ATMOSPHERIC MERCURY MEASUREMENTS AT CAPE POINT, SOUTH AFRICA

Over the 1995-2009 period the gaseous elemental mercury (GEM) concentrations have decreased by about 0.04 ng m-3 yr-1 -at Cape Point (CPT). A reduction of the same magnitude is indicated by measurements during intermittent ship cruises, implying a homogeneous distribution of GEM concentrations in the Southern Hemisphere (SH) and a 30% reduction of its atmospheric burden. Almost all GEM measurements in the Northern Hemisphere (NH) point to a substantial decrease but the trends are inhomogeneous, most likely due to a variable source distribution. However, measurements in the NH during ship cruises suggest a trend of similar magnitude. A decrease in the total atmospheric GEM burden by about 30% is inconsistent with the current mercury budgets. The most probable explanation for this is subsiding re-emissions from the legacy of large past emissions. High-resolution data since 2007 revealed depletion (DES) as well as pollution events (PEs). Both types are embedded in air masses ranging from marine background to continental. The DES observed at Cape Point are a local phenomenon (<100 km) and are the first mercury depletion events reported outside the Polar Regions. In contrast to polar DES, the DES at CPT are not accompanied by concurrent O3 depletion. They mostly appear at wind speeds < 10 m s-1 and their predominating occurrence between 11 and 18 hours suggests a photochemical destruction mechanism which could not be explained yet. GEM correlates with CO, C02, and CH4 during most PES at CPT (GEM levels > 1.3 ng m-3) and with 222Rn during about half the events. Most of the observed GEM/CO emission ratios are within the range bracketed by values reported for biomass burning and industrial/urban emissions, thus suggesting a mixture of both. No significant differences of GEM/CO and GEM/C02 could be found between different source regions defined by backward trajectories. This implies that exceptionally high emissions ascribed to the Gauteng region in global mercury inventories are overestimated

Continuous measurements of greenhouse gases and atmospheric oxygen at the Namib Desert atmospheric observatory

A new coastal background site has been established for observations of greenhouse gases (GHGs) in the central Namib Desert at Gobabeb, Namibia. The location of the site was chosen to provide observations for a data-poor region in the global sampling network for GHGs. Semi-automated continuous measurements of carbon dioxide, methane, nitrous oxide, carbon monoxide, atmospheric oxygen, and basic meteorology are made at a height of 21 m a.g.l., 50 km from the coast at the northern border of the Namib Sand Sea. Atmospheric oxygen is measured with a differential fuel cell analyzer (DFCA). Carbon dioxide and methane are measured with an early-model cavity ring-down spectrometer (CRDS); nitrous oxide and carbon monoxide are measured with an off-axis integrated cavity output spectrometer (OA-ICOS). Instrument-specific water corrections are employed for both the CRDS and OA-ICOS instruments in lieu of drying. The performance and measurement uncertainties are discussed in detail. As the station is located in a remote desert environment, there are some particular challenges, namely fine dust, high diurnal temperature variability, and minimal infrastructure. The gas handling system and calibration scheme were tailored to best fit the conditions of the site. The CRDS and DFCA provide data of acceptable quality when base requirements for operation are met, specifically adequate temperature control in the laboratory and regular supply of electricity. In the case of the OA-ICOS instrument, performance is significantly improved through the implementation of a drift correction through frequent measurements of a reference cylinder