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1.
J Phys Chem Lett ; 10(23): 7433-7438, 2019 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-31725306

RESUMO

The interaction of water with TiO2 is of substantial scientific and technological interest as it determines the activity of TiO2 in photocatalytic and environmental applications in nanoparticle suspensions in water, in complex appliances, or in pure form interacting with water vapor. The influence of TiO2 nanoparticles on the hydrogen bonding structure of water molecules is an important factor that controls hydration of other species, reactions, or nucleation processes. We use a combination of ambient-pressure X-ray photoelectron spectroscopy and electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the oxygen K-edge to investigate the hydrogen bonding structure of adsorbed water on titania nanoparticles in equilibrium with nearly saturated water vapor at 235 K. The results clearly show that the net NEXAFS spectrum of adsorbed water resembles that of liquid, disordered water at 235 K, a temperature at which both homogeneous and heterogeneous freezing of bulk water is anticipated.

2.
Philos Trans A Math Phys Eng Sci ; 377(2146): 20190138, 2019 06 03.
Artigo em Inglês | MEDLINE | ID: mdl-30982453
3.
Environ Sci Process Impacts ; 21(1): 63-73, 2019 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-30534711

RESUMO

The reaction of ozone with bromide is an initiation process in bromine activation resulting in the formation of reactive bromine species with impacts on the fate of compounds in the lower atmosphere. Environmental halide sources often contain organics, which are known to influence aqueous bulk reactivity. Here, we present a study investigating the temperature dependence of bromide oxidation by ozone using a coated wall flow tube reactor coated with an aqueous mixture of citric acid, as a proxy for oxidized secondary organic matter, and sodium bromide. Using the resistor model formulation, we quantify changes in the properties of the aqueous bulk relevant for the observed reactivity. The reactive uptake coefficient decreased from 2 × 10-6 at 289 K to 0.5 × 10-6 at 245 K. Our analysis indicates that the humidity-driven increase in concentration with a corresponding increase in the pseudo-first order reaction rate was countered by the colligative change in ozone solubility and the effect of the organic fraction via increased viscosity and decreased diffusivity of ozone as the temperature decreased. From our parameterization, we provide an extension of the temperature dependence of the reaction rate coefficients driving the oxidation of bromide, and assess the temperature-dependent salting effects of citric acid on ozone solubility. This study shows the effects of the organic species at relatively mild temperatures, between the freezing point and eutectic temperature of sea as is typical for the Earth's cryosphere. Thus, this study may be relevant for atmospheric models at different scales describing halogen activation in the marine boundary layer or free troposphere including matrices such as sea-spray aerosol and brine in sea ice, snow, and around mid-latitude salt lakes.


Assuntos
Atmosfera/química , Brometos/química , Bromo/química , Ozônio/química , Ácido Cítrico/química , Compostos Orgânicos , Oxirredução , Ozônio/análise , Sais , Compostos de Sódio/química , Solubilidade , Temperatura Ambiente , Viscosidade
4.
Phys Chem Chem Phys ; 20(37): 24408-24417, 2018 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-30221299

RESUMO

Interactions between trace gases and ice are important in environmental chemistry and for Earth's climate. In particular, the adsorption of trace gases to ice surfaces at temperatures approaching the melting point has raised interest in the past, because of the prevailing pre-melting. Here, we present Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy data at ambient partial pressure of water to better define the onset temperature of pre-melting at the interfacial region of ice. Further, this study directly compares the interaction between an organic acid common in the atmosphere, formic acid, and that of an aliphatic carbon with ice at 253 K. It makes use of X-ray Photoelectron Spectroscopy (XPS) with its inherent narrow probing depth covering both the surface and near-surface bulk region when detecting electrons. We use the tender X-ray range for excitation to locate the organic species within the interfacial region with an extended probing depth compared to published XPS work. Electron kinetic energy dependent C1s photoemission data indicate that, at low coverage of a few 1014 molecules cm-2, the presence of formic acid is restricted to the upper ice layers of the interfacial region. Increasing the dosage, formic acid penetrates 6-7 nm into the air-ice interface. The presence of the more hydrophobic aliphatic carbon is restricted to the upper ice monolayers. This direct comparison of an organic acid with an aliphatic compound confirms the emerging picture where solutes enter the interfacial region of ice at a depth related to their specific tendency to form solvation shells.

5.
Environ Sci Technol ; 52(14): 7680-7688, 2018 07 17.
Artigo em Inglês | MEDLINE | ID: mdl-29898357

RESUMO

Atmospheric aerosol particles may contain light absorbing (brown carbon, BrC), triplet forming organic compounds that can sustain catalytic radical reactions and thus contribute to oxidative aerosol aging. We quantify UVA induced radical production initiated by imidazole-2-carboxaldehyde (IC), benzophenone (BPh). and 4-benzoylbenzoic acid (BBA) in the presence of the nonabsorbing organics citric acid (CA), shikimic acid (SA), and syringol (Syr) at varying mixing ratios. We observed a maximum HO2 release of 1013 molecules min-1 cm-2 at a mole ratio XBPh < 0.02 for BPh in CA. Mixtures of either IC or BBA with CA resulted in 1011-1012 molecules min-1 cm-2 of HO2 at mole ratios ( XIC and XBBA) between 0.01 and 0.15. HO2 release was affected by relative humidity ( RH) and film thickness suggesting coupled photochemical reaction and diffusion processes. Quantum yields of HO2 formed per absorbed photon for IC, BBA and BPh were between 10-7 and 5 × 10-5. The nonphotoactive organics, Syr and SA, increased HO2 production due to the reaction with the triplet excited species ensuing ketyl radical production. Rate coefficients of the triplet of IC with Syr and SA measured by laser flash photolysis experiments were kSyr = (9.4 ± 0.3) × 108 M-1 s-1 and kSA = (2.7 ± 0.5) × 107 M-1 s-1. A simple kinetic model was used to assess total HO2 and organic radical production in the condensed phase and to upscale to ambient aerosol, indicating that BrC induced radical production may amount to an upper limit of 20 and 200 M day-1 of HO2 and organic radical respectively, which is greater or in the same order of magnitude as the internal radical production from other processes, previously estimated to be around 15 M per day.


Assuntos
Compostos Orgânicos , Aerossóis , Difusão , Cinética , Oxirredução
6.
Nat Commun ; 8(1): 700, 2017 09 26.
Artigo em Inglês | MEDLINE | ID: mdl-28951540

RESUMO

Oxidation of bromide in aqueous environments initiates the formation of molecular halogen compounds, which is important for the global tropospheric ozone budget. In the aqueous bulk, oxidation of bromide by ozone involves a [Br•OOO-] complex as intermediate. Here we report liquid jet X-ray photoelectron spectroscopy measurements that provide direct experimental evidence for the ozonide and establish its propensity for the solution-vapour interface. Theoretical calculations support these findings, showing that water stabilizes the ozonide and lowers the energy of the transition state at neutral pH. Kinetic experiments confirm the dominance of the heterogeneous oxidation route established by this precursor at low, atmospherically relevant ozone concentrations. Taken together, our results provide a strong case of different reaction kinetics and mechanisms of reactions occurring at the aqueous phase-vapour interface compared with the bulk aqueous phase.Heterogeneous oxidation of bromide in atmospheric aqueous environments has long been suspected to be accelerated at the interface between aqueous solution and air. Here, the authors provide spectroscopic, kinetic and theoretical evidence for a rate limiting, surface active ozonide formed at the interface.

7.
J Phys Chem Lett ; 8(19): 4757-4762, 2017 Oct 05.
Artigo em Inglês | MEDLINE | ID: mdl-28902513

RESUMO

The interfacial ionization of strong acids is an essential factor of multiphase and heterogeneous chemistry in environmental science, cryospheric science, catalysis research and material science. Using near ambient pressure core level X-ray photoelectron spectroscopy, we directly detected a low surface coverage of adsorbed HCl at 253 K in both molecular and dissociated states. Depth profiles derived from XPS data indicate the results as physisorbed molecular HCl at the outermost ice surface and dissociation occurring upon solvation deeper in the interfacial region. Complementary X-ray absorption measurements confirm that the presence of Cl- ions induces significant changes to the hydrogen bonding network in the interfacial region. This study gives clear evidence for nonuniformity across the air-ice interface and questions the use of acid-base concepts in interfacial processes.

10.
Environ Sci Technol ; 51(13): 7486-7495, 2017 Jul 05.
Artigo em Inglês | MEDLINE | ID: mdl-28581723

RESUMO

Irradiated nitrophenols can produce nitrite and nitrous acid (HONO) in bulk aqueous solutions and in viscous aqueous films, simulating the conditions of a high-solute-strength aqueous aerosol, with comparable quantum yields in solution and viscous films (10-5-10-4 in the case of 4-nitrophenol) and overall reaction yields up to 0.3 in solution. The process is particularly important for the para-nitrophenols, possibly because their less sterically hindered nitro groups can be released more easily as nitrite and HONO. The nitrophenols giving the highest photoproduction rates of nitrite and HONO (most notably, 4-nitrophenol and 2-methyl-4-nitrophenol) could significantly contribute to the occurrence of nitrite in aqueous phases in contact with the atmosphere. Interestingly, dew-water evaporation has shown potential to contribute to the gas-phase HONO levels during the morning, which accounts for the possible importance of the studied process.


Assuntos
Aerossóis , Nitritos , Processos Fotoquímicos , Nitrofenóis , Ácido Nitroso
11.
Top Curr Chem ; 339: 1-53, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-23247500

RESUMO

Sunlight is a major driving force of atmospheric processes. A detailed knowledge of atmospheric photochemistry is therefore required in order to understand atmospheric chemistry and climate. Considerable progress has been made in this field in recent decades. This contribution will highlight a set of new and emerging ideas (and will therefore not provide a complete review of the field) mainly dealing with long wavelength photochemistry both in the gas phase and on a wide range of environmental surfaces. Besides this, some interesting bulk photochemistry processes are discussed. Altogether these processes have the potential to introduce new chemical pathways into tropospheric chemistry and may impact atmospheric radical formation.

13.
J Phys Chem A ; 117(2): 401-9, 2013 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-23252403

RESUMO

Ice plays a key role in the environment, and the ice-air interface influences heterogeneous chemical reactions between snowpack or cirrus clouds and the surrounding air. Soluble gases have been suspected to affect the topmost, disordered layer on ice (often referred to as a quasiliquid layer, QLL). Changes are especially expected in the hydrogen-bonding structure of water in the presence of solutes at the ice surface. Here, we used ambient-pressure X-ray photoelectron spectroscopy (XPS) to detect acetic acid at the ice surface at 230-240 K under atmospheric conditions for the first time. Electron-kinetic-energy-dependent C 1s spectra indicate that acetic acid remains confined to the topmost ice surface layers. Spectral analysis provides information about the protonation state of acetate at the ice surface. Surface-sensitive Auger-electron-yield C-edge near-edge X-ray absorption fine structure (NEXAFS) spectra were recorded to probe the molecular state of the adsorbed species. The O-edge NEXAFS spectra show only minor differences between clean ice and ice with adsorbed acetic acid and thus indicate that acetic acid does not lead to an extended disordered layer on the ice surface between 230 and 240 K.

14.
Photochem Photobiol Sci ; 10(10): 1680-90, 2011 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-21811730

RESUMO

Photochemical processes on ground and airborne surfaces have been suspected to lead to production of HONO in the sunlit lower troposphere, e.g. upon light activation of humic acids followed by reaction with adsorbed NO(2). Here, we used tannic and gentisic acids as proxies for atmospheric polyphenolic compounds to obtain further insights into the photoenhanced NO(2) conversion to HONO, which is a significant tropospheric hydroxyl radical (OH) source. The coated wall flow tube technique was used in combination with online detection of gas-phase HONO and NO(x) under different irradiation conditions. Photoenhanced HONO formation rates of 0.1 to 2 ppbv s(-1) were measured upon NO(2) (0-400 ppbv) uptake on tannic and gentisic acid coatings under irradiation with UV light. The data allow identification of three pathways of light-induced HONO formation: (I) photolysis of a nitroaromatic intermediate formed by a non-photochemical process in the dark, with a photolysis frequency of 10(4) s(-1) at 2 × 10(20) photons m(-2) photon flux; (II) direct photo-oxidation, presumably through electron or hydrogen transfer of the excited substrate; and (III) sensitized electron or hydrogen transfer as suggested before but also demonstrated for visible irradiation here. Aging of tannic acid under oxygen in the dark led to products which promoted light-induced HONO formation in the visible.

15.
Chemosphere ; 82(2): 199-203, 2011 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-21044797

RESUMO

Reduction of divalent mercury and subsequent emission to the atmosphere has been identified as loss process from surface snow, but its mechanism and importance are still unclear. The amount of mercury that stays in the snow pack until spring is of significance, because during snow melt it may be released to the aquatic environment and enter the food web. Better knowledge of its fate in snow might further assist the interpretation of ice core data as paleo-archive. Experiments were performed under well-controlled laboratory conditions in a coated wall flow tube at atmospheric pressure and irradiated with light between 300 nm and 420 nm. Our results show that the presence of benzophenone and of oxalic acid significantly enhances the release of mercury from the ice film during irradiation, whereas humic acid is less potent to promote the reduction. Further it was found that oxygen or chloride, and acidic conditions lowered the photolytically induced mercury release in the presence of benzophenone, while the release got larger with increasing temperatures.


Assuntos
Cátions Bivalentes/química , Poluentes Ambientais/química , Gelo , Mercúrio/química , Processos Fotoquímicos , Benzofenonas/química , Substâncias Húmicas , Fotólise , Neve/química
16.
Phys Chem Chem Phys ; 12(26): 7194-202, 2010 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-20485807

RESUMO

The co-adsorption of acetic and nitrous acids was investigated in a packed ice bed flow tube at atmospheric pressure in the temperature range of 213-243 K. The uptake of acetic acid (CH3COOH) on ice was probed by means of a chemical ionisation quadrupole mass spectrometer. For the interaction of HONO with ice, molecules labelled with the short-lived radioisotope 13N (t1/2 = 10 min) were used. First, the retention time of acetic acid in the packed ice beds was measured in the absence of HONO and used to calculate the linear partition coefficient, K(Lin,C,AA) and the dimensionless adsorption partition coefficient, K0(p,AA). The temperature dependence of KLin,C,AA was found to be K(Lin,C,AA)[m] = 1.9 x 10(-14) exp(7.4 x 10(3)/T) in good agreement with previous studies. No evidence of diffusion of acetic acid in the bulk of the ice matrix was observed. The co-adsorption measurements are well reproduced by a competitive Langmuir adsorption model and showed that the partition coefficients derived for single compounds can also be applied to this more complex system. Further, the measurements revealed that the bulk uptake of HONO is not influenced by the presence of acetic acid. The results are discussed in terms of partitioning of trace gases in snowpacks.

17.
Phys Chem Chem Phys ; 9(11): 1362-9, 2007 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-17347709

RESUMO

Aqueous aerosols and other water surfaces in the environment may be coated with organic films, which can give rise to significant effects on gas-solution transport and surface reactivity. We have used acridine as a molecular fluorescent pH probe to examine the hydration of nitric acid and ammonia at both the uncoated and the organic-coated air-water interface. For uncoated samples, a transient decrease in pH is observed at the interface upon introduction of nitric acid vapour, followed by a relaxation to a final pH which is lower than the initial value. This long-time final change in pH is also measured in bulk pH measurements. Solutions having monolayer and sub-monolayer films of 1-octanol do not display the transient, but do show the same long-time change in pH. The degree of suppression of the surface pH transient depends directly on the amount of octanol present at the surface. Hydrolysis of ammonia at the water surface is also indicated by a surface pH transient which is also suppressed when a monolayer of octanol is present at the surface. Monolayers of butanol and of uncompressed stearic acid at the surface show little difference from the clean interface. The results are related to the concentration of available water at the interface.


Assuntos
Acridinas/química , Amônia/química , Materiais Revestidos Biocompatíveis/química , Ácido Nítrico/química , Compostos Orgânicos/química , Água/química , Hidrólise , Interações Hidrofóbicas e Hidrofílicas , Solubilidade , Propriedades de Superfície
18.
J Phys Chem A ; 109(20): 4531-9, 2005 May 26.
Artigo em Inglês | MEDLINE | ID: mdl-16833789

RESUMO

An atmospheric pressure variant of the coated-wall flow-tube technique in combination with a Monte Carlo simulation is presented. In a performance test of simple first-order wall loss, the Monte Carlo simulation, which uses a simplified model of transport in laminar flow, reproduced results of an analytical solution of the transport equations in a flow tube. This technique was then used to investigate the reversible adsorption of acetone on ice films between 203 and 223 K and a surface coverage of below 5% of a formal monolayer. Simulation of the experimental uptake traces allowed retrieving an adsorption enthalpy of -46 +/- 3 kJ mol(-1) for acetone on ice, which is in good agreement with other static and flow-tube methods. For the experimental conditions adopted here, the transport of acetone molecules along the ice film is governed by equilibrium thermodynamics. Therefore, the surface accommodation coefficient, S(0), and the preexponential factor, tau(0), for the activated desorption cannot be independently determined. These two main microphysical parameters describing partitioning can rather be estimated through their relation to the adsorption entropy. A first estimate for S(0) of acetone on ice in the range of 0.004-0.043 is given.

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