The winter/summer ratio of airborne 210Pb – a possible way to quantify the significance of long-range transport of natural radionuclides to the Arctic

Abstract

In the environment, the main sources of naturally occurring radionuclides come from radionuclides in the uranium decay series. Activity concentrations of uranium decay series radionuclides may vary considerably from place to place depending on the geological characteristics at the location. Their releases to the atmosphere are mainly through radon (222Rn), a radioactive noble gas occurring naturally as an indirect decay product of uranium in soils and rocks. Due to the abundance of uranium, radon continuously emanates from continental land masses. With radon as the main source of naturally occurring radioactivity in the environment, one would think that the Arctic should be an area of low background radiation, because a considerable area of the Arctic is covered by glaciers and permafrost, and the presence of permafrost entraps radon in the ground and serves as an effective barrier to radon emanation. However, over the past decades, ground level air monitoring data from stations in the Arctic region (UNSCEAR, 2000, AMAP, 2010, IAEA, 2012) and many special designed field studies (Lockhart, 1962, Wilkening et al., 1972, Larson et al., 1972, Polian et al., 1986, Paatero et al., 2010) have demonstrated that radon and radon progeny concentrations in the Arctic are significantly higher during the cold winter than in summer. The elevated level of naturally occurring radioactivity in the Arctic, especially during winter months, is due to radon and radon progeny produced from natural sources as well as industrial activities (such as mining, hydraulic fracturing and coal burning for power generation) outside of the Arctic, and carried to the Arctic mainly through long-range atmospheric transport (Turekian et al., 1977, Tracy and Prantl, 1985, Samuelsson et al., 1986, IAEA, 2003, Piliposian and Appleby, 2003, AMAP, 2010, Baskaran, 2011, Ram and Sarin, 2012, Charro and Pena, 2013, Persson and Holm, 2011, Papastefanou, 2010, Persson and Holm, 2014). The long-lived radon decay products 210Pb and 210Po can accumulate to relatively high levels and become a health concern or a limiting factor of country food consumption (UNSCEAR, 2000, Van Oostdam et al., 2005, AMAP, 2010). Airborne concentration of 210Pb has been a routine measurement in almost all air monitoring stations around the globe. Because it has a long half-life (22.3 years), 210Pb can be transported over vast distances and remain in the atmosphere for extended periods of time. The Arctic monitoring data reported in the literature have shown consistently that 210Pb activity concentrations in air are higher during mid-winter and lower in summer months, such as the measurements of 210Pb activity concentrations in air at Mount Zeppelin Global Atmosphere Watch station (78°58′N, 11°53′E) in Svalbard from 2001 to 2005 (Paatero et al., 2010) and at two monitoring stations in northern Canada, Whitehorse (60.73°N, 135.10°W) and Resolute (74.71°N, 94.97°W), from 2003 to 2009 (AMAP, 2010). These naturally occurring radionuclides being long-range transported to the Arctic can be seen as “natural contaminants” to distinguish them from the traditional meaning of contamination, the “artificial contaminants” which are attributable to industrial or man-made sources (Chen, 2014). This letter analysed existing long-term atmospheric monitoring data in the Canadian high Arctic, sub-Arctic and mid-latitude sites, and demonstrated that the winter to summer ratios of 210Pb can serve as an operational quantity to quantify the significance of naturally occurring radionuclides transported to the Arctic.