C31F-01 INVITED
Black Carbon Measurements in Arctic Snow
A survey of the black carbon (BC) content of Arctic snow is underway, updating and expanding the 1983/84
survey of Clarke and Noone. Samples of snow are collected in mid to late spring when the entire winter
snowpack is accessible. The samples are melted and filtered, and the filters are analyzed for absorptive
impurities. Snow has been sampled on tundra, glaciers, ice caps, and sea ice, and in forests. To date about
one thousand snow samples have been melted and filtered.
The sampling effort has been assisted by IPY collaborations with S. Gerland (Svalbard), K. Steffen and C.
Boeggild (Greenland), M. Sturm (Canada), V. Radionov (Russia), and J. Morison (North Pole), as well as
several other volunteers. Two expeditions to arctic Russia were carried out, across longitudes 50-170 E, to
cover a region that had not been sampled in the 1983/84 survey.
The filters are examined with a spectrophotometer, scanning wavelengths 450-900 nm. The relative
contributions of BC and soil dust to the absorption can be estimated from the spectral dependence of
transmission. Calibration is achieved with use of several standard filters containing measured amounts of a
commercial soot with a mass absorption cross-section of about 6 square meters per gram.
Preliminary results indicate that the snow cover in Alaska, Canada, and the Arctic Ocean has lower BC
concentrations now than 20 years ago (5-10 ppb instead of 15-30 ppb), consistent with the declining trend of
BC found in air samples at Alert. Background levels of BC in arctic Russia, distant from sources of local
pollution, have median values 20-30 ppb, but with higher concentrations at the surface at some locations,
and lower concentrations in newly fallen snow. In some regions, particularly the Canadian Arctic islands and
the Arctic coast of northeast Siberia, the snow cover, even at its maximum depth in April before melting
began, was thin and patchy; in these regions the albedo is determined more by snow thickness than by
impurities. Also in these regions the major impurity is often dust rather than soot.
The Greenland ice sheet has the cleanest snow of the Arctic, with median values about 2 ppb in surface
snow in springtime, but the BC apparently is left behind at the surface during melting. In the percolation zone
of South Greenland at the end of July, the subsurface snow had a BC content of 2 ppb but the top 5 cm had
10-20 ppb.
Chemical analyses of filters and meltwater, input to a receptor model, indicate that the major source of BC in
all regions is biomass burning.
The effect of natural amounts of BC on snow albedo is small, and depends on the vertical variation of snow
grain size, so it is computed with a radiative transfer model rather than measured. Nevertheless, some
coincident measurements of spectral albedo and BC content are essential to test assumptions made in the
modeling. Therefore, experiments are underway with artificial uniform snowpacks containing large amounts
of soot, to obtain a large measurable reduction of albedo.
The quantitative values of BC are not yet definitive because (1) the calibration soot needs to be related to
the absorption and size distribution of ambient BC aerosols, and (2) discrepancies in comparisons with other
methods of measuring BC have not been resolved.
http://www.atmos.washington.edu/sootinsnow/
C31F-02 INVITED
Continuous high-temporal resolution black carbon ice core records from Antarctica
The Antarctic ice cap is a unique vantage point from which to observe the global background of black carbon aerosol (BC). Far removed from sources, BC in the Antarctic atmosphere is largely due to biomass burning at low- to mid-latitudes modulated by upper tropospheric (and perhaps stratospheric) transport, climate variability and human activity. BC aerosols have been investigated at several locations in Antarctica including the coastal stations Halley, Syowa and Neumayer, Amundsen-Scott at the South Pole and the South Shetland islands north of the Antarctic Peninsula. Beyond these time series little is known regarding the history of BC over Antarctica. Pioneering research by Petr Chylek demonstrated that it was possible to develop BC records from Antarctic ice cores, albeit with great difficulty and at low temporal resolution. We have recently developed an extremely sensitive analytical method capable of determining BC in Antarctic ice cores at sub annual resolution. This method has allowed us to build upon the research of Chylek and reconstruct BC deposition to Antarctica over the past 200 years at ~ monthly time scales. These "new- generation" records will be presented and the extent of which they reflect large scale BC aerosol variability discussed.
C31F-03 INVITED
Effects of Post-Depositional Processes on Chemical Species in a Snow-Firn Pack on Urumqi Glacier No. 1 in Eastern Tianshan, China
Chemical records from alpine ice cores provide an invaluable source of paleoclimatic and environmental information. However, not only are the atmospheric chemical composition and depositional processes recorded, but post-depositional processes within the snow/firn stratum, especially when melt occurs, are recorded as well. To investigate the effects of depositional processes and meltwater-related post- depositional processes on chemical species in the snow-firn pack, so as to answer the question that how much snow chemical information can be preserved as ice core record under the effect of those post- depositional processes on a alpine glacier, a research program, Program for Glacier Processes Investigation (PGPI), was launched in July 2002 by the Tianshan Glaciological Station (TGS), Chinese Academy of Sciences (CAS). An observational and experimental site (the PGPI site) was carefully located in a percolation zone at an altitude of 4,130 m a. s. l. on Urumqi glacier No. 1 in eastern Tianshan, China. Aerosol, surface snow, and snowpit samples were collected on a weekly basis at the PGPI site using established techniques to prevent contamination. Over the duration of the sampling campaign from 2002 to 2008, around 200 snowpit profiles were sampled which produced about 5000 snow/firn sample. In addition, six 10-20 m ice cores were retrieved in 2006 around the PGPI site. At the same time, a variety of observations of the snow-firn pack physical properties and experiments on meltwater were made. Snow-firn density and temperature were measured along a snow pit wall and in-situ air temperature was continuously observed using an Automatic Weather Station (AWS). Most of the samples were analyzed for major ions and insoluble microparticles while oxygen isotopic ratios, trace metals, and carbonaceous particles (organic and black carbon, OC and BC, respectively) were analyzed for selected samples. Up to the present, some primary results have been obtained as followings. The development of the microparticle and magnesium and Calcium stratigraphy in the snow pit are closely related to the physical development of the snow-firn pack. In addition to the effects of sublimation and wind erosion, melting plays a crucial role in both the physical and chemical evolution processes in the snow pack. The elution of ions is found to be driven primarily by air temperature and become evident when a diurnal mean temperature of -3.6 oC is attained. At 0.3 oC all of the year-round new ionic input can be leached from the snow. A significant linear relationship between ammonium concentrations in surface snow and aerosol during spring and summer indicates that the warm-wet condition facilitates the air-snow exchange of ammonium. Humidity is found to be a significant meteorological factor influencing ammonium in deposition in autumn and winter. The ammonium concentration in aerosol evidently shows a trend similar to that in surface snow, suggesting that the variation of atmospheric ammonium may have been preserved in the surface snow. Vertical nitrate stratigraphy in the upper snowpack is formed during wither by dry deposition and compaction and sublimation of snowpack. During the summer the snowpack nitrate is redistributed through the percolation of meltwater. Nitrate is found to be highly mobile in the snowpack relative to magnesium.
C31F-04 INVITED
Effects of Snow and Ice Impurities and Atmospheric Aerosols on Radiative Forcing
The widely accepted Milankovitch theory connects glacial-interglacial cycles to changes in orbital forcing. A large climate response to weak initial orbital forcing implies strong positive feedbacks. This strong amplification is dominated by an increase in greenhouse gases and surface albedo (due to ice sheet growth and biofeedbacks). The role of aerosols in glacial-interglacial transition is supposed to be minor and has been occasionally neglected altogether. We present an alternative picture of the glacial-interglacial transitions in which aerosols in the atmosphere and impurities on the surface of ice sheets play an important role in radiative forcing that is then amplified by greenhouse gases and other feedbacks.
C31F-05 INVITED
Arctic Snow Melt Hydrology: Observations, Modeling and Implications to Runoff Chemistry
During the last few decades considerable progress has been made in our understanding of the physical processes controlling snowcover formation, melt, and runoff. This work provides the basis required for a better understanding of, and ability to predict, the complex role snow plays in alpine and northern environments, including, for example, streamflow, surface-atmosphere interactions, and the fluxes of contaminants through the snowpack. The interaction of meltwater with strata within the snow, the resulting formation of preferential flow fingers, and re-freezing of meltwater as ice layers and ice columns within the snow, complicates our ability to model both the release of meltwater and contaminants from the snow. This paper will provide a synthesis of the current state of our understanding of, and ability to model, snow melt and snow melt runoff. A major focus will be to review progress to date on the importance of the heterogeneous nature of the snowcover over a wide range of scales, and implications to contaminant runoff.
C31F-06 INVITED
An Introduction to Atmosphere - Snow/Ice Interactions
Discoveries in the last decade have shown that the snow pack and, on sea ice, the underlying ice, are not simply forming a white blanket isolating the land surfaces or ocean waters from the atmosphere. A wide range of active chemical exchanges occurs right at the snow/ice surface. These interactions significantly influence the overlying atmosphere: Polar boundary layer ozone depletion, halogen chemistry, mercury deposition, emissions of reactive nitrogen compounds, OH production, and reactions of organic species are all related to chemical and physical properties of the snow and ice surfaces. This introduction to the topic will highlight these phenomena and will discuss possible interactions and feed- backs between them and their relevance for Global Climate Change. The presentation will also indicate today's research challenges in the field and potential future directions.
C31F-07 INVITED
Persistent Organic Pollutants in Snow: Scavenging, Aging, and Implications for Remote Ecosystems
Snow is an efficient scavenger of both gas-phase and particulate-phase persistent organic pollutants (POPs) and other anthropogenic semi-volatile organic compounds (SOCs). In addition, in some remote high elevation and high latitude ecosystems, snow is the predominant form of annual precipitation and the primary route of POP deposition to these ecosystems. Recent laboratory and field studies have begun to elucidate the mechanisms of POP scavenging from the atmosphere, the physical and chemical aging of POPs in annual snowpack, and the release of POPs from melting snowpack to the ecosystem. These recent results suggest that POP deposition to remote ecosystems can result in bioaccumulation in the aquatic food web and, in some ecosystems, the concentrations of POPs in fish exceed human and wildlife health criteria.
C31F-08
Laboratory Study of the Diffusivity of NOx and HONO in Snow.
The migration of NOx and HONO through the air/snow interface was experimentally investigated using a unique combination of techniques namely, X-ray micro-tomography and nitrogen oxides labeled with the short-lived radioactive tracer 13N in a snow diffusion chamber. The experimental set up allows blowing a gas mixture containing NO, NO2 or HONO into the headspace over a snow sample to mimic low wind air/snow interactions. The overall loss into the snow can be determined using a chemiluminescence detector at the outlet of the headspace. The concentration of HONO entering the headspace is moreover monitored by a long path absorption photometer (LOPAP). The migration of the labeled nitrogen oxide molecules along the snow sample can be observed by two γ-detectors, which can move along the sample during the experiment. The snow used for the experiment was produced artificially in a specially designed snow maker. The micro structure of the artificially produce snow is measured and visualized by micro-tomography and is identical to natural snow. For the experiment presented here, two types of snow were used, fresh snow and rounded snow. The rounded snow was obtained from a fresh snow sample which was kept during more than a month at -~10°C to allow for isothermal metamorphism. For NO and NO2 the observed profiles were consistent with gas phase diffusion through the open volume of the snow without any significant interaction with the ice surface, e.g. through heterogeneous hydrolysis of NO2. In the case of HONO, the strong interaction with the ice led to a reduced penetration of the molecules into the snow sample. The observed temperature dependence of the depth of penetration and of the apparent Henry constant, i.e., the ratio gas phase to snow phase of HONO and the clear difference seen between the different snow samples is a strong indication that bulk diffusion in the ice phase is the dominant process involved in the migration and that it takes place mainly into grain boundaries. Therefore, for HONO, surface adsorption alone is not properly describing migration into snow. This will also be true in the case of migration of HONO produced within the snow pack out into the atmosphere.