Elsevier

Atmospheric Environment

Volume 262, 1 October 2021, 118634
Atmospheric Environment

Mercury in precipitated and surface snow at Dome C and a first estimate of mercury depositional fluxes during the Austral summer on the high Antarctic plateau

https://doi.org/10.1016/j.atmosenv.2021.118634Get rights and content

Highlights

  • The daily collection of snow accumulation at Dome C station and from this an estimation of the depositional fluxes of mercury.

  • Surface snow sublimation is apparently linked with the net release of mercury from the snowpack in the summer.

  • Estimated of the net dry deposition velocity of mercury , the results show a net release of mercury from the snowpack.

  • We have made a first estimation of the snow scavenging factor for mercury on the high Antarctic plateau.

Abstract

The role of deposition fluxes on the mercury cycle at Concordia station, on the high Antarctic plateau have been investigated over the Austral summer between December 2017 to January 2018. Wet/frozen deposition was collected daily from specially sited tables, simultaneously with the collection of surface (0–3 cm) and subsurface (3–6 cm) snow and the analysis of Hg0 in the ambient air. Over the course of the experiment the atmospheric Hg0 concentrations ranged from 0.58 ± 0.19 to 1.00 ± 0.33 ng m−3, surface snow Hg concentrations varied between (0–3 cm) 0.006 ± 0.003 to 0.001 ± 0.001 ng cm−3 and subsurface snow (3–6 cm) concentrations varied between 0.001 ± 0.001 to 0.003 ± 0.002 ng cm−3. The maximum daily wet deposition flux was found to be 23 ng m−2 d−1. Despite the low temporal resolution of our measurements combined with their potential errors, the linear regression of the Hg deposition fluxes against the snow accumulation rates allowed us to estimate the mean dry deposition rate from the intercept of the graph as −0.005 +- 0.008 ng m−2 d−1. From this analysis, we conclude that wet deposition accounts for the vast majority of the Hg deposition fluxes at Concordia Station. The number of snow events, together with the continuous GEM measurements have allowed us to make a first estimation of the mean snow scavenging factor at Dome C. Using the slope of the regression of mercury flux on snow accumulation we obtained a snow scavenging factor that ranges from 0.21 to 0.22 ± 0.02 (ngHg/g snow)/(ngHg/m3 air). Our data indicate that the boundary layer height and local meteorological effects influence Hg0 reemission from the top of (0–3 cm) the snowpack into the atmosphere and into the deeper snowpack layer (3–6 cm). These data will help constrain numerical models on the behaviour of mercury in Antarctica.

Introduction

Mercury once emitted into the atmosphere from natural or anthropogenic sources can travel around the globe to be deposited onto soils, into waters (in liquid or solid form) and plants (United Nations Environment Program, 2018). The dominant form in the atmosphere is Hg0, which is estimated to have an atmospheric lifetime of 0.5–1.7 years (Holmes et al., 2006) allowing plenty of time to reach the Poles (Angot et al., 2016a) and the high Antarctic plateau.

The role of the Antarctic plateau in the global mercury cycle was relatively unknown until Brooks (Brooks et al., 2008) carried out the first measurements between November and December 2003 and in November 2005 at the South Pole. They found near surface atmospheric concentrations of gaseous elemental mercury (GEM) of 539 ± 189 pg (Hg) m−3, 344 ± 151 pg m−3 of reactive gaseous mercury (RGM) and mean Hg concentrations in surface snow of 198 ng L−1 that diminished to 10 ng L−1 at a depth of 25 cm. From above and below surface measurements they showed an estimated GEM surface emission flux of +8.1 ng m−2 h−1 and an RGM deposition flux of – 10.8 ng m−2 h−1. From these measurements they were able to estimate a mercury lifetime in the snow of 16 days and calculated that the entire Antarctic plateau could sequester up to 60 tonnes of Hg per year, effectively acting as a giant sink. These high concentrations of Hg in surface snow were confirmed by Angot et al. (2016c) at Concordia station, the joint French/Italian base on the East Antarctic ice sheet at Dome C. The authors reported evidence of intense oxidation of atmospheric Hg0 during the austral summer with photochemical reactions and exchanges happening at the air-snow interface. They observed multi-day to weeklong atmospheric Hg depletion events in summer that were not associated with ozone loss, indicating possible dry deposition during air mass stagnation events. These air masses then descend towards the coast with the katabatic winds (Angot et al., 2016b) provoking large variability in summer Hg0 values and elevated deposition of mercury in surface snow, suggesting that the Antarctic ice sheet can affect the mercury cycle on a continental scale.

Studies on temporal variations of Hg concentrations in the snowpack at Dome C (Spolaor et al., 2018) have shown that within 6 h after precipitation, surface snow Hg concentrations can drop from 200 pg g−1 to 20 pg g−1, showing that high concentrations can be deposited by snowfall and then be rapidly released back into the atmosphere. This is supported by concurrent atmospheric Hg0 increases from 0.5 to 1.5 ng m−3 under stable low wind speed atmospheric conditions. The converse was also found to be true, during a diamond dust deposition event. It should be noted that diamond dust is a type of precipitation composed of slowly falling, very small, unbranched crystals of ice that form under very low air temperatures such as those found at Dome C. During such an event, atmospheric Hg0 dropped from 1.5 to 0.5 ng m−3, accompanied by an increase in surface snow concentrations from 20 to 80 pg g−1, suggesting the role of diamond dust as an Hg scavenger. These combined results showed the importance of freshly deposited snow/diamond dust or frost as potential Hg atmospheric scavengers on the Antarctic plateau.

The aim of this study is to build on this work and investigate the contribution of the mercury deposition flux to mercury concentrations in surface and subsurface snow over a summer campaign. This was done by determining the Hg concentrations in wet deposition (snow, frost and diamond dust) and surface and subsurface snow on a daily basis. The effects of local meteorological conditions on variations in mercury concentrations were then studied. Since the Antarctic plateau is essentially a frozen desert, it is important to collect freshly deposited snow to investigate whether dry deposition is an important process, or whether evasion of Hg from the snowpack is dominant. This work tries to estimate whether net dry deposition occurred during the 2017–2018 sampling campaign and provides a first estimate for the snow Hg scavenging factor to help with modelling efforts in the future.

Section snippets

Concordia station

Concordia station is a research station jointly run by France and Italy. It is located at Dome C, one of the “summits” of the East Antarctic ice sheet at an elevation of 3220 m above sea level and over 1200 km from the coast (Fig. 1a). The mean annual temperature at the site is −54.5 °C (Stenni et al., 2016) but meteorological observations show a maximum of −14 °C during summer and a minimum of −85 °C during winter. Generally the weather at Dome C is dominated by a high pressure system

Surface snow mercury concentrations, temperature, and meteorological data

The results for the daily surface snow sampling can be seen in Fig. 2, showing that surface snow has mercury concentrations greater than subsurface snow until around the 28th of December, except for a spike between the 17th to 19th of December. This trend breaks with subsurface snow being more concentrated in mercury than surface snow from the 28th of December to the 8th of January. After this, to the end of the experiment, the surface and subsurface values are similar.

These results can be

Conclusions and future work

During the 2017–2018 Antarctic summer, surface and subsurface snow as well as snow deposition was sampled every day. A mean surface snow Hg concentration of 9.5 ± 13 (standard deviation (SD) n = 82) ng kg−1 was found, and the subsurface snow (3–6 cm deep) had a mean concentration of 6.7 ± 11 ng kg−1 (1 SD n = 81). These concentrations were then corrected with the snow density to make them more comparable. At the same time, atmospheric Hg (Hg0 and RM), ozone and other parameters were measured.

Funding

QAed/Qced DMC GEM data, accessible in GMOS-FR national database (https://gmos.aeris-data.fr/) and from the G-DQM central database system of GMOS/GOS4M (https://www.gmos.eu/sdi/), have been collected through fundings obtained by the European Union 7th Framework Programme project Global Mercury Observation System (GMOS, 2010–2015), LabEX OSUG@2020 (ANR10 LABX56), LEFE CNRS/INSU as well as from French Polar Institute (IPEV) via GMOStral-1028 IPEV program since 2012.

This project has received

Data

Concordia (DCC) L2 GEM data are freely available at https://gmos.aeris-data.fr/. Automatic weather station, radiosounding and sub surface temperature data are freely available at www.climantartide.it under request.

Using the mercury flux model in section 3.2, the fit intercept q represents J dry = v dry *c air while the slope m = k*c air. m has units of ng Hg g−1 of snow while q has units of ng Hg m −2 day −1.

CRediT authorship contribution statement

Clara Turetta: Investigation. Niccolò Maffezzoli: Investigation, Validation. Olivier Magand: Investigation, Writing – original draft. Beatriz Ferreira Araujo: Investigation. Hélène Angot: Writing – original draft. Delia Segato: Formal analysis. Paolo Cristofanelli: Investigation, Writing – original draft. Francesca Sprovieri: Funding acquisition. Claudio Scarchilli: Writing – original draft, Validation. Paolo Grigioni: Investigation. Virginia Ciardini: Investigation. Carlo Barbante: Project

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

These observations contribute to the GEO GOS4M (Global Observation System for Mercury (www.gos4m.org). It is aimed to support the UN Global Partnership on Mercury Fate and Transport Research (UN F&T) of the UN environment in the implementation of the Minamata Convention (www.mercuryconvention.org) by providing a Knowledge Platform on mercury in environment and the human health. It will support UN environment and Nations to assess the effectiveness of measures that will be undertaken. Data and

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