our results suggest that at high NO(2) concentrations encountered, for example, in some sampling systems, adsorption and reaction of NO(2) at the surface may contribute to the formation of gas-phase HONO.

A stochastic, local mode study of neon-liquid surface collision dynamics.

Equations of motion for a fast, light rare gas atom passing over a liquid surface are derived and used to infer the dynamics of neon collisions with squalane and perfluorinated polyether surfaces from experimental data. The equations incorporate the local mode model of a liquid surface via a stochastic process and explicitly account for impulsive collisional energy loss to the surface. The equations predict angular distributions for scattering of neon that are in good quantitative agreement with experimental data. Our key dynamical conclusions are that experimental angular distributions derive mainly from local mode surface topography rather than from structural features of individual surface molecules, and that the available data for these systems can be accounted for almost exclusively by single collisions between neon atoms and the liquid surface.

Photolysis in aqueous aerosols: 300 nm yields of Fe2+ from a ferrioxalate actinometer and of OH radical from nitrate ions.

300 nm photolysis yields of Fe(2+) from potassium ferrioxalate and of OH from nitrate ion have been measured in aqueous aerosols, the yield from ferrioxalate in a bulk solution being used to measure the light intensity. Mie theory was used to calculate effective cross-sections for absorption and scattering of light by the aerosol droplets. Yields of OH from nitrate ion have been measured with benzoic acid and carbon monoxide radical scavengers. The photolysis yield of Fe(2+) from ferrioxalate was found to be enhanced in the aerosol by a factor of 48 +/- 17. This enhancement is believed to be real, and is attributed to surfactant behaviour that results in the presence of a high concentration of ferrioxalate in a region of high light intensity near the droplet surface. The experiments with benzoic acid indicate that the yield of OH from nitrate in aerosol droplets is not significantly different from the yield in bulk solution. The CO experiments appear to indicate that the total OH production in the aerosol is enhanced over that in bulk solution by a factor of 10 +/- 3, but this number is not considered reliable.

A stochastic, local mode treatment of high-energy gas-liquid collisions.

The scattering angle distributions of high-energy molecular beams at the surfaces of three different liquids are treated in terms local mode theory. This is achieved by setting up a stochastic process modeling the effect of a superposition of local mode surface displacements on the incoming particle's trajectory. The results are found to be in good qualitative agreement with experiment, and directions for further work are indicated.

Complexes of HNO3 and NO3 - with NO2 and N2O4, and their potential role in atmospheric HONO formation.

Calculations were performed to determine the structures, energetics, and spectroscopy of the atmospherically relevant complexes (HNO(3)).(NO(2)), (HNO(3)).(N(2)O(4)), (NO(3)(-)).(NO(2)), and (NO(3)(-)).(N(2)O(4)). The binding energies indicate that three of the four complexes are quite stable, with the most stable (NO(3)(-)).(N(2)O(4)) possessing binding energy of almost -14 kcal mol(-1). Vibrational frequencies were calculated for use in detecting the complexes by infrared and Raman spectroscopy. An ATR-FTIR experiment showed features at 1632 and 1602 cm(-1) that are attributed to NO(2) complexed to NO(3)(-) and HNO(3), respectively. The electronic states of (HNO(3)).(N(2)O(4)) and (NO(3)(-)).(N(2)O(4)) were investigated using an excited state method and it was determined that both complexes possess one low-lying excited state that is accessible through absorption of visible radiation. Evidence for the existence of (NO(3)(-)).(N(2)O(4)) was obtained from UV/vis absorption spectra of N(2)O(4) in concentrated HNO(3), which show a band at 320 nm that is blue shifted by 20 nm relative to what is observed for N(2)O(4) dissolved in organic solvents. Finally, hydrogen transfer reactions within the (HNO(3)).(NO(2)) and (HNO(3)).(N(2)O(4)) complexes leading to the formation of HONO, were investigated. In both systems the calculated potential profiles rule out a thermal mechanism, but indicate the reaction could take place following the absorption of visible radiation. We propose that these complexes are potentially important in the thermal and photochemical production of HONO observed in previous laboratory and field studies.

Raman spectra of complexes of HNO3 and NO3- with NO2 at surfaces and with N2O4 in solution.

Raman spectra of HNO(3).NO(2) have been detected on liquid and solid surfaces in the presence of concentrated HNO(3) and NO(2) gas. The Raman spectrum of HNO(3) solutions containing N(2)O(4) has been partly reinterpreted in terms of contributions from HNO(3).N(2)O(4) and N(2)O(4).NO(3)(-) complexes.

Onsager heat of transport for water vapour at the surface of water and ice: thermal accommodation coefficients for water vapour on a stainless-steel surface.

The Onsager heat of transport Q* has been measured for water vapour at the surface of water, supercooled water, and ice, over the temperature range -8 to +10 degrees C. For liquid water, Q* is constant at -24.7 +/- 3.6 kJ mol(-1) (two standard deviations) over the pressure range 4-9.5 Torr. Provided the ice is suitably aged, the |Q*| values are very similar for water and ice, a result which is consistent with the presence of a liquid-like layer at the surface of ice. The values are slightly larger for ice, in proportion to the ratio of the heat of sublimation of ice to the heat of vaporization of the liquid. Departures from linearity of plots of P against DeltaT are attributed to temperature jumps at the surface of the dry upper plate. Hence jump coefficients and thermal accommodation coefficients have been derived as a function of temperature for collisions of water molecules with type-304 stainless steel.

Enhanced photolysis in aerosols: evidence for important surface effects.

While there is increasing evidence for unique chemical reactions at interfaces, there are fewer data on photochemistry at liquid-vapor junctions. This paper reports a comparison of the photolysis of molybdenum hexacarbonyl, Mo(CO)(6), in 1-decene either as liquid droplets or in bulk-liquid solutions. Mo(CO)(6) photolysis is faster by at least three orders of magnitude in the aerosols than in bulk-liquids. Two possible sources of this enhancement are considered: (1) increased light intensity due to either Morphology-Dependent Resonances (MDRs) in the spherical aerosol particles and/or to increased pathlengths for light inside the droplet due to refraction, which are termed physical effects in this paper; and (2) interface effects such as an incomplete solvent-cage at the gas-liquid boundary and/or enhanced interfacial concentrations of Mo(CO)(6), which are termed chemical effects. Quantitative calculations of the first possibility were carried out in which the light intensity distribution in the droplets averaged over 215-360 nm was obtained for 1-decene droplets. Calculations show that the average increase in light intensity over the entire droplet is 106%, with an average increase of 51% at the interface. These increases are much smaller than the observed increase in the apparent photolysis rate of droplets compared to the bulk. Thus, chemical effects, i.e., a decreased solvent-cage effect at the interface and/or enhancement in the surface concentration of Mo(CO)(6), are most likely responsible for the dramatic increase in the photolysis rate. Similar calculations were also carried out for broadband (290-600 nm) solar irradiation of water droplets, relevant to atmospheric conditions. These calculations show that, in agreement with previous calculations by Mayer and Madronich [B. Mayer and S. Madronich, Atmos. Chem. Phys., 2004, 4, 2241] MDRs produce only a moderate average intensity enhancement relative to the corresponding bulk-liquid slabs when averaged over a range of wavelengths characteristic of solar radiation at the Earth's surface. However, as in the case of Mo(CO)(6) in 1-decene, chemical effects may play a role in enhanced photochemistry at the aerosol-air interface for airborne particles.

Surfing the nanowaves: progress in understanding the gas-liquid interface.

Recent work on the gas-liquid interface is discussed with emphasis on theoretical studies of the small-scale motion of the capillary-wave zone, experimental and theoretical studies of the ultra-small-scale surface roughness of liquids, experimental measurements of the Onsager heat of transport at a gas-liquid interface, the likely origin of the heat of transport at a molecular level, and the resolution of several types of paradoxical behavior that have been observed or predicted to occur at a liquid-vapor interface.