A new reconstruction of atmospheric gaseous elemental mercury trend over the last 60 years from Greenland firn records
Introduction
The Arctic troposphere receives mercury (Hg) through air mass transporting anthropogenic emissions (mining, fuel combustions) from Asian, Northern American and European sources (Durnford et al., 2010). Hg mainly reaches the Arctic troposphere as gaseous elemental mercury (GEM), as it has a longer atmospheric lifetime than other Hg species. GEM is also emitted by evasion from the Arctic Ocean (Dastoor and Durnford, 2013) and by re-emission of deposited Hg from snow and ice surfaces (Durnford and Dastoor, 2011). The oceanic Hg source, fed by circumpolar riverine inputs, was recently shown to dominate at coastal Arctic sites, where it could cause a summertime atmospheric GEM maximum (Fisher et al., 2012).
Atmospheric GEM concentrations in the Arctic troposphere are one of the key parameters that controls Hg deposition. How the Arctic atmosphere responded to historical changes in anthropogenic emissions is an important question to answer in order to understand the present-day biogeochemical cycle of Hg in the Arctic. This understanding is also a prerequisite for predicting how the Hg cycle will be affected by future Hg emission policies (e.g. the Minamata Convention).
The longest monitoring record of atmospheric GEM in the Arctic began in 1995 at Alert (82°N, Canada) and revealed important seasonal variations (Cole and Steffen, 2010, Steffen et al., 2008). The High Arctic springtime atmosphere is characterized by extremely low GEM values (<1 ng m−3) due to a photochemically-driven rapid oxidation process involving bromine, while summer months exhibit a return to higher GEM values (∼2 ng m−3). The so-called springtime atmospheric mercury depletion events (AMDE) are an important pathway by which atmospheric GEM can be converted into more labile species (Larose et al., 2011). It delivers large quantities of oxidized mercury (Hg(II)) on environmental surfaces for a short period of time (Douglas et al., 2012). Hg(II) species are susceptible to photoreduction and a fraction – which is large in the case of seasonal terrestrial snowpacks – is rapidly reemitted back to the atmosphere as GEM (Poulain et al., 2004). The air monitoring records at Alert and at Zeppelin station (79°N, Svalbard) show either slowly declining or stable GEM levels in the past decade (Berg et al., 2008, Berg et al., 2013, Cole and Steffen, 2010, Cole et al., 2013). These observations, as well as others from sub-Arctic sites (Cole et al., 2013), appear to contradict rising global Hg emission trends (Streets et al., 2011). However, recent modeling studies have highlighted the potentially important role played by the ocean (Chen et al., 2015, Soerensen et al., 2012) in modulating atmospheric GEM levels (Fisher et al., 2013).
An indirect method to reconstruct long-term trends in atmospheric gases consists of measuring their concentration profile in the interstitial air (also called firn air) of the upper layers of polar ice sheets (e.g. Schwander et al., 1993, Witrant et al., 2012). Applying this method at Summit in central Greenland, Faïn et al. (2009) reconstructed a history of Arctic GEM levels over 60 years. Their measurements revealed that atmospheric GEM concentrations peaked in the 1970s and were in line with global Hg production. In this paper, we present GEM measurements obtained in the upper 80 m of the northeastern Greenland ice sheet cap at the international drilling site NEEM (North Greenland Eemian Ice Drilling). Using a new and robust modeling approach, we are able to propose a new atmospheric scenario of GEM for the last 60 years. GEM data obtained at Summit (Faïn et al., 2009) are re-evaluated and compared to our results.
Section snippets
Firn air sampling and GEM analysis
GEM measurements were carried out during the NEEM program from July 16th to July 29th, 2009. The NEEM site is situated in northeastern Greenland (Fig. 1) at an altitude of 2452 m above sea level. Experiments were conducted 1.8 km away from the main camp (77°25.8 N, 51°06.4 W).
The sampling of firn air was first documented by Schwander et al. (1993). Here we describe a modified setup for the extraction of large samples using a Firn Air Sampling Device (FASD). Drilling progressed stepwise with
Laboratoire de Glaciologie et Géophysique de l’Environnement and Grenoble Image Parole Signal Automatique (LGGE-GIPSA) model
Firn is an open porosity medium where atmospheric gases move mainly by diffusion. Its density increases from the surface to the firn-ice transition, and the firn diffusivity decreases in consequence. Variations of atmospheric GEM concentrations occurring at the surface propagate into the firn by molecular diffusion, gravitational settling and advection (due to the fact that the firn sinks progressively as snow accumulates). The physics of our model are described in detail in Rommelaere et al.
GEM concentrations in firn air and in the atmosphere at NEEM
GEM concentrations measured at 14 depths in the firn at NEEM are presented in Fig. 2. Average GEM concentrations at these depths varied between 0.85 and 1.39 ng m−3 with maximum levels found in the upper layers and between 67 and 73 m. The mean surface atmospheric GEM concentration was 1.38 ± 0.10 ng m−3 (n = 327) during the 6 days of sampling. Fig. 2 also shows measurements made both in firn air and in the atmosphere at Summit Greenland in 2006 (Faïn et al., 2009).
Due to gravitational
Comparing the different factors influencing atmospheric GEM signals recorded in the Arctic
Possible explanations of the differences between NEEM and Summit data are geographical variations, experimental bias, and atmospheric reactivity.
Atmospheric GEM measurements were conducted during different and short time periods and the instrumental configuration differ from one site to another. Atmospheric GEM was 1.38 ± 0.10 ng m−3 at NEEM 2009 during the field campaign while mean GEM was 1.77 ± 0.20 ng m−3 at Summit in 2006 (Faïn et al., 2009). Brooks et al. (2011) showed average values of
Implications for the atmospheric GEM history in the Arctic
As shown on Fig. 5, our optimal atmospheric GEM scenario is consistent with both historical anthropogenic Hg emissions and atmospheric Hg concentrations simulated by a fully coupled global biogeochemical box model presented in Horowitz et al. (2014).
Our optimal scenario show an increase of GEM concentrations during the 1950s and 1960s which can be primarily related to the global rise in Hg emissions after the Second World War (Faïn et al., 2009, Streets et al., 2011). At NEEM, GEM
Acknowledgements
This research was funded by the IPEV Program NEEM Mercury 1205. We thank our NEEM collaborators for their assistance during the field campaign, especially Mauro Rubino of CSIRO, Australia and the Centre for Ice and Climate, University of Copenhagen, now at the Second University of Naples, Italy, Vas Petrenko of UCSD, USA, now at The University of Rochester, USA, Zoe Courville of USACE, USA and Thomas Blunier (Niels Bohr Institute) for their support. NEEM is directed and organized by the Center
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