Anthropogenic mercury emissions are transported through the atmosphere as gaseous elemental mercury (Hg(0)) prior to deposition to Earth's surface. Strong seasonality in atmospheric Hg(0) concentrations in the Northern Hemisphere has been explained by two factors: anthropogenic Hg(0) emissions are thought to peak in winter due to higher energy consumption, and atmospheric oxidation rates of Hg(0) are faster in summer. Oxidationdriven Hg(0) seasonality should be equally pronounced in the Southern Hemisphere, which is inconsistent with observations of constant year-round Hg(0) levels. Here, we assess the role of Hg(0) uptake by vegetation as an alternative mechanism for driving Hg(0) seasonality. We find that at terrestrial sites in the Northern Hemisphere, Hg(0) co-varies with CO2, which is

The atmospheric deposition of mercury (Hg) occurs via several mechanisms, including dry and wet scavenging by precipitation events. In an effort to understand the atmospheric cycling and seasonal depositional characteristics of Hg, wet deposition samples were collected for approximately 5 years at 17 selected GMOS monitoring sites located in the Northern and Southern hemispheres in the framework of the Global Mercury Observation System (GMOS) project. Total mercury (THg) exhibited annual and seasonal patterns in Hg wet deposition samples. Interannual differences in total wet deposition are mostly linked with precipitation volume, with the greatest deposition flux occurring in the wettest years. This data set provides a new insight into baseline concentrations of THg concentrations in precipitation worldwide, particularly in regions such as the Southern Hemisphere and tropical areas where wet deposition as well as atmospheric Hg species were not investigated before, opening the way for future and additional simultaneous measurements across the GMOS network as well as new findings in future modeling studies

Abstract. The Minamata Convention on Mercury (Hg) entered into force in 2017, committing its 116 parties (as of January 2019) to curb anthropogenic emissions. Monitoring of atmospheric concentrations and trends is an important part of the effectiveness evaluation of the convention. A few years ago (in 2017) we reported an increasing trend in atmospheric Hg concentrations at the Cape Point Global Atmosphere Watch (GAW) station in South Africa (34.3535∘ S, 18.4897∘ E) for the 2007–2015 period. With 2 more years of measurements at Cape Point and the 2012–2017 data from Amsterdam Island (37.7983∘ S, 77.5378∘ E) in the remote southern Indian Ocean, a more complex picture emerges: at Cape Point the upward trend for the 2007–2017 period is still significant, but no trend or a slightly downward trend was detected for the period 2012–2017 at both Cape Point and Amsterdam Island. The upward trend at Cape Point is driven mainly by the Hg concentration minimum in 2009 and maxima in 2014 and 2012. Using ancillary data on 222Rn, CO, O3, CO2, and CH4 from Cape Point and Amsterdam Island, the possible reasons for the trend and its change are investigated. In a companion paper this analysis is extended for the Cape Point station by calculations of source and sink regions using backward-trajectory analysis.

Mercury measurements were concurrently made in air (Gaseous Elemental Mercury, i.e. GEM) as well as in precipitation samples (Total mercury, i.e. TotHg) over a seven year period (2007-2013) at Cape Point, South Africa, during the rainy seasons (May-October). Eighty-five rain events, almost exclusively associated with cold fronts, have been identified of which 75% reached the Cape Point observatory directly across the Atlantic Ocean from the south, while 19% moved in to the measuring site via the Cape Town metropolitan region. In statistic terms the GEM, TotHg, CO and ²²²Rn levels within the urban-marine events do not differ from those seen in the marine rain episodes. Over the 2007-2013 period, the May till Oct averages for GEM ranged from 0.913 ng m^-3 to 1.108 ng m^-3, while TotHg concentrations ranged from 0.03 to 52.5 ng L^-1 (overall average: 9.91 ng L^-1). A positive correlation (R² = 0.49, n = 7) has been found between the average annual (May till October) GEM concentrations in air and TotHg concentration in rainwater suggesting a close relationship between the two species. The wetter years are normally associated with higher GEM and TotHg levels. Both GEM and TotHg annual means correlate positively with total annual (May till October) rain depths. If one or two outlier years are removed from the data set, the R² values increase from 0.23 to 0.10 for GEM and TotHg to 0.97 (n = 5) and 0.89 (n = 5), respectively. The relationship between annual mean GEM and annual precipitation depth also holds for the period 1996-2004 (R² = 0.6, n = 8) when GEM was measured manually (low resolution data). A positive correlation was also seen between annual average GEM concentrations and the El Nifio Southern Oscillation (ENSO) Index (SOI), for the 1996-2004 period (R² = 0.7, n = 8). For the 2007-2013 periods this relationship was also positive but less pronounced. The relationship between annual precipitation depth and annual SOI suggests that the inter-annual variations of GEM (Hg0) concentration might be caused by large-scale meteorological processes. (C) 2015 Elsevier Ltd. All rights reserved

<p><strong>Abstract.</strong> The atmospheric deposition of mercury (Hg) occurs via several mechanisms including dry and wet scavenging by precipitation events. In an effort to understand the atmospheric cycling and seasonal depositional characteristics of Hg, wet deposition samples were collected for approximately five years at 17 selected GMOS monitoring sites located in the Northern and Southern Hemispheres in the framework of the Global Mercury Observation System (GMOS) project. Total mercury (THg) exhibited annual and seasonal patterns in Hg wet deposition samples. Inter-annual differences in total wet deposition are mostly linked with precipitation volume, with the greatest deposition flux occurring in the wettest years. This data set provides a new insight into baseline concentrations of THg concentrations in precipitation worldwide, particularly in regions, such as the Southern Hemisphere and tropical areas where wet deposition as well as atmospheric Hg species were not investigated before, opening the way for future and additional simultaneous measurements across the GMOS network as well as new findings in future modeling studies.</p>

South Africa's Cape Point (CPT) trace gas observatory, operated by the South African Weather Service (SAWS), has been monitoring mole fractions (mol/vol) of ambient greenhouse gases (GHG), e.g. carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and selected chlorofluorocarbons (CFCs) since the late 1970s. In addition, the Cape Point trace gas observatory boasts the longest atmospheric carbon monoxide (CO) dataset continuously from 1977 in the Southern Hemisphere (SH). Instruments for the measurement of radiation and other selected gases, such as ozone (O3), have gradually been added since the inception of measurements. The year 1995 heralded a milestone when the Cape Point observatory became part of the World Meteorological Organization's (WMO) Global Atmosphere Watch (GAW) programme. Besides supplying data to the World Data Centres, it also supports the local database, South African Air Quality Information System (SAAQIS). These affiliations, which require strict measurement protocols, make the Cape Point data records available within the global data centres. Due to its location in a mostly pristine marine environment and its proximity to the Southern Ocean, Cape Point measurements are highly valued in the United Nations Environment Programme (UNEP)/WMO Ozone and IPCC Climate Assessments and serves as a bellwether for regional changes in southern African atmospheric composition. A review of selected climate change relevant trace gas measurements acquired at Cape Point is presented here.

Figure 1 (left): CO 2 time series (1993-2017) showing background monthly means and smoothed growth rate. Figure 1 (right): CH 4 time series (1983-2017) for background data. Red line highlights the 2-yr smoothed growth rate.

This commentary paper from the recently formed International Global Atmospheric Chemistry (IGAC) Southern Hemisphere Working Group outlines key issues in atmospheric composition research that particularly impact the Southern Hemisphere. In this article, we present a broad overview of many of the challenges for understanding atmospheric chemistry in the Southern Hemisphere, before focusing in on the most significant factors that differentiate it from the Northern Hemisphere. We present sections on the importance of biogenic emissions and fires in the Southern Hemisphere, showing that these emissions often dominate over anthropogenic emissions in many regions. We then describe how these and other factors influence air quality in different parts of the Southern Hemisphere. Finally, we describe the key role of the Southern Ocean in influencing atmospheric chemistry and conclude with a description of the aims and scope of the newly formed IGAC Southern Hemisphere Working Group.