High variability of atmospheric mercury in the summertime boundary layer through the central Arctic Ocean

OPEN SUBJECT AREAS: ENVIRONMENTAL CHEMISTRY ATMOSPHERIC SCIENCE OCEAN SCIENCES High variability of atmospheric mercury in the summertime boundary layer through the central Arctic Ocean Juan Yu1, Zhouqing Xie1, Hui Kang1, Zheng Li1, Chen Sun1, Lingen Bian2 & Pengfei Zhang1,3 1 Received 26 March 2014 Accepted 30 July 2014 Published 15 August 2014 Correspondence and requests for materials should be addressed to Z.Q.X. (zqxie@ustc. edu.cn) Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China, 2Chinese Academy of Meteorological Sciences, Beijing 100081, China, 3Department of Earth and Atmospheric Sciences, City College of New York, New York, NY 10031, USA. The biogeochemical cycles of mercury in the Arctic springtime have been intensively investigated due to mercury being rapidly removed from the atmosphere. However, the behavior of mercury in the Arctic summertime is still poorly understood. Here we report the characteristics of total gaseous mercury (TGM) concentrations through the central Arctic Ocean from July to September, 2012. The TGM concentrations varied considerably (from 0.15 ng/m3 to 4.58 ng/m3), and displayed a normal distribution with an average of 1.23 6 0.61 ng/m3. The highest frequency range was 1.0–1.5 ng/m3, lower than previously reported background values in the Northern Hemisphere. Inhomogeneous distributions were observed over the Arctic Ocean due to the effect of sea ice melt and/or runoff. A lower level of TGM was found in July than in September, potentially because ocean emission was outweighed by chemical loss. A tmospheric mercury is an atmospheric pollutant with two main sources: anthropogenic and natural. Commonly, anthropogenic sources include coal combustion, waste incineration, metal smelting, refining and manufacturing, and gold mining. Natural sources include emissions from oceans, surface soils, water bodies (both fresh and salty water), vegetation surfaces, wild fires, crustal out-gassing, volcanoes, and geothermal sources1,2. Due to its relatively high vapor pressure, low solubility and relatively long atmospheric residence time (about 1 year), mercury can be globally transported3, even to remote areas such as the Arctic4 and Antarctica5. During its long-range transport, gaseous elemental mercury can deposit on surfaces through both wet and dry processes acting on Hg(II) and Hg(p) species1. Once deposited into the environment, inorganic mercury species are likely to convert into highly toxic methyl mercury (MeHg) species, which are then enriched in the aquatic food chain and may pose threats to human health eventually1. At present, the seasonality of atmospheric mercury based on the long term site observation is obvious in the Arctic. The pattern of variability for atmospheric mercury in the Arctic area presented the spring minimum and summer maximum6,7. During the polar spring, the depletion of Hg0 correlated with the loss of O38. This depletion event was initiated by halogen species originating from sea salt linked to sea ice9 or snow on the sea ice10 after polar sunrise in the Arctic springtime11. The deposited mercury in the Arctic can undergo both reduction and oxidation processes in the snow and some may be re-emitted to the atmosphere12. However, the variable characteristics of atmospheric mercury in the Arctic summer, especially over the Arctic Ocean, are poorly understood. It is known that oceans play an important role in the cycle of global mercury because they can serve as sources or sinks for atmospheric mercury through air-sea exchange13,14. Modeling work showed that the maximum mercury in Arctic summer may be driven by river sources based upon a few site observations15. The Beringia 2005 expedition over the Arctic Ocean in the Northwest passage presented a rapid increase of mercury in air when entering the ice-covered waters4. As the Arctic Ocean has undergone a shift in decreasing sea ice extent in summer, the effect of this shift on the cycle of atmospheric mercury remains unknown. During the 5th Chinese National Arctic Research Expedition (CHINARE 2012), the Chinese vessel Xuelong passed through the Northeast Passage and the central Arctic Ocean. Moreover, in CHINARE 2010 Xuelong cruised the same track in the Chukchi Sea. This provides opportunity to investigate the spatial and temporal distribution of atmospheric mercury over the Arctic Ocean and to examine the complex interrelations. SCIENTIFIC REPORTS | 4 : 6091 | DOI: 10.1038/srep06091 1 www.nature.com/scientificreports Figure 1 | Spatial distribution of TGM in air (ng/m3) along the cruise paths during CHINARE 2012(a) and 2010(b), respectively, generated by Ocean Data View 4.0. The cruise dates were UTC time. The dashed line without color means the period without data. Base map is from Ocean Data View 4.0. Results General observation. Figure 1 shows the TGM concentrations in the marine boundary layer (MBL) during both CHINARE 2012 and CHINARE 2010 along the cruise paths. During CHINARE 2012, the cruise started on July 18th in the Chukchi Sea, passed through the Northeast Passage, arrived at Ice Island, then returned to the Chukchi Sea on September 8th through the central Arctic. The instrument was turned off when the ship passed through the Northeast Passage near the Russia coast due to political reasons. The TGM concentrations for the sites around the Ice Island but outside the Arctic Ocean were not considered in this study. The instrument was also turned off over the Greenland Sea due to the carrier gas for instrument being exhausted. Details of the cruise legs during CHINARE 2012 are shown in Table S1. For CHINARE 2010, the cruise started on July 20th in the Chukchi Sea, reached the central Arctic Ocean and then returned to the Chukchi Sea on August 31th. For CHINARE 2012, the concentrations varied considerably from 0.15 ng/m3 to 4.58 ng/m3, with an average of 1.23 6 0.61 ng/m3 (median: 1.15 ng/m3). The average is lower than the reported data over the western Arctic Ocean in 2005 (1.72 6 0.35 ng/m3)4. However, it is somewhat higher than the average over the North Atlantic Ocean in 2008 (about 1.15 ng/m3) and 2009 (about 1.12 ng/m3)2. TGM concentrations displayed an inhomogeneous distribution over the oceans, similar to the results obtained during the expedition of Galathea 3 cruise16, which covered the North Atlantic Ocean etc. The spatial distribution of TGM over the Arctic Ocean is notably in agreement with the recent observation of higher Hg in biota in the western Arctic than in the east17,18. The frequency distribution of TGM concentrations is given in Figure 2a, and the Kolmogorov-Smirnov test suggested a normal distribution (P , 0.001). The highest frequency is in the range from 1.0 to 1.5 ng/ m3. For CHINARE 2010, the TGM concentrations ranged from 0.73 to 4.78 ng/m3, with an average of 1.81 6 0.45 ng/m3 (median: 1.72 ng/ m3). Again, Kolmogorov-Smirnov test sugge (...truncated)