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«Interactions of Peroxynitric Acid and Hydrogen Peroxide with Ice and the Envrionmental Implications Inauguraldissertation der ...»

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Interactions of Peroxynitric Acid and Hydrogen

Peroxide with Ice and the Envrionmental

Implications

Inauguraldissertation

der Philosophisch-naturwissenschaftlichen Fakult¨t

a

der Universit¨t Bern

a

vorgelegt von

Thomas Ulrich

aus Deutschland

Leiter der Arbeit:

Prof. Dr. Samuel Leutwyler

Departement f¨r Chemie und Biochemie

u

Interactions of Peroxynitric Acid and Hydrogen

Peroxide with Ice and the Envrionmental

Implications

Inauguraldissertation der Philosophisch-naturwissenschaftlichen Fakult¨t a der Universit¨t Bern a vorgelegt von Thomas Ulrich aus Deutschland

Leiter der Arbeit:

Prof. Dr. Samuel Leutwyler Departement f¨r Chemie und Biochemie u Von der Philosophisch-naturwissenschaftlichen Fakult¨t angenommen.

a

Bern, 31. Mai 2013 Der Dekan:

Prof. Dr. Silvio Decurtins La gravitation de l’esprit nous fait tomber vers le haut.

Simone Weil (1909-43) Abstract Peroxynitric acid (HNO4 ) and hydrogen peroxide (H2 O2 ) are important trace gases with strong links to the oxidative capacity of the troposphere.

Their adsorption to ice and snow in the troposphere can lead to a scavenging of both trace gas species from the atmosphere. Also their adsorption on snow packs and ice on the ground a↵ect the atmospheric chemistry of the overlaying boundary layer. In this thesis results of laboratory experiments are presented and discussed, investigating the uptake of HNO4 and H2 O2 to ice.

A new gas phase synthesis for HNO4 is presented. This gas phase synthesis comprises a purification step, consisting of a Ti(IV) denuder and a cooling trap. The synthesis was successfully tested in packed bed flow tub experiments with di↵erent nitrogen oxides, resulting in a partitioning from the gas phase towards the ice surface in the order of HNO3 HNO4 = HNO2 NO2.

In a subsequent study, the new synthesis was used in coated wall flow tube experiments. For the first time ice adsorption measurements low in impuritieswere possible and the temperature dependency of the partition constant K LinC from gas phase to ice surface was derived. The temperature dependence follows a relation ship given by 3.74 10 12 e(7098/T) [cm]. The results di↵er from a previous study, our results show a much lower reversible adsorption process on the ice surface. Uptake of HNO4 to ice was often compared to that of HNO3 in literature. Using the lower partition constant derived in this study a di↵erent adsorption behavior of HNO4 as compared to HNO3 in upper tropospheric cirrus clouds becomes evident. The uptake of HNO4 to ice particles in those clouds only becomes important lower temperatures. Below 220 K and with very dense clouds present less than 10 % of HNO4 is adsorbed on the ice particles. Considering snow on the ground, snow packs represent a sink for HNO4 due to the high specific surface area of ice there.

Literature results of the second trace gas species investigated in this thesis, H2 O2, di↵ered three orders of magnitude considering partitioning towards the ice surface. The older studies investigated H2 O2 uptake to ice on short time scales to smooth ice surfaces. The results presented here agree with the higher, more temperature dependent partitioning reported earlier. In addition it is shown in this thesis, that the uptake of H2 O2 to ice also includes a long term uptake to the ice, which is beyond the surface adsorption process. The long term uptake agrees quantitatively with an older packed bed study. Our results in the coated wall flow tube experiments suggest a di↵usion process into the smooth polycrystalline ice. The environmental relevance of this bulk uptake exceeds that of the surface adsorption process when the chemistry above snow packs is concerned.

On a more fundamental approach on the uptake of trace gases to ice, grain boundaries in polycrystalline ice have been proposed as a major reservoir for the bulk uptake of trace gases. In the last part of this thesis the development of a new flow through reactor for grain boundary dependent uptake of trace gases is presented. With this reactor di↵erent types of ices, varying in a grain boundary content by a factor of five, can be produced. With this setup the uptake of trace gases into grain boundaries could be disentangled from the uptake into the ice crystal in follow-up studies. Preliminary results with nitrous acid (HONO) in the reactor are presented.

Contents

–  –  –

3.1 Solubility, acidity and adsorption enthalpies for di↵erent trace gases. 67

4.1 Molecules lost from the gas phase compared to molecules desorbed from the ice after equivalent times.................... 97 16 LIST OF TABLES Chapter 1 Introduction The topic of this thesis is the interaction of trace gases with ice surfaces. In the following laboratory studies are presented considering trace gas uptake on ice. The resulting environmental consequences are discussed for each topic. Ice surfaces are ubiquitously present in the environment. In February the land ice and seasonal snow on ther northern hemisphere covers 46 million km2 and the Antarctic land ice coverage alone is 14 million km2 (Washburn, 1980). In the atmosphere ice is present in clouds, for example in cirrus clouds in the upper troposphere which consist mostly of ice particles. They can cover up to 40 % area fraction of earth (Popp et al., 2004).





Atmospheric trace gases interact with the ice surfaces both in the atmosphere and on the ground. The interactions in the upper troposphere and on the ground are summarized in a simplified version in Figure 1.1. Trace gases can be scavenged by falling snow or adsorb directly on snow or ice on the ground. In the interstitial air of snow packs di↵usion can take place. Snow packs can act as a chemical reactor by provdining sites for heterogeneous reactions or photo chemistry.

In this chapter the relevant environments and the tropospheric chemistry are introduced, followed by the scientific context of this study and discussion of the open questions. The introduction ends with a summary of the main goals of this thesis.

1.1 Troposphere To give the reader the context of this study, the relevance of tropospheric and polar environments for trace gas ice interactions are highlighted. Regarding ice surfaces in the troposphere, ice clouds are of major relevance. It has been proposed that the interactions of the trace gases with ice in cirrus clouds play a role in O3 depletion in the upper troposphere and lower stratosphere through scavenging O3 precursors (Jaegle et al., 1998; Roumeau et al., 2000). The ice particles of such clouds scavenge

18 Introduction

–  –  –

Figure 1.1: Interaction of trace gases with ice particles in the troposphere and subsequent scavenging.

In the lower part processes in the snow pack are shown.

Left side: Tomography picture (adapted from Pinzer et al., 2010). Right side:

Microscopy picture with individual ice grains and grain boundaries (adapted from Riche et al., 2012).

trace gas species with a high a nity to the ice surface and remove them from the atmosphere by deposition. The reduction of ozone, driven by the uptake of precursor trace gases to cirrus clouds has recently been confirmed using atmospheric chemistrytransport models (Marecal et al., 2010; Neu and Prather, 2012).

1.2 Polar environments A snow pack with its ice surface and interstitial air acts as a multi-phase chemical reactor (Domine and Shepson, 2002; Bartels-Rausch et al., 2013a). Photochemical production of NOX for example can be enhanced compared to production in the gas-phase in the photic zone of a snow pack. Also glacial ice can be formed from the snow packs after firn to ice transfer, making chemical and physical processes in the snowpack relevant for ice core studies. Climatic change has a big impact on the snow cover of our planet. For example the arctic sea is estimated to be ice free during the summer season from 2030 plus minus 10 years (Wang and Overland, 2012). The lower extend in snow covers leads to less heterogeneous chemisty in snow packs.

1.3 Choice of trace gas species 19 The Arctic and Antarctic represent important examples of snow and ice environments in contact with the atmosphere. Both environments are covered with extensive areas of snow or ice, yet the conditions present di↵er for each of them.

The Arctic boundary layer is influenced by the transport of polluted air masses from the northern American and Eurasian regions. Air pollution is of major concern for human health. The world health organization (WHO) stated that in 2002 around 865 000 people died due to bad air quality in their 191 member states (WHO, 2007).

The greatest part of this death toll is due to small airborne particulate matter; but also a significant part is due to high ozone (O3 ) and NOX (= NO + NO2 ) concentrations. The Environmental Protection Agency (EPA) reported air pollution related health costs from 2005 to 2007 of 1 768 833 due to Ozone health e↵ects in California (Romley et al., 2010). Air pollution is primarily important in urban zones, where its sources are ubiquitous. But polluted air masses transported from northern American and northern Eurasian regions towards the Arctic impact the local inhabitants and wild life population (Barrie et al., 1981). As discussed in a later section chemistry in snow packs also impacts reactive nitrogen budgets, which contribute to the local air pollution. The Antarctic is a very prisinte envrionment, but also very high NOX and O3 concnterations have been measured above the Antarctic plateau.

1.3 Choice of trace gas species This study focuses on two atmospheric trace gases: Peroxynitric acid (HNO4 or HO2 NO2 ) and hydrogen peroxide (H2 O2 ). HNO4 is an important trace gas species in the polar environments. In Antarctica for example HNO4 has been measured in concentrations comparable in magnitude to other important trace gases like HNO3 and HONO (Slusher et al., 2010) (3 1010 molecules / cm3 ). Also in the cold upper troposphere HNO4 has been measured in significant concentrations (Kim et al., 2007) (6 108 molecules / cm3 ).

H2 O2 is present in the global atmosphere in significant mixing ratios. Global satellite observations (Allen et al., 2013) measured mixing ratios of 600 ppt – 700 ppt in the low to mid latitudes up to an altitude of 6.5 km. At the higher latitudes mixing ratios of 100 ppt – 300 ppt were measured up to an altitude of 6 km.

In the following the links of HNO4 and H2 O2 to some important atmospheric cycles are discussed. The atmospheric reaction circles of NOX,,O3 and VOCs are closely connected (Finlayson, Pitts and Pitts, 1997; Atkinson, 2000). Central to these networks is the oxidative capacity of the atmosphere and with it the OH radical (Atkinson, 2000). The OH radical acts like a cleaning agent of the atmosphere. This cleaning agent degrades volatile organic compounds (VOCs), which are emitted into the atmosphere by both anthropogenic and natural sources. OH attacks the VOCs and oxidizes them; a process which produces many intermediate species. As OH

20 Introduction

oxidizes VOCs in a first step; alkyl radicals (R ), alkyl peroxy and alkoxy radicals (RO + RO2 = ROX ) are produced and OH is converted to HO2 as shown in equation (1.1 - 1.4).

–  –  –

1.4 Ice surfaces as a site for trace gas interactions The aim of this study is to elucidate uptake processes of trace gas by snow and ice, the focus being laid on the ice surface in the experiments. This gives an approach

1.4 Ice surfaces as a site for trace gas interactions 21

–  –  –

to the identification of the physical and chemical aspects of the interaction without the complexity of snow packs with their micro and macro structures.

To understand the physical and chemical processes involving ice surfaces, the properties of the ice surface itself have to be understood. The water molecules in the environmentally relevant ice crystal phase (I h ), are structured in bi-layers ordered in a hexagonal matrix. Ice exists in the environment roughly between 190 K and 273 K. At these temperatures ice has a very high vapor pressure up to 6 mbar near the melting point (Marti and Mauersberger, 1993). This results in a highly dynamic ice surface. For example at 180 K, 100 mono-layers of the ice evaporate and re-condense per second; at 240 K 10 000 mono-layers evaporate and re-condense per second (Abbatt, 2003).

1.4.1 Disordering of the topmost ice surface Apart from the high dynamics, the ice surface also has very interesting properties, when the topmost layers in the nano-scale are considered. If we imagine the structure of the ice surface without considering additional e↵ects, there would be H-bonds dangling towards the gas phase, providing a very polar surface. The ordered structure of those H bonds is however energetically unfavorable. The result is that the water molecules in the topmost layers of the ice crystal are disordered at OF ICE... PHYSICAL REVIEW B 66, 085401 2002 or/ice interface.

plane being the input or output surface monoFigure 1.4: Visualization of grain boundaries marked by blue areas in a crystal matrix. Defects of the crystal structure are visible at the grain boundaries. Adapted from G. Lisensky, University of Wisconsin.

about three magnitudes higher in grain boundaries compared to single crystalline ice (Lu et al., 2009). Also for other molecules beside water, for example HCl, higher di↵usivities in grain boundaries have been observed (e.g. Domine et al., 1994).



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