AS2.12

EDI
Surface Exchange Processes in the Polar Regions: Physics, Chemistry, Isotopes, and Aerosols

This session is intended to provide an interdisciplinary forum to bring together researchers working in the areas of high-latitude meteorology, atmospheric chemistry, stable isotope research, oceanography, and climate. The emphasis is on the role of boundary layer processes that mediate exchange of heat, momentum and mass between the Earth's surface (snow, sea-ice, ocean and land) and the atmosphere as well as the local to large-scale influences on these exchanges. An adequate understanding and quantification of these processes is necessary to improve modeling and prediction of future changes in the polar regions and their teleconnections with mid-latitude weather and climate, including meridional transport of heat, moisture, chemical trace species, aerosols and isotopic tracers (indicating airmass origins and atmospheric processes); and regional emission and vertical mixing of climate active trace gases and aerosol, such as cloud-forming particles (CCN/INP) and their precursors. It is expected that the recent implementation of new measurements such as those from pan-Arctic water vapor isotope networks, observations such as those obtained during the MOSAiC field program, and data from existing networks will help diagnose long-range moisture and aerosol sources and the coupling between local and large-scale dynamics. We encourage submissions such as (but not limited to):
(1) External controls on the boundary layer such as clouds, radiation and long-range transport processes
(2) Results from field programs, such as MOSAiC, and routine observatories, insights from laboratory studies, and advances in modeling and reanalysis,
(3) Use of data from pan-Arctic and Antarctic observing networks,
(4) Surface processes involving snow, sea-ice, ocean, land/atmosphere chemical and isotope exchanges, and natural aerosol sources
(5) The role of boundary layers in polar climate change and implications of climate change for surface exchange processes, especially in the context of reduced sea ice, wetter snowpacks, increased glacial discharge and physical and chemical changes associated with an increasing fraction of first year ice and increasing open water.

Co-organized by CR7
Convener: William Neff | Co-conveners: Markus Frey, Michael Tjernström, Sonja WahlECSECS, Gillian YoungECSECS
vPICO presentations
| Fri, 30 Apr, 09:00–10:30 (CEST)

vPICO presentations: Fri, 30 Apr

Chairperson: Markus Frey
09:00–09:05
|
EGU21-4137
|
solicited
|
Highlight
Julia Schmale, Lubna Dada, Ivo Beck, Tuija Jokinen, Lauriane Quélélever, and Tii Laurila

The Arctic aerosol and trace gas regime features strong seasonal differences. The haze season in winter is dominated by long-range transported mid-latitude anthropogenic emissions, while the cleaner summer season is characterized by more local and natural trace gas and aerosol sources. Aerosols and trace gases have been shown to be important for the Arctic radiative balance, inducing an overall net positive radiative forcing through direct radiation interactions.

Aerosols and trace gases are fundamentally different between seasons in terms of chemical composition and microphysical properties. In winter, accumulation mode particles - with a concentration between 100 and 300 cm-3- composed mainly of sulfate, sodium and organics occur, and trace gases relevant for aerosol formation have very low concentrations. In summer, the aerosol number concentration is highly variable reaching from a few to thousands per cubic centimeter, in case of new particle formation. Their size distribution contains nucleation, Aitken and accumulation modes. Trace gases become more abundant, particularly in the marginal ice zone where marine microbial activity emits dimethylsulfide, which leads to the formation of the trace gases sulfuric acid and methanesulfonic acid, which in turn form secondary aerosol mass.

The transition seasons, i.e. spring and autumn, have been studied much less in terms of aerosol and trace gas chemical and microphysical properties in the past , except for ozone depletion events in spring where halogenated trace gases are predominantly involved. The transition between the two aerosol regimes is relatively short. Focusing on spring, the season is characterized by the arrival of warmer and moister air masses from the south, which transport different aerosols and trace gases up north. Cloud formation and precipitation en route have a strong impact on the aerosol number concentrations and size distribution, as well as on the chemical composition due to heterogeneous chemistry in cloud droplets.

Here, we present first results from observations of warm air mass intrusions reaching the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in mid-April 2020. The period before arrival was characterized by persistent northerly winds, hence aged and dry Arctic air masses, where a very stable accumulation mode composed of sulfate and organics with traces of halogens was measured. With the arrival of southerly air masses, the size distribution started featuring several modes, which increased and decreased in diameter and concentration. Moreover, the chemical composition was significantly changed, featuring methanesulfonic acid from algal blooms in the north Atlantic.

How to cite: Schmale, J., Dada, L., Beck, I., Jokinen, T., Quélélever, L., and Laurila, T.: Impact of warm air mass intrusions on atmospheric chemistry and microphysics – Observations during MOSAiC, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4137, https://doi.org/10.5194/egusphere-egu21-4137, 2021.

09:05–09:07
|
EGU21-11175
|
ECS
Andrew Elvidge, Ian Renfrew, Ian Brooks, Piyush Srivastava, Margaret Yelland, and John Prytherch

Aircraft observations from two Arctic field campaigns are used to derive ice surface characteristics and make recommendations for the parametrisation of surface heat and moisture exchange over sea ice and the marginal ice zone (MIZ). The observations were gathered in the Barents Sea and Fram Strait as part of the Aerosol–Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) project, and off the south-east coast of Greenland as part of the Iceland-Greenland Seas Project (IGP). Estimates of roughness lengths for momentum (z0), heat (z0T) and moisture (z0q) are derived from turbulent wind velocity, temperature and humidity measurements; while sea ice concentration is derived from albedo measurements. The two data sets cover a range of sea ice characteristics, being much rougher in general during IGP than during ACCACIA. Large fluxes of heat and moisture were observed in the vicinity of the MIZ during both field campaigns. We show that z0T and z0q over 100 % sea ice (z0Ti and z0qi) vary as a function of a roughness Reynolds number (R*; which itself is a function of z0 and wind stress), with a peak at the transition between the aerodynamically smooth (R*<0.135) and aerodynamically rough (R*>2.5) regimes. One of the few theory-based parameterisations available for z0Ti and z0qi (that of Andreas et al., 1987) reproduces these peaks, in contrast to the simple treatments currently employed in two leading numerical weather and climate prediction models – the Met Office Unified Model (MetUM) and the Integrated Forecast System (IFS) – which do not. The MetUM and IFS schemes perform adequately in smooth conditions, but greatly overestimate heat and moisture exchange in rough conditions. We develop a new parameterisation for heat and moisture exchange as a function of sea ice concentration, which blends the Andreas et al. (1987) scheme over sea ice with exchange over the ocean. This new parameterisation performs much better than the current MetUM and IFS schemes for the rough conditions observed during IGP, at least halving the bias and root-mean-square errors in sensible and latent heat fluxes; and is also marginally better for the comparatively smooth conditions observed during ACCACIA, suggesting further evaluation is warranted. However, it should be noted that representing heat and moisture exchange over sea ice is currently limited by the variability in z0 over 100 % sea ice, which is unrepresented in weather and climate models.

How to cite: Elvidge, A., Renfrew, I., Brooks, I., Srivastava, P., Yelland, M., and Prytherch, J.: Observations of surface heat and moisture exchange in the marginal sea ice and implications for model parameterization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11175, https://doi.org/10.5194/egusphere-egu21-11175, 2021.

09:07–09:09
|
EGU21-8303
|
ECS
Shaddy Ahmed, Jennie Thomas, Katie Tuite, Jochen Stutz, Frank Flocke, John Orlando, Rebecca Hornbrook, and Eric Apel

Polar halogen chemistry has long been known to be active, especially in spring, and is known to have an important influence on the lifetime of some volatile organics, ozone and mercury. Our understanding of polar halogen chemistry is changing, including the recognition that there is active chlorine, bromine and iodine chemistry occurring within the polar boundary. Recently, very high concentrations of molecular chlorine (Cl2) were recorded at Utqiaġvik, Alaska during the Ocean-Atmosphere-Sea Ice-Snowpack (OASIS) campaign in spring 2009, with a correlation between daytime Cl2 mixing ratios, ozone concentrations and sunlight. However, the chlorine radical concentrations inferred from these Cl2 measurements, with the observed VOC abundances and lifetimes, cannot yet be fully explained via chemical box modelling alone. To explain these discrepancies, modelling that includes surface snow Cl2 formation processes, subsequent atmospheric chemistry and vertical mixing is needed and is an essential tool in quantifying impacts on VOC lifetimes and the role of vertical mixing in controlling boundary layer chemistry.

In this work, we use a one-dimensional atmospheric chemistry and transport model (Platform for Atmospheric Chemistry and Transport in 1-Dimension, PACT-1D) to investigate surface Cl2 production from snow, snowpack recycling, vertical transport and reactivity with VOCs at Utqiaġvik, Alaska during the OASIS campaign. We implement a new surface parameterization of chlorine emissions from the snowpack based on the solar irradiance and surface ozone levels and consider the role of vertical mixing processes. By considering both production and transport mechanisms, we are able to obtain good agreement between the model predicted Cl2 mixing ratios and observations at 1.5 meters. The model predicts that nearly all reactive chlorine resides within the lowest 15 m of the boundary layer, resulting in increased chemical reactivities and oxidation rates in the lowest part of the atmosphere. VOC abundances near the surface that are co-located with elevated chlorine can be explained by downward mixing of VOCs from aloft, which replenishes VOCs from free tropospheric reservoirs. The proposed surface emission parameterization of chlorine in this work could be used to develop current 3D numerical models in order to explore chlorine emissions and reactivity over the entire Arctic as well as the effects of future Arctic climate scenarios on atmospheric halogen chemistry.

How to cite: Ahmed, S., Thomas, J., Tuite, K., Stutz, J., Flocke, F., Orlando, J., Hornbrook, R., and Apel, E.: Using 1D-modelling to study Arctic chlorine activation, transport and VOC oxidation during Arctic springtime, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8303, https://doi.org/10.5194/egusphere-egu21-8303, 2021.

09:09–09:11
|
EGU21-13650
Michael Lawler, Eric Saltzman, Linn Karlsson, Paul Zieger, Matthew Salter, Andrea Baccarini, Julia Schmale, and Caroline Leck

The summertime high Arctic is an extremely low-aerosol region, where even small inputs of particles can have significant impacts on cloud formation and therefore on the surface energy budget. The relative importance of new particle formation from gas phase precursors and primary sea spray production in this region remains uncertain, as does the role of atmospheric transport. We made direct, time-resolved composition measurements of Aitken mode (~20-60 nm diameter) aerosol over the high Arctic pack ice in August-September 2018, including during an intense Aitken mode formation event on August 30-31. The event particles contained both primary sea spray materials (sodium, potassium, and polysaccharide-like organics) and secondary components (non-sea-salt sulfate, methanesulfonic acid, non-sea-salt iodine, and secondary organics), most of which could be quantified on the basis of analytical standards. The composition is consistent with primary sea spray that had been atmospherically processed, and the aerosol size distribution dynamics imply the action of a process by which larger atmospheric particles or aggregates broke up to form smaller particles. Hypotheses to explain the results will be discussed.

How to cite: Lawler, M., Saltzman, E., Karlsson, L., Zieger, P., Salter, M., Baccarini, A., Schmale, J., and Leck, C.: An Aitken mode aerosol formation event in the high Arctic: evidence for aggregate breakup, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13650, https://doi.org/10.5194/egusphere-egu21-13650, 2021.

09:11–09:13
|
EGU21-8966
|
ECS
Radiance Calmer, Gijs de Boer, Jonathan Hamilton, John Cassano, Gina Jozef, Dale Lawrence, Steve Borenstein, Abhiram Doddi, Brian Argrow, Matthew Shupe, and Christopher Cox

The University of Colorado, Boulder, deployed unmanned aerial systems (UAS) over the sea ice during Leg 4 (June-August 2020) of the MOSAiC expedition. Among the different UAS platforms operated, a hexacopter, the HELiX, was dedicated for characterizing the surface properties, such as the surface albedo and the sea ice/melt pond fractions. The HELiX was equipped with two pyranometers to measure incoming and reflected broadband shortwave irradiance, and a multispectral camera to map the surface of the ice floe. Three flight plans were conducted with this platform, including (1) grid patterns at 10 m.asl to map out the distribution of albedo at this altitude, (2) hovering flights at 3 m.asl over identified surfaces (sea ice, melt pond, ocean, ridge, etc.) to get a detailed look at the albedo of each surface individually, and (3) profiles up to 100 m.asl. to evaluate the convergence height where surface heterogeneity is obscured when using a hemispheric sensor. In total, 34 flights took place in varied weather conditions, from clear sky to foggy weather with very low visibility. The UAS observations bring complementary results to a variety of other albedo observations collected during MOSAiC (albedo lines, sled-based, tethered balloon-based, and ship-based measurements).  These observations spanned the majority of the melt season, capturing seasonal evolution in surface reflectivity, as well as melt pond fraction and resulting impact on surface albedo.  In this presentation, we will present results from these flight activities and offer perspectives on the evolving sea ice pack during the summer portion of the MOSAiC expedition.

How to cite: Calmer, R., de Boer, G., Hamilton, J., Cassano, J., Jozef, G., Lawrence, D., Borenstein, S., Doddi, A., Argrow, B., Shupe, M., and Cox, C.: Unmanned Aerial System measurements of surface albedo for the melting season during the MOSAiC expedition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8966, https://doi.org/10.5194/egusphere-egu21-8966, 2021.

09:13–09:15
|
EGU21-1683
Günther Heinemann, Sascha Willmes, Lukas Schefczyk, Alexander Makshtas, Vasilii Kustov, and Irina Makhotina

The parameterization of ocean/sea-ice/atmosphere interaction processes is a challenge for regional climate models (RCMs) of the Arctic, particularly for wintertime conditions, when small fractions of thin ice or open water cause strong modifications of the boundary layer. Thus, the treatment of sea ice and sub-grid flux parameterizations in RCMs is of crucial importance. However, verification data sets over sea ice for wintertime conditions are rare. In the present paper, data of the ship-based experiment Transarktika 2019 during the end of the Arctic winter for thick one-year ice conditions are presented. The data are used for the verification of the regional climate model CCLM. In addition, Moderate Resolution Imaging Spectroradiometer (MODIS) data are used for the comparison of ice surface temperature (IST) simulations of the CCLM sea ice model. CCLM is used in a forecast mode (nested in ERA5) for the Norwegian and Barents Seas with 5km resolution and is run with different configurations of the sea ice model and sub-grid flux parameterizations. The use of a new set of parameterizations yields improved results for the comparisons with in-situ data. Comparisons with MODIS IST allow for a verification over large areas and show also a good performance of CCLM. The comparison with twice-daily radiosonde ascents during Transarktika 2019, hourly microwave water vapor measurements of first 5 km in the atmosphere and hourly temperature profiler data shows a very good representation of the temperature, humidity and wind structure of the whole troposphere for CCLM.

How to cite: Heinemann, G., Willmes, S., Schefczyk, L., Makshtas, A., Kustov, V., and Makhotina, I.: Observations and simulations of meteorological conditions over Arctic thick sea ice in late winter during the Transarktika 2019 expedition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1683, https://doi.org/10.5194/egusphere-egu21-1683, 2021.

09:15–09:17
|
EGU21-13363
|
ECS
Ben Kopec, Martin Werner, Kyle Mattingly, Eric Klein, David Noone, Pete Akers, Hannah Bailey, Jean-Louis Bonne, Camilla Brunello, Kaisa-Riikka Mustonen, Alun Hubbard, Bjørn Kløve, and Jeffrey Welker

One of the key changes of the global climate system is the loss of Arctic sea ice, particularly through its impact on ocean-atmosphere interactions. Enhanced evaporation under open-water conditions is widespread from places and periods previously precluded by perennial sea ice cover, leading to an increase in vapor uptake across the Arctic. However, the response of ocean-atmosphere system to sea ice loss varies significantly over time and space. To quantify these variations, the Arctic Water Isotope Network (AWIN) has been established to make continuous water vapor isotope measurements (δD, δ18O, and d-excess) at seven land-based stations from Barrow, Alaska to Ny Alesund, Svalbard. This network has been supplemented by continuous mobile isotope data from the CiASOM project on the Polarstern ice-breaker throughout the MOSAiC “Arctic-drift” expedition. With this network, we comprehensively track water vapor from its source to sink, thereby demonstrating how it varies simultaneously across the entire Arctic Basin.

Here, we utilize AWIN measurements to specifically quantify how variations in sea ice extent and distribution affect moisture content, water vapor isotope traits, and transport along several critical storm tracks. By monitoring vapor isotopic changes in air masses advected from one site to another, we are able to track how much moisture is added along a given trajectory. We investigate several primary vapor transport pathways into the Arctic, including the North Atlantic/Greenland Sea, Baffin Bay, and the Bering Strait, and track the geochemical signature of this vapor as it transits along these well-established storm pathways into and within the Arctic. By quantifying isotopic changes between our sites we: 1) identify the distinct isotopic fingerprint of moisture sourced by evaporation from Arctic seas that is critically dependent on variable sea ice conditions, 2) detect moisture addition into critical storm tracks as they transit across the Arctic, and 3) determine the spatial variability of this enhanced Arctic-sourced evaporation and moisture. We find that for every major storm track observed, the Arctic Ocean and surrounding seas are significant sources of enhanced moisture uptake, acting within an amplified water cycle.

How to cite: Kopec, B., Werner, M., Mattingly, K., Klein, E., Noone, D., Akers, P., Bailey, H., Bonne, J.-L., Brunello, C., Mustonen, K.-R., Hubbard, A., Kløve, B., and Welker, J.: Sea ice controls on Arctic water vapor content and transport: Discoveries from MOSAiC’s pan Arctic Water Isotope Network (AWIN), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13363, https://doi.org/10.5194/egusphere-egu21-13363, 2021.

09:17–09:19
|
EGU21-13119
Von Walden, Heather Guy, Christopher Cox, William Neff, Ryan Neely, and Matthew Shupe

Above freezing temperatures and melting surface snow have occurred at Summit Station (3250 m asl), atop the Greenland Ice Sheet, only five times in the last 800 years, including once in 2012 and twice 2019 (June 12; July 29-31). Such events are linked to southerly advection of continental air that cross the North Atlantic as atmospheric rivers (ARs). The specific mechanisms that are responsible for these rare events appear to be varied and complex. While the 2012 event was supported by anomalous cloud forcing caused by thin, liquid-bearing clouds, the two events in 2019 occurred under both clear and cloudy conditions. The net surface radiation measured during the 2019 events was actually similar between clear (~47 W m-2) and cloudy (~52 W m-2) conditions, and, surprisingly, these values are unremarkable for afternoon conditions in summer at Summit Station.

Observations from the ICECAPS-ACE project at Summit Station (including radiative and turbulent fluxes, surface skin temperature, snow pit stratigraphy) allow a process-level analysis of the mechanisms that transfer energy from the AR events into local melting. By combining the measurements with a finite-volume diffusion model of the sub-surface temperature field, we find that a concentration of energy in the surface layer caused by converging fluxes toward the surface from both directions (upward from within the snowpack and downward from the atmosphere) led to initiation of both of the 2019 melt events. Thus, coupling between the atmosphere and the snowpack, and the timing of atmospheric fluctuations, appear important, and are suggestive that preconditioning of the snowpack from events prior to the day of melt may be a factor. Both sensible and latent heat fluxes were relatively small during these melt events while several regimes of commonly occurring radiative processes were observed. Under cloudy conditions, longwave cloud radiative forcing played a role, while under clear skies, lower surface albedo was sufficient to make up the difference of the absence of cloud forcing. For example, during the July 2019 event, the surface snow albedo decreased significantly from 0.86 to 0.80, which facilitated greater absorption of solar radiation. These findings are supportive of the notion that longwave processes are triggers of melt while shortwave processes persist melt. he co-dependent roles of the radiative and subsurface heat fluxes during the 2019 events suggest that the rarity of melt at Summit Station may be explained by preconditioning processes, and that a particular sequence of events over several days leading up to melt may be important.

How to cite: Walden, V., Guy, H., Cox, C., Neff, W., Neely, R., and Shupe, M.: Mechanisms of multiple, anomalous melt events at Summit Station, Greenland in summer 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13119, https://doi.org/10.5194/egusphere-egu21-13119, 2021.

09:19–09:21
|
EGU21-16503
|
ECS
Laura Dietrich, Hans Christian Steen-Larsen, Cécile Agosta, Xavier Fettweis, Anne-Katrine Faber, and Sonja Wahl

Precipitation along with sublimation and deposition are the main contributors to the surface mass balance (SMB) in the accumulation area of the Greenland Ice Sheet (GrIS). However, precipitation events are rare and intermittent. In between precipitation events the surface snow continuously undergoes sublimation and deposition. Recent studies imply that these surface exchange processes influence the isotopic composition of the surface snow which is later archived as a climate record in ice cores. In order to understand the possible implications on the recorded climate signal, sublimation needs to be quantified on a local scale.

Here we present simulated SMB components for eight ice core drilling sites on the GrIS using the regional climate model MAR (Modèle Atmosphérique Régional). We validated MAR against in-situ flux observations at the East Greenland Ice Core Project site and found a high sensitivity of sublimation to the downward long wave flux and to the parameterization of the surface roughness length. We propose a surface roughness length optimized for the interior of the GrIS which is supported by our observations.

Our results show that in the GrIS accumulation area the mass turnover via sublimation and deposition can reach the same order of magnitude as precipitation. This highlights the importance of a better understanding of how the climate signal is imprinted in the surface snow isotopic composition.

How to cite: Dietrich, L., Steen-Larsen, H. C., Agosta, C., Fettweis, X., Faber, A.-K., and Wahl, S.: The Role of Sublimation on the Surface Mass Balance of the Interior Greenland Ice Sheet, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16503, https://doi.org/10.5194/egusphere-egu21-16503, 2021.

09:21–09:23
|
EGU21-9701
|
ECS
Sonja Wahl, Alexandra Zuhr, Maria Hörhold, Anne-Katrine Faber, and Hans Christian Steen-Larsen

Post-depositional processes affect the stable water isotope signal of surface snow between precipitation events. Combined vapor-snow exchange processes and isotope diffusion influence the top layer of snow as well as buried layers below. This implies, that ice core isotope climate proxy records can not be interpreted as a precipitation weighted temperature signal alone.

Here we present to what extend surface sublimation can explain in-situ observed changes of the stable water isotope signal in the snow.
We use direct observations of the isotopic composition of the sublimation flux together with surface snow samples taken in the North-East of the Greenland Ice Sheet accumulation zone throughout the summer months of 2019 to demonstrate sublimation impacts.
We show that, contrary to the understanding of effectless layer-by-layer removal of snow, sublimation involves fractionation and therefore influences the isotopic composition of the snow. Complementary measurements of humidity as well as isotope fluxes constrain the local vapor snow exchange and allow for the quantification of post-depositional influences while the snow is exposed to the atmosphere.
This improved process understanding of the formation of the climate signal found in snow is important for merging climate modeling and ice core proxies. 

How to cite: Wahl, S., Zuhr, A., Hörhold, M., Faber, A.-K., and Steen-Larsen, H. C.: The Effect of Surface Sublimation on the Snow Isotope Signal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9701, https://doi.org/10.5194/egusphere-egu21-9701, 2021.

09:23–09:25
|
EGU21-10834
Mathieu Casado, Christophe Leroy-Dos Santos, Elise Fourré, Vincent Favier, Cécile Agosta, Laurent Arnaud, Frédéric Prié, Pete D Akers, Leoni Janssen, Christoph Kittel, Joël Savarino, and Amaëlle Landais

Stable water isotopes are effective hydrological tracers due to fractionation processes throughout the water cycle, and thus, the stable isotopes from ice cores can serve as valuable proxies for past changes in the climate and local environment of polar regions. Proper interpretation of these isotopes requires to understand the influence of each potential fractionating process, such as initial evaporation over the ocean and precipitation events, but also the effects of post-depositional exchange between snow and moisture in the atmosphere. Thanks to new developments in infrared spectroscopy, it is now possible to continuously monitor the isotopic composition of atmospheric water vapor in coordination with discrete snow sampling. This allows us to readily document the isotopic and mass exchanges between snow and vapor as well as the stability of the atmospheric boundary layer, as has recently been shown on the East Antarctic Plateau at Kohnen (Ritter et al., TC, 2016) and Dome C (Casado et al., ACP, 2016) stations where substantial diurnal isotopic variations have been recorded.

In this study, we present the first vapor monitoring of an East Antarctic transect that covered more than 3600 km over a period of 3 months from November 2019 to February 2020 as part of the EAIIST mission. The isotopic record therefore describes the evolution from typical coastal values to highly depleted values deep inside the continent on the high-altitude plateau. In parallel, we also monitored the vapor isotopic composition at two stations: the coastal starting point of Dumont D’Urville (DDU) and the plateau halfway point of Dome C. Two automatic weather stations (at Paleo and Megadunes sites) were also installed in a previously unexplored region of the East Antarctic plateau that was covered by this transect. This suite of cross-calibrated vapor isotope observations and weather stations, coupled with Modele Atmospherique Régional (MAR) climate modeling, offers a unique opportunity to compare the spatial and temporal gradients of humidity, temperature, and water vapor isotopic composition in East Antarctica during the summer season, and to estimate how the water vapour isotope measurements at Dome C and DDU are representative of the conditions in East Antarctica. The quantitative agreement between the EAIIST record and those recorded at DDU and Dome C stations at the times the raid was nearby, gives confidence in the quality of the results acquired on this traverse. Although further comparisons with the surface snow isotopic composition are required to quantify the impact of the snow-atmosphere exchanges on the local surface mass balance, these initial results of vapor isotopic composition show the potential of using water stables isotopes to evaluate hydrological processes in East Antarctica and better reconstruct past climate changes through ice cores.

How to cite: Casado, M., Leroy-Dos Santos, C., Fourré, E., Favier, V., Agosta, C., Arnaud, L., Prié, F., Akers, P. D., Janssen, L., Kittel, C., Savarino, J., and Landais, A.: Water vapor isotopic signature along the EAIIST traverse (East Antarctica Plateau), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10834, https://doi.org/10.5194/egusphere-egu21-10834, 2021.

09:25–09:27
|
EGU21-13076
Cécile Agosta, Cécile Davrinche, Christophe Leroy-Dos Santos, Antoine Berchet, Amaëlle Landais, Elise Fourré, Anaïs Orsi, Frédéric Prié, Charles Amory, Vincent Favier, Xavier Fettweis, Christophe Genthon, Christoph Kittel, Dana Veron, and Jonathan Wille

On December 19-21, 2018, an atmospheric river hit the French-Italian Concordia station, located at Dome C, East Antarctic Plateau, 3 269 m above sea level. It induced an extreme surface warming (+ 15°C in 3 days), combined with high specific humidity (multiplied by 3 in 3 days) and a strong isotopic anomaly in water vapor (+ 15 ‰ for δ18O). The isotopic composition of water vapor monitored during the event may be explained by (1) the isotopic signature of long-range water transport, and by (2) local moisture uptake during the event. In this study we quantify the influence of each of these processes.

To estimate the isotopic composition of water vapor advected by long-range transport, we perform back-trajectories with the FLEXible PARTicle dispersion model FLEXPART. We retrieve meteorological conditions along different trajectories between the moisture uptake area and Concordia, and use them to compute isotopic fractionation during transport with the mixed cloud isotope model MCIM. While intermediate conditions along the trajectory do not seem to have a major impact on the final isotopic composition (less than 0.1 ‰), the latter appears sensitive to surface conditions (temperature, pressure and relative humidity) in the moisture uptake area (±5.1 ‰). As the event is characterized by the presence of liquid water clouds above Concordia, we show additional sensitivity tests exploring the impact of mixed phase clouds on the water vapor isotopic composition.

Finally, we perform a water vapor mass budget in the boundary layer using observations and simulations from the regional atmospheric model MAR, ran with and without drifting snow. The presence of mixed-phase clouds during the event induced a significant increase in downward longwave radiative fluxes, which led to high turbulent mixing in the boundary layer and to heavy drifting snow (white-out conditions). Using MAR simulations, we show that a significant part of the atmospheric water vapor originates from sublimation of drifting snow particles removed from the snowpack. Consequently, the isotopic signal monitored in water vapor during this atmospheric river event reflects both long-range moisture advection and interactions between the boundary layer and the snowpack. Only specific meteorological conditions driven by the atmospheric river, and their associated intense poleward moisture transport, can explain these strong interactions.

How to cite: Agosta, C., Davrinche, C., Leroy-Dos Santos, C., Berchet, A., Landais, A., Fourré, E., Orsi, A., Prié, F., Amory, C., Favier, V., Fettweis, X., Genthon, C., Kittel, C., Veron, D., and Wille, J.: Isotopic anomalies in water vapor during an atmospheric river event at Dome C, East Antarctic plateau, controlled by large-scale advection and boundary layer processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13076, https://doi.org/10.5194/egusphere-egu21-13076, 2021.

09:27–09:29
|
EGU21-672
|
ECS
Pete D. Akers, Joël Savarino, Nicolas Caillon, Mark Curran, and Tas Van Ommen

Precise Antarctic snow accumulation estimates are needed to understand past and future changes in global sea levels, but standard reconstructions using water isotopes suffer from competing isotopic effects external to accumulation. We present here an alternative accumulation proxy based on the post-depositional photolytic fractionation of nitrogen isotopes (d15N) in nitrate. On the high plateau of East Antarctica, sunlight penetrating the uppermost snow layers converts snow-borne nitrate into nitrogen oxide gas that can be lost to the atmosphere. This nitrate loss favors 14NO3- over 15NO3-, and thus the d15N of nitrate remaining in the snow will steadily increase until the nitrate is eventually buried beneath the reach of light. Because the duration of time until burial is dependent upon the rate of net snow accumulation, sites with lower accumulation rates have a longer burial wait and thus higher d15N values. A linear relationship (r2 = 0.86) between d15N and net accumulation-1 is calculated from over 120 samples representing 105 sites spanning East Antarctica. These sites largely encompass the full range of snow accumulation rates observed in East Antarctica, from 25 kg m-2 yr-1 at deep interior sites to >400 kg m-2 yr-1 at near coastal sites. We apply this relationship as a transfer function to an Aurora Basin ice core to produce a 700-year record of accumulation changes. Our nitrate-based estimate compares very well with a parallel reconstruction for Aurora Basin that uses volcanic horizons and ice-penetrating radar. Continued improvements to our database may enable precise independent estimates of millennial-scale accumulation changes using deep ice cores such as EPICA Dome C and Beyond EPICA-Oldest Ice.

How to cite: Akers, P. D., Savarino, J., Caillon, N., Curran, M., and Van Ommen, T.: Reconstructing Antarctic snow accumulation using nitrogen isotopes of nitrate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-672, https://doi.org/10.5194/egusphere-egu21-672, 2021.

09:29–09:31
|
EGU21-9425
|
ECS
Albane Barbero, Roberto Grilli, Camille Blouzon, Ghislain Picard, Markus Frey, Nicolas Caillon, and Joel Savarino

Previous Antarctic summer campaigns have shown unexpectedly high levels of oxidants in the continental interior as well as at coastal regions, with atmospheric hydroxyl radical (OH) concentrations up to 4 x 106 cm-3. It is now well established that such high reactivity of the summer Antarctic boundary layer results in part from the emissions of nitrogen oxides (NOx ≡ NO + NO2) produced during the photo-denitrification of the snowpack. Despite the numerous observations collected at various sites during previous campaigns such as ISCAT 1998, 2000, ANTCI, NITE-DC and OPALE, a robust quantification of the NOx emissions on a continental scale over Antarctica is still lacking. Only NO emissions were measured during ISCAT and the ratio NO2:NO was measured during NITE-DC and OPALE using indirect NO2 measurements. This leaves significant uncertainties on the snow-air-radiation interaction. To overcome this crucial lack of information, direct NO2 measurements are needed to estimate the NOx flux emissions with reduced uncertainties.

For the first time, new developed optical instruments based on the IBB-CEAS technique and allowing direct measurement of NO2 with detection limit of 10 x 10-12 mol mol-1, (1σ), (Barbero et al., 2020) were deployed on the field during the 2019–2020 summer campaign at Dome C (75°06'S, 123°20'E, 3233m a.s.l). They were coupled with new designed dynamic flux chamber experiments. Snows of different ages ranging from newly formed drift snow to 16-20 year-old firn were sampled. Unexpectedly, the same daily average photolysis constant rate of (2.18 ± 0.38) x 10-8 s-1 (1σ) was estimated for the different type of snow samples, suggesting that the photolabile nitrate behaves as a single-family source with common photochemical properties. Daily summer NOx fluxes were estimated to be (4.4 ± 2.3) x 107 molec cm-2 s-1, 10 to 70 times less than what has been estimated in previous studies at Dome C and with uncertainties reduced by a factor up to 30. Using these results, we extrapolated an annual continental snow source NOx budget of 0.025 ± 0.013 Tg.N y-1, more than three times the N-budget of the stratospheric denitrification estimated to be 0.008 ± 0.003 Tg.N y-1 for Antarctica (Savarino et al., 2007), making the snowpack source a rather significant source in Antarctica. This innovative approach for the parameterization of nitrate photolysis using flux chamber experiments could  significantly improve future global atmospheric models.

How to cite: Barbero, A., Grilli, R., Blouzon, C., Picard, G., Frey, M., Caillon, N., and Savarino, J.: Innovative approach for new estimation of NOx snow-source on the Antarctic Plateau, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9425, https://doi.org/10.5194/egusphere-egu21-9425, 2021.

09:31–09:33
|
EGU21-15775
|
ECS
Ghislain Motos, Paraskevi Georgakaki, Paul Zieger, Jörg Wieder, Ulrike Lohmann, and Athanasios Nenes

The Arctic region suffers an extreme vulnerability to climate change, with an increase in surface air temperatures that have reached twice the global rate during several decades (McBean et al., 2005). The role of clouds, and in particular low-levels clouds and fog, in this arctic amplification by regulating the energy transport from and to space has recently gained interest among the scientific community. The NASCENT 2019-2020 campaign (Ny-Ålesund AeroSol Cloud ExperimeNT) based in Ny-Ålesund, Svalbard (79º North) aimed at studying the microphysical and chemical properties of low-level clouds using measurements both at the sea level and at the Zeppelin station (475 m a.s.l.). Specifically, the susceptibility of droplet formation, which has recently been shown to be highly dependent on aerosol levels in European alpine valleys (Georgakaki et al., under review), could strongly vary between the fall to winter months, with pristine-like conditions, and the higher particle concentrations generally found in spring, known as the arctic haze. First results using a scanning mobility particle sizer (SMPS) and a cloud condensation nuclei counter (CCNC) confirmed that aerosol concentrations in the range 10 < Dpart [nm] < 500 were approximatively 4-5 times higher during the months of spring 2021 compared to those of fall 2020. In addition, we found relatively low values of the aerosol hygroscopic parameter κ, generally below 0.3, consistently with previous studies in the arctic region (Moore et al., 2011).

 

Georgakaki, P., Bougiatioti, A., Wieder, J., Mignani, C., Kanji, Z. A., Henneberger, J., Hervo, M., Berne, A. and Nenes, A.: On the drivers of droplet variability in Alpine mixed-phase clouds, , 34, under review.

McBean, G., Alekseev, G., Chen, D., Førland, E., Fyfe, Groisman, J., P. Y., King, R., Melling, H., Voseand, R., Whitfield, P. H.: Arctic climate: past and present. Arctic Climate Impacts Assessment (ACIA), C. Symon, L. Arris and B. Heal, Eds., Cambridge University Press, Cambridge, 21-60, 2005.

Moore, R. H., Bahreini, R., Brock, C. A., Froyd, K. D., Cozic, J., Holloway, J. S., Middlebrook, A. M., Murphy, D. M. and Nenes, A.: Hygroscopicity and composition of Alaskan Arctic CCN during April 2008, Atmospheric Chemistry and Physics, 11(22), 11807–11825, https://doi.org/10.5194/acp-11-11807-2011, 2011.

How to cite: Motos, G., Georgakaki, P., Zieger, P., Wieder, J., Lohmann, U., and Nenes, A.: In-situ observations of aerosol-cloud interactions in Ny-Ålesund, Svalbard, during fall 2019 and spring 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15775, https://doi.org/10.5194/egusphere-egu21-15775, 2021.

09:33–09:35
|
EGU21-15351
Tjarda Roberts, Meeta Cesler-Maloney, William Simpson, Jingqui Mao, Brice Barret, Slimane Bekki, Brice Temine-Roussel, Barbara d'Anna, Julia Maillard, Francois Ravetta, Jean-Christophe Raut, Andy Woods, Eleftherios Ioannidis, and Kathy Law

During the Arctic winter, local emissions (e.g. from home-heating, traffic, power station or industry plumes) coupled to poor dispersion caused by strong temperature inversions can lead to severe air pollution events. For example, each winter, Fairbanks (Alaska) experiences high abundances of gaseous pollutants and particulate matter (PM), leading to air-quality exceedances. However, there is still limited knowledge on the coupled physico-chemical and dynamical processes that cause wintertime Arctic pollution and aerosol formation under the very cold and low light conditions, and where levels of oxidants such as ozone at the surface can become depleted under limited vertical mixing. Here, we demonstrate novel deployment of low cost small sensors measuring PM2.5, gases (CO, NO, NO2, O3) and meteorological parameters (P, T, RH) to characterize Arctic atmospheric composition and properties, including mapping vertical distributions.

Our three-week pre-ALPACA (Alaskan Layered Pollution and Chemical Analysis) intensive field-campaign took place in downtown Fairbanks in Nov-Dec 2019. Small sensor temperature-dependencies were characterized by instrument cross-comparisons and correction-algorithms developed. Sensors were then deployed near-ground, on the roof of a 19 m building, and on a vertical pulley system set-up along the side of the building for vertical profiling. The small sensors show a strong capability to capture temporal variations in PM2.5, CO, NO and NO2 and O3, across a wide temperature range: surface gas and particle abundances became elevated during a cold-polluted period (temperatures as low as -30 C) and again became elevated during a subsequent warm-polluted period (temperatures around -3 C). Vertical profiling during the warm-polluted period identified strong temperature inversions associated with near-surface layers of high PM2.5 and CO that are distinct from an overlying clean, warm, humid air-mass. During the cold-polluted period, temperature inversions were present but less strong, there was little vertical structure in composition, and PM2.5 was often greater at 20m than at the surface. This finding contrasts with a full winter-season analysis that shows cold surface temperatures typically associated with strong inversions and PM highest at the surface. We invoke plume-rise modelling to show how buoyant plumes from local emissions (e.g. home-heating) can reach heights of about 10-20 m, allowing polluted emissions to rise and accumulate at altitude unless inversions are sufficiently strong to constrain the plume-rise. Causes of the temperature inversions include radiative cooling and advection of overlying warm-air. Our study highlights how small sensor measurements and vertical profiling can help elucidate the coupled processes of atmospheric chemistry, physics, dynamics and emissions that lead to surface air pollution episodes at high latitudes.

This study forms part of the Alaskan Layered Pollution and Chemical Analysis (ALPACA) project https://alpaca.community.uaf.edu/. We are grateful for technical support from Alaska-DEC, LPC2E, UAF, SEOSS, Alphasense and SouthCoastScience.

How to cite: Roberts, T., Cesler-Maloney, M., Simpson, W., Mao, J., Barret, B., Bekki, S., Temine-Roussel, B., d'Anna, B., Maillard, J., Ravetta, F., Raut, J.-C., Woods, A., Ioannidis, E., and Law, K.: In-situ characterization of layered pollution in the wintertime Arctic atmosphere by small sensors, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15351, https://doi.org/10.5194/egusphere-egu21-15351, 2021.

09:35–09:37
|
EGU21-15477
|
ECS
Eleftherios Ioannidis, Kathy S. Law, Jean-Christophe Raut, Tatsuo Onishi, Louis Marelle, Tjarda J. Roberts, Brice Barret, Barbara D'Anna, Brice Temine-Roussel, Nicole Mölders, Jingqiu Mao, and William R. Simpson

The wintertime Arctic is influenced by air pollution transported from mid-latitudes, leading to formation of Arctic Haze, as well as local emissions such as combustion for heating and power production in very cold winter conditions. This contributes to severe air pollution episodes, with enhanced aerosol concentrations, inter-dispersed with cleaner periods. However, the formation of secondary aerosol particles (sulphate, organics, nitrate) in cold/dark wintertime Arctic conditions, which could contribute to these pollution episodes, is poorly understood.

In this study, which contributes to the Air Pollution in the Arctic: Climate, Environment and Societies - Alaskan Layered Pollution and Arctic Chemical Analysis (PACES-ALPACA) initiative, the Weather Research Forecasting Model with chemistry (WRF-Chem) is used to investigate wintertime pollution over central Alaska focusing on the Fairbanks region, during the pre-ALPACA campaign in winter 2019-2020. Fairbanks is the most polluted city in the United States during wintertime, due to high local emissions and the occurrence of strong surface temperature inversions trapping pollutants near the surface.

Firstly, different WRF meteorological and surface schemes were tested over Alaska with a particular focus on improving simulations of the wintertime boundary layer structure including temperature inversions. An optimal WRF set-up, with increased vertical resolution below 2km, was selected based on evaluation against available data.

Secondly, a quasi-hemispheric WRF-Chem simulation, using the improved WRF setup, was used to assess large-scale synoptic conditions and to evaluate background aerosols originating from remote anthropogenic and natural sources affecting central Alaska during the campaign. The model was run with Evaluating the Climate and Air Quality Impacts of Short-Lived Pollutants (ECLIPSE) v6b anthropogenic emissions and improved sea-spray aerosol emissions. Discrepancies in modelled aerosols compared available data are being investigated (e.g. missing dark formation mechanisms, treatment of removal processes).

Thirdly, fine resolution simulations, using high resolution emissions (e.g. 2019 CAMS inventory), including local point sources, over the Fairbanks region, were used to investigate chemical and dynamical processes influencing aerosols under different meteorological conditions observed during the field campaign including a cold stable episode and a period with possible mixing of air masses from aloft. The model was evaluated against available aerosol, oxidant (ozone) and aerosol precursor data from surface monitoring sites and collected during the pre-campaign, including vertical profile data collected in the lowest 20m. The sensitivity of modelled aerosols to meteorological factors, such as relative humidity, temperature gradients and vertical mixing under winter conditions are investigated.

How to cite: Ioannidis, E., Law, K. S., Raut, J.-C., Onishi, T., Marelle, L., Roberts, T. J., Barret, B., D'Anna, B., Temine-Roussel, B., Mölders, N., Mao, J., and Simpson, W. R.: Wintertime Arctic Air Pollution over central Alaska: pre-ALPACA campaign, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15477, https://doi.org/10.5194/egusphere-egu21-15477, 2021.

09:37–10:30