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We present a city-scale inversion over Cape Town, South Africa. Measurement sites for atmospheric CO2 concentrations were installed at Robben Island and Hangklip lighthouses, located downwind and upwind of the metropolis. Prior estimates of the fossil fuel fluxes were obtained from a bespoke inventory analysis where emissions were spatially and temporally disaggregated and uncertainty estimates determined by means of error propagation techniques. Net ecosystem exchange (NEE) fluxes from biogenic processes were obtained from the land atmosphere exchange model CABLE (Community Atmosphere Biosphere Land Exchange). Uncertainty estimates were based on the estimates of net primary productivity. CABLE was dynamically coupled to the regional climate model CCAM (Conformal Cubic Atmospheric Model), which provided the climate inputs required to drive the Lagrangian particle dispersion model. The Bayesian inversion framework included a control vector where fossil fuel and NEE fluxes were solved for separately.Due to the large prior uncertainty prescribed to the NEE fluxes, the current inversion framework was unable to adequately distinguish between the fossil fuel and NEE fluxes, but the inversion was able to obtain improved estimates of the total fluxes within pixels and across the domain. The median of the uncertainty reductions of the total weekly flux estimates for the inversion domain of Cape Town was 28 %, but reach as high as 50 %. At the pixel level, uncertainty reductions of the total weekly flux reached up to 98 %, but these large uncertainty reductions were for NEE-dominated pixels. Improved corrections to the fossil fuel fluxes would be possible if the uncertainty around the prior NEE fluxes could be reduced. In order for this inversion framework to be operationalised for monitoring, reporting, and verification (MRV) of emissions from Cape Town, the NEE component of the CO2 budget needs to be better understood. Additional measurements of Δ14C and δ13C isotope measurements would be a beneficial component of an atmospheric monitoring programme aimed at MRV of CO2 for any city which has significant biogenic influence, allowing improved separation of contributions from NEE and fossil fuel fluxes to the observed CO2 concentration.

This paper describes and discusses fifteen years (1999–2013) of continuous hourly atmospheric radon (222Rn) monitoring at the coastal low-altitude Southern Hemisphere Cape Point Station in South Africa. A strong seasonal cycle is evident in the observed radon concentrations, with maxima during the winter months, when air masses arriving at the Cape Point station from over the African continental surface are more frequently observed, and minima during the summer months, when an oceanic fetch is predominant. An atmospheric mean radon activity concentration of 676 ± 2 mBq/m3 is found over the 15-year record, having a strongly skewed distribution that exhibits a large number of events falling into a compact range of low values (corresponding to oceanic air masses), and a smaller number of events with high radon values spread over a wide range (corresponding to continental air masses). The mean radon concentration from continental air masses (1 004 ± 6 mBq/m3) is about two times higher compared to oceanic air masses (479 ± 3 mBq/m3). The number of atmospheric radon events observed is strongly dependent on the wind direction. A power spectral Fast Fourier Transform analysis of the 15-year radon time series reveals prominent peaks at semi-diurnal, diurnal and annual timescales. Two inter-annual radon periodicities have been established, the diurnal 0.98 ± 0.04 day−1 and half-diurnal 2.07 ± 0.15 day−1. The annual peak reflects major seasonal changes in the patterns of offshore versus onshore flow associated with regional/hemispheric circulation patterns, whereas the diurnal and semi-diurnal peaks together reflect the influence of local nocturnal radon build-up over land, and the interplay between mesoscale sea/land breezes. The winter-time diurnal radon concentration had a significant decrease of about 200 mBq/m3 (17%) while the summer-time diurnal radon concentration revealed nearly no changes. A slow decline in the higher radon percentiles (75th and 95th) for the winter and spring seasons is found over the 15-year data set, with most of the change occurring in the first 9 years (1999–2007). This observed inter-annual decline appears to be associated with changes in the frequency of air masses having originated from over the African continental surfaces, and no significant trend is found in the lower radon percentiles associated with oceanic air masses. The general decrease of atmospheric radon-associated with continental air-masses at Cape Point could be attributed to changing meteorological conditions, possibly driven by climate change.

Long-term measurements of gaseous elemental mercury (GEM) concentrations at Cape Point, South Africa, reveal a downward trend between September 1995 and December 2005 and an upward one from March 2007 until June 2015, implying a change in trend sign between 2004 and 2007. The trend change is qualitatively consistent with the trend changes in GEM concentrations observed at Mace Head, Ireland, and in mercury wet deposition over North America, suggesting a change in worldwide mercury emissions. Seasonally resolved trends suggest a modulation of the overall trend by regional processes. The trends in absolute terms (downward in 1995–2004 and upward in 2007–2015) are highest in austral spring (SON), coinciding with the peak in emissions from biomass burning in South America and southern Africa. The influence of trends in biomass burning is further supported by a biennial variation in GEM concentration found here and an El Niño–Southern Oscillation (ENSO) signature in GEM concentrations reported recently.

International audience; Sulphur dioxide (SO 2), nitrogen dioxide (NO 2) and ozone (O 3) are considered typical inorganic gaseous pollutants in the atmosphere. The aim of this study was to assess long-term seasonal and inter-annual trends of a 21-year SO 2 , NO 2 and O 3 passive sampling (monthly means) dataset collected at the Cape Point Global Atmosphere Watch (CPT GAW) station. Comparison of the concentrations of these gaseous species with other IGAC DEBITS Africa measurement sites indicated that levels of these species were generally similar to other African inland ecosystems, but lower compared to an industrial site in southern Africa. The SO 2 , NO 2 and O 3 monthly mean concentrations showed seasonal patterns, which can be attributed to various factors influencing levels of these species at CPT GAW. These factors are generally season specific, which include changes in meteorological conditions and source contributions. Higher NO 2 concentrations were also attributed to increased microbial activity in the wet season. The O 3 seasonal pattern corresponded to the NO 2 seasonality, which was attributed to their related chemistry. SO 2 and NO 2 concentrations displayed inter-annual variability, while O 3 did not indicate significant inter-annual fluctuations. The seasonal and inter-annual variability was explored with a multilinear regression model, in which global, regional and local meteorological factors, as well as population growth were included. Modelling results indicated that variances in SO 2 concentrations were predominantly influenced by changes in global forcing factors. Global, regional and local factors played a significant role in NO 2 trends, which included the influence of population growth and associated increased anthropogenic activities. It was also established that variances in O 3 concentrations were predominantly associated with regional and local factors. Trend analysis indicated that SO 2 , NO 2 and O 3 concentrations remained relatively constant over the 21-year sampling period at this background site in the Southern Hemisphere.

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 con-tinuously 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 At-mosphere 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.

A ground-based aerosol optical depth (AOD) climatology is presented for Cape Point (CPT) station, South Africa, for the 2008–2013 period. CPT is part of the Global Atmosphere Watch–Precision Filter Radiometer network which conducts long-term AOD measurements at remote background sites. AOD (λ = 500 nm) and Angstrom exponent (368 to 862 nm; α368–862) averages for the entire period were 0.059 and 0.68, displaying only a weak seasonality. Based on an established method for air mass classification using the in situ wind direction and 222Rn concentration, the following four air mass types were used to further investigate AOD: background marine, marine, mixed, and continental. AOD was similar for all types, but α368–862 was distinctly lower (0.43) for background marine and higher (1.07) for continental air masses, illustrating the presence of coarse mode and anthropogenic aerosols, respectively. Trajectory cluster analysis of 5 day back trajectories confirmed/augmented this classification. AOD for background marine and marine air mass types were consistent with ship-based (Maritime Aerosol Network) and island (AErosol RObotic NETwork) measurements, suggesting that CPT is a suitable site to monitor pristine conditions in the South Atlantic and Southern Oceans when 222Rn concentrations are

CITATION: Botha, R. et al. 2018. Radon-222 measurements at Cape Point : a characterization of a 15-year time series. Clean Air Journal, 28(2):19-20, doi:10.17159/2410-972X/2018/v28n2a11.

The International Halocarbons in Air Comparison Experiment (IHALACE) was conducted to document relationships between calibration scales among various laboratories that measure atmospheric greenhouse and ozone depleting gases. This study included trace gases such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs), as well as nitrous oxide, methane, sulfur hexafluoride, very short-lived halocompounds, and carbonyl sulfide. Many of these gases are present in the unpolluted atmosphere at pmol mol−1 (parts per trillion) or nmol mol−1 (parts per billion) levels. Six stainless steel cylinders containing natural and modified natural air samples were circulated among 19 laboratories. Results from this experiment reveal relatively good agreement (within a few percent) among commonly used calibration scales. Scale relationships for some gases, such as CFC-12 and CCl4, were found to be consistent with those derived from estimates of global mean mole fractions, while others, such as halon-1211 and CH3Br, revealed discrepancies. The transfer of calibration scales among laboratories was problematic in many cases, meaning that measurements tied to a particular scale may not, in fact, be compatible. Large scale transfer errors were observed for CH3CCl3 (10–100%) and CCl4 (2–30%), while much smaller scale transfer errors (< 1%) were observed for halon-1211, HCFC-22, and HCFC-142b. These results reveal substantial improvements in calibration over previous comparisons. However, there is room for improvement in communication and coordination of calibration activities with respect to the measurement of halogenated and related trace gases.

Longer-term (i.e., 20–40 years) tropospheric ozone (O3) time series obtained from surface and ozonesonde observations have been analyzed to assess possible changes with time through 2010. The time series have been selected to reflect relatively broad geographic regions and where possible minimize local scale influences, generally avoiding sites close to larger urban areas. Several approaches have been used to describe the changes with time, including application of a time series model, running 15-year trends, and changes in the distribution by month in the O3 mixing ratio. Changes have been investigated utilizing monthly averages, as well as exposure metrics that focus on specific parts of the distribution of hourly average concentrations (e.g., low-, mid-, and high-level concentration ranges). Many of the longer time series (∼30 years) in mid-latitudes of the Northern Hemisphere, including those in Japan, show a pattern of significant increase in the earlier portion of the record, with a flattening over the last 10–15 years. It is uncertain if the flattening of the O3 change over Japan reflects the impact of O3 transported from continental East Asia in light of reported O3 increases in China. In the Canadian Arctic, declines from the beginning of the ozonesonde record in 1980 have mostly rebounded with little overall change over the period of record. The limited data in the tropical Pacific suggest very little change over the entire record. In the southern hemisphere subtropics and mid-latitudes, the significant increase observed in the early part of the record has leveled off in the most recent decade. At the South Pole, a decline observed during the first half of the 35-year record has reversed, and O3 has recovered to levels similar to the beginning of the record. Our understanding of the causes of the longer-term changes is limited, although it appears that in the mid-latitudes of the northern hemisphere, controls on O3 precursors have likely been a factor in the leveling off or decline from earlier O3 increases.

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.

Concern about the adverse effects of mercury on human health and ecosystems has led to tightening emission controls since the mid 1980s. But the resulting mercury emissions reductions in many parts of the world are believed to be offset or even surpassed by the increasing emissions in rapidly industrializing countries. Consequently, concentrations of atmospheric mercury are expected to remain roughly constant. Here we show that the worldwide atmospheric mercury concentrations have decreased by about 20 to 38 % since 1996 as indicated by long-term monitoring at stations in the Southern and Northern Hemispheres combined with intermittent measurements of latitudinal distribution over the Atlantic Ocean. The total reduction of the atmospheric mercury burden of this magnitude within 14 years is unusually large among most atmospheric trace gases and is at odds with the current mercury emission inventories with nearly constant anthropogenic emissions over this period. This suggests a major shift in the biogeochemical cycle of mercury including oceans and soil reservoirs. Decreasing reemissions from the legacy of historical mercury emissions are the most likely explanation for this decline since the hypothesis of an accelerated oxidation rate of elemental mercury in the atmosphere is not supported by the observed trends of other trace gases. Acidification of oceans, climate change, excess nutrient input and pollution may also contribute by their impact on the biogeochemistry of ocean and soils. Consequently, models of the atmospheric mercury cycle have to include soil and ocean mercury pools and their dynamics to be able to make projections of future trends.

The authors evaluated continuous high-resolution gaseous elemental mercury (GEM) data from the Cape Point Global Atmosphere Watch (CPT GAW) station with different statistical analysis techniques. GEM data were evaluated by cluster analysis and the results indicated that two clusters, separated at 0.904 ng m−3, existed. The air mass history for the two-cluster solution was investigated by means of back-trajectory analysis. The air mass back-trajectory net result showed lower GEM concentrations originating from the sparsely populated semi-arid interior of South Africa and the marine environment, whereas higher GEM concentrations originated predominately along the coast of South Africa that most likely coincide with trade routes and industrial activities in urban areas along the coast. Considering the net result from the air mass back-trajectories, it is evident that not all low GEM concentrations are from marine origin, and similarly, not all high GEM concentrations have a terrestrial origin. Equations were developed by means of multi-linear regression (MLR) analysis that allowed for the estimation and/or prediction of atmospheric GEM concentrations from other atmospheric parameters measured at the CPT GAW station. These equations also provided some insight into the relation and interaction of GEM with other atmospheric parameters. Both measured and MLR calculated data confirm a decline in GEM concentrations at CPT GAW over the period evaluated.

Measurements of surface ozone in the southern African region are limited to a few active and passive monitoring sites. Over the region, the mean surface ozone concentrations exhibit strong seasonal and diurnal variations. The seasonal maximum generally occurs in the spring months from August to November and the minimum occurs in December and January. With the exception of Cape Point, a strong diurnal variation is observed at all sites. Ozone concentrations increase from a minimum near sunrise to a maximum in the afternoon, then decrease again to the early morning minimum. The highest ozone concentrations occur over Botswana and the Mpumalanga highveld. In both regions the springtime maximum is between 40 and 60 ppb, but reached more than 90 ppb as a mean in October 2000. In these two regions the monthly minimum is between 20 and 30 ppb. The mean daytime ozone concentrations in Botswana and on the highveld reach 40 ppb as early as 10:00 and remain above this level for up to 10 h. At the background stations at Cape Point, in Namibia and areas adjacent to the highveld the maximum concentrations are between 20 and 30 ppb with minimums between 10 and 20 ppb. In the Cross border Air Pollution Impact Assessment project, CAPIA, a threshold value of 40 ppb is used to assess the potential risk of damage to maize by ozone. Measured data show that this threshold is exceeded over Botswana and on the South African highveld.

Concentrations of 222Rn (March 1999–August 2002) from a recently installed analyser at the station Cape Point (34°S, 18°E), South Africa, were analysed statistically. The combination of 222Rn with information on wind direction and carbon monoxide (CO) permitted a classification of air masses into continental, marine, and mixtures of both. The ability to select trace gas data representing purely maritime conditions is shown through application to carbon dioxide (CO2) data. 222Rn levels at Cape Point ranged from near zero to above 5000 mBq m−3. Monthly percentiles show practically no seasonal dependence for values up to the 25th percentile (P25), corresponding to 222Rn

The Monitoring Atmospheric Composition and Climate (MACC) project represents the European Union's Copernicus Atmosphere Monitoring Service (CAMS) (http://www.copernicus.eu/), which became fully operational during 2015. The global near-real-time MACC model production run for aerosol and reactive gases provides daily analyses and 5-day forecasts of atmospheric composition fields. It is the only assimilation system worldwide that is operational to produce global analyses and forecasts of reactive gases and aerosol fields. We have investigated the ability of the MACC analysis system to simulate tropospheric concentrations of reactive gases covering the period between 2009 and 2012. A validation was performed based on carbon monoxide (CO), nitrogen dioxide (NO2) and ozone (O3) surface observations from the Global Atmosphere Watch (GAW) network, the O3 surface observations from the European Monitoring and Evaluation Programme (EMEP) and, furthermore, NO2 tropospheric columns, as well as CO total columns, derived from satellite sensors. The MACC system proved capable of reproducing reactive gas concentrations with consistent quality; however, with a seasonally dependent bias compared to surface and satellite observations – for northern hemispheric surface O3 mixing ratios, positive biases appear during the warm seasons and negative biases during the cold parts of the year, with monthly modified normalised mean biases (MNMBs) ranging between −30 and 30 % at the surface. Model biases are likely to result from difficulties in the simulation of vertical mixing at night and deficiencies in the model's dry deposition parameterisation. Observed tropospheric columns of NO2 and CO could be reproduced correctly during the warm seasons, but are mostly underestimated by the model during the cold seasons, when anthropogenic emissions are at their highest level, especially over the US, Europe and Asia. Monthly MNMBs of the satellite data evaluation range from values between −110 and 40 % for NO2 and at most −20 % for CO, over the investigated regions. The underestimation is likely to result from a combination of errors concerning the dry deposition parameterisation and certain limitations in the current emission inventories, together with an insufficiently established seasonality in the emissions.

Specific factors of a baseline 222Rn detector installed at Cape Point, South Africa, were studied with the aim of improving its performance. Direct sunlight caused air turbulence within the instrument, resulting in 13.6% variability of the calibration factor. Shading the instrument eliminated this effect. A residual temperature dependence of the calibration factor was reduced to negligible levels with an improved photomultiplier tube. A superior detector head permits field servicing of the instrument, and has reduced one component of the instrumental background by a factor of 2. The other component probably constitutes thoron emissions from the stainless steel walls. The detection limit of the instrument could be reduced from its current 33 to 20 mBq m−3 if the thoron were to be eliminated.

The monitoring of total gaseous Hg (TGM) was established at the Cape Point Global Atmospheric Watch (GAW) station in September 1995. This paper presents the first data obtained until June 1999. Atmospheric Hg concentrations were found to be fairly homogeneous fluctuating between 1.2–1.4 ng m −3 . Whilst no significant diurnal variation is detectable, a slight seasonal variation with a TGM minimum in March–May and maximum in June–August was observed. A minimum annual TGM concentration was detected in 1997, 1 year later than observed at the Wank summit in the Northern Hemisphere (NH). The Cape Point GAW station was found to constitute a suitable site for monitoring TGM concentrations in the Southern Hemisphere (SH).

During the continuous monitoring of atmospheric parameters at the station Cape Point (343S, 183E), a smoke plume originating from a controlled "re of 30-yr-old fynbos was observed on 6 May 1997. For this episode, which was associated with a nocturnal inversion and o!shore air#ow, atmospheric parameters (solar radiation and meteorological data) were considered and the levels of various trace gases compared with those measured at Cape Point in maritime air. Concentration maxima in the morning of 6 May for CO 2 , CO, CH 4 and O 3 amounted to 370.3 ppm, 491 ppb, 1730 ppb and 47 ppb, respectively, whilst the mixing ratios of several halocarbons (F-11, F-12, F-113, CCl 4 and CH 3 CCl 3 ) remained at background levels. In the case of CO, the maritime background level for this period was exceeded by a factor of 9.8. Di!erences in ozone levels of up to 5 ppb between air intakes at 4 and 30 m above the station (located at 230 m above sea level) indicated strati"cation of the air advected to Cape Point during the plume event. Aerosols within the smoke plume caused the signal of global solar radiation and UV}A to be attenuated from 52.4 to 13.0 mW cm~2 and from 2.3 to 1.3 mW cm~2, respectively, 5 h after the trace gases had reached their maxima. Emission ratios (ERs) calculated for CO and CH 4 relative to CO 2 mixing ratios amounted to 0.042 and 0.0040, respectively, representing one of the "rst results for "res involving fynbos. The CO ER is somewhat lower than those given in the literature for African savanna "res (average ER"0.048), whilst for CH 4 the ER falls within the range of ERs reported for the #aming (0.0030) and smouldering phases (0.0055) of savanna "res. Non-methane hydrocarbon (NMHC) data obtained from a grab sample collected during the plume event were compared to background levels. The highest ERs (*NMHC/*CH 4 ) have been obtained for the C 2 }C 3 hydrocarbons (e.g. ethene at 229.3 ppt ppb~1), whilst the C 4 }C 7 hydrocarbons were characterised by the lowest ERs (e.g. n-hexane at 1.0 and n-pentane at 0.8 ppt ppb~1). ( 2001 Elsevier Science Ltd. All rights reserved.

Using a set of selected surface ozone (nine stations) and ozone vertical profile measurements (from six stations), we have documented changes in tropospheric ozone at a number of locations. From two stations at high northern hemisphere (NH) latitudes there has been a significant decline in ozone amounts throughout the troposphere since the early 1980s. At midlatitudes of the NH where data are the most abundant, on the other hand, important regional differences prevail. The two stations in the eastern United States show that changes in ozone concentrations since the early 1970s have been relatively small. At the two sites in Europe, however, ozone amounts increased rapidly into the mid-1980s, but have increased less rapidly (or in some places not at all) since then. Increases at the Japanese ozonesonde station have been largest in the lower troposphere, but have slowed in the recent decade. The tropics are sparsely sampled but do not show significant changes. Small increases are suggested at southern hemisphere (SH) midlatitudes by the two surface data records. In Antarctica large declines in the ozone concentration are noted in the South Pole data, and like those at high latitudes of the NH, seem to parallel the large decreases in the stratosphere.

Gaseous elemental mercury (GEM) has been measured at the WMO Global Atmospheric Watch station at Cape Point, South Africa since September 1995. Two techniques were used: a low resolution manual technique till the end of 2004 and a high resolution automated technique since March 2007. The GEM measurements at Cape Point constitute only one component of the GAW monitoring program consisting of continuous measurements of CO, CH4 , CO2 , O3 , N2 O, and since March 1999 also of 222 Rn. The seasonality and trend of GEM concentrations from the low resolution data was analyzed by Slemr et al. (2008) and the trend of the combined low and high resolution data until the end of 2009 by Slemr et al. (2011). In this paper we will present an updated analysis of the trend and seasonality of GEM data until the end of 2011 and compare these to measurements made at Troll, a Norwegian research station in Antarctica (Pfaffhuber et al., 2012).

Gaseous elemental mercury (GEM) and 222Rn, a radioactive gas of primarily terrestrial origin with a half-life of 3.8 days, have been measured simultaneously at Cape Point, South Africa, since March 2007. Between March 2007 and December 2011 altogether 191 events with high 222Rn concentrations were identified. GEM correlated with 222Rn in 94 of the events and was constant during almost all the remaining events without significant correlation. The average GEM/222Rn emission ratio of all events including the non-significant ones was −0.0001 &pm; 0.0030 pg mBq−1, with 0.0030 pg mBq−1 being the standard error of the average. With an emission rate of 1.1 222Rn atoms cm−2 s−1 and a correction for the transport duration, this emission ratio corresponds to a radon calibrated flux of about −0.01 ng GEM m−2 h−1 with a standard error of &pm;0.34 ng GEM m−2 h−1 (n = 191). With wet deposition, which is not included in this estimate, the terrestrial surface of southern Africa seems to be a net mercury sink of about −1.01 ng m−2 h−1.

Mercury emissions in South Africa have so far been estimated only by a bottom-up approach from activities and emission factors for different processes. In this paper we derive GEM/CO (GEM being gaseous elemental mercury, Hg0), GEM/CO2, GEM/CH4, CO/CO2, CH4/CO2, and CH4/CO emission ratios from plumes observed during long-term monitoring of these species at Cape Point between March 2007 and December 2009. The average observed GEM/CO, GEM/CO2, GEM/CH4, CO/CO2, CH4/CO2, and CH4/CO emission ratios were 2.40 ± 2.65 pg m−3 ppb−1 (n = 47), 62.7 ± 80.2 pg m−3 ppm−1 (n = 44), 3.61 ± 4.66 pg m−3 ppb−1 (n = 46), 35.6 ± 25.4 ppb ppm−1 (n = 52), 20.2 ± 15.5 ppb ppm−1 (n = 48), and 0.876 ± 1.106 ppb ppb−1 (n = 42), respectively. The observed CO/CO2, CH4/CO2, and CH4/CO emission ratios agree within the combined uncertainties of the observations and emissions with the ratios calculated from EDGAR (version 4.2) CO2, CO, and CH4 inventories for South Africa and southern Africa (South Africa, Lesotho, Swaziland, Namibia, Botswana, Zimbabwe, and Mozambique) in 2007 and 2008 (inventories for 2009 are not available yet). Total elemental mercury emission of 13.1, 15.2, and 16.1 t Hg yr−1 are estimated independently using the GEM/CO, GEM/CO2, and GEM/CH4 emission ratios and the annual mean CO, CO2, and CH4 emissions, respectively, of South Africa in 2007 and 2008. The average of these independent estimates of 14.8 t GEM yr−1 is much less than the total emission of 257 t Hg yr−1 shown by older inventories which are now considered to be wrong. Considering the uncertainties of our emission estimate, of the emission inventories, and the fact that emission of GEM represents 50–78 % of all mercury emissions, our estimate is comparable to the currently cited GEM emissions in 2004 and somewhat smaller than emissions in 2006. A further increase of mercury emissions due to increasing electricity consumption will lead to a more pronounced difference. A quantitative assessment of the difference and its significance, however, will require emission inventories for the years of observations (2007–2009) as well as better data on the speciation of the total mercury emissions in South Africa.

The recent increase of atmospheric methane is investigated by using two atmospheric inversions to quantify the distribution of sources and sinks for the 2006–2008 period, and a process-based model of methane emissions by natural wetland ecosystems. Methane emissions derived from the two inversions are consistent at a global scale: emissions are decreased in 2006 (−7 Tg) and increased in 2007 (+21 Tg) and 2008 (+18 Tg), as compared to the 1999–2006 period. The agreement on the latitudinal partition of the flux anomalies for the two inversions is fair in 2006, good in 2007, and not good in 2008. In 2007, a positive anomaly of tropical emissions is found to be the main contributor to the global emission anomalies (~60–80%) for both inversions, with a dominant share attributed to natural wetlands (~2/3), and a significant contribution from high latitudes (~25%). The wetland ecosystem model produces smaller and more balanced positive emission anomalies between the tropics and the high latitudes for 2006, 2007 and 2008, mainly due to precipitation changes during these years. At a global scale, the agreement between the ecosystem model and the inversions is good in 2008 but not satisfying in 2006 and 2007. Tropical South America and Boreal Eurasia appear to be major contributors to variations in methane emissions consistently in the inversions and the ecosystem model. Finally, changes in OH radicals during 2006–2008 are found to be less than 1% in inversions, with only a small impact on the inferred methane emissions.

The recent increase of atmospheric methane is investigated by using two atmospheric inversions to quantify the global distribution of sources and sinks for the 2006–2008 period, and a process-based model of CH4 emissions by natural wetland ecosystems. Global emissions derived from inversions are found to have increased by 19 Tg on average in 2007 (16 to 21 Tg) and by 13 Tg in 2008 (6 to 20 Tg), as compared to the 1999–2006 period. A positive anomaly of tropical emissions is found to be the main contributor to the global emission anomaly of 2007 (~60–75%), with a dominant share attributed to natural wetlands (~66%). Abnormally high wetlands emissions from high latitudes are also detected by both inversions in 2007, contributing 15–30% of the global anomaly. Good agreement is found between the results of the wetland ecosystem model and the inversions for 2007. The inferred distribution of the source anomaly in 2007 is shown to be consistent with the observation of a more pronounced increase in near surface methane atmospheric growth rate at high latitudes, because the dilution of surface fluxes by convection is strong in the tropics and weak at high latitudes. The source anomaly in 2008 is found to be much larger in the wetland ecosystem model than in the inversions, suggesting a too strong sensitivity of bottom-up modeled emissions to precipitation. Changes in OH radicals during 2006–2008 are found to be less than 1% in inversions, with only a small impact on the inferred methane emissions.

Gaseous mercury in the marine boundary layer has been measured with a 15 min temporal resolution at the Global Atmosphere Watch station Cape Point since March 2007. The most prominent features of the data until July 2008 are the frequent occurrences of pollution (PEs) and depletion events (DEs). Both types of events originate mostly within a short transport distance (up to about 100 km), which are embedded in air masses ranging from marine background to continental. The Hg/CO emission ratios observed during the PEs are within the range reported for biomass burning and industrial/urban emissions. The depletion of gaseous mercury during the DEs is in many cases almost complete and suggests an atmospheric residence time of elemental mercury as short as a few dozens of hours, which is in contrast to the commonly used estimate of approximately 1 year. The DEs observed at Cape Point are not accompanied by simultaneous depletion of ozone which distinguishes them from the halogen driven atmospheric mercury depletion events (AMDEs) observed in Polar Regions. Nonetheless, DEs similar to those observed at Cape Point have also been observed at other places in the marine boundary layer. Additional measurements of mercury speciation and of possible mercury oxidants are hence called for to reveal the chemical mechanism of the newly observed DEs and to assess its importance on larger scales.

This paper documents a global Bayesian variational inversion of CO2 surface fluxes during the period 1988–2008. Weekly fluxes are estimated on a 3.75° × 2.5° (longitude-latitude) grid throughout the 21 years. The assimilated observations include 128 station records from three large data sets of surface CO2 mixing ratio measurements. A Monte Carlo approach rigorously quantifies the theoretical uncertainty of the inverted fluxes at various space and time scales, which is particularly important for proper interpretation of the inverted fluxes. Fluxes are evaluated indirectly against two independent CO2 vertical profile data sets constructed from aircraft measurements in the boundary layer and in the free troposphere. The skill of the inversion is evaluated by the improvement brought over a simple benchmark flux estimation based on the observed atmospheric growth rate. Our error analysis indicates that the carbon budget from the inversion should be more accurate than the a priori carbon budget by 20% to 60% for terrestrial fluxes aggregated at the scale of subcontinental regions in the Northern Hemisphere and over a year, but the inversion cannot clearly distinguish between the regional carbon budgets within a continent. On the basis of the independent observations, the inversion is seen to improve the fluxes compared to the benchmark: the atmospheric simulation of CO2 with the Bayesian inversion method is better by about 1 ppm than the benchmark in the free troposphere, despite possible systematic transport errors. The inversion achieves this improvement by changing the regional fluxes over land at the seasonal and at the interannual time scales. This work was performed using HPC resources from GENCI‐ (CCRT/CINES/IDRIS; grant 2009‐ t2009012201). It was cofunded by the European Commission under the EU Seventh Research Framework Programme (grant agreements 212196, COCOS, and 218793, MACC).

[1] Total gaseous mercury (TGM) has been measured at the WMO Global Atmosphere Watch (GAW) station at Cape Point, South Africa, since September 1995, representing the only long term TGM measurement in the Southern Hemisphere (SH) so far. Annual medians suggest a small but significant decrease of TGM concentrations from 1.29 ng m−3 in 1996 to 1.19 ng m−3 in 2004. Background TGM concentrations at Cape Point varied seasonally, with a summer maximum and a winter minimum. Comparison with the seasonal variation of CO concentrations in both hemispheres calls into question the influence of mercury sink by the Hg0 + OH reaction. If confirmed at other sites in the SH, the observed TGM seasonal variation may pose an important constraint on the global models of atmospheric mercury.

Since 1978, a measuring station has been operated at Cape Point (34°21′ S, 18°29′ E). In this article, results of measurements of CO, CFCl3, CCl4, O3, N2O and CH4 are presented as monthly means and analyzed with respect to long-term trends and seasonal variations. For CO and CH4, very similar seasonal variations have been observed, indicating strong interrelations between these two gases. For CO and O3, no significant changes of the mean annual concentrations can be established for the observation periods of 10 and 5 years, respectively. The measurements yield a growth rate of 9.1 pptv yr-1 for CFCl3 (1980–1987) and 0.6 ppbv yr-1 for N2O (1983–1987). The concentration increases of CH4 (10.3 ppbv yr-1 for 1983–1987) and of CCl4 (2.1 pptv yr-1 for 1980–1988) are analyzed for temporal changes during the last years.

The results of CO, CH4 and N2O measurements performed over several years at the station Cape Point (34°21′S, 18dg29′E), South Africa, are presented. The analytical technique and data filtering procedure are described. The results indicate seasonal variations of the CO and CH4 mixing ratios with maximum values around October and minimum values in February/March. The patterns and the amplitudes of the seasonal variations of both gases compare suprisingly well, indicating interrelations between the source-sink and/or transport mechanisms of CH4 and CO in the Southern Hemisphere. CO measurements performed over a period of 9 years do not show any significant trend of the mean annual CO mixing ratios while the N2O mixing ratios increase at a rate of 0.2% per year for the observation period from 1983 to 1987. The data obtained for CH4 between 1983 and 1987 yield an increase rate of 0.7% per year indicating a reduction of the CH4 increase in the Southern Hemisphere during recent years.