Scientific Highlights from Research Division "Biology and Chemistry" (BIO)

BIO Scientific Highlights

The competition between organics and bromide at the aqueous solution – air interface as seen from ozone uptake kinetics and X-ray photoelectron spectroscopy

Research Division Biology and Chemistry (BIO), surface chemistry Group, Head Markus Ammann. A more detailed understanding of the heterogeneous chemistry of halogenated species in the marine boundary layer is required. Here, we studied the reaction of ozone (O3) with NaBr solutions in presence and absence of citric acid (C6H8O7) under ambient conditions. Citric acid is used as a proxy for oxidized organic material present at the ocean surface or in sea spray aerosol. On neat NaBr solutions, the observed kinetics is consistent with bulk reaction limited uptake, and a second order rate constant for the reaction of O3 + Br- is 57±10 M-1 s-1. On mixed NaBr citric acid aqueous solutions the uptake kinetics was faster than that predicted by bulk reaction limited uptake and also faster than expected based on an acid catalyzed mechanism. X-ray photoelectron spectroscopy (XPS) on a liquid microjet of the same solutions at 1.0 x 10^3 - 1.0 x 10^4 mbar was used to obtain quantitative insight into the interfacial composition relative to that of the bulk solutions. It revealed that bromide anion becomes depleted by 30±10 % while the sodium cation gets enhanced by 40±20 % at the aqueous solution-air interface of a 0.12 M NaBr solution mixed with 2.5 M citric acid in the bulk, attributed to the role of citric acid as a weak surfactant. Therefore, the enhanced reactivity of bromide solutions observed in presence of citric acid is not necessarily attributable to a surface reaction but could also result from an increased solubility of ozone at higher citric acid concentrations. Whether the acid catalyzed chemistry may have a larger effect on the surface than in the bulk to offset the effect of bromide depletion also remains open.
Citation: M.-T. Lee, M. A. Brown, S. Kato, A. Kleibert, A. Türler and M. Ammann, The Journal of Physical Chemistry A (2015)

Publication: http://dx.doi.org/10.1021/jp510707s

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The nature of nitrate at the ice surface

Surface Chemistry Research Group, Head Markus Ammann. Trace contaminants such as strong acids have been suggested to affect the thickness of the quasi-liquid layer at the ice/air interface, which is at the heart of heterogeneous chemical reactions between snowpacks or cirrus clouds and the surrounding air. We used X-ray photoelectron spectroscopy (XPS) and electron yield near edge X-ray absorption fine structure (NEXAFS) spectroscopy at the Advanced Light Source (ALS) to probe the ice surface in the presence of HNO3 at 230 K. We studied the nature of the adsorbed species at the ice/vapor interfaces as well as the effect of HNO3 on the hydrogen bonding environment at the ice surface. The NEXAFS spectrum of ice with adsorbed HNO3 can be represented as linear combination of the clean ice and nitrate solution spectrum, thus indicating that in the presence of HNO3 the ice surface consists of a mixture of clean ice and nitrate ions that are coordinated as in a concentrated solution at the same temperature but higher HNO3 pressures.
Citation: Křepelová, K., Newberg, J., Huthwelker, T., Bluhm, H., and Ammann, M. (2010). "The nature of nitrate at the ice surface studied by XPS and NEXAFS" Physical Chemistry Chemical Physics 12(31), 8870-8880.

Publication: http://dx.doi.org/10.1039/C0CP00359J

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Recent increase in black carbon concentrations
from a Mt. Everest ice core spanning 1860–2000 AD

Research Division Biology and Chemistry (BIO), Analytical Chemistry Group, Head Margit Schwikowski. A Mt. Everest ice core spanning 1860–2000 AD and analyzed at high resolution for black carbon (BC) using a Single Particle Soot Photometer demonstrates strong seasonality, with peak concentrations during the winter‐spring, and low concentrations during the summer monsoon season. BC concentrations from 1975–2000 relative to 1860–1975 have increased approximately threefold, indicating that BC from anthropogenic sources is being transported to high elevation regions of the Himalaya. The timing of the increase in BC is consistent with BC emission inventory data from South Asia and the Middle East, however since 1990 the ice core BC record does not indicate continually increasing BC concentrations. The Everest BC and dust records provide information about absorbing impurities that can contribute to glacier melt by reducing the albedo of snow and ice. There is no increasing trend in dust concentrations since 1860, and estimated surface radiative forcing due to BC in snow exceeds that of dust in snow. This suggests that a reduction in BC emissions may be an effective means to reduce the effect of absorbing impurities on snow albedo and melt, which affects Himalayan glaciers and the availability of water resources in major Asian rivers.
Citation: S.D. Kaspari, M. Schwikowski, M. Gysel, M.G. Flanner, S. Kang, S. Hou, and P.A. Mayewski, Geophysical Research Letters 38, L04703 (2011)

Publication: http://dx.doi.org/10.1029/2010GL046096

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The role of long-lived reactive oxygen intermediates in the reaction of ozone with aerosol particles

Research Division Biology and Chemistry (BIO), Surface Chemistry Group, Head Markus Ammann. The heterogeneous reactions of ozone with aerosol particles are of central importance to air quality. They are studied extensively, but the molecular mechanisms and kinetics remain unresolved. Based on new experimental data and calculations, we show that long-lived reactive oxygen intermediates (ROIs) are formed. The chemical lifetime of these intermediates exceeds 100 seconds, which is much longer than the surface residence time of molecular ozone (~ ns). The ROIs explain and resolve apparent discrepancies between earlier quantum mechanical calculations and kinetic experiments. They play a key role in the chemical transformation and adverse health effects of toxic and allergenic air-particulate matter, such as soot, polycyclic aromatic hydrocarbons and proteins. ROIs may also be involved in the decomposition of ozone on mineral dust and in the formation and growth of secondary organic aerosols. Moreover, ROIs may contribute to the coupling of atmospheric and biospheric multiphase processes.
Citation: M. Shiraiwa, Y. Sosedova, A. Rouvière, H. Yang, Y. Zhang, J. P. D. Abbatt, M. Ammann and U. Pöschl, Nat Chem advance online publication, (2011)

Publication: http://dx.doi.org/10.1038/nchem.988

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Gas uptake and chemical aging of semisolid organic aerosol particles

Research Division Biology and Chemistry (BIO), Surface Chemistry Group, Head Markus Ammann. Organic substances can adopt an amorphous solid or semisolid state, influencing the rate of heterogeneous reactions and multiphase processes in atmospheric aerosols. Here we demonstrate how molecular diffusion in the condensed phase affects the gas uptake and chemical transformation of semisolid organic particles. Flow tube experiments show that the ozone uptake and oxidative aging of amorphous protein is kinetically limited by bulk diffusion. The reactive gas uptake exhibits a pronounced increase with relative humidity, which can be explained by a decrease of viscosity and increase of diffusivity due to hygroscopic water uptake transforming the amorphous organic matrix from a glassy to a semisolid state (moisture-induced phase transition). The reaction rate depends on the condensed phase diffusion coefficients of both the oxidant and the organic reactant molecules, which can be described by a kinetic multilayer flux model but not by the traditional resistor model approach of multiphase chemistry. The chemical lifetime of reactive compounds in atmospheric particles can increase from seconds to days as the rate of diffusion in semisolid phases can decrease by multiple orders of magnitude in response to low temperature or low relative humidity. The findings demonstrate that the occurrence and properties of amorphous semisolid phases challenge traditional views and require advanced formalisms for the description of organic particle formation and transformation in atmospheric models of aerosol effects on air quality, public health, and climate.
Citation: M. Shiraiwa, M. Ammann, T. Koop and U. Pöschl, Proceedings of the National Academy of Sciences, published online before print (2011)

Publication: http://dx.doi.org/10.1073/pnas.1103045108

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Three centuries of eastern european and Altai lead emissions recorded in a belukha ice core

Research Division Biology and Chemistry (BIO), Analytical Chemistry Group, Head Margit Schwikowski. Human activities have significantly altered atmospheric Pb concentrations and thus, its geochemical cycle, for thousands of years. Whereas historical Pb emissions from Western Europe, North America, and Asia are well documented, there is no equivalent data for Eastern Europe. Here, we present ice-core Pb concentrations for the period 1680–1995 from Belukha glacier in the Siberian Altai, assumed to be representative of emissions in Eastern Europe and the Altai. Pb concentrations and Pb-207/Pb-206 ratios were strongly enhanced during the period 1935–1995 due to the use of Pb additives in Russian gasoline mined in the Rudny Altai. Comparable to Western Europe and North America, Eastern European Pb emissions peaked in the 1970s. However, the subsequent downward trend in Eastern Europe was mainly caused by the economic crisis in the U.S.S.R. and not by a phase-out of leaded gasoline. Pb concentrations in the period 1680–1935, preceding the era of intensified industrialization in Russia, reflect the history of local emissions from Rudny Altai mining and related metallurgical processing primarily for the production of Russian coins. During this time, Altai ore Pb contributed about 40% of the regional atmospheric Pb.
Citation: A. Eichler, L. Tobler, S. Eyrikh, G. Gramlich, N. Malygina, T. Papina and M. Schwikowski, Environmental Science & Technology 46, 4323 (2012)

Publication: http://dx.doi.org/10.1021/es2039954

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Variations in diesel soot reactivity along the exhaust after-treatment system, based on the morphology and nanostructure of primary soot particles

Research Division Biology and Chemistry (BIO), Surface Chemistry Group, Head Markus Ammann. The reactivity of soot at different sites of the exhaust after-treatment system of a diesel engine (upstream and downstream of the diesel oxidation catalyst (DOC), downstream of the diesel particulate filter (DPF), as well as inside the DPF) was investigated on the basis of morphology and structure of primary soot particles by high resolution transmission electron microscopy (HRTEM). The results indicate that combustion-formed soot particles are susceptible to further transformations of their morphology within the exhaust system. The same primary soot particles can possess both oxidation-promoting and oxidation-inhibiting morphological features, the particle cores being highly reactive. Most reactivity-promoting features are encountered in pre-DOC and post-DOC primary particles, suggesting that soot can be more easily oxidised before it enters the DPF. The residence time of soot in the DPF contributes to modification of its reactivity by affecting size distribution and nanostructure of primary particles. Partial NO2 oxidation and high temperatures during active regeneration modify the morphology of outer particle shells, thus rendering post-DOC and post-DPF primary soot particles less reactive in this respect. Primary soot particles that pass through the DPF and reach the atmosphere are characterised by the highest graphitisation degree and sizes larger than those entering the DPF. Complementary Near-Edge X-ray Absorption Fine Structure (NEXAFS) analyses proved not as relevant regarding soot reactivity but indicate higher chemical inhomogeneity of pre-DOC than of post-DOC and post-DPF soot and high contents of carboxyl carbon in post-DPF particles.
Citation: A. Liati, P. Dimopoulos Eggenschwiler, D. Schreiber, V. Zelenay and M. Ammann, Combustion and Flame, in press (2012)

Publication: http://dx.doi.org/10.1016/j.combustflame.2012.10.024

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Chemistry: Ten things we need to know about ice and snow

Research Division Biology and Chemistry (BIO), surface chemistry Group, Head Markus Ammann. Understanding the molecular behaviour of frozen water is essential for predicting the future of our planet, says Thorsten Bartels-Rausch.
Citation: T. Bartels-Rausch, Nature 494, 27 (2013)

Publication: http://dx.doi.org/10.1038/494027a

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Effect of surface charge density on the affinity of oxide nanoparticles for the vapor–water interface

Research Division Biology and Chemistry (BIO), Surface Chemistry Group, Head Markus Ammann. Using in-situ X-ray photoelectron spectroscopy at the vapor-water interface, the affinity of nanometer-sized silica colloids to adsorb at the interface is shown to depend on colloid surface charge density. In aqueous suspensions at pH 10 corrected Debye-Hückel theory for surface complexation calculations predict that smaller silica colloids have increased negative surface charge density that originates from enhanced screening of deprotonated silanol groups by counterions in the condensed ion layer. The increased negative surface charge density results in an electrostatic repulsion from the vapor-water interface that is seen to a lesser extent for larger particles that have a reduced charge density in the XPS measurements. We compare the results and interpretation of the in-situ XPS and corrected Debye-Hückel theory for surface complexation calculations with traditional surface tension measurements. Our results show that controlling the surface charge density of colloid particles can regulate their adsorption to the interface between two dielectrics.
Citation: M. A. Brown, N. Duyckaerts, A. B. Redondo, I. Jordan, F. Nolting, A. Kleibert, M. Ammann, H. J. Wörner, J. A. van Bokhoven and Z. Abbas, Langmuir 29, 5023 (2013)

Publication: http://dx.doi.org/10.1021/la4005054

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Ice-core based assessment of historical anthropogenic heavy metal (Cd, Cu, Sb, Zn) emissions in the Soviet Union

Research Division Biology and Chemistry (BIO), Analytical Chemistry Group, Head Margit Schwikowski. The development of strategies and policies aiming at the reduction of environmental exposure to air pollution requires the assessment of historical emissions. Although anthropogenic emissions from the extended territory of the Soviet Union (SU) considerably influenced concentrations of heavy metals in the Northern Hemisphere, Pb is the only metal with long-term historical emission estimates for this region available, whereas for selected other metals only single values exist. Here we present the first study assessing long-term Cd, Cu, Sb, and Zn emissions in the SU during the period 1935-1991 based on ice-core concentration records from Belukha glacier in the Siberian Altai and emission data from 12 regions in the SU for the year 1980. We show that Zn primarily emitted from the Zn production in Ust-Kamenogorsk (East Kazakhstan) dominated the SU heavy metal emission. Cd, Sb, Zn (Cu) emissions increased between 1935 and the 1970s (1980s) due to expanded non-ferrous metal production. Emissions of the four metals in the beginning of the 1990s were as low as in the 1950s, which we attribute to the economic downturn in industry, changes in technology for an increasing metal recovery from ores, the replacement of coal and oil by gas, and air pollution control.
Citation: A. Eichler, L. Tobler, S. Eyrikh, N. Malygina, T. Papina and M. Schwikowski, Environmental Science & Technology ASAP (2014)

Publication: http://dx.doi.org/10.1021/es404861n

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