scholarly journals Modelling the physical multiphase interactions of HNO<sub>3</sub> between snow and air on the Antarctic Plateau (Dome C) and coast (Halley)

2018 ◽  
Vol 18 (3) ◽  
pp. 1507-1534 ◽  
Author(s):  
Hoi Ga Chan ◽  
Markus M. Frey ◽  
Martin D. King
Keyword(s):  
Atmospheric Chemistry ◽  
Lower Troposphere ◽  
Solute Concentration ◽  
Threshold Temperature ◽  
Liquid Volume ◽  
Snow Layer ◽  
Antarctic Plateau ◽  
Mixing Ratios ◽  
Surface Snow ◽  
The Antarctic

Abstract. Emissions of nitrogen oxide (NOx  =  NO + NO2) from the photolysis of nitrate (NO3−) in snow affect the oxidising capacity of the lower troposphere especially in remote regions of high latitudes with little pollution. Current air–snow exchange models are limited by poor understanding of processes and often require unphysical tuning parameters. Here, two multiphase models were developed from physically based parameterisations to describe the interaction of nitrate between the surface layer of the snowpack and the overlying atmosphere. The first model is similar to previous approaches and assumes that below a threshold temperature, To, the air–snow grain interface is pure ice and above To a disordered interface (DI) emerges covering the entire grain surface. The second model assumes that air–ice interactions dominate over all temperatures below melting of ice and that any liquid present above the eutectic temperature is concentrated in micropockets. The models are used to predict the nitrate in surface snow constrained by year-round observations of mixing ratios of nitric acid in air at a cold site on the Antarctic Plateau (Dome C; 75°06′ S, 123°33′ E; 3233 m a.s.l.) and at a relatively warm site on the Antarctic coast (Halley; 75°35′ S, 26°39′ E; 35 m a.s.l). The first model agrees reasonably well with observations at Dome C (Cv(RMSE)  =  1.34) but performs poorly at Halley (Cv(RMSE)  =  89.28) while the second model reproduces with good agreement observations at both sites (Cv(RMSE)  =  0.84 at both sites). It is therefore suggested that in winter air–snow interactions of nitrate are determined by non-equilibrium surface adsorption and co-condensation on ice coupled with solid-state diffusion inside the grain, similar to Bock et al. (2016). In summer, however, the air–snow exchange of nitrate is mainly driven by solvation into liquid micropockets following Henry's law with contributions to total surface snow NO3− concentrations of 75 and 80 % at Dome C and Halley, respectively. It is also found that the liquid volume of the snow grain and air–micropocket partitioning of HNO3 are sensitive to both the total solute concentration of mineral ions within the snow and pH of the snow. The second model provides an alternative method to predict nitrate concentration in the surface snow layer which is applicable over the entire range of environmental conditions typical for Antarctica and forms a basis for a future full 1-D snowpack model as well as parameterisations in regional or global atmospheric chemistry models.

scholarly journals Modeling the Physical Multi-Phase Interactions of HNO<sub>3</sub> Between Snow and Air on the Antarctic Plateau (Dome C) and coast (Halley)

2016 ◽  
Author(s):  
Hoi Ga Chan ◽  
Markus M. Frey ◽  
Martin D. King
Keyword(s):  
Atmospheric Chemistry ◽  
Lower Troposphere ◽  
Solute Concentration ◽  
Physical Models ◽  
Threshold Temperature ◽  
Liquid Volume ◽  
Pollution Levels ◽  
Surface Snow ◽  
Atmospheric Nitrate ◽  
Multi Phase

Abstract. Nitrogen oxides (NOx = NO + NO2) emissions from nitrate (NO3−) photolysis in snow affect the oxidising capacity of the lower troposphere especially in remote regions of the high latitudes with low pollution levels. The porous structure of snowpack allows the exchange of gases with the atmosphere driven by physicochemical processes, and hence, snow can act as both source and sink of atmospheric chemical trace gases. Current models are limited by poor process understanding and often require tuning parameters, for example the recently developed air-snow exchange model by Bock et al. (2016) requires an unrealistically large growth rate of snow grains to explain the NO3− peak in surface snow at Dome C in the summer. Here, two multi-phase physical models were developed from first principles constrained by observed atmospheric nitrate, HNO3, to describe the air-snow interaction of nitrate. Similar to most of the previous approaches, the first model assumes that below a threshold temperature, To, the air-snow grain interface is pure ice and above To, a disordered interface (DI) emerges assumed to be covering the entire grain surface. The second model assumes that Air-Ice interactions dominate over the entire temperature range below melting and that only above the eutectic temperature, liquid is present in the form of micropockets in grooves. The models are validated with available year-round observations of nitrate in snow and air at a cold site on the Antarctica Plateau (Dome C, 75°06′ S, 123°33′ E, 3233 m a.s.l.) and at a relatively warm site on the Antarctica coast (Halley, 75°35′ S, 26°39′ E, 35 m a.s.l). The first model agrees reasonably well with observations at Dome C (Cv(RMSE) = 1.34), but performs poorly at Halley (Cv(RMSE) = 89.28) while the second model reproduces with good agreement observations at both sites without any tuning (Cv(RMSE) = 0.84 at both sites). It is therefore suggested that air-snow interactions of nitrate in the winter are determined by non-equilibrium surface adsorption and co-condensation on ice coupled with solid-state diffusion inside the grain, similar to Bock et al. (2016). In summer, however, the air-snow exchange of nitrate is mainly driven by solvation into liquid micropockets following Henry’s law with contributions to total NO3− concentrations of 75 % and 80 % at Dome C and Halley respectively. It is also found that liquid volume of the snow grain and air-micropocket partitioning of HNO3 are sensitive to total solute concentration and pH. In conclusion, the second model can be used to predict nitrate concentration in surface snow over the entire range of environ- mental conditions typical for Antarctica and forms a basis for parameterisations in regional or global atmospheric chemistry models.

scholarly journals The Weddell Sea region: an important precipitation channel to the interior of the Antarctic ice sheet as revealed by glaciochemical investigation of surface snow along the longest trans-Antarctic route

1999 ◽  
Vol 29 ◽  
pp. 55-60 ◽  
Author(s):  
Qin Dahe ◽  
Paul A. Mayewski ◽  
Ren Jiawen ◽  
Xiao Cunde ◽  
Sun Junying
Keyword(s):  
Ice Sheet ◽  
Weddell Sea ◽  
Sea Salt ◽  
East Antarctica ◽  
Antarctic Ice Sheet ◽  
Air Masses ◽  
Antarctic Plateau ◽  
Salt Ions ◽  
Surface Snow ◽  
The Antarctic

AbstractGlaciochemical analysis of surface snow samples, collected along a profile crossing the Antarctic ice sheet from the Larsen Ice Shelf, Antarctic Peninsula, via the Antarctic Plateau through South Pole, Vostok and Komsomolskaya to Mirny station (at the east margin of East Antarctica), shows that the Weddell Sea region is an important channel for air masses to the high plateau of the Antarctic ice sheet (>2000 m a.s.l.). This opinion is supported by the following. (1) The fluxes of sea-salt ions such as Na+, Mg2 + and CF display a decreasing trend from the west to the east of interior Antarctica. In |eneral, as sea-salt aerosols are injected into the atmosphere over the Antarctic ice sheet from the Weddell Sea, large aerosols tend to decrease. For the inland plateau, few large particles of sea-salt aerosol reach the area, and the sea-salt concentration levels are low (2) The high altitude of the East Antarctic plateau, as well as the polar cold high-pressure system, obstruct the intrusive air masses mainly from the South Indian Ocean sector. (3) For the coastal regions of the East Antarctic ice sheet, the elevation rises to 2000 m over a distance from several to several tens of km. High concentrations of sea salt exist in snow in East Antarctica but are limited to a narrow coastal zone. (4) Fluxes of calcium and non-sea-salt sulfate in snow from the interior plateau do not display an eastward-decreasing trend. Since calcium is mainly derived from crustal sources, and nssSO42- is a secondary aerosol, this again confirms that the eastward-declining tendency of sea-salt ions indicates the transfer direction of precipitation vapor.

scholarly journals Air–snow transfer of nitrate on the East Antarctic Plateau – Part 2: An isotopic model for the interpretation of deep ice-core records

2015 ◽  
Vol 15 (20) ◽  
pp. 12079-12113 ◽  
Author(s):  
J. Erbland ◽  
J. Savarino ◽  
S. Morin ◽  
J. L. France ◽  
M. M. Frey ◽  
...  
Keyword(s):  
Mass Fraction ◽  
Ice Core ◽  
Ice Cores ◽  
Reactive Nitrogen ◽  
Antarctic Plateau ◽  
Surface Snow ◽  
Rate Building ◽  
The Antarctic ◽  
The Impact ◽  
Ice Core Records

Abstract. Unraveling the modern budget of reactive nitrogen on the Antarctic Plateau is critical for the interpretation of ice-core records of nitrate. This requires accounting for nitrate recycling processes occurring in near-surface snow and the overlying atmospheric boundary layer. Not only concentration measurements but also isotopic ratios of nitrogen and oxygen in nitrate provide constraints on the processes at play. However, due to the large number of intertwined chemical and physical phenomena involved, numerical modeling is required to test hypotheses in a quantitative manner. Here we introduce the model TRANSITS (TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow), a novel conceptual, multi-layer and one-dimensional model representing the impact of processes operating on nitrate at the air–snow interface on the East Antarctic Plateau, in terms of concentrations (mass fraction) and nitrogen (δ15N) and oxygen isotopic composition (17O excess, Δ17O) in nitrate. At the air–snow interface at Dome C (DC; 75° 06' S, 123° 19' E), the model reproduces well the values of δ15N in atmospheric and surface snow (skin layer) nitrate as well as in the δ15N profile in DC snow, including the observed extraordinary high positive values (around +300 ‰) below 2 cm. The model also captures the observed variability in nitrate mass fraction in the snow. While oxygen data are qualitatively reproduced at the air–snow interface at DC and in East Antarctica, the simulated Δ17O values underestimate the observed Δ17O values by several per mill. This is explained by the simplifications made in the description of the atmospheric cycling and oxidation of NO2 as well as by our lack of understanding of the NOx chemistry at Dome C. The model reproduces well the sensitivity of δ15N, Δ17O and the apparent fractionation constants (15&amp;varepsilon;app, 17Eapp) to the snow accumulation rate. Building on this development, we propose a framework for the interpretation of nitrate records measured from ice cores. Measurement of nitrate mass fractions and δ15N in the nitrate archived in an ice core may be used to derive information about past variations in the total ozone column and/or the primary inputs of nitrate above Antarctica as well as in nitrate trapping efficiency (defined as the ratio between the archived nitrate flux and the primary nitrate input flux). The Δ17O of nitrate could then be corrected from the impact of cage recombination effects associated with the photolysis of nitrate in snow. Past changes in the relative contributions of the Δ17O in the primary inputs of nitrate and the Δ17O in the locally cycled NO2 and that inherited from the additional O atom in the oxidation of NO2 could then be determined. Therefore, information about the past variations in the local and long-range processes operating on reactive nitrogen species could be obtained from ice cores collected in low-accumulation regions such as the Antarctic Plateau.

scholarly journals Biases in modeled surface snow BC mixing ratios in prescribed aerosol climate model runs

2014 ◽  
Vol 14 (9) ◽  
pp. 13167-13196 ◽  
Author(s):  
S. J. Doherty ◽  
C. M. Bitz ◽  
M. G. Flanner
Keyword(s):  
Climate Model ◽  
Aerosol Deposition ◽  
Deposition Flux ◽  
Snow Layer ◽  
Mixing Ratios ◽  
Mass Fluxes ◽  
Alternative Approach ◽  
Surface Snow ◽  
Snow Water ◽  
High Bias

Abstract. A series of recent studies have used prescribed aerosol deposition flux fields in climate model runs to assess forcing by black carbon in snow. In these studies, the prescribed mass deposition flux of BC to surface snow is decoupled from the mass deposition flux of snow water to the surface. Here we use a series of offline calculations to show that this approach results, on average, in a~factor of about 1.5–2.5 high bias in annual-mean surface snow BC mixing ratios in three key regions for snow albedo forcing by BC: Greenland, Eurasia and North America. These biases will propagate directly to positive biases in snow and surface albedo reduction by BC. The bias is shown to be due to coupling snowfall that varies on meteorological timescales (daily or shorter) with prescribed BC mass deposition fluxes that are more temporally and spatially smooth. The result is physically non-realistic mixing ratios of BC in surface snow. We suggest that an alternative approach would be to prescribe BC mass mixing ratios in snowfall, rather than BC mass fluxes, and we show that this produces more physically realistic BC mixing ratios in snowfall and in the surface snow layer.

scholarly journals Biases in modeled surface snow BC mixing ratios in prescribed-aerosol climate model runs

2014 ◽  
Vol 14 (21) ◽  
pp. 11697-11709 ◽  
Author(s):  
S. J. Doherty ◽  
C. M. Bitz ◽  
M. G. Flanner
Keyword(s):  
Black Carbon ◽  
Radiative Forcing ◽  
Climate Model ◽  
Aerosol Deposition ◽  
Deposition Flux ◽  
Snow Layer ◽  
Mixing Ratios ◽  
Mass Fluxes ◽  
Surface Snow ◽  
Snow Water

Abstract. Black carbon (BC) in snow lowers its albedo, increasing the absorption of sunlight, leading to positive radiative forcing, climate warming and earlier snowmelt. A series of recent studies have used prescribed-aerosol deposition flux fields in climate model runs to assess the forcing by black carbon in snow. In these studies, the prescribed mass deposition flux of BC to surface snow is decoupled from the mass deposition flux of snow water to the surface. Here we compare prognostic- and prescribed-aerosol runs and use a series of offline calculations to show that the prescribed-aerosol approach results, on average, in a factor of about 1.5–2.5 high bias in annual-mean surface snow BC mixing ratios in three key regions for snow albedo forcing by BC: Greenland, Eurasia and North America. These biases will propagate directly to positive biases in snow and surface albedo reduction by BC. The bias is shown be due to coupling snowfall that varies on meteorological timescales (daily or shorter) with prescribed BC mass deposition fluxes that are more temporally and spatially smooth. The result is physically non-realistic mixing ratios of BC in surface snow. We suggest that an alternative approach would be to prescribe BC mass mixing ratios in snowfall, rather than BC mass fluxes, and we show that this produces more physically realistic BC mixing ratios in snowfall and in the surface snow layer.

scholarly journals Air–snow transfer of nitrate on the East Antarctic plateau – Part 2: An isotopic model for the interpretation of deep ice-core records

2015 ◽  
Vol 15 (5) ◽  
pp. 6887-6966 ◽  
Author(s):  
J. Erbland ◽  
J. Savarino ◽  
S. Morin ◽  
J. L. France ◽  
M. M. Frey ◽  
...  
Keyword(s):  
Mass Fraction ◽  
Ice Core ◽  
Ice Cores ◽  
Reactive Nitrogen ◽  
Antarctic Plateau ◽  
Surface Snow ◽  
Rate Building ◽  
The Antarctic ◽  
The Impact ◽  
Ice Core Records

Abstract. Unraveling the modern budget of reactive nitrogen on the Antarctic plateau is critical for the interpretation of ice core records of nitrate. This requires accounting for nitrate recycling processes occurring in near surface snow and the overlying atmospheric boundary layer. Not only concentration measurements, but also isotopic ratios of nitrogen and oxygen in nitrate, provide constraints on the processes at play. However, due to the large number of intertwined chemical and physical phenomena involved, numerical modelling is required to test hypotheses in a~quantitative manner. Here we introduce the model "TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow" (TRANSITS), a~novel conceptual, multi-layer and one-dimensional model representing the impact of processes operating on nitrate at the air–snow interface on the East Antarctic plateau, in terms of concentrations (mass fraction) and the nitrogen (δ15N) and oxygen isotopic composition (17O}-excess, Δ17O) in nitrate. At the air–snow interface at Dome C (DC, 75°06' S, 123°19' E), the model reproduces well the values of δ15N in atmospheric and surface snow (skin layer) nitrate as well as in the δ15N profile in DC snow including the observed extraordinary high positive values (around +300 ‰) below 20 \\unit{cm}. The model also captures the observed variability in nitrate mass fraction in the snow. While oxygen data are qualitatively reproduced at the air–snow interface at DC and in East Antarctica, the simulated Δ17O values underestimate the observed Δ17O values by a~few~‰. This is explained by the simplifications made in the description of the atmospheric cycling and oxidation of NO2. The model reproduces well the sensitivity of δ15N, Δ17O and the apparent fractionation constants (15&amp;varepsilon;app, 17Eapp) to the snow accumulation rate. Building on this development, we propose a~framework for the interpretation of nitrate records measured from ice cores. Measurement of nitrate mass fractions and δ15N in the nitrate archived in an ice core, may be used to derive information about past variations in the total ozone column and/or the primary inputs of nitrate above Antarctica as well as in nitrate trapping efficiency (defined as the ratio between the archived nitrate flux and the primary nitrate input flux). The Δ17O of nitrate could then be corrected from the impact of cage recombination effects associated with the photolysis of nitrate in snow. Past changes in the relative contributions of the Δ17O in the primary inputs of nitrate and the Δ17O in the locally cycled NO2 could then be determined. Therefore, information about the past variations in the local and long range processes operating on reactive nitrogen species could be obtained from ice cores collected in low accumulation regions such as the Antarctic plateau.

scholarly journals Dynamic recycling of gaseous elemental mercury in the boundary layer of the Antarctic Plateau

2012 ◽  
Vol 12 (7) ◽  
pp. 18133-18161 ◽  
Author(s):  
A. Dommergue ◽  
M. Barret ◽  
J. Courteaud ◽  
P. Cristofanelli ◽  
C. P. Ferrari ◽  
...  
Keyword(s):  
Mixing Layer ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
Photochemical Reactions ◽  
Solar Irradiation ◽  
The Arctic ◽  
Gaseous Elemental Mercury ◽  
Antarctic Plateau ◽  
Mixing Ratios ◽  
The Antarctic

Abstract. Gaseous elemental mercury (Hg(0)) was investigated in the troposphere and in the interstitial air extracted from the snow at Dome Concordia station (alt. 3320 m) on the Antarctic Plateau during January 2009. Measurements showed evidence of a very dynamic and daily cycling of Hg(0) inside the mixing layer with a range of values from 0.2 ng m−3 up to 2.3 ng m−3. During low solar irradiation periods, fast Hg(0) oxidation processes in a confined layer were observed leading to an enrichment of the upper snow layers in divalent Hg. Unexpectedly high Hg(0) concentrations for such a remote place were measured under higher solar irradiation due to the reemission of Hg(0) by the snowpack via photochemical reactions. Hg(0) concentrations showed a negative correlation with ozone mixing ratios, which contrasts with atmospheric mercury depletion events observed during the Arctic spring. It remains unclear whether halogens are involved in Hg(0) oxidation. We suggest that snow surfaces may play a role in promoting the heterogeneous oxidation of Hg(0). The cycling of other oxidants should be investigated together with Hg in order to clarify the complex reactivity on the Antarctic Plateau.

scholarly journals Large mixing ratios of atmospheric nitrous acid (HONO) at Concordia (East Antarctic Plateau) in summer: a strong source from surface snow?

2014 ◽  
Vol 14 (18) ◽  
pp. 9963-9976 ◽  
Author(s):  
M. Legrand ◽  
S. Preunkert ◽  
M. Frey ◽  
Th. Bartels-Rausch ◽  
A. Kukui ◽  
...  
Keyword(s):  
Nitrous Acid ◽  
Austral Summer ◽  
Vertical Gradient ◽  
No Oxidation ◽  
Local Source ◽  
Mixing Ratio ◽  
Model Calculations ◽  
Antarctic Plateau ◽  
Mixing Ratios ◽  
Surface Snow

Abstract. During the austral summer 2011/2012 atmospheric nitrous acid (HONO) was investigated for the second time at the Concordia site (75°06' S, 123°33' E), located on the East Antarctic Plateau, by deploying a long-path absorption photometer (LOPAP). Hourly mixing ratios of HONO measured in December 2011/January 2012 (35 ± 5.0 pptv) were similar to those measured in December 2010/January 2011 (30.4 ± 3.5 pptv). The large value of the HONO mixing ratio at the remote Concordia site suggests a local source of HONO in addition to weak production from oxidation of NO by the OH radical. Laboratory experiments demonstrate that surface snow removed from Concordia can produce gas-phase HONO at mixing ratios half that of the NOx mixing ratio produced in the same experiment at typical temperatures encountered at Concordia in summer. Using these lab data and the emission flux of NOx from snow estimated from the vertical gradient of atmospheric concentrations measured during the campaign, a mean diurnal HONO snow emission ranging between 0.5 and 0.8 × 109 molecules cm−2 s−1 is calculated. Model calculations indicate that, in addition to around 1.2 pptv of HONO produced by the NO oxidation, these HONO snow emissions can only explain 6.5 to 10.5 pptv of HONO in the atmosphere at Concordia. To explain the difference between observed and simulated HONO mixing ratios, tests were done both in the field and at lab to explore the possibility that the presence of HNO4 had biased the measurements of HONO.

scholarly journals Large mixing ratios of atmospheric nitrous acid (HONO) at Concordia (East Antarctic plateau) in summer: a strong source from surface snow?

2014 ◽  
Vol 14 (8) ◽  
pp. 11749-11785 ◽  
Author(s):  
M. Legrand ◽  
S. Preunkert ◽  
M. Frey ◽  
T. Bartels-Rausch ◽  
A. Kukui ◽  
...  
Keyword(s):  
Nitrous Acid ◽  
Austral Summer ◽  
Vertical Gradient ◽  
No Oxidation ◽  
Local Source ◽  
Mixing Ratio ◽  
Model Calculations ◽  
Antarctic Plateau ◽  
Mixing Ratios ◽  
Surface Snow

Abstract. During the austral summer 2011/2012 atmospheric nitrous acid was investigated for the second time at the Concordia site (75°06' S, 123°33' E) located on the East Antarctic plateau by deploying a long path absorption photometer (LOPAP). Hourly mixing ratios of HONO measured in December 2011/January 2012 (35 ± 5.0 pptv) were similar to those measured in December 2010/January 2011 (30.4 ± 3.5 pptv). The large value of the HONO mixing ratio at the remote Concordia site suggests a local source of HONO in addition to weak production from oxidation of NO by the OH radical. Laboratory experiments demonstrate that surface snow removed from Concordia can produce gas phase HONO at mixing ratios half that of NOx mixing ratio produced in the same experiment at typical temperatures encountered at Concordia in summer. Using these lab data and the emission flux of NOx from snow estimated from the vertical gradient of atmospheric concentrations measured during the campaign, a mean diurnal HONO snow emission ranging between 0.5 and 0.8 × 109 molecules cm−2 s−1 is calculated. Model calculations indicate that, in addition to around 1.2 pptv of HONO produced by the NO oxidation, these HONO snow emissions can only explain 6.5 to 10.5 pptv of HONO in the atmosphere at Concordia. To explain the difference between observed and simulated HONO mixing ratios, tests were done both in the field and at lab to explore the possibility that the presence of HNO4 had biased the measurements of HONO.

scholarly journals The diurnal variability of atmospheric nitrogen oxides (NO and NO<sub>2</sub>) above the Antarctic Plateau driven by atmospheric stability and snow emissions

2013 ◽  
Vol 13 (6) ◽  
pp. 3045-3062 ◽  
Author(s):  
M. M. Frey ◽  
N. Brough ◽  
J. L. France ◽  
P. S. Anderson ◽  
O. Traulle ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Nitrogen Oxides ◽  
Atmospheric Chemistry ◽  
Atmospheric Stability ◽  
Polar Regions ◽  
Atmospheric Nitrogen ◽  
Diurnal Variability ◽  
Antarctic Plateau ◽  
Mixing Ratios ◽  
Early Evening

Abstract. Atmospheric nitrogen oxides (NO and NO2) were observed at Dome C, East Antarctica (75.1° S, 123.3° E, 3233 m), for a total of 50 days, from 10 December 2009 to 28 January 2010. Average (±1σ) mixing ratios at 1.0 m of NO and NO2, the latter measured for the first time on the East Antarctic Plateau, were 111 (±89) and 98 (±89) pptv, respectively. Atmospheric mixing ratios are on average comparable to those observed previously at South Pole, but in contrast show strong diurnal variability: a minimum around local noon and a maximum in the early evening coincide with the development and collapse of a convective boundary layer. The asymmetric diurnal cycle of NOx concentrations and likely any other chemical tracer with a photolytic surface source is driven by the turbulent diffusivity and height of the atmospheric boundary layer, with the former controlling the magnitude of the vertical flux and the latter the size of the volume into which snow emissions are transported. In particular, the average (±1σ) NOx emission flux from 22 December 2009 to 28 January 2010, estimated from atmospheric concentration gradients, was 8.2 (±7.4) × 1012 molecule m−2 s−1 belongs to the largest values measured so far in the polar regions and explains the 3-fold increase in mixing ratios in the early evening when the boundary layer becomes very shallow. Dome C is likely not representative for the entire East Antarctic Plateau but illustrates the need of an accurate description of the boundary layer above snow in atmospheric chemistry models. A simple nitrate photolysis model matches the observed median diurnal NOx flux during the day but has significant low bias during the night. The difference is significant taking into account the total random error in flux observations and model uncertainties due to the variability of NO3− concentrations in snow and potential contributions from NO2− photolysis. This highlights uncertainties in the parameterization of the photolytic NOx source in natural snowpacks, such as the poorly constrained quantum yield of nitrate photolysis. A steady-state analysis of the NO2 : NO ratios indicates that peroxy (HO2 + RO2) or other radical concentrations in the boundary layer of Dome C are either higher than measured elsewhere in the polar regions or other processes leading to enhanced NO2 have to be invoked. These results confirm the existence of a strongly oxidising canopy enveloping the East Antarctic Plateau in summer.

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