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Microchemistry of HONO in snow under non-equilibrium conditions

Applicant Ammann Markus
Number 121779
Funding scheme Project funding (Div. I-III)
Research institution Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Physical Chemistry
Start/End 01.11.2008 - 31.10.2009
Approved amount 56'950.00
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All Disciplines (2)

Discipline
Physical Chemistry
Climatology. Atmospherical Chemistry, Aeronomy

Keywords (12)

nitrous acid; snow pack; arctic boundar layer; snow metamorphosis; X-ray tomography; radioactive tracer; acetic acid; competitive adsorption; growing ice; snowpack; diffusivity; Arctic

Lay Summary (English)

Lead
Lay summary
Snow can cover a significant fraction of the land mass on Earth. The snowpack itself is an active chemical reactor under the influence of trace gas exchange and sunlight. Understanding the partitioning behaviour of trace gases to ice helps assessing the impact of the chemical processes in snow on atmospheric composition, it helps assessing the impact of deposition of trace gases to the local environment, and it helps elucidating the relation between atmospheric composition and the composition of ice core archives used to reconstruct past climates. Snow is a highly dynamic medium that undergoes continuous metamorphosis induced by non-equilibrium processes due to always present and changing temperature gradients. This project is heading towards understanding these non-equilibrium processes in more detail.Nitrous acid (HONO) is an important atmospheric nitrogen oxide species. It is easily photolysed by sunlight and thereby is an important source of OH radicals in the troposphere. Its sources in the troposphere are still not fully established. In the snowpack it can be produced from photolysis of nitrate and through surface reactions of nitrogen dioxide. Its release from the snowpack into the air aloft depends on temperature and snow properties. We have chosen HONO to probe the chemical aspects of non-equilibrium processes due its significance for the oxidant budget in the polar boundary layer.In a collaborative project with the Snow and Avalanche Research Institut (SLF) in Davos, the physical and chemical aspects of metamorphosing snow are being explored since about 3 years. The project combines X-ray micro-tomography of metamorphosing snow samples and chemical uptake experiments under equilibrium and non-equilibrium conditions. We first investigated partitioning of nitrous acid and acetic acid under equilibrium conditions alone and in competition with each other. Acetic acid should help assessing the effects of other acidic species typically present in snow. Using the combination of tomography with chemical experiments, we have observed that the ice surface is smooth up to the scale of tens of micrometers, i.e., it does usually not contain nanostructures. With regard to non-equilibrium conditions, we have performed first successful combined experiments with a novel snow diffusion chamber that allows monitoring diffusion of nitrous acid (labeled with 13N) within a snow sample under the same geometrical and temperature gradient conditions as in a natural snow cover. The last task will be to perform uptake experiments of nitrous acid to ice in a new setup that allows varying ice growth rates over a large range to get more fundamental insights into the dynamics at the air - ice interface.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Associated projects

Number Title Start Funding scheme
108219 Dynamics of snow metamorphism: observations of physical and chemical processes and microstructural multi-phase numerical simulation 01.11.2005 Project funding (Div. I-III)

Abstract

Snow can cover a significant fraction of the land mass on Earth. Wind pumping leads to exchange of trace gases between the snowpack and the air mass above it. The snowpack itself is an active chemical reactor under the influence of trace gas exchange and sunlight. Understanding the partitioning behaviour of trace gases to ice helps assessing the impact of the chemical processes in snow on atmospheric composition, it helps assessing the impact of deposition of trace gases to the local environment, and it helps elucidating the relation between atmospheric composition and the composition of ice core archives used to reconstruct past climates. Snow is a highly dynamic medium that undergoes continuous metamorphosis induced by non-equilibrium processes due to always present and changing temperature gradients, i.e., locally ice is always either growing or evaporating. While even partitioning between air and ice under equilibrium conditions is only reasonably well understood for a few gases, the partitioning behaviour under non-equilibrium conditions, i.e., on ice that is either growing or evaporating, is not at all clear. Nitrous acid (HONO) is an important atmospheric nitrogen oxide species. It is easily photolysed by sunlight and thereby is an important source of OH radicals in the troposphere. Its sources in the troposphere are still not fully established. In the snowpack it can be produced from photolysis of nitrate and through surface reactions of nitrogen dioxide. Its release from the snowpack into the air aloft depends on temperature and snow properties. We have chosen HONO to probe the chemical aspects of non-equilibrium processes due its significance for the oxidant budget in the polar boundary layer.In a collaborative project with the Snow and Avalanche Research Institut (SLF) in Davos, the physical and chemical aspects of metamorphosing snow have been started to be explored since about 2 years, funded by SNF. The objective has been to create well-defined experiments of controlled snow metamorphism, to make simultaneous structural, physical, and chemical measurements with high temporal resolution. The complexity of these tasks is accomplished by joining forces between the two institutes, each with a high reputation in its discipline. The project combines time-lapse X-ray micro-tomography of metamorphosing snow samples and chemical uptake experiments under equilibrium and non-equilibrium conditions. Two PhD students, one located at SLF and one at PSI, work on the physical and chemical aspects, respectively, though in close collaboration for experimental design and parallel measurements on snow samples. The tasks foreseen for the PhD thesis in Davos can be completed within the project period of three years. The chemical uptake measurements under metamorphosis conditions are a first ever and have required the development of two new experiments. This is the reason to ask for extension of the project at PSI by one year.While developing the design for the non-equilibrium experiments, we have started to investigate partitioning of nitrous acid and acetic acid under equilibrium conditions alone and in competition with each other. Acetic acid should help assessing the effects of other acidic species typically present in snow. The aim of these experiments has been to derive adsorption equilibrium constants and rates of transfer into the bulk of polycrystalline ice. Next, it has been shown that the ice surface area of snow accessible to gases retrieved from adsorption isotherm measurements is identical to that retrieved from computed X-ray tomography with about 30µm resolution that can be used to follow snow structure with time under metamorphosis conditions. With regard to non-equilibrium conditions, we have set up a new experiment allowing the measurement of HONO uptake rates to growing ice as a function of the growth rate. This experiment combines a Knudsen cell working in the molecular flow regime with radioactivity detection from HONO labeled with the short-lived radioactive tracer N-13. Finally, we have performed first successful combined experiments with a novel snow diffusion chamber that allows monitoring diffusion of nitrous acid (labeled with N-13) within a snow sample under the same geometrical and temperature gradient conditions as the physical tomography experiments.While the tasks related to the equilibrium uptake on ice will be completed within the preceding project, during the additional year, for which funds are requested from SNF with this proposal, we would like to complete the non-equilibrium uptake experiments and their analysis in both the snow diffusion chamber and the low pressure cell.
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