No Job Name Seasonal variation and source areas of airborne lead-210 at Ny-Ålesund in the High Arcticpor_185 345..352 Jussi Paatero,1 Murat Buyukay,1 Kim Holmén,2 Juha Hatakka1 & Yrjö Viisanen1 1 Finnish Meteorological Institute, PO Box 503, FI-00101 Helsinki, Finland 2 Norwegian Polar Institute, Fram Centre, NO-9296 Tromsø, Norway Abstract High-volume aerosol particle samples were collected onto glass-fibre filters at Mount Zeppelin Global Atmosphere Watch station, Ny-Ålesund, Svalbard, in 2001–05. The filters were assayed for lead-210 (210Pb) by measuring the alpha particles of its in-grown daughter nuclide polonium-210 (210Po). The observed 210Pb activity concentrations at Mount Zeppelin vary between <4 and 1060 mBq m-3, with an arithmetic mean of 130 mBq m-3 and a median of 74 mBq m-3. The lowest 210Pb activity concentrations are found during summer and the highest are found in winter. This variation is caused by seasonal differences in the mixing conditions of the troposphere, the level of precipita- tion and the speed of atmospheric chemistry induced by solar radiation. The performed source area analysis, which is based on air mass back trajectories, indicated that in summer, 210Pb can be used as a tracer for air masses coming into contact with land areas within the past 5 days. In winter this cannot be performed because of the accumulation of 210Pb-carrying aerosol particles into the Arctic atmosphere during the Arctic night. But even in winter a low 210Pb activity concentration indicates that the associated air mass has had little if any contact with land areas. Keywords High Arctic; lead-210; Spitsbergen; trajectory analysis; troposphere. Correspondence Jussi Paatero, Finnish Meteorological Institute, PO Box 503, FI-00101 Helsinki, Finland. E-mail: jussi.paatero@fmi.fi doi:10.1111/j.1751-8369.2010.00185.x The Arctic region is experiencing environmental changes as a result of climate change and pressures by direct human activities, including air pollutants, as presented in the environmental assessment reports by the Arctic Climate Impact Assessment (Symon et al. 2005) and the Arctic Monitoring and Assessment Programme (AMAP 2002). The ice cover of the Arctic Ocean has decreased by 7% per decade during the last 30 years. This phenom- enon has a huge impact on the albedo, and thus the atmospheric radiation balance in the area. In the conti- nental Arctic regions the depth of the permafrost can extend to several hundreds of metres. During the summer season the uppermost 0.3–2.5 m of the soil melts. The depth of this active layer is increasing by almost 1 cm per year owing to the warming climate. The melting of the permafrost will cause dramatic changes to the environment. For example, lakes can dry out, bogs producing peat can turn into ponds, and methane and carbon dioxide fluxes between the soil and the atmo- sphere can increase. All these phenomena can change the large-scale atmospheric flow patterns affecting the transport of airborne pollutants, including radionuclides, to the Arctic. Lead-210 (210Pb) is formed in the atmosphere from the radioactive noble gas radon-222 (222Rn) emanating from the Earth’s crust. Of the airborne 222Rn, 99% originates from land and only 1% originates from the sea (Baskaran et al. 1993). In the air, 222Rn, being a noble gas, exists as single atoms. Its progeny, including lead-210, are, however, heavy metal atoms, and are rapidly attached to ambient aerosol particles. Most of the airborne 210Pb activity is attached to accumulation-mode aerosol par- ticles with an aerodynamic diameter of a few hundred nanometres (Sanak et al. 1981; Papastefanou & Bondietti 1991; Suzuki et al. 1996; Suzuki et al. 1999). Owing to its long half-life (22 years), 210Pb is removed from the atmo- sphere by the various scavenging processes, especially wet deposition, of aerosol particles carrying it rather than its radioactive decay. The amount of 210Pb in the air is not affected by anthropogenic activities (Hötzl & Winkler 1987). Mean aerosol residence times can be cal- culated from the activity ratio of 210Pb and its progeny, Polar Research 29 2010 345–352 © 2010 the authors, journal compilation © 2010 Blackwell Publishing Ltd 345 mailto:paatero@fmi.fi bismuth-210 (210Bi) or polonium-210 (210Po) (Papaste- fanou & Bondietti 1991). The observed residence times vary between 0 days and over 5 weeks. Generally there is a tendency towards longer residence times with decreas- ing temperature (Mattsson 1975; Samuelsson et al. 1986; Baskaran & Shaw 2001). The practically exclusive forma- tion mechanism of airborne 210Pb facilitates its use as a tracer for air masses with a recent contact to land areas (Preiss et al. 1996). The Comprehensive Test Ban Treaty Organization gathers information about the 210Pb as well as beryllium-7 (7Be) activity content of the ground-level air around the globe as a side product to nuclear test monitoring. In this project we have measured the activity concen- tration of 210Pb in the air at the Mount Zeppelin Global Atmosphere Watch (GAW) station. The GAW programme is coordinated by the World Meteorological Organization. The GAW monitoring programme includes a co-ordinated global network of some 20 observing stations, and it provides data for scientific assessments and for early warnings of changes in the chemical composition and related physical characteristics of the atmosphere. GAW monitors, among other things, greenhouse gases, ozone and ultraviolet radiation, certain reactive gases and aerosol particles. The data obtained in this study during the 5-year period 2001–05 can be used as a tracer to help identify variations in transport behaviour of air masses, and therefore also of air pollutants, in the Arctic region. Materials and methods The sampling site was at Mount Zeppelin GAW station, Ny-Ålesund, (78°58′N, 11°53′E), on the western coast of Spitssbergen, the largest island in the Svalbard archi- pelago (Paatero & Holmen 2004). The station is located 474 m a.s.l. (Fig. 1). High-volume aerosol particle samples were collected onto glass-fibre filters (Munktell, Falun Sweden). The sampler is made of stainless steel. The flow rate was ca. 120 m3 h–1, and was measured with a pressure differ- ence gauge over a throat. Three samples per week were collected with filter changes on Mondays, Wednesdays and Fridays. One of the 25 filters was left unexposed, and was used as a field blank sample. All the filters, both samples and field blanks, were stored in cardboard boxes at room temperature (ca. 20°C) before measurement. The indoor 222Rn diffusing into the filters during storage has not been found to have a discernible effect on the 210Pb content of the filters (Paatero et al. 2005). The measurement of 210Pb content of the aerosol filters is based on the alpha counting of the in-grown daughter nuclide 210Po. The exposed filters and the field blanks were assayed for 210Pb 6 months after sampling with an automatic alpha/beta analyser. The technical details of the instrument were described in Mattsson et al. (1996). The detector arrangement consists of five gas-flow pro- portional counters. The flow gas is P-10, a mixture of argon (90%) and methane (10%). The effective window area is 625 cm2. The counter immediately above the filter has a thin plastic window and measures alpha particles. The alpha counting efficiency is 41.5%. The other four counters are for beta particle counting and for anti- coincidence background suppression. The results were handled as follows. First the alpha count rate of the field blank was subtracted from the sample alpha count rate. In most cases the field blank did not deviate from the instrumental background. Next, a fixed value of 0.5 cpm was subtracted from the alpha count rate to take into consideration the 210Po already on the filter at the end of sampling. The alpha count rate was converted to 210Po activity by dividing it with the counting efficiency. Then the 210Pb activity at the end of sampling was calculated from the delay time between the sampling Fig. 1 Location of the sampling site, Mount Zeppelin Global Atmosphere Watch station, Ny-Ålesund, Svalbard. Airborne lead-210 at Ny-Ålesund, High Arctic J. Paatero et al. Polar Research 29 2010 345–352 © 2010 the authors, journal compilation © 2010 Blackwell Publishing Ltd346 and the measurement, and from the in-grown 210Po activ- ity. Finally, the 210Pb activity was divided by the air volume of the sample to obtain an activity concentration. The method can lead to an overestimation of the 210Pb activity concentration because of the uncertainty related to the level of 210Po already present on the filter at the end of sampling. This overestimation is expected to be, even in the worst case, less than 20%. The 210Po/210Pb activity ratio is at its seasonal maximum in late winter because of the long residence time of aerosol particles in the air, allowing time for 210Po to accumulate. The effect of unsupported 210Po in the air is expected to be a rare incident related to atmospheric long-range transport from volcano eruptions or biomass burning events (Stohl et al. 2007; Paatero et al. 2009). To check the stability of the instrument, reference samples (plutonium-242 [242Pu], strontium-90 [90Sr] and iron-55 [55Fe]) are measured daily. The measurement uncertainty depends on the level of 210Pb present on the filter and counting time. Usually the 1s standard devia- tion of the radioassay varies between 5 and 10%, but can be significantly larger if the activity on the filter is very low. Results and discussion Altogether 666 aerosol filter samples were collected at the Mount Zeppelin GAW station in 2001–05. The observed 210Pb activity concentrations varied between <4 and 1060 mBq m-3. The median, arithmetic mean and geo- metric mean activity concentrations were 74, 130 and 74 mBq m-3, respectively. The 10, 25, 75 and 90% percen- tiles were 16, 33, 180 and 330 mBq m-3, respectively. Suzuki et al. (1996) reported 210Pb activity concentrations ranging from 83 to 1204 mBq m-3, and an average con- centration of 325 mBq m-3 at Ny-Ålesund in February– March 1995. Samuelsson et al. (1986) reported an average concen- tration of 75 mBq m-3 during a Swedish ice-breaker expedition in July–September 1980 between 75° and 83°N, and between Greenland and Franz Josef Land. Mysłek-Laurikainen et al. (2006) reported that the 210Pb activity concentration in the air at the Polish research station at Hornsund, some 200 km south of Ny-Ålesund, varied between 0 and 1200 mBq m-3. The values found at Svalbard are very low compared with continental areas, but are clearly higher than in Antarctic areas. For example, the average and maximum monthly 210Pb activ- ity concentrations in Belgrade, former Yugoslavia, were 1200 and 3170 mBq m-3, respectively (Todorovic et al. 2000). At Marambio research station (64°14′S, 56°43′W), close to the Antarctic Peninsula, the mean annual 210Pb activity concentration was only 4.5 mBq m-3 in 2005 (Paatero et al. 2007). The contribution of Svalbard itself as a source of air- borne 210Pb is likely to be very small because of the associated time scales. The area of the islands is only 63 000 km2, and 60% of it is covered with glaciers, with a negligible radon exhalation potential. An air mass travel- ling from east over the islands with a speed of, e.g., 4 m s–1 would spend only about 12 h above ground before arriv- ing at the west coast of Svalbard. The radon accumulated into the air would, on average, still have 5 days left before decaying into 210Pb. During these 5 days the air mass would have moved 1700 km away from Svalbard. The observed 210Pb activity concentrations present a clear seasonal variation, with highest concentrations in winter (Fig. 2; Table 1). This is attributed to the small Fig. 2 Airborne 210Pb (mBq m-3) at Mount Zeppelin Global Atmosphere Watch station, Svalbard, 2001–05. The sampler was broken from May to December 2004. Airborne lead-210 at Ny-Ålesund, High ArcticJ. Paatero et al. Polar Research 29 2010 345–352 © 2010 the authors, journal compilation © 2010 Blackwell Publishing Ltd 347 level of precipitation, reduced air chemistry and stagnant mixing conditions in the troposphere during the Arctic night. These factors increase the aerosol residence time, and thus the accumulation of 210Pb in the air. The phe- nomenon is similar to Arctic haze, the accumulation of soot and sulphate particles in the Arctic atmosphere during winter (e.g., Shaw 1983). The maximum concen- trations are only moderately lower in the High Arctic compared with Finland. For example, in northern Finland the average January to March activity concentra- tion was about 280 mBq m-3, and in southern Finland 300 mBq m-3 in 1995–97 (Paatero & Hatakka 2002). The minimum 210Pb activity concentrations occur in the High Arctic in summer, when the continuous solar radia- tion induces efficient vertical mixing of the troposphere. At the same time, the level of precipitation, which causes wet deposition and atmospheric chemistry induced by solar radiation, are at their seasonal maximum. This is in agreement with observations in Finland. For example, in northern Finland the average May to July activity con- centration was about 140 mBq m-3, and in southern Finland 200 mBq m-3 in 1995–97 (Paatero & Hatakka 2002). However, the concentrations are even lower in the High Arctic. In southern Germany the seasonal variation is much less profound than in Svalbard, even though the monthly mean concentrations in southern Germany are much higher: 400–700 mBq m-3 (Winkler & Rosner 2000). To analyse the 210Pb observations at Mount Zeppelin, a set of three-dimensional 5-day air mass back trajectories were calculated using the trajectory model FLEXTRA (Stohl & Seibert 1998). The wind fields were obtained from the European Centre for Medium-Range Weather Forecasts. The trajectories were utilized by two different methods. Figure 3 depicts the starting points of trajecto- ries coincident with the lowest 5% of the 210Pb activity concentrations: <10 mBq m-3. Five days earlier, most of the air masses with a low 210Pb content were situated over the Arctic Ocean, the North Atlantic Ocean or Greenland. Only a few starting points are over northern Europe or the coastal regions of Siberia. On the other hand, most of the trajectory starting points coincident with the highest 5% of 210Pb activity concentrations, >400 mBq m-3, are located over Siberia (Fig. 4). A number of starting points are located between the Bering Strait and the North Pole, contrary to preliminary expectations. This kind of analy- sis does not, of course, take into account curvatures that the air masses make over the oceans or continents. It is therefore not a proof for particular types of air masses, either continental or maritime. But it gives a general picture of source areas of air masses with high or low activity concentrations of 210Pb in the air. The uncertainty related to air mass trajectory calculations has been reviewed by Stohl (1998). Secondly, a statistical evaluation of the 210Pb activity concentrations in different air masses was performed. The 120 hourly locations of each trajectory arriving at Mount Zeppelin were combined with the 210Pb activity concen- trations observed during the moment of arrival. Next, the concentration–location pairs of all the chosen trajectories were averaged over cells of 1° latitude and 1° longitude. The cell in which Mount Zeppelin is situated thereby obtains a value close to the average concentration. The activity concentrations obtained in the cells should not be interpreted as absolute concentration values, but rather as concentrations relative to the average concentration. The statistical accuracy decreases with increasing distance from the sampling site, as the averaged area contains fewer and fewer concentration–location pairs. A similar method, but with certain statistical enhancements, has been used previously at the Finnish Meteorological Insti- tute to identify the sources of airborne substances, e.g., atmospheric fine particles and sulphur dioxide, 210Pb and 7Be in Finnish Lapland (Virkkula et al. 1995; Virkkula et al. 1997; Paatero & Hatakka 2000). Two subsets of trajectories and 210Pb observations were used in the sub- sequent analyses: summer months (June–September) and winter months (October–April). These periods were chosen on the basis of the seasonal variation of 210Pb activity concentration in Fig. 2. The trajectory analysis of summertime source areas of airborne 210Pb in Svalbard is presented in Fig. 5. The highest 210Pb activity concentrations are found in air masses coming from northern Europe and the north- western coastal region of Russia. Somewhat elevated concentrations are associated with air masses coming from Arctic Canada, too. The lowest concentrations are found in air masses coming from Greenland, the North Table 1 Average monthly activity concentration of 210Pb in the air (mBq m-3), measured at the Mount Zeppelin Global Atmosphere Watch station, Svalbard, 2001–05. Month 210Pb (mBq m-3) Average Standard error of mean Jan. 202 18 Feb. 169 15 Mar. 237 19 Apr. 184 18 May 112 9 Jun. 37 4 Jul. 33 4 Aug. 42 7 Sep. 44 5 Oct. 93 13 Nov. 136 16 Dec. 229 37 Airborne lead-210 at Ny-Ålesund, High Arctic J. Paatero et al. Polar Research 29 2010 345–352 © 2010 the authors, journal compilation © 2010 Blackwell Publishing Ltd348 Fig. 3 Starting points (open red squares) of the 5-day-long air mass back trajectories coin- cident with the lowest 5% of the 210Pb activity concentrations, <10 mBq m-3, at Mount Zeppe- lin Global Atmosphere Watch station (solid black square). Fig. 4 Starting points (open red squares) of the 5-day-long air mass back trajectories coin- cident with the highest 5% of the 210Pb activity concentrations, >400 mBq m-3, at Mount Zep- pelin Global Atmosphere Watch station (solid black square). Airborne lead-210 at Ny-Ålesund, High ArcticJ. Paatero et al. Polar Research 29 2010 345–352 © 2010 the authors, journal compilation © 2010 Blackwell Publishing Ltd 349 Atlantic Ocean and the Central Arctic Ocean. This is in agreement with the classical view that airborne 210Pb is a tracer for recent contact of air masses with land areas. In winter, the highest 210Pb activity concentrations are found in air masses coming from Russia, Alaska and Arctic Canada (Fig. 6). High concentrations are also associated with air masses coming from the Arctic Ocean between Alaska, north-eastern Siberia and the North Pole. Air masses containing slightly elevated 210Pb activity concen- trations arrive in Svalbard from northern Europe and Greenland. The lowest concentrations are found in air masses coming from the North Atlantic Ocean. The accumulation of 210Pb-carrying aerosol particles into the Arctic atmosphere during winter explains why the Arctic Ocean apparently is a source area of 210Pb. An interesting detail is the area of lower 210Pb activity con- centrations between Norway and Iceland, and the region immediately north-west of Norway. This phenomenon can be attributed to the Gulf Stream going northwards along the coast of Norway. This warm sea current seems to cause enough convection in the air aloft that airborne 210Pb is diluted to a bigger air volume. Conclusions In this report we have presented a 5-year observation series of airborne 210Pb at Mount Zeppelin GAW station. To the best of our knowledge this is the longest data set on airborne 210Pb in the High Arctic. In winter the 210Pb activity concentrations found in Svalbard are comparable with those found in Finland. In summer, however, the concentrations are much lower than in continental areas. The performed source area analysis, which is based on air mass back trajectories, indicated that in summer 210Pb can be used as a tracer for air masses having been in contact with land areas within the past 5 days. In winter this cannot be used because of the accumulation of 210Pb- carrying aerosol particles in the Arctic atmosphere during the Arctic night. But even in winter a low 210Pb activity concentration indicates that the associated air mass has had little if any contact with land areas. Airborne 210Pb has been used in verifications of atmo- spheric general circulation models (Feichter et al. 1991). The data produced in this study might find use in verifi- cation activities, as there are few data sets on airborne Fig. 5 Source areas of airborne 210Pb (mBq m-3) at Mount Zeppelin Global Atmo- sphere Watch station (solid black square), from June to September 2001–05. Airborne lead-210 at Ny-Ålesund, High Arctic J. Paatero et al. Polar Research 29 2010 345–352 © 2010 the authors, journal compilation © 2010 Blackwell Publishing Ltd350 210Pb available from 79°N. As a contribution to the Inter- national Polar Year 2007–08, the recent filters have now also been measured for 7Be. 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