Arctic mercury depletion and its quantitative link with halogens

Abstract

Gas phase elemental mercury (Hg°) was measured aboard the NASA DC-8 research aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign conducted in spring 2008 primarily over the North American Arctic. We examined the vertical distributions of Hg°and ozone (O 3 ) together with tetrachloroethylene (C 2 Cl 4 ), ethyne (C 2 H 2 ), and alkanes when Hg°-and O 3 -depleted air masses were sampled near the surface (<1 km). This study suggests that Hg°a nd O 3 depletions commonly occur over linear distances of~20-200 km. Horizontally there was a sharp decreasing gradient of~100 ppqv in Hg°over <10 km in going from the bay near Ellesmere Island to the frozen open ocean. There was a distinct land-ocean difference in the vertical thickness of the Hg°-depleted layer -being variable but around a few 100 meters over the ocean whereas occurring only very near the surface over land. Data J Atmos Chem (2010)

Temperature and sunlight controls of mercury oxidation and deposition atop the Greenland ice sheet

“…In general, it has been determined that polar mercury chemistry results from GEM oxidation by halogens , and is confined to the shallow atmospheric boundary layer (typically less then a few hundreds of meters; Banic et al, 2003;Tackett et al, 2007;Mao et al, 2010). RGM production and deposition is considered the predominant pathway for mercury deposition to the polar regions as GEM itself does not condense or significantly dry deposit (dry deposit is very slow), and is not significantly adsorbed onto snow and ice surfaces (Lindberg et al, 2002;BartelsRauch et al, 2002;Ferrari et al, 2004).…”

Thermodynamics of reactions of ClHg and BrHg radicals with atmospherically abundant free radicals

“…Calvert and Lindberg (2005) questioned the role of OH radical under atmospheric conditions. Atomic halogens (Br and Cl) have been invoked to explain the simultaneous decrease in concentrations of both O 3 and Hg(0) in mercury depletion events (Steffen et al 2008, references therein;Obrist et al, 2011;Mao et al, 2011). Field data in the marine boundary layer has been analyzed to suggest a major role for Br in initiating Hg(0) oxidation (Sprovieri et al, 2010a), and recent models suggest that Br plays a dominant role in mercury cycling in the global troposphere (Holmes et al, 2009).…”

Sources and Dynamic Processes Controlling Background and Peak Concentrations of TGM in Nanjing, China

“…Additionally, there has been only one study on atmospheric mercury in the arctic mid-troposphere during the spring and summer, but no measurements have been made during the fall. During this measurement campaign, the concentrations of mercury were near the tropospheric background [27], which would have had a strong dilution capacity over Nanjing. By contrast, concentrations over the South China Sea and Taiwan measured by Sheu et al [12] were near 2.0 ng•m −3 , and GEM in Okinawa [20] was much higher than the northern hemispheric background, close to the concentrations observed in the July episode when air masses arrived from over the East China Sea and the Pacific passing between Taiwan and Okinawa.…”

“…This is supported by values reported for widely varied geographical locations in the literature. Free tropospheric mercury concentrations over the Arctic range from less than 0.5 ng•m −3 during Arctic mercury depletion events to 2.2 ng•m −3 [27], while at altitudes from the surface to the tropopause over diverse areas including Mexico City, Houston, TX, Alaska, and Hawaii concentrations ranged from below detection limits to greater than 1.5 ng•m −3 [29]. Ground-based measurements within the free troposphere also show diverse average concentrations from 2.1 ng•m −3 in Taiwan to ~1.5 ng•m −3 on Storm Peak, CO [12,26].…”