Physical, Chemical, and Optical Properties of Snow

While photochemical reactions in polar snowpacks can alter the composition and chemistry of the snow and overlying air, only a few of these reactions have been characterized, such as the release of NOx due to photolysis of nitrate on snow grains. Our understanding of snowpack reactions is limited, in part, because little is known about the chromophores (light absorbing species) that are present on snow grains. These species likely initiate most of snow photochemistry, but they are largely uncharacterized except for H2O2, NO3–, and NO2–.
Recent research has shown that unidentified, likely organic, chromophores account for the bulk of sunlight absorption by soluble species on snow grains at Summit, Greenland and Dome C, Antarctica. To more fully understand photochemical reactions in snowpacks will require identifying these unknown chromophores, characterizing their contribution to light absorption in snowpacks, determining how their concentrations vary with snow physical properties, and examining how they affect chemistry in the snow and overlying boundary layer.

Objectives: This research has four main science objectives:
1) Measure the amount of light absorption by soluble species in/on snow grains, including H2O2, NO3–, NO2–, and unknown chromophores.
2) Measure the physical and chemical properties of the snow.
3) Determine the optical properties of the snow.
4) Integrate these results to characterize the contributions of organic chromophores to light absorption in snow, determine the physical and chemical factors affecting light fluxes and light absorption in snow, examine the potential effect of snow photochemistry on snow and boundary layer composition, and model how snow photochemistry will be altered by climate change.

This integrated effort will yield a better and more complete understanding of snow properties.  It is central to a deeper insight into surface atmosphere exchanges.

PIs Cort Anastasio & Harry Beine:
Light absorption by soluble species in/on snow grains

Department of Land, Air & Water Resources
University of California, 1 Shields Ave.
Davis, CA 95616-8627

Cor

Example of previous work:

Figure.  (a) Light absorption coefficients in an afternoon, June surface snow from Summit showing the contributions from hydrogen peroxide, nitrate, and unknown chromophores.  The upper (solid) line is the total light absorption coefficient of the sample.  The difference between this line and the middle (dashed) line represents the absorption coefficient from H2O2, while the difference between the middle line and lower (solid) line represents the absorption coefficient due to NO3–.  The lowest (solid) line represents the absorption coefficients of the unknown chromophores.  The dotted line is the surface actinic flux at Summit for the time when this snow sample was collected.  (b) Fraction of total light absorption coefficient at each wavelength that is due to unknown chromophores (solid line), H2O2 (dotted line), and NO3- (dashed line) in the same sample.

Anastasio & Robles, JGR, 112, 2007

Methods:

Snow samples will be collected from the clean air sector.  The samples will be slowly melted in the lab at the BARC and then divided into several aliquots for various analyses, including light absorption and the chemical measurements (below). The light absorption aliquot will be filtered after melting and then analyzed using a liquid-core-waveguide UV-vis spectrophotometer to measure total light extinction. The scattering component of the spectrum will be mathematically removed.  The contribution of known chromophores to the spectrum will be derived, and the remaining spectrum analyzed.

Florent Domine:  Snow Photochemistry in the Ocean-Atmosphere-Sea Ice-Snow International Project

PI : Florent Domine

Glaciology Laboratory (LGGE), CNRS and Université Joseph Fourier, Grenoble, France
(33) 476 82 42 69
florent@lgge.obs.ujf-grenoble.fr
Other participants: Didier Voisin, Stephan Houdier, Hans-Werner Jacobi, Manuel Barret, Josué Bock
Our contribution to the OASIS Barrow 2009 campaign will be focused on snow physical properties and chemical composition. We propose to study in detail organic compounds in the snow and to measure snowpack physical properties. These data will be used to develop the first coupled physical and chemical model of snowpack evolution. This model, in conjunction with atmospheric models, will be used to understand polar atmospheric chemistry, climate-chemistry interactions, and to improve ice core interpretation.

Methods

Snow studies will start as always by observation of snow stratigraphy on land and on sea ice, to identify layers and their spatial variability. Selected layers will be sampled regularly to observe the evolution of their chemical composition and of their physical properties.
Different analytical techniques will be used to analyze organic compounds. Organic acids will be identified and quantified by ion chromatography. Other light organic molecules and in particular aldehydes  will be analyzed by derivatization/HPLC/fluorescence detection. Organic macromolecules such as Humic Like Substances (HULIS) will be separated on an ion exchange resin and quantified by IR spectroscopy after combustion.
The physical properties of the snow layers that we will measure are thickness, density, heat conductivity and specific surface area. Permleability will be calculated from density and specific surface area. Density will be measured by weighing a snow core of known volume. Heat conductivity will be measured using a heated needle probe. The principle is to monitor the rate of dissipation of a  heat pulse in the snow. This rate is proportional to the heat conductivity of the snow. The specific surface area is a measure of the ice-air interface, and is therefore a crucial variable for snow photochemistry, which mostly takes place at the ice-air interface. To measure that variable, we will use a novel technique based on a new optical system (DUFISSS) where the hemispherical infra-red reflectance of snow is measured with an integrating sphere (see schematic and picture). We found that there was an excellent correlation between IR hemispherical reflectance and specific surface area.

prototype of DUFISSS being tested during the 2007 TARA campaign, near the North Pole.

Hans-Werner Jacobi:  Snow physics and chemistry model

PI : Hans-Werner Jacobi
Phone ++33 476 824277
e-mail jacobi@lgge.obs.ujf-grenoble.fr

Other participants: Jean-Luc Jaffrezo

Methods:

Another goal of the project will be the development of a fully coupled snow physics and chemistry model for the Arctic. The physical part relies on the existing snow physics model CROCUS developed for the avalanche forecasting in the French Alps. This model will be expanded to simulate conditions encountered in the tundra snowpack in the Arctic. Moreover, the model will be extended to calculate further physical parameters, which are crucial for a successful modeling of the chemical processes in the snow. Parallel to the improvement of the snow physics model, we will develop a reaction mechanism describing major chemical processes in the snow. This mechanism will be based on photochemical reactions involving inorganic compounds like nitrogen-containing compounds and hydrogen peroxide. While a sufficiently comprehensive understanding of the inorganic chemistry in the snow is existing, the role of organic compounds in snowpack chemistry remains unresolved: acting as light-absorbers initiating photolytical reactions; constituting sinks for highly reactive radicals reducing chemical reactivity; and acting as precursors for more volatile compounds forming reactive species, which can be released to the atmosphere. However, which of these processes dominate in the arctic snow is currently not known and will be investigated in detail using the snowpack model.

Fig: Schematic description of the coupled snowpack physics and chemistry model

Acknowledgements:
Funding for the work is received from

INSU (Institut national des sciences de l'universe) "The development of an Arctic Snow Physics and Chemistry Model (Arctic SPACMod) within the Ocean-Atmosphere-Sea Ice-Snow (OASIS) international project", Program LEFE-CHAT

Martin D. King:  Optical properties of snow

Lecturer; Director of M.Sc. Environmental Analysis and Assessment
Department of Earth Sciences
Royal Holloway University of London
Egham, Surrey
TW20 0EX, UK
Phone: +44 (0)1784 414038
email: m.king@gl.rhul.ac.uk
http://www.gl.rhul.ac.uk/staff/mdk.html

Goals of this campaign

The aim of the work is to determine if natural organic matter in sunlit snowpacks (and sea ice) is responsible for fluxes of HONO gas from snowpacks and for generating hydrogen peroxide in snowpack. The natural organic matter is acting as a  photocatalytic photosensitizer.

This work has five specific objectives: 
 
1) Measure the visible optical properties (spectrally resolved albedos (350-700nm) and extinction or e-folding depths) of  three types of Polar snow and sea-ice using a tried and tested technique previously used to study the UV optical  properties of polar snow and seaice. The snows will be coastally impacted snow, soil impacted snow and snow on seaice.  The selected snows have very different optical properties owing to snow structure and content of chromatic organic  material. Studies of the optical properties of sea-ice  will be undertaken at a dedicated ice camp. The optical properties of  snow overlying seaice will be studied photochemically for the first time.
 
2) Adapt a coupled atmosphere-snow radiative transfer model to include visible wavelengths (using field data) and known  photophysical data for natural organic matter photosensitizers (from the very recent literature). As well as a Fortran model  the grant will produce simple plots of photosensitizing rates of organic versus  snowpack depth and solar zenith angle for  use by non-expert workers. The extended TUV model will be used to characterize the polar snow samples and sea ice.  For the first time the TUV model will also be adapted to study light fluxes in layers (of various thickness) of snow and seaice columns of snow on seaice.

3) Calculation of the flux of nitrous acid and hydrogen peroxide from snow to the atmosphere using known photophysical data and data from objectives 1-3. This objective will determine if the visible organic photcatalysis mechanism is plausible in real snow or not 
 
 5) Calculate PAR (photosynthetic active radiation) depth profile in polar snow and sea-ice. The calculation of PAR in snow is an extra to the work described here that the model, developed for (objective 2) will produce as an extremely useful by-product. Present measurement of PAR in snow are inaccurate because previous work in biological literature assumes, incorrectly, a Beer-Lambert relationship in the top of the seaice.

Measurement technique

The equipment to measure the penetration of light into snow and ice consists of some very small and inexpensive spectrometers that can be operated from batteries with a ruggedized laptop and can be all placed in a rucksack. The probes will measure irradiance through a flat Teflon surface with a cosine response, a design that has been used three times previously.  Whilst the fibre optic probes record the irradiance in the snowpack two spectral radiometers, GER1500 (hired from NERC FSF), will record the albedo (nadir reflectance) of the snowpack and provide a travelling irradiance standard.  A TUVR and Metcon radiometer will be present to record downwelling atmospheric radiation

Acknowledgements
Funding for this work is received from

NSF ATM-0807702

NERC (natural Environmental Research Council)" Photochemical 
oxidation in snow and sea-ice by organic matter photocatalysis in the 
visible: A international field and modelling investigation",  NE/
F010788/1

NERC Field spectroscopy facility "Photochemical oxidation in snow 
and sea-ice by organic matter photocatalysis in the visible: A 
international field and modelling investigation"  555.0608, loan of 
GER1500 spectrometers.

Royal Holloway Unversity of London: Thomas Holloway scholarship and 
RSF fund.