222 Rn calibrated mercury fluxes from terrestrial surface of southern Africa derived from observations at Cape Point, South Africa
Abstract: Overview: The manuscript entitled “222Rn calibrated mercury fluxes from terrestrial surface of southern Africa derived from observations at Cape Point, South Africa”, by F. Slemr et al. describes observationally constrained estimates of terrestrial Hg fluxes from southern Africa. The analysis draws upon nearly 5 years (2007–2011) of measurements of gaseous elemental mercury (GEM) and 222Rn made at the Global Atmospheric Watch (GAW) station Cape Point. Estimated terrestrial 222Rn fluxes are combined with correlations between GEM and 222Rn during periods with elevated 222Rn to derive terrestrial GEM fluxes. The derived fluxes are corrected for 222Rn decay using an assumed transport time. As the authors point out, terrestrial Hg fluxes are poorly constrained by existing observations, particularly outside of northern mid-
Summary (1 min read)
- Mercury poses a serious environmental issue because of its transformation to methyl mercury, which is a potent toxin to humans and animals (Mergler et al., 2007; Scheuhammer et al., 2007).
- Of primary concern are thus the emissions of mercury, which due to the long atmospheric residence time of elemental mercury (Lindberg et al., 2007) can be distributed all over the world.
- Whereas anthropogenic emissions are believed to be known with an uncertainty of±30 %, the uncertainties of the emissions from oceans and terrestrial surfaces are consid red to be±50 % and more (Lin et al., 2006; Lindberg et al, 2007).
- Its emission rate from soil is relatively evenly distributed (Zhang et al., 2011 and references therein) making222Rn a good tracer for studies of emissions from terrestrial surfaces (Zahorowski et al., 2004).
- The major advantage of the Hg/222Rn method is its capability to estimate regional fluxes and by this its capability to avoid shortcomings related to up-scaling of point measurements in the field (Wilson et al., 1997; Obrist et al., 2006).
- The Cape Point station (34◦21′ S, 18◦29′ E) is part of the World Meteorological Organization’s (WMO) Global Atmosphere Watch (GAW) network.
- The site is occasionally also subjected to air from the northern to north-eastern sector (mainly during austral winter), which is influenced by anthropogenic emissions from the greater Cape Town area and/or by other continental sources (both local and regional).
- Continuous measurements of gaseous mercury are made using a Tekran 2537A vapour-phase mercury analyser (Tekran Inc., Toronto, Canada).
- The so-called two-filter instrument is described in detail by Whittlestone and Zahorowski (1998) and Brunke et al. (2002) and was run with 30 min resolution.
3 Results and discussion
- Altogether 191 events with222Rn concentrations above 1000 mBq m−3, which lasted usually for more than a day, have been identified between March 2007 and December 2011.
- Figure 2 shows the frequency distribution of the GEM /222Rn slopes from the correlations.
- Speciated mercury measurements and more data on wet deposition in southern Africa would further constrain the uncertainty of the net mercury deposition in this area.
- Radon-calibrated fluxes of mercury over the terrestrial surface of southern Africa were derived from concurrent measurements of GEM and222Rn at Cape Point between March www.atmos-chem-phys.net/13/6421/2013/.
- By including wet mercury deposition (−1.01 ng m−2 h−1), but discounting the RGM dry deposition, the terrestrial surface of southern Africa thus represents a significant net sink for atmospheric mercury of −1.55 ng m−2 h−1.
- Furthermore, information about RGM fluxes is missing altogether.
- By the same token the assistance rendered by B. Parker and T. Mkololo in maintaining the Tekran 2537A analyser at Cape Point in good working order has also been greatly appreciated.
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Q1. What contributions have the authors mentioned in the paper "222rn-calibrated mercury fluxes from terrestrial surface of southern africa" ?
In this paper, the average GEM / 222Rn flux ratio was −0.0048 ± 0.0001 with a standard error of ±0.6422 F.