# A 60 Year Record of Atmospheric Aerosol Depositions Preserved in a High-Accumulation Dome Ice Core, Southeast Greenland

Articlein · December 2017with 61 Reads
Abstract
The Southeastern Greenland Dome (SE-Dome) has both a high elevation and a high accumulation rate (1.01 m w.e. yr-1), which are suitable properties for reconstructing past environmental changes with a high time resolution. For this study, we measured the major ion fluxes in a 90-m ice core drilled from the SE-Dome region in 2015, and present the records of annual ion fluxes from 1957 to 2014. From 1970 to 2010, the trend of non-sea-salt (nss) SO42- flux decreases, whereas that for NH4+ increases, tracking well with the anthropogenic SOx and NH3 emissions mainly from North America. The result suggests that these fluxes reflect histories of the anthropogenic SOx and NH3 emissions. In contrast, the decadal trend of NO3- flux differs from the decreasing trend of anthropogenic NOx emissions. Although the cause of this discrepancy remains unclear, it may be related to changes in particle formation processes and chemical scavenging rates caused by an increase in sea-salt and dust and/or a decrease in nssSO42-. We also find a high average NO3- flux (1.13 mmol m-2 yr-1) in the ice core, which suggests a negligible effect from post-depositional NO3- loss. Thus, the SE-Dome region is an excellent location for reconstructing nitrate fluxes. Over a decadal timescale, our NO3- flux record is similar to those from other ice cores in Greenland high-elevation sites, suggesting that NO3- concentrations records from these ice cores are reliable.
• ... A number of tracers (e.g., inorganic: potassium and ammonium, formate; organic: levoglucosan, dehydroabietic acid, and vanillic acid; and the isotopic compositions of trace gases) have been proposed to reconstruct changes in the BB activity in the past ( Bock et al., 2017;Legrand et al., 2016Legrand et al., , 1992Rubino et al., 2016;Simoneit et al., 1999) and have been applied to ice cores ( Iizuka et al., 2018;Kawamura et al., 2012;Zennaro et al., 2014). Of these tracers, organic tracers are be- coming increasingly common tools because they are produced solely by https://doi.org/10.1016/j.atmosenv.2018.10.012 ...
... From 1958From , 2014, the SEIS2016 age scale, which is determined via the oxygen isotope matching method, was used. The SEIS2016 age scale was carefully evaluated with independent age markers, and its preci- sion is within two months ( Furukawa et al., 2017;Iizuka et al., 2018). The average accumulation rate of the SE-Dome ice core was 1.01 ± 0.22 m/yr, which is the highest rate of any ice core reported in Greenland. ...
... In addition, melt layers are relatively infrequent in the SE- Dome ice core. Therefore, the post depositional alteration of the de- posited aerosols due to ice melt and low accumulation is thought to be minimal ( Iizuka et al., 2018). This ice core is ideal to assess the pa- leoclimatic utility of aerosol tracers in ice cores by a precise comparison with observationally based records. ...
Article
We provide continuous records of biomass burning molecular tracers (levoglucosan and dehydroabietic acid) in a Greenland ice core collected from the Southeastern Dome (the SE-Dome ice core) over the past several decades to assess the paleoclimatic utility of these tracers in Greenland ice cores. An air mass backward-trajectory analysis indicates that eastern Canada is likely the primary source region of the biomass burning tracers. Comparisons of levoglucosan and dehydroabietic acid data in the SE-Dome ice core and area burned (vegetation fire) events in Canada suggests that the biomass burning tracers in the ice core document most of the pronounced biomass burning events in eastern Canada over the past several decades, confirming that analyses of biomass burning molecular tracers in Greenland ice cores are useful to reconstruct the frequency of significant biomass burning events in a local region. However, our study also highlights that the wind pattern when the biomass burning occurs is decisive for the registration of a biomass burning event in an ice core even though long-term changes in the wind regime associated with decadal-scale climate oscillations do not significantly influence the transport and deposition of biomass burning tracers on the Greenland ice sheet.
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On May 2015, we drilled a 90.45 m ice core in a high accumulation area of the southeastern Greenland Ice Sheet. The drilling site (SE-Dome; 67.18°N, 36.37°W, 3170 m a.s.l.) is located 185 km north of the town of Tasiilaq in southeastern Greenland [1]. Then we measure physical and chemical properties of the SE-Dome ice core. Based on the measurements, we show the general characteristics of the SE-Dome ice core. I) As for dating of the ice core [2], we propose a dating method based on matching the δ 18 O variations between ice-core records and records simulated by isotope-enabled climate models. We applied this method to a δ 18 O record from the SE-Dome ice core. The close similarity between the δ 18 O records from the ice core and models enables correlation and the production of a precise age scale, with an accuracy of a few months. II) As for physical property [3], the ice was-20.9 ºC at 20-m depth. The close-off density of 830 kg m-3 occurs at 83.4-86.8-m depth, which is about 20-m shallower than that obtained from empirical models, indicating that the firn with a higher density is softer than that from empirical result. We interpret that the high accumulation rate creates a high overburden pressure in a short time. The relative softness of the firn may arise from 1) there being not enough time to form bonds between grains as strong as those in a lower accumulation-rate area, and similarly, 2) the dislocation density in the firn being relatively high. III) As for chemical property [4], we measured the major ion fluxes, and obtained records of annual ion fluxes from 1957 to 2014. We find a high average NO 3-flux (1.13 mmol m-2 yr-1) in the ice core, which suggests a negligible effect from post-depositional NO 3-loss, indicating the SE-Dome region is an excellent location for reconstructing nitrate fluxes. For the non-sea-salt (nss) SO 4 2-and NH 4 + fluxes, a decreasing and increasing trend from 1970 to 2010, respectively, tracks well with the anthropogenic SO x and NH 3 emissions. In contrast, the decadal trend of NO 3-flux differs from the decreasing trend of anthropogenic NO x emissions. We continue to investigate the paleoenvironment with multi proxies from several analyses (e.g. [5]) of the high-time-resolution and
• Article
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Ice core nitrate concentrations peak in the summer in both Greenland and Antarctica. Two nitrate concentration peaks in one annual layer have been observed some years in ice cores in Greenland from samples dating post-1900, with the additional nitrate peak occurring in the spring. The origin of the spring nitrate peak was hypothesized to be pollution transport from the mid-latitudes in the industrial era. We performed a case study on the origin of a spring nitrate peak in 2005 measured from a snowpit at Summit, Greenland, covering 3 years of snow accumulation. The effect of long-range transport of nitrate on this spring peak was excluded by using sulfate as a pollution tracer. The isotopic composition of nitrate (δ15N, δ18O and Δ17O) combined with photochemical calculations suggest that the occurrence of this spring peak is linked to a significantly weakened stratospheric ozone (O3) layer. The weakened O3 layer resulted in elevated UVB (ultraviolet-B) radiation on the snow surface, where the production of OH and NOx from the photolysis of their precursors was enhanced. Elevated NOx and OH concentrations resulted in enhanced nitrate production mainly through the NO2 + OH formation pathway, as indicated by decreases in δ18O and Δ17O of nitrate associated with the spring peak. We further examined the nitrate concentration record from a shallow ice core covering the period from 1772 to 2006 and found 19 years with double nitrate peaks after the 1950s. Out of these 19 years, 14 of the secondary nitrate peaks were accompanied by sulfate peaks, suggesting long-range transport of nitrate as their source. In the other 5 years, low springtime O3 column density was observed, suggesting enhanced local production of nitrate as their source. The results suggest that, in addition to direct transport of nitrate from polluted regions, enhanced local photochemistry can also lead to a spring nitrate peak. The enhanced local photochemistry is probably associated with the interannual variability of O3 column density in the Arctic, which leads to elevated surface UV radiation in some years. In this scenario, enhanced photochemistry caused increased local nitrate production under the condition of elevated local NOx abundance in the industrial era.
• Article
A precise age scale based on annual layer counting is essential for investigating past environmental changes from ice core records. However, sub-annual scale dating is hampered by the irregular intra-annual variabilities of oxygen isotope (δ18O) records. Here, we propose a dating method based on matching the δ18O variations between ice-core records and records simulated by isotope-enabled climate models. We applied this method to a new δ18O record from an ice core obtained from a dome site in southeast Greenland. The close similarity between the δ18O records from the ice core and models enables correlation and the production of a precise age scale, with an accuracy of a few months. A missing δ18O minimum in the 1995/1996 winter is an example of an indistinct δ18O seasonal cycle. Our analysis suggests that the missing δ18O minimum is likely caused by a combination of warm air temperature, weak moisture transport, and cool ocean temperature. Based on the age scale, the average accumulation rate from 1960 to 2014 is reconstructed as 1.02 m yr-1 in water equivalent. The annual accumulation rate shows an increasing trend with a slope of 3.6 mm year-1, which is mainly caused by the increase in the autumn accumulation rate of 2.6 mm year-1. This increase is likely linked to the enhanced hydrological cycle caused by the decrease in Arctic sea ice area. Unlike the strong seasonality of precipitation amount in the ERA re-analysis data in the southeast dome region, our reconstructed accumulation rate suggests a weak seasonality.
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Triple oxygen isotopic compositions (Δ¹⁷O = δ¹⁷O − 0.52 × δ¹⁸O) of atmospheric sulfate (SO4²⁻) and nitrate (NO3⁻) in the atmosphere reflect the relative contribution of oxidation pathways involved in their formation processes, which potentially provides information to reveal missing reactions in atmospheric chemistry models. However, there remain many theoretical assumptions for the controlling factors of Δ¹⁷O(SO4²⁻) and Δ¹⁷O(NO3⁻) values in those model estimations. To test one of those assumption that Δ¹⁷O values of ozone (O3) have a flat value and do not influence the seasonality of Δ¹⁷O(SO4²⁻) and Δ¹⁷O(NO3⁻) values, we performed the first simultaneous measurement of Δ¹⁷O values of atmospheric sulfate, nitrate, and ozone collected at Dumont d'Urville (DDU) Station (66°40′ S, 140°01′ E) throughout 2011. Δ¹⁷O values of sulfate and nitrate exhibited seasonal variation characterized by minima in the austral summer and maxima in winter, within the ranges of 0.9–3.4 and 23.0–41.9 ‰, respectively. In contrast, Δ¹⁷O values of ozone showed no significant seasonal variation, with values of 26 ± 1 ‰ throughout the year. These contrasting seasonal trends suggest that seasonality in Δ¹⁷O(SO4²⁻) and Δ¹⁷O(NO3⁻) values is not the result of changes in Δ¹⁷O(O3), but of the changes in oxidation chemistry. The trends with summer minima and winter maxima for Δ¹⁷O(SO4²⁻) and Δ¹⁷O(NO3⁻) values are caused by sunlight-driven changes in the relative contribution of O3 oxidation to the oxidation by HOx, ROx, and H2O2. In addition to that general trend, by comparing Δ¹⁷O(SO4²⁻) and Δ¹⁷O(NO3⁻) values to ozone mixing ratios, we found that Δ¹⁷O(SO4²⁻) values observed in spring (September to November) were lower than in fall (March to May), while there was no significant spring and fall difference in Δ¹⁷O(NO3⁻) values. The relatively lower sensitivity of Δ¹⁷O(SO4²⁻) values to the ozone mixing ratio in spring compared to fall is possibly explained by (i) the increased contribution of SO2 oxidations by OH and H2O2 caused by NOx emission from snowpack and/or (ii) SO2 oxidation by hypohalous acids (HOX = HOCl + HOBr) in the aqueous phase.
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Rationale: Triple oxygen and nitrogen isotope ratios in nitrate are powerful tools for assessing atmospheric nitrate formation pathways and their contribution to ecosystems. N2 O decomposition using microwave-induced plasma (MIP) has been used only for measurements of oxygen isotopes to date, but it is also possible to measure nitrogen isotopes during the same analytical run. Methods: The main improvements to a previous system are (i) an automated distribution system of nitrate to the bacterial medium, (ii) N2 O separation by gas chromatography before N2 O decomposition using the MIP, (iii) use of a corundum tube for microwave discharge, and (iv) development of an automated system for isotopic measurements. Three nitrate standards with sample sizes of 60, 80, 100, and 120 nmol were measured to investigate the sample size dependence of the isotope measurements. Results: The δ17 O, δ18 O, and Δ17 O values increased with increasing sample size, although the δ15 N value showed no significant size dependency. Different calibration slopes and intercepts were obtained with different sample amounts. The slopes and intercepts for the regression lines in different sample amounts were dependent on sample size, indicating that the extent of oxygen exchange is also dependent on sample size. The sample-size-dependent slopes and intercepts were fitted using natural log (ln) regression curves, and the slopes and intercepts can be estimated to apply to any sample size corrections. When using 100 nmol samples, the standard deviations of residuals from the regression lines for this system were 0.5‰, 0.3‰, and 0.1‰, respectively, for the δ18 O, Δ17 O, and δ15 N values, results that are not inferior to those from other systems using gold tube or gold wire. Conclusions: An automated system was developed to measure triple oxygen and nitrogen isotopes in nitrate using N2 O decomposition by MIP. This system enables us to measure both triple oxygen and nitrogen isotopes in nitrate with comparable precision and sample throughput (23 min per sample on average), and minimal manual treatment. Copyright © 2016 John Wiley & Sons, Ltd.
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The formation and recycling of reactive nitrogen (NO, NO2, HONO) at the air–snow interface has implications for air quality and the oxidation capacity of the atmosphere in snow-covered regions. Nitrate (NO3−) photolysis in snow provides a source of oxidants (e.g., hydroxyl radical) and oxidant precursors (e.g., nitrogen oxides) to the overlying boundary layer, and alters the concentration and isotopic (e.g., δ15N) signature of NO3− preserved in ice cores. We have incorporated an idealized snowpack with a NO3− photolysis parameterization into a global chemical transport model (Goddard Earth Observing System (GEOS) Chemistry model, GEOS-Chem) to examine the implications of snow NO3− photolysis for boundary layer chemistry, the recycling and redistribution of reactive nitrogen, and the preservation of ice-core NO3− in ice cores across Antarctica and Greenland, where observations of these parameters over large spatial scales are difficult to obtain. A major goal of this study is to examine the influence of meteorological parameters and chemical, optical, and physical snow properties on the magnitudes and spatial patterns of snow-sourced NOx fluxes and the recycling and redistribution of reactive nitrogen across Antarctica and Greenland. Snow-sourced NOx fluxes are most influenced by temperature-dependent quantum yields of NO3− photolysis, photolabile NO3− concentrations in snow, and concentrations of light-absorbing impurities (LAIs) in snow. Despite very different assumptions about snowpack properties, the range of model-calculated snow-sourced NOx fluxes are similar in Greenland (0.5–11 × 108 molec cm−2 s−1) and Antarctica (0.01–6.4 × 108 molec cm−2 s−1) due to the opposing effects of higher concentrations of both photolabile NO3− and LAIs in Greenland compared to Antarctica. Despite the similarity in snow-sourced NOx fluxes, these fluxes lead to smaller factor increases in mean austral summer boundary layer mixing ratios of total nitrate (HNO3+ NO3−), NOx, OH, and O3 in Greenland compared to Antarctica because of Greenland's proximity to pollution sources. The degree of nitrogen recycling in the snow is dependent on the relative magnitudes of snow-sourced NOx fluxes versus primary NO3− deposition. Recycling of snow NO3− in Greenland is much less than in Antarctica Photolysis-driven loss of snow NO3− is largely dependent on the time that NO3− remains in the snow photic zone (up to 6.5 years in Antarctica and 7 months in Greenland), and wind patterns that redistribute snow-sourced reactive nitrogen across Antarctica and Greenland. The loss of snow NO3− is higher in Antarctica (up to 99 %) than in Greenland (up to 83 %) due to deeper snow photic zones and lower snow accumulation rates in Antarctica. Modeled enrichments in ice-core δ15N(NO3−) due to photolysis-driven loss of snow NO3− ranges from 0 to 363 ‰ in Antarctica and 0 to 90 ‰ in Greenland, with the highest fraction of NO3− loss and largest enrichments in ice-core δ15N(NO3−) at high elevations where snow accumulation rates are lowest. There is a strong relationship between the degree of photolysis-driven loss of snow NO3− and the degree of nitrogen recycling between the air and snow throughout all of Greenland and in Antarctica where snow accumulation rates are greater than 130 kg m−2 a−1 in the present day.
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Particle acidity affects aerosol concentrations, chemical composition and toxicity. Sulfate is often the main acid component of aerosols, and largely determines the acidity of fine particles under 2.5 μm in diameter, PM2.5. Over the past 15 years, atmospheric sulfate concentrations in the southeastern United States have decreased by 70%, whereas ammonia concentrations have been steady. Similar trends are occurring in many regions globally. Aerosol ammonium nitrate concentrations were assumed to increase to compensate for decreasing sulfate, which would result from increasing neutrality. Here we use observed gas and aerosol composition, humidity, and temperature data collected at a rural southeastern US site in June and July 2013 (ref.), and a thermodynamic model that predicts pH and the gas-particle equilibrium concentrations of inorganic species from the observations to show that PM2.5 at the site is acidic. pH buffering by partitioning of ammonia between the gas and particle phases produced a relatively constant particle pH of 0-2 throughout the 15 years of decreasing atmospheric sulfate concentrations, and little change in particle ammonium nitrate concentrations. We conclude that the reductions in aerosol acidity widely anticipated from sulfur reductions, and expected acidity-related health and climate benefits, are unlikely to occur until atmospheric sulfate concentrations reach near pre-anthropogenic levels.
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During spring 2014, we drilled an ice core on the northwestern Greenland Ice Sheet, recovering a core of total length 225m. We also conducted stratigraphic observations, measurements of the density of the ice core, near-infrared photography of the ice core, preparation of liquid samples for chemical analysis, and measurements of borehole temperature. The pore close-off depth was 60m, and the temperature in the borehole was −25.6°C at a depth of 10m. In addition, we conducted snow-pit observations, ice-velocity and surface-elevation measurements using the global positioning system (GPS), meteorological observations, and installation of an automated weather station (AWS).
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We use the Total Ozone Mapping Spectrometer (TOMS) sensor on the Nimbus 7 satellite to map the global distribution of major atmospheric dust sources with the goal of identifying common environmental characteristics. The largest and most persistent sources are located in the Northern Hemisphere, mainly in a broad "dust belt" that extends from the west coast of North Africa, over the Middle East, Central and South Asia, to China. There is remarkably little large-scale dust activity outside this region. In particular, the Southern Hemisphere is devoid of major dust activity. Dust sources, regardless of size or strength, can usually be associated with topographical lows located in arid regions with annual rainfall under 200-250 mm. Although the source regions themselves are arid or hyperarid, the action of water is evident from the presence of ephemeral streams, rivers, lakes, and playas. Most major sources have been intermittently flooded through the Quaternary as evidenced by deep alluvial deposits. Many sources are associated with areas where human impacts are well documented, e.g., the Caspian and Aral Seas, Tigris-Euphrates River Basin, southwestern North America, and the loess lands in China. Nonetheless, the largest and most active sources are located in truly remote areas where there is little or no human activity. Thus, on a global scale, dust mobilization appears to be dominated by natural sources. Dust activity is extremely sensitive to many environmental parameters. The identification of major sources will enable us to focus on critical regions and to characterize emission rates in response to environmental conditions. With such knowledge we will be better able to improve global dust models and to assess the effects of climate change on emissions in the future. It will also facilitate the interpretation of the paleoclimate record based on dust contained in ocean sediments and ice cores.
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Climate changes in the North Atlantic region during the last glacial cycle were dominated by the slow waxing and waning of the North American ice sheet as well as by intermittent, millennial-scale Dansgaard–Oeschger climate oscillations. However, prior to the last deglaciation, the responses of North American vegetation and biomass burning to these climate variations are uncertain. Ammonium in Greenland ice cores, a product from North American soil emissions and biomass burning events, can help to fill this gap. Here we use continuous, high-resolution measurements of ammonium concentrations between 110,000 to 10,000 years ago from the Greenland NGRIP and GRIP ice cores to reconstruct North American wildfire activity and soil ammonium emissions. We find that on orbital timescales soil emissions increased under warmer climate conditions when vegetation expanded northwards into previously ice-covered areas. For millennial-scale interstadial warm periods during Marine Isotope Stage 3, the fire recurrence rate increased in parallel to the rapid warmings, whereas soil emissions rose more slowly, reflecting slow ice shrinkage and delayed ecosystem changes. We conclude that sudden warming events had little impact on soil ammonium emissions and ammonium transport to Greenland, but did result in a substantial increase in the frequency of North American wildfires.
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Records of ice-core nitrate and its isotopes hold the potential to assess past atmospheric conditions regarding NOx and oxidant levels. However, relating such records to past atmospheric conditions requires a site-specific understanding of the post-depositional processing of snow nitrate. We report δ15N(NO3-) records from the Greenland Ice Sheet Project 2 (GISP2) ice core over major climate transitions. Model calculations and comparison with records of parameters influencing UV-driven post-depositional processing of snow nitrate suggest that the observed variability in GISP2 δ15N(NO3-) over major climate transitions is primarily driven by changes in the degree of post-depositional loss of snow nitrate. Estimates of the fractional loss of snow nitrate is (16 - 23) % in the Holocene and (45 - 53) % in the glacial period, suggesting a (41 ± 32) % lower nitrate depositional flux to Greenland during the glacial period relative to the Holocene.
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HYSPLIT, developed by NOAA’s Air Resources Laboratory, is one of the most widely used models for atmospheric trajectory and dispersion calculations. We present the model’s historical evolution over the last 30 years from simple hand drawn back trajectories to very sophisticated computations of transport, mixing, chemical transformation, and deposition of pollutants and hazardous materials. We highlight recent applications of the HYSPLIT modeling system, including the simulation of atmospheric tracer release experiments, radionuclides, smoke originated from wild fires, volcanic ash, mercury, and wind-blown dust.
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Southern Greenland is characterized by a number of low-level high wind speed weather systems that are the result of topographic flow distortion. These systems include barrier winds and katabatic flow that occur along its southeast coast. Global atmospheric reanalyses have proven to be important tools in furthering our understanding of these orographic winds and their role in the climate system. However, there is evidence that the mesoscale characteristics of these systems may be missed in these global products. Here we show that the Arctic System Reanalysis, a higher resolution regional reanalysis, is able to capture mesoscale features of barrier winds and katabatic flow that are missed or under-represented in ERA-I, a leading modern global reanalysis. This suggests that our understanding of the impact of these wind systems on the coupled climate system can be enhanced through the use of higher resolution regional reanalyses or model data.
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We quantify source contributions to springtime (April 2008) surface black carbon (BC) in the Arctic by interpreting surface observations of BC at five receptor sites (Denali, Barrow, Alert, Zeppelin, and Summit) using a global chemical transport model (GEOS-Chem) and its adjoint. Contributions to BC at Barrow, Alert, and Zeppelin are dominated by Asian anthropogenic sources (40–43 %) before 18 April and by Siberian open biomass burning emissions (29–41 %) afterward. In contrast, Summit, a mostly free tropospheric site, has predominantly an Asian anthropogenic source contribution (24–68 %, with an average of 45 %). We compute the adjoint sensitivity of BC concentrations at the five sites during a pollution episode (20–25 April) to global emissions from 1 March to 25 April. The associated contributions are the combined results of these sensitivities and BC emissions. Local and regional anthropogenic sources in Alaska are the largest anthropogenic sources of BC at Denali (63 % of total anthropogenic contributions), and natural gas flaring emissions in the western extreme north of Russia (WENR) are the largest anthropogenic sources of BC at Zeppelin (26 %) and Alert (13 %). We find that long-range transport of emissions from Beijing–Tianjin–Hebei (also known as Jing–Jin–Ji), the biggest urbanized region in northern China, contribute significantly (∼ 10 %) to surface BC across the Arctic. On average, it takes ∼ 12 days for Asian anthropogenic emissions and Siberian biomass burning emissions to reach the Arctic lower troposphere, supporting earlier studies. Natural gas flaring emissions from the WENR reach Zeppelin in about a week. We find that episodic transport events dominate BC at Denali (87 %), a site outside the Arctic front, which is a strong transport barrier. The relative contribution of these events to surface BC within the polar dome is much smaller (∼ 50 % at Barrow and Zeppelin and ∼ 10 % at Alert). The large contributions from Asian anthropogenic sources are predominately in the form of chronic pollution (∼ 40 % at Barrow, 65 % at Alert, and 57 % at Zeppelin) on about a 1-month timescale. As such, it is likely that previous studies using 5- or 10-day trajectory analyses strongly underestimated the contribution from Asia to surface BC in the Arctic.
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[1] There is great interest in using nitrate (NO3−) isotopic composition in ice cores to track the history of precursor nitrogen oxides (NOx = NO + NO2) in the atmosphere. NO3−, however, can be lost from the snow by surface processes, such as photolysis back to NOx upon exposure to sunlight, making it difficult to interpret records of NO3− as a tracer of atmospheric NOx loading. In a campaign consisting of two field seasons (May–June) at Summit, Greenland, high temporal frequency surface snow samples were collected and analyzed for the oxygen isotopic composition of NO3−. The strong, linear relationship observed between the oxygen isotopes of NO3−, in both 2010 and 2011, is difficult to explain in the presence of significant postdepositional processing of NO3−, unless several unrelated variables change in concert. Therefore, the isotopic signature of NO3− in the snow at Summit is most feasibly explained as preserved atmospheric NO3− deposition.
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Atmospheric nitrate is preserved in Antarctic snow firn and ice. However, at low snow accumulation sites, post-depositional processes induced by sunlight obscure its interpretation. The goal of these studies (see also Paper I by Meusinger et al. ["Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry," J. Chem. Phys. 140, 244305 (2014)]) is to characterize nitrate photochemistry and improve the interpretation of the nitrate ice core record. Naturally occurring stable isotopes in nitrate ((15)N, (17)O, and (18)O) provide additional information concerning post-depositional processes. Here, we present results from studies of the wavelength-dependent isotope effects from photolysis of nitrate in a matrix of natural snow. Snow from Dome C, Antarctica was irradiated in selected wavelength regions using a Xe UV lamp and filters. The irradiated snow was sampled and analyzed for nitrate concentration and isotopic composition (δ(15)N, δ(18)O, and Δ(17)O). From these measurements an average photolytic isotopic fractionation of (15)ɛ = (-15 ± 1.2)‰ was found for broadband Xe lamp photolysis. These results are due in part to excitation of the intense absorption band of nitrate around 200 nm in addition to the weaker band centered at 305 nm followed by photodissociation. An experiment with a filter blocking wavelengths shorter than 320 nm, approximating the actinic flux spectrum at Dome C, yielded a photolytic isotopic fractionation of (15)ɛ = (-47.9 ± 6.8)‰, in good agreement with fractionations determined by previous studies for the East Antarctic Plateau which range from -40 to -74.3‰. We describe a new semi-empirical zero point energy shift model used to derive the absorption cross sections of (14)NO3 (-) and (15)NO3 (-) in snow at a chosen temperature. The nitrogen isotopic fractionations obtained by applying this model under the experimental temperature as well as considering the shift in width and center well reproduced the values obtained in the laboratory study. These cross sections can be used in isotopic models to reproduce the stable isotopic composition of nitrate found in Antarctic snow profiles.
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Radiative forcing by aerosols and tropospheric ozone could play a significant role in recent Arctic warming. These species are in general poorly accounted for in climate models. We use the GEOS-Chem global chemical transport model to construct a 3-D representation of Arctic aerosols and ozone that is consistent with observations and can be used in climate simulations. We focus on 2008, when extensive observations were made from different platforms as part of the International Polar Year. Comparison to aircraft (ARCTAS), surface, and ship cruise (ICEALOT, ASCOS) observations suggests that GEOS-Chem provides in general a successful year-round simulation of Arctic black carbon (BC), organic carbon (OC), sulfate, and dust aerosol. BC has major fuel combustion and boreal fire sources, OC is mainly from fires, sulfate has a mix of anthropogenic and natural sources, and dust is mostly from the Sahara. The model is successful in simulating aerosol optical depth (AOD) observations from AERONET stations in the Arctic; the sharp drop from spring to summer appears driven in part by the smaller size of sulfate aerosol in summer. The anthropogenic contribution to Arctic AOD is a factor of 4 larger in spring than summer and is mainly sulfate. Simulation of absorbing aerosol optical depth (AAOD) indicates that non-BC aerosol (OC and dust) contributed 24% of Arctic AAOD at 550 nm and 37% of absorbing mass deposited to the snow pack in 2008. Open fires contributed half of AAOD at 550 nm and half of deposition to the snowpack.
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Biomass burning is a major source of greenhouse gases and influences regional to global climate. Pre-industrial fire-history records from black carbon, charcoal and other proxies provide baseline estimates of biomass burning at local to global scales spanning millennia, and are thus useful to examine the role of fire in the carbon cycle and climate system. Here we use the specific biomarker levoglucosan together with black carbon and ammonium concentrations from the North Greenland Eemian (NEEM) ice cores (77.49° N, 51.2° W; 2480 m a.s.l) over the past 2000 years to infer changes in boreal fire activity. Increases in boreal fire activity over the periods 1000–1300 CE and decreases during 700–900 CE coincide with high-latitude NH temperature changes. Levoglucosan concentrations in the NEEM ice cores peak between 1500 and 1700 CE, and most levoglucosan spikes coincide with the most extensive central and northern Asian droughts of the past millennium. Many of these multi-annual droughts are caused by Asian monsoon failures, thus suggesting a connection between low- and high-latitude climate processes. North America is a primary source of biomass burning aerosols due to its relative proximity to the Greenland Ice Cap. During major fire events, however, isotopic analyses of dust, back trajectories and links with levoglucosan peaks and regional drought reconstructions suggest that Siberia is also an important source of pyrogenic aerosols to Greenland.
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Volcanic eruptions are an important cause of natural climate variability. In order to improve the accuracy of climate models, precise dating and magnitude of the climatic effects of past volcanism are necessary. Here we present a 2000-yr record of Southern Hemisphere volcanism recorded in ice cores from the high accumulation Law Dome site, East Antarctica. The ice cores were analysed for a suite of chemistry signals and are independently dated via annual layer counting, with 11 ambiguous years at 23 BCE, which has presently the lowest error of all published long Antarctic ice cores. Independently dated records are important to avoid circular dating where volcanic signatures are assigned a date from some external information rather than using the date it is found in the ice core. Forty-five volcanic events have been identified using the sulphate chemistry of the Law Dome record. The low dating error and comparison with the NGRIP (North Greenland Ice Core Project) volcanic records (on the GICC05 timescale) suggest Law Dome is the most accurately dated Antarctic volcanic dataset, which will improve the dating of individual volcanic events and potentially allow better correlation between ice core records, leading to improvements in global volcanic forcing datasets. One of the most important volcanic events of the last two millennia is the large 1450s CE event, usually assigned to the eruption of Kuwae, Vanuatu. In this study, we review the evidence surrounding the presently accepted date for this event, and make the case that two separate eruptions have caused confusion in the assignment of this event. Volcanic sulphate deposition estimates are important for modelling the climatic response to eruptions. The largest volcanic sulphate events in our record are dated at 1458 CE (Kuwae?, Vanuatu), 1257 and 422 CE (unidentified).
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We examine the formation of nitrate and ammonium on five types of externally mixed pre-existing aerosols using the hybrid dynamic method in a global chemistry transport model. The model developed here predicts a similar spatial pattern of total aerosol nitrate and ammonium to that of several pioneering studies, but separates the effects of nitrate and ammonium on pure sulfate, biomass burning, fossil fuel, dust and sea salt aerosols. Nitrate and ammonium boost the scattering efficiency of sulfate and organic matter but lower the extinction of sea salt particles since the hygroscopicity of a mixed nitrate-ammonium-sea salt particle is less than that of pure sea salt. The direct anthropogenic forcing of particulate nitrate and ammonium at the top of the atmosphere (TOA) is estimated to be -0.12 W m-2. Nitrate, ammonium and nitric acid gas also affect aerosol activation and the reflectivity of clouds. The first aerosol indirect forcing by anthropogenic nitrate (gas plus aerosol) and ammonium is estimated to be -0.09 W m-2 at the TOA, almost all of which is due to condensation of nitric acid gas onto growing droplets (-0.08 W m-2).
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Here we report the measurement of the comprehensive isotopic composition (δ15N, Δ17O and δ18O) of nitrate at the air-snow interface at Dome C, Antarctica (DC, 75° 06' S, 123° 19' E) and in snow pits along a transect across the East Antarctic Ice Sheet (EAIS) between 66° S and 78° S. For each of the East Antarctic snow pits in most of which nitrate loss is observed, we derive apparent fractionation constants associated with this loss as well as asymptotic values of nitrate concentration and isotopic ratios below the photic zone. Nitrate collected from snow pits on the plateau have average apparent fractionation constants of (-59±10)‰, (+2.0±1.0)‰ and (+8.7±2.4)‰, for δ15N, Δ17O and δ18O, respectively. In contrast, snow pits sampled on the coast show distinct isotopic signatures with average apparent fractionation constants of (-16±14)‰, (-0.2±1.5)‰ and (+3.1±5.8)‰, for δ15N, Δ17O and δ18O, respectively. From a lab experiment carried out at DC in parallel to the field investigations, we find that the 15N/14N fractionation associated with the physical release of nitrate is (-8.5±2.5)‰, a value significantly different from the modelled estimate previously found for photolysis (-48‰, Frey et al., 2009) when assuming a Rayleigh-type process. Our observations corroborate that photolysis is the dominant nitrate loss process on the East Antarctic Plateau, while on the coast the loss is less pronounced and could involve both physical release and photochemical processes. Year-round isotopic measurements at DC show a close relationship between the Δ17O of atmospheric nitrate and Δ17O of nitrate in skin layer snow, suggesting a photolytically-driven isotopic equilibrium imposed by nitrate recycling at this interface. The 3-4 weeks shift observed for nitrate concentration in these two compartments may be explained by the different sizes of the nitrate reservoirs and by deposition from the atmosphere to the snow. Atmospheric nitrate deposition may lead to fractionation of the nitrogen isotopes and explain the almost constant shift on the order of 25‰ between the δ15N values in the atmospheric and skin layer nitrate at DC. Asymptotic δ15N(NO3-) values and the inverse of snow accumuation rates are correlated (ln(δ15N(as.) + 1) = (5.76±0.47) · (kg m-2 a-1/A) + (0.01±0.02)) confirming the strong relationship between the snow accumulation rate on the residence time of nitrate in the photic zone and the degree of isotopic fractionation, consistent with with previous observations by Freyer et al. (1996). Asymptotic Δ17O(NO3-) values on the plateau are smaller compared to the values found in the skin layer most likely due to oxygen isotope exchange between the nitrate photo-products and water molecules from the surrounding ice. However, the overall fractionation in Δ17O is small thus allowing the preservation of an atmospheric signal.
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Widespread retreat of glaciers has been observed along the southeastern margin of Greenland. This retreat has been associated with increased air and ocean temperatures. However, most observations are from the satellite era; presatellite observations of Greenlandic glaciers are rare. Here we present a unique record that documents the frontal positions for 132 southeast Greenlandic glaciers from rediscovered historical aerial imagery beginning in the early 1930s. We combine the historical aerial images with both early and modern satellite imagery to extract frontal variations of marine- and land-terminating outlet glaciers, as well as local glaciers and ice caps, over the past 80 years. The images reveal a regional response to external forcing regardless of glacier type, terminal environment and size. Furthermore, the recent retreat was matched in its vigour during a period of warming in the 1930s with comparable increases in air temperature. We show that many land-terminating glaciers underwent a more rapid retreat in the 1930s than in the 2000s, whereas marine-terminating glaciers retreated more rapidly during the recent warming.
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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 (tau(wind) (pumping)) from the snowpack, and compare this to the calculated lifetime of NOx against chemical conversion to HNO3 (tau(chemical)) to determine whether the NOx produced at a given depth can escape from the snowpack to the overlying atmosphere. Comparison of tau(wind) (pumping) and tau(chemical) 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 4.4x10(8)-3.8x10(9) molecules cm(-2) s(-1) in remote polar locations and 3.2-8.2x10(8) 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 under-estimating the amount of NOx emitted from the clean polar snowpack.
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European scale harmonized monitoring of atmospheric composition was initiated in the early 1970s, and the activity has generated a comprehensive dataset (available at http://www.emep.int) which allows the evaluation of regional and spatial trends of air pollution during a period of nearly 40 yr. Results from the monitoring made within EMEP, the European Monitoring and Evaluation Programme, show large reductions in ambient concentrations and deposition of sulphur species during the last decades. Reductions are in the order of 70-90% since the year 1980, and correspond well with reported emission changes. Also reduction in emissions of nitrogen oxides (NOx) are reflected in the measurements, with an average decrease of nitrogen dioxide and nitrate in precipitation by about 23% and 25% respectively since 1990. Only minor reductions are however seen since the late 1990s. The concentrations of total nitrate in air have decreased on average only by 8% since 1990, and fewer sites show a significant trend. A majority of the EMEP sites show a decreasing trend in reduced nitrogen both in air and precipitation on the order of 25% since 1990. Deposition of base cations has decreased during the past 30 yr, and the pH in precipitation has increased across Europe. Large inter annual variations in the particulate matter mass concentrations reflect meteorological variability, but still there is a relatively clear overall decrease at several sites during the last decade. With few observations going back to the 1990s, the observed chemical composition is applied to document a change in particulate matter (PM) mass even since 1980. These data indicate an overall reduction of about 5 μg m−3 from sulphate alone. Despite the significant reductions in sulphur emissions, sulphate still remains one of the single most important compounds contributing to regional scale aerosol mass concentration. Long-term ozone trends at EMEP sites show a mixed pattern. The year-to-year variability in ozone due to varying meteorological conditions is substantial, making it hard to separate the trends caused by emission change from other effects. For the Nordic countries the data indicate a reduced occurrence of very low concentrations. The most pronounced change in the frequency distribution is seen at sites in the UK and the Netherlands, showing a reduction in the higher values. Smaller changes are seen in Germany, while in Switzerland and Austria, no change is seen in the frequency distribution of ozone. The lack of long-term data series is a major obstacle for studying trends in volatile organic compounds (VOC). The scatter in the data is large, and significant changes are only found for certain components and stations. Concentrations of the heavy metals lead and cadmium have decreased in both air and precipitation during the last 20 yr, with reductions in the order of 80–90% for Pb and 64–84% for Cd (precipitation and air respectively). The measurements of total gaseous mercury indicate a dramatic decrease in concentrations during 1980 to about 1993. Trends in hexachlorocyclohexanes (HCHs) show a significant decrease in annual average air concentrations. For other persistent organic pollutants (POPs) the patterns is mixed, and differs between sites and between measurements in air versus precipitation.
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ERA-40 is a re-analysis of meteorological observations from September 1957 to August 2002 produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) in collaboration with many institutions. The observing system changed considerably over this re-analysis period, with assimilable data provided by a succession of satellite-borne instruments from the 1970s onwards, supplemented by increasing numbers of observations from aircraft, ocean-buoys and other surface platforms, but with a declining number of radiosonde ascents since the late 1980s. The observations used in ERA-40 were accumulated from many sources. The first part of this paper describes the data acquisition and the principal changes in data type and coverage over the period. It also describes the data assimilation system used for ERA-40. This benefited from many of the changes introduced into operational forecasting since the mid-1990s, when the systems used for the 15-year ECMWF re-analysis (ERA-15) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis were implemented. Several of the improvements are discussed. General aspects of the production of the analyses are also summarized.
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ABSTRACT Annual and monthly,snow,accumulation,for the Greenland,Ice Sheet was derived from ECMWF fore- casts [mainly 40-yr ECMWR Re-Analysis (ERA-40)] and further meteorological modeling. Modeled,ac- cumulation,was validated using 58 ice core accumulation,datasets across the ice sheet and was found to be 95% of the observed accumulation on average, with a mean correlation of 0.53 between modeled and observed. Many of the ice core datasets are new,and are presented,here for the first time. Central and northern interior parts of the ice sheet were found to be 10%–30% too dry in ERA-40, in line with earlier ECMWF analysis, although too much (50% locally) snow accumulation was modeled for interior southern parts of Greenland. Nevertheless, 47 of 58 sites show significant correlation in temporal variability of modeled,with observed,accumulation. The model,also captures the absolute amount,of snow,accumulation at several sites, most notably Das1 and Das2 in southeast Greenland. Mean modeled accumulation over the ice sheet was 0.279 (standard deviation 0.034) m yr, for 1958–2003 with no significant trend for either the ice sheet or any of the core sites. Unusually high accumulation,in southeast Greenland in 2002/03 leads the authors to study meteorological,synoptic forcing patterns and comment,on the prospect of enhanced,climate variability leading to more,such events as a result of global warming. There is good,agreement,between precipitation measured,at coastal meteorological,stations in southern Greenland,and accumulation,modeled for adjacent regions of the ice sheet. There is no significant persistent relation between,the North Atlantic Oscillation index and whole,or southern,Greenland,accumulation.
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Recent studies have shown that the spring dust storm frequency (DSF) in northern China exhibits an obvious downward trend over the past 50 years concurrently with the recent global warming. We found that the decline of DSF is significantly correlated with the increase of the surface air temperature (SAT) in the region of 70°E-130°E, 45°N-65°N around Lake Baikal, where anthropogenic forcing induces prominent warming in the recent decades. Corresponding to the SAT rise in this region, an anomalous dipole circulation pattern is found in the troposphere that consists of a warm anti-cyclone centered at 55°N and a cold cyclone centered around 30°N. The DSF is positively correlated to the activity of Mongolian cyclones. The warming trend around Lake Baikal possibly induces a weakening of the westerly jet stream and the atmospheric baroclinicity in northern China and Mongolian regions, which suppress the frequency of occurrence and the intensity of the Mongolian cyclones and result in the decreasing DSF in North China. This mechanism will likely further reduce the spring DSF in the future global warming scenario.
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NH4+ to SO4−− ratio of aerosols and snows are investigated in Greenland. Data suggest that the atmospheric NH4+ and SO4−− signals are well preserved in snow and that previous discrepancies sometimes observed between the composition of the air and that of the snow were probably due to NH4+ artifacts. This study leads to the conclusion that NH3 is not able to neutralize the acidity of the high latitude atmosphere in particular in the Southern Hemisphere.
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Mineralogical and isotopic composition (Sr and Nd) of six dust samples, obtained from six widely spread ice-coring sites in Greenland, were analyzed in order to investigate the regional geographic variability of dust provenance. We show that long-range transport from eastern Asian deserts provides mineral dust with essentially the same composition to all elevated interior sites (Dye 3, Site A, GRIP, and NorthGRIP), while most material deposited at sites located closer to the edge and at lower altitude (Hans Tausen and Renland) derives from proximal source regions. No contribution from other sources is apparent at any of the interior sites from the mineralogical and isotopic composition of the dust samples, each of which represents several decades of dust deposition during the 17th–18th century. These results provide additional evidence that African and North American deserts do not play a significant role in the dust deposited over Greenland, which has implications for ice core record interpretation and atmospheric dust transport model validation.
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By chemically analyzing snow samples at Dome Fuji, Antarctica, we found that the snowdrift deposited just after snowfall and the snow layer down to 3.4 m have summer minima in the non-sea-salt (nss)-SO42− and Na+ concentrations and summer maxima of Cl−/Na+. Such a summer nss-SO42− minimum in either snowdrift or the snow layer has not been reported at any other site in Antarctica and arises in spite of the known maximum in the nss-SO42− concentration in aerosol at Dome Fuji in summer. We then did laboratory experiments to better understand the phenomenon. The results supported the following mechanism for the summer nss-SO42− minimum in the snowdrift and snow layer. In summer, water vapor sublimates from within the snow in the daytime and condenses on the surface as frost in the nighttime, resulting in a dilution of the nss-SO42− concentration. This sublimation-condensation process likely occurs at other cold inland regions. In addition, the results might be useful for obtaining a high-resolution dating method for Dome Fuji deep ice cores by counting the number of layers with low nss-SO42− concentration.
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Massive post-depositional processes alter the nitrate concentration in polar firn where the annual snow accumulation is low. This hinders a direct atmospheric interpretation of the ice core nitrate record. Fractionation of nitrate isotopes during post-depositional nitrate loss may allow estimating the amount of nitrate loss in the past. We measured δ15N of nitrate in two Antarctic surface cores from the Dome C area. In concert with the known concentration decrease with depth we observe an increase in the isotopic signature. Assuming a Rayleigh type process we find an isotope effect of $\varepsilon$ = −54‰. We measured the fractionation factor for photolysis in the laboratory and obtained $\varepsilon$ = −11.7 ± 1.4‰. As the observed fractionation factor in the firn is much lower this rules out that photolysis in the surface snow is the main process leading to the dramatic nitrate loss in the top centimeters of the firn.
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Weekly measurements of surface height change were made at an accumulation forest of 100 stakes at Summit, Greenland, over a 2-year period (17 August 2000 to 8 August 2002). On average, the surface height relative to the stakes increased 64 (±4.8) cm in the first year and 65 (±5.3) cm in the second, identical to the average (65 ± 4.5 cm yr-1) previously reported for the period 1991-1995 in a similar forest 28 km to the southwest. The continuous 2-year data set indicates that the rate of surface rise was not constant, with the summers of 2001 and 2002 both showing markedly slower increases. On-site weather observations suggest that more new snow fell during the summer months than in any other season, consistent with results from previous snow pit and modeling studies yet apparently at odds with the slow rate of height increase. Density profiles from a series of 1-m-deep snow pits sampled monthly reveal that the thickness of the most recent year of accumulated snow (25 cm water equivalent) decreased rapidly between late May and early July, and the layers remained thin through early September. The thinning of the top year is clearly due to compaction in the snowpack. Combining the observed variations in annual layer thickness with a linear height increase based on assumed constant accumulation at 0.18 cm d-1 explains much of the variation in surface height found in the stake measurements. Estimated surface height changes can be forced to exactly match the stake measurements by combining changes in annual layer thickness with a variable accumulation rate over the intervals between pits. This exercise suggests that during the 2 years of this study a consistent seasonal pattern in accumulation was not apparent, rather the intervals indicated to have had enhanced accumulation in the first year (August-October and March-April) apparently had reduced accumulation in the second year.
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Sulfur isotopes of sulfate have been measured in a discontinuous set of polar ice core samples from Summit, central Greenland, covering the preindustrial (from the fourteenth to the eighteenth century) and industrial (from 1872 to 1969 A.D.) periods. Results have been used to estimate the different source contributions to the deposited sulfate and their evolution along the last centuries. They indicate that the preindustrial background sulfate budget is slightly dominated on a year-round average by marine biogenic emissions, amounting to close to half of the non-sea-salt sulfate (49%). The second contribution is provided by continental sources of secondary sulfate, including background volcanism and, to a lesser extent, continental biota (44% of the non-sea-salt sulfate). Sulfur emitted by relatively weak eruptions is found to be largely depleted in 34S compared to bulk volcanic S, suggesting an efficient washout of the heavier isotope during the tropospheric transport. The impact of human-driven emissions on the sulfate deposited in central Greenland ice is visible in isotope data as early as 1870 A.D. The isotopic signature of anthropogenic sulfur deposited during the twentieth century is found to be constant (delta34S ~ +3.0 +/- 1.50/00), regardless of the changes of dominant source regions and emission processes. This signature is slightly but measurably lighter than the one reported for Arctic haze pollution events.
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Significant natural artifacts in ice chemical records have been pointed out in recent preliminary glaciochemical studies carried out in central Antarctic areas with very low snow accumulation rates (generally less than 5 gcm-2yr-1). Several deep drilling operations are underway in these regions for long-term paleoclimatic reconstructions. A detailed glaciochemical study has been carried out at Vostok Station in order to investigate post deposition changes of ion concentrations in the snow and firn layers. The results show that, in general, concentration profiles of species such as Cl, F, and NO3, partly deposited as gases, exhibit a rapid decrease in the first few meters, indicating that a fraction, sometimes major, of these compounds is expelled back in the atmosphere after deposition. Some redeposition process of the gases is likely in the upper firn layers. Surprisingly, a similar effect is found for methanesulfonate (MS), suggesting that this compound could have a gaseous component in central Antarctic regions. The data also show that Cl, F, NO3, and MSA may be slowly but significantly displaced in the firn layers by high sulfuric acid levels of volcanic origin. The drastic changes observed in the surface snow layers may severely question current interpretations of certain chemical data recovered in these areas and point out an urgent need for new field and laboratory experiments on the air-to-ice transfer processes prevailing under central Antarctic conditions.
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Intercomparison of three new chemical ice core records from northern Greenland (covering the time span from approximately 1500 A.D. to present) with previously published records for southern and central Greenland reveals a uniform timing of anthropogenic changes in sulfate and nitrate firn concentrations over the entire ice sheet. The anthropogenic sulfate increase started around 1890, was interrupted by a transient decrease in the 1930s, and has resumed a major increase since 1950. Since the late 1970s though, a significant 30% decline in Greenland sulfate firn levels can be documented. The maximum anthropogenic increase in northern Greenland sulfate firn concentrations (up to 200-230ppb) is 2-3 times larger than in southern and central Greenland. Nitrate records show an essentially steady increase since 1950 and, documented for the first time, a slight reduction during most recent years. Maximum nitrate firn levels of 100-130ppb exceed the preindustrial background by 100% all over the Greenland ice sheet. Comparison with anthropogenic SO2 and NOx emission records indicates that the major increase in sulfate firn concentrations since 1950 can be attributed to Eurasian sources, while firn levels during the first half of this century appear to be dominated by North American emissions. A stronger North American source contribution is indicated over the entire 20th century in the case of nitrate. Applying a macroscopic deposition model separate time series for wet and dry deposition were derived which revealed a close correspondence of wet deposited sulfate with the timing of U.S. emissions, while the temporal evolution of Eurasian emissions is mainly reflected in the dry sulfate deposition record. During this century wet sulfate deposition increased by a factor of two while the total dry sulfate deposition flux increased by more than 500%. Wet and dry nitrate deposition both increased by 100% during the same period.
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The Greenland Ice Sheet Project 2 (GISP2) and Greenland Ice Core Project (GRIP) deep drilling programs at Summit, Greenland included support (both logistical and scientific) of extensive investigation of atmospheric transport and air-snow exchange processes of gases and particles relevant to the interpretation of the ice-core records. Much of the sampling for the air-snow exchange investigations was conducted at a unique solar-powered camp 30 km southwest of the GISP2 drill camp (even further from the GRIP camp) and was characterized by a high degree of international collaboration and cooperation. The wide range of expertise and analytical capabilities of the 20-plus investigators participating in these studies has provided important insight into the meteorological, physical, and chemical processes which interact to determine the composition of snow and firn at Summit. Evolving understanding of this system will allow improved reconstruction of the composition of the atmosphere over Greenland in the past from the detailed Summit ice-core records. This paper provides an overview of air-snow exchange investigations at Summit, including their development through the course of the drilling programs (1989-1993), significant findings related to both air-snow exchange issues and the present state of the Arctic free troposphere, as well as the major outstanding questions which are being addressed in ongoing experiments at Summit.
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A 2083 m deep ice core from Vostok Station (East Antarctica) has been used for a comprehensive study of all major ions (i.e. Na+, NH4+, K+, Ca2+, Mg2+, H+, Cl−, NO3− and SO42− originating from aerosols deposited over the last climatic cycle (160,000 a), as depicted from the isotopic composition of the ice.For the first time in deep ice core studies, a good balance between anions and cations is obtained throughout the profile. This allows the clear identification of marine salts (i.e. sea salt and Na2SO4), terrestrial salts (calcium and magnesium associated with nitrates and sulfates) and strong mineral acids (HNO3, H2SO4 and HCl).Concentration profiles confirm that both marine and terrestrial aerosol inputs were higher during cold climatic conditions (from 110 to 15 ka B.P.) than during the Last Interglacial (centered around 130 ka B.P.) and the Holocene (the last 10,000 a) stages. High concentration peaks (up to 5 and 30 times the Holocene values of marine and terrestrial contents, respectively) are in particular observed during the very cold climate characterizing the end of the penultimate glacial age (⋍ 160 ka B.P.) and the Last Glacial Maximum which terminated around 15 ka B.P. These peaks reflect strengthened sources and transport during full glacial conditions, linked to higher wind speeds, more extensive arid areas on the continents and the greater exposure of continental shelves. More generally, marine and terrestrial aerosol concentrations measured in ice are strongly affected by climatic conditions of global (source strength and atmospheric transport efficiency) and local (rate of snow accumulation) concern.As opposed to marine and terrestrial inputs, acidic gas-derived impurity concentrations (HNO3, H2SO4) remain relatively stable over the whole climatic cycle. In particular there is no correlation between observed H2SO4 fluctuations and the isotope-temperature profile. This would indicate the absence of a long-term relationship between volcanism and climate.The mineral acid contribution represents a large part (over 50%) of ice impurities deposited during interglacial periods. For glacial ice the contribution of marine and terrestrial salts becomes preponderant (up to 75% of total soluble impurities).During interglacial stages and the relatively warm periods of the Last Glacial Age, significant quantities of either Na2SO4 or HCl are found, possibly resulting from marine aerosol alteration during atmospheric transport from sea sources towards Antarctica. On the other hand, the Cl/Nam ratio values indicate the presence of non-fractionated marine aerosols during full glacial conditions, confirming faster transport from sea sources towards Antarctica.
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Daily samples of the surface snow at Summit, Greenland were collected from June 1997 to April 1998 and then from August 2000 to August 2002. Concentrations of nine soluble ions (only eight in the first year) were determined in order to assess the validity of seasonal variations in snow composition at this site inferred from earlier snowpit and core studies. Strong and consistently sharp spring (April) peaks in dust, and broader summer (June–August) enhancements of NH4+ and excess Cl- in the surface snow fully support the timing of these signals inferred from the pit profiles. Sea-salt reached maximum concentrations in the surface snow in late winter (February–March), based on averaging all three years of monthly means, but showed different patterns in winter each of the years. This is also consistent with the variable late-winter to spring timing inferred from pits. Simulated snowpit profiles constructed from the surface snow samples compared well with the ion profiles recovered from well dated snowpits sampled in this investigation, suggesting that early postdepositional changes do not greatly impact the glaciochemical records preserved at Summit. Nitrate in the real snowpits was approximately 25% lower than in simulated pits, this was the worst agreement for any ion but is consistent with several processes being known to deplete NO3- from near-surface snow.