Snowpack Nitrate Research Articles

An inverse model to correct for the effects of post-depositional processing on ice-core nitrate and its isotopes: model framework and applications at Summit, Greenland, and Dome C, Antarctica

Abstract. Comprehensive evaluation of the effects of post-depositional processing is a prerequisite for appropriately interpreting ice-core records of nitrate concentration and isotopes. In this study, we developed an inverse model that uses archived snow/ice-core nitrate signals to reconstruct primary nitrate flux (i.e., the deposition flux of nitrate to surface snow that originates from long-range transport or stratospheric input) and its isotopes (δ15N and Δ17O). The model was then applied to two polar sites, Summit, Greenland, and Dome C, Antarctica, using measured snowpack nitrate concentration and isotope profiles in the top few meters. At Summit, the model successfully reproduced the observed atmospheric δ15N(NO3-) and Δ17O(NO3-) and their seasonality. The model was also able to reasonably reproduce the observed snowpack nitrate profiles at Dome C as well as the skin layer and atmospheric δ15N(NO3-) and Δ17O(NO3-) at the annual scale. The calculated Fpri at Summit was 6.9 × 10−6 kgN m2 a−1, and the calculated Δ17O(NO3-) of Fpri is consistent with atmospheric observations in the Northern Hemisphere. However, the calculated δ15N(NO3-) of Fpri displays an opposite seasonal pattern to atmospheric observations in the northern mid-latitudes, but it is consistent with observations in two Arctic coastal sites. The calculated Fpri at Dome C varies from 1.5 to 2.2 × 10−6 kgN m−2 a−1, with δ15N(NO3-) of Fpri varying from 6.2 ‰ to 29.3 ‰ and Δ17O(NO3-) of Fpri varying from 48.8 ‰ to 52.6 ‰. The calculated Fpri at Dome C is close to the previous estimated stratospheric denitrification flux in Antarctica, and the high δ15N(NO3-) and Δ17O(NO3-) of Fpri at Dome C also point towards the dominant role of stratospheric origin of primary nitrate to Dome C.

Snowpack nitrate photolysis drives the summertime atmospheric nitrous acid (HONO) budget in coastal Antarctica

Abstract. Measurements of atmospheric nitrous acid (HONO) amount fraction and flux density above snow were carried out using a long-path absorption photometer at Halley station in coastal Antarctica between 22 January and 3 February 2022. The mean ±1σ HONO amount fraction was (2.1 ± 1.5) pmol mol−1 and showed a diurnal cycle (range of 1.0–3.2 pmol mol−1) with a maximum at solar noon. These HONO amount fractions are generally lower than have been observed at other Antarctic locations. The flux density of HONO from the snow, measured between 31 January and 1 February 2022, was between 0.5 and 3.4×1012 m-2s-1 and showed a decrease during the night. The measured flux density is close to the calculated HONO production rate from photolysis of nitrate present in the snow. A simple box model of HONO sources and sinks showed that the flux of HONO from the snow makes a >10 times larger contribution to the HONO budget than its formation through the reaction of OH and NO. Ratios of these HONO amount fractions to NOx measurements made in summer 2005 are low (0.15–0.35), which we take as an indication of our measurements being comparatively free from interferences. Further calculations suggest that HONO photolysis could produce up to 12 pmolmol-1h-1 of OH, approximately half that produced by ozone photolysis, which highlights the importance of HONO snow emissions as an OH source in the atmospheric boundary layer above Antarctic snowpacks.

New Estimation of the NOx Snow‐Source on the Antarctic Plateau

AbstractTo fully decipher the role of nitrate photolysis on the atmospheric oxidative capacity in snow‐covered regions, NOx flux must be determined with more precision than existing estimates. Here, we introduce a method based on dynamic flux chamber measurements for evaluating the NOx production by photolysis of snowpack nitrate in Antarctica. Flux chamber experiments were conducted for the first time in Antarctica, at the French‐Italian station Concordia, Dome C (75°06'S, 123°20’E, 3233 m a.s.l) during the 2019–2020 summer campaign. Measurements were gathered with several snow samples of different ages ranging from newly formed drifted snow to 6‐year‐old firn. Contrary to existing literature expectations, the daily average photolysis rate coefficient, , did not significantly vary between differently aged snow samples, suggesting that the photolabile nitrate in snow behaves as a single‐family source with common photochemical properties, where a = (2.37 0.35) × 10−8 s−1 (1) has been calculated from December 10th 2019 to January 7th 2020. At Dome C summer daily average NOx flux, , based on measured NOx production rates was estimated to be (4.3 1.2) × 108 molecules cm−2 s−1, which is 1.5–7 times less than the net NOx flux observed previously above snow at Dome C using the gradient flux method. Using these results, we extrapolated an annual continental snow sourced NOx budget of 0.017 0.003 TgN y−1, 2 times the nitrogen budget, (N‐budget), of the stratospheric denitrification previously estimated for Antarctica. These quantifications of nitrate photolysis using flux chamber experiments provide a road‐map toward a new parameterization of the product that can improve future global and regional models of atmospheric chemistry.

HONO, Particulate Nitrite, and Snow Nitrite at a Midlatitude Urban Site during Wintertime

Nitrous acid (HONO) plays an important role in the oxidative capacity of the atmosphere during wintertime via photolysis to produce hydroxyl radicals (OH). While it is known that HONO is emitted from the Arctic snowpack, sparse observations of HONO in the midlatitude urban wintertime environment have hindered our understanding of cold-season atmospheric chemistry. In this study, measurements of ambient HONO, particulate nitrite (pN(III); N(III) = NO2(aq)– + HONO(aq) + H2ONO(aq)+), and snow nitrite (sN(III)) were conducted in Kalamazoo, Michigan during January–February 2018. Elevated levels of HONO and particulate nitrite were observed over snow-covered ground, likely due to emissions of HONO from the snowpack, as well as weak turbulent mixing in the atmospheric boundary layer. The noontime peak in HONO of 87 ± 60 (1σ) parts per trillion (ppt) over snow-covered ground suggests photochemical snowpack HONO production, likely in part through snowpack nitrate photolysis, with only a minor contribution from par...

Nitrate Photochemistry at the Air-Ice Interface and in Other Ice Reservoirs.

The photolysis of snowpack nitrate (NO3-) is an important source of gaseous reactive nitrogen species that affect atmospheric oxidants, particularly in remote regions. However, it is unclear whether nitrate photochemistry differs between the three solute reservoirs in/on ice: in liquid-like regions (LLRs) in the ice; within the solid ice matrix; and in a quasi-liquid layer (QLL) at the air-ice interface, where past work indicates photolysis is enhanced. In this work, we explore the photoformation of nitrite in these reservoirs using laboratory ices. Nitrite quantum yields, Φ(NO2-), at 313 nm for aqueous and LLR ice samples agree with previous values, e.g., 0.65 ± 0.07% at -10 °C. For ice samples made via flash-freezing solution in liquid nitrogen, where nitrate is possibly present as a solid solution, the nitrite quantum yield is 0.57 ± 0.05% at -10 °C, similar to the LLR results. In contrast, the quantum yield at the air-ice interface is enhanced by a factor of 3.7 relative to LLRs, with a value of 2.39 ± 0.24%. These results indicate nitrate photolysis is enhanced at the air-ice interface, although the importance of this enhancement in the environment depends on the amount of nitrate present at the interface.

Spatial variation of isotopic compositions of snowpack nitrate related to post-depositional processes in eastern Dronning Maud Land, East Antarctica

Spatial variation of isotopic compositions of snowpack nitrate related to post-depositional processes in eastern Dronning Maud Land, East Antarctica

Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry

Post-depositional processes alter nitrate concentration and nitrate isotopic composition in the top layers of snow at sites with low snow accumulation rates, such as Dome C, Antarctica. Available nitrate ice core records can provide input for studying past atmospheres and climate if such processes are understood. It has been shown that photolysis of nitrate in the snowpack plays a major role in nitrate loss and that the photolysis products have a significant influence on the local troposphere as well as on other species in the snow. Reported quantum yields for the main reaction spans orders of magnitude - apparently a result of whether nitrate is located at the air-ice interface or in the ice matrix - constituting the largest uncertainty in models of snowpack NOx emissions. Here, a laboratory study is presented that uses snow from Dome C and minimizes effects of desorption and recombination by flushing the snow during irradiation with UV light. A selection of UV filters allowed examination of the effects of the 200 and 305nm absorption bands of nitrate. Nitrate concentration and photon flux were measured in the snow. The quantum yield for loss of nitrate was observed to decrease from 0.44 to 0.003 within what corresponds to days of UV exposure in Antarctica. The superposition of photolysis in two photochemical domains of nitrate in snow is proposed: one of photolabile nitrate, and one of buried nitrate. The difference lies in the ability of reaction products to escape the snow crystal, versus undergoing secondary (recombination) chemistry. Modeled NOx emissions may increase significantly above measured values due to the observed quantum yield in this study. The apparent quantum yield in the 200nm band was found to be ∼1%, much lower than reported for aqueous chemistry. A companion paper presents an analysis of the change in isotopic composition of snowpack nitrate based on the same samples as in this study.

The influence of snow grain size and impurities on the vertical profiles of actinic flux and associated NO<sub>x</sub> emissions on the Antarctic and Greenland ice sheets

Abstract. We use observations of the absorption properties of black carbon and non-black carbon impurities in near-surface snow collected near the research stations at South Pole and Dome C, Antarctica, and Summit, Greenland, combined with a snowpack actinic flux parameterization to estimate the vertical profile and e-folding depth of ultraviolet/near-visible (UV/near-vis) actinic flux in the snowpack at each location. We have developed a simple and broadly applicable parameterization to calculate depth and wavelength dependent snowpack actinic flux that can be easily integrated into large-scale (e.g., 3-D) models of the atmosphere. The calculated e-folding depths of actinic flux at 305 nm, the peak wavelength of nitrate photolysis in the snowpack, are 8–12 cm near the stations and 15–31 cm away (>11 km) from the stations. We find that the e-folding depth is strongly dependent on impurity content and wavelength in the UV/near-vis region, which explains the relatively shallow e-folding depths near stations where local activities lead to higher snow impurity levels. We calculate the lifetime of NOx in the snowpack interstitial air produced by photolysis of snowpack nitrate against wind pumping (τwind pumping) from the snowpack, and compare this to the calculated lifetime of NOx against chemical conversion to HNO3 (τchemical) to determine whether the NOx produced at a given depth can escape from the snowpack to the overlying atmosphere. Comparison of τwind pumping and τchemical suggests efficient escape of photoproduced NOx in the snowpack to the overlying atmosphere throughout most of the photochemically active zone. Calculated vertical actinic flux profiles and observed snowpack nitrate concentrations are used to estimate the potential flux of NOx from the snowpack. Calculated NOx fluxes of 4.4 × 108–3.8 × 109 molecules cm−2 s−1 in remote polar locations and 3.2–8.2 × 108 molecules cm−2 s−1 near polar stations for January at Dome C and South Pole and June at Summit suggest that NOx flux measurements near stations may be underestimating the amount of NOx emitted from the clean polar snowpack.

The effect of the novel HO2 + NO → HNO3 reaction channel at South Pole, Antarctica

AbstractIt is well established that the reaction of HO2 with NO plays a central role in atmospheric chemistry, by way of OH/HO2 recycling and reduction of ozone depletion by HOx cycles in the stratosphere and through ozone production in the troposphere. Utilizing a photochemical box model, we investigate the impact of the recently observed HNO3 production channel (HO2+NO → HNO3) on NOx (NO + NO2), HOx (OH + HO2), HNO3, and O3 concentrations in the boundary layer at the South Pole, Antarctica. Our simulations exemplify decreases in peak O3, NO, NO2, and OH and an increase in HNO3. Also, mean OH is in better agreement with observations, while worsening the agreement with O3, HO2, and HNO3 concentrations observed at the South Pole. The reduced concentrations of NOx are consistent with expected decreases in atmospheric NOx lifetime as a result of increased sequestration of NOx into HNO3. Although we show that the inclusion of the novel HNO3 production channel brings better agreement of OH with field measurements, the modelled ozone and HNO3 are worsened, and the changes in NOx lifetime imply that snowpack NOx emissions and snowpack nitrate recycling must be re-evaluated.

Exclusion of Nitrate to the Air–Ice Interface During Freezing

During freezing, the majority of solutes are rejected from the growing ice lattice and are concentrated at grain boundaries or nodes within the ice crystal or at the ice crystal surface itself. The degree of solute enrichment as well as the location of the rejected solutes has important consequences for reactions occurring in or on frozen media. We have used glancing-angle Raman spectroscopy to probe the exclusion of nitrate to the air–ice interface during freezing. This work represents the first use of this technique to measure solutes at the ice surface. Our results show that nitrate is excluded to the ice surface but not to the extent predicted by equilibrium thermodynamics. These findings have important implications for understanding the mechanism of snowpack nitrate photolysis.

Isotopic ratios in gas‐phase HNO3 and snow nitrate at Summit, Greenland

Measurements of the isotope ratios of snow nitrate and atmospheric gas‐phase HNO3 (15N/14N and 18O/16O) from Summit, Greenland provide new observational constraints on postdepositional processing of snowpack nitrate. The δ15N of NO3− is similar in surface snow and in air sampled 1.5 m above the snow surface during Spring and Summer 2006, ranging between −15 and +6‰ versus N2. The δ18O of NO3−, however, is ∼40‰ higher in surface snow than air samples. Photochemical box modeling of HNO3 chemistry at Summit suggests that the δ18O of the air samples is derived predominantly from gas‐phase HNO3 originating from photolyzed snow nitrate. Measurements of isotopically labeled nitrate in surface snow confirm that photolytic recycling of snowpack nitrate, in which photolyzed nitrate products recombine to form HNO3 that is subsequently redeposited to the snow surface, does occur at Summit. Calculations incorporating these measurements suggest that photolytic recycling of snow nitrate at Summit will have a maximum −7.5‰ change on the δ18O of nitrate buried in snow, while photolytic loss of snow nitrate will decrease the δ18O of buried nitrate by 0.9‰. Photolytic recycling and loss will increase the δ15N of buried snow nitrate, the degree to which depends on the nitrogen isotope fractionation of nitrate photolysis. Finally, a comparison of surface snow and snowpit samples demonstrates the preservation of the seasonal cycle in isotope ratios of snow nitrate.

Atmosphere-to-snow-to-firn transfer of NO3− on Ürümqi glacier No. 1, eastern Tien Shan, China

AbstractIn order to investigate the relationship between atmospheric nitrate and nitrate records in glacial snow and firn, nitrate was investigated in atmospheric aerosol, surface snow and the snow–firn pack on Ürümqi glacier No. 1 , eastern Tien Shan, China, from September 2002 to September 2004. The study found that vertical nitrate stratigraphy in the upper snowpack is formed during the winter by dry deposition and compaction and sublimation of the snowpack. During the summer the snowpack nitrate is redistributed through the percolation of meltwater. Nitrate was found to be highly mobile in the snowpack relative to magnesium.

Oxygen isotopic fractionation in the photochemistry of nitrate in water and ice

We recently reported the first multiple oxygen isotope composition of nitrate (NO3−) in ice cores (Alexander et al., 2004). Postdepositional photolysis and volatilization may alter the isotopic signatures of snowpack nitrate. Therefore the precise assessment of the geochemical/atmospheric significance of O‐isotopic signatures requires information on the relative rates of photolysis (λ > 300 nm) of N16O3−, N16O217O−, and N16O218O− in ice. Here we report on 17O‐ and 18O‐fractionation in the 313‐nm photolysis of 10‐mM aqueous solutions of normal Fisher KNO3 (i.e., Δ17O = −0.2 ± 0.2‰) and 17O‐enriched USGS‐35 NaNO3 (Δ17O = 21.0 ± 0.4‰) between −30° and 25°C. We found that Fisher KNO3 undergoes mass‐dependent O‐fractionation, i.e., a process that preserves Δ17O = 0. In contrast, Δ17O in USGS‐35 NaNO3 decreased by 1.6 ± 0.4‰ and 2.0 ± 0.4‰ at 25°C, 1.2 ± 0.4‰ and 1.3 ± 0.4‰ at −5°C, and 0.2 ± 0.4‰ and 1.1 ± 0.4‰ at −30°C, after 12 and 24 hours, respectively. Since the small quantum yield (∼0.2%) of NO3− photodecomposition into (NO2 + OH) is due to extensive cage recombination of the primary photofragments rather than to intramolecular processes, the observed Δ17O decreases likely reflect competitive O‐isotope exchange of geminate OH‐radicals with H2O (Δ17O = 0) and escape from the solvent cage, in addition to residual O‐isotope mixing of the final photoproducts NO, NO2, NO2−, with H2O. At the prevailing low temperatures, photochemical processing will not impair the diagnostic value of O‐isotopic signatures in tracing the chemical ancestry of nitrate in polar ice.

Postdepositional change in snowpack nitrate from observation of year‐round near‐surface snow in coastal Antarctica

Postdepositional loss of nitrate in near‐surface snowfall is well known, with mean levels of nitrate in ice cores of around 20 to 80 ng g−1, while nitrate in surface snow may occasionally reach over 300 ng g−1. This has been explained as reemission of nitrate (as nitric acid) during aging, via processes that are not yet clear. However, clear seasonal cycles remain in nitrate profiles from higher accumulation rate ice cores and, across Antarctica, the mean concentration of nitrate is remarkably similar despite widely varying deposition conditions, marking nitrate out as quite different to other major ice core species. This paper examines the year‐round deposition of nitrate at the snow surface at a coastal Antarctic site and discusses the degree and timing of nitrate loss. At Halley Station, Antarctica, the mean concentration of near‐surface snow was 96 ng g−1 over a 2.6 year daily sampling period, while the mean concentration in newly accumulated snow was 79 ng g−1. At the end of this period, a shallow core integrating the sampling period had a mean nitrate concentration of 65 ng g−1 taken over 2 full years of accumulation. Nitrate concentrations in the surface layer were in general highest during the summer period reaching 400 ng g−1 with a mean of about 150 ng g−1 in each of December, January, and February, and lowest during the winter with a mean of around 50 ng g−1 in June. Fresh snow data from Neumayer Station shows a similar seasonal signal, with a mean nitrate concentration of 77 ng g−1, while year‐round aerosol data shows total nitrate (particulate and gas phase) in the air is at a minimum in April to June and reaches a maximum in late November [Wagenbach et al., this issue], slightly out of phase with snowfall nitrate. The observation of a reduction in high nitrate concentrations in new snowfall over a few days does not appear to be general, and at Halley, there is evidence of both uptake and loss of nitrate in the surface snow layer, possibly indicating an equilibrium with changing air concentrations. However, there is attenuation of the nitrate signal over the longer period, with concentrations in the ice core taken at the end of sampling never reaching values seen in the upper surface layer.

Nitrate deposition to the Alaskan snowpack

Snowpack samples were collected from interior and arctic Alaska during March 1988 and analysed for pH, conductivity, NO 3 −, SO 4 2− and other constituents. The mean snowpack NO 3 − and SO 4 2− concentrations in the interior Alaska snowpack were found to be 160 and 179 mg g −1, respectively. The interior snowpack was observed to have concentrations and deposition fluxes of NO 3 − which are approximately 1.5 and 1–3 times, respectively, those observed in Greenland. In the arctic samples, collected in the Sagavanirktok River Valley, wind-deposited loess substantially increases both pH and SO 4 2− concentrations in the snowpack. Snowpack nitrate in these samples is unaffected by the windborne loess and had a mean NO 3 − concentration of 688 ng g −1. The NO 3 − deposition flux in the Arctic is approximately two times that found in the interior snowpack. The most plausible explanation for the elevated NO 3 − deposition flux is that the snowpack deposition is strongly influenced by the presence of the “arctic front”, a meteorological boundary which acts to contain the polluted, arctic air mass. Alternatively, local NO x emissions on Alaska's arctic coast or substantial changes in the scavenging efficiencies may also influence the observe north-south gradient in NO 3 − concentrations in the snowpack.