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- Volume 18, Issue 1, 2010
Clean Air Journal = Tydskrif vir Skoon Lug - Volume 18, Issue 1, 2010
Volume 18, Issue 1, 2010
Author Hanlie Liebenberg-EnslinSource: Clean Air Journal = Tydskrif vir Skoon Lug 18 (2010)More Less
For the past two years the NACA Journal has been silent. This was due to the attempt to provide our members with the NACA Magazine, a combination of the traditional journal content and more contemporary air quality matters. The financial outset proved not viable and I am pleased to welcome back our trusted Journal. This publication covers feedback from the 2009 Annual Conference and the 2009 winning papers.
Comparisons of meso-scale air pollution dispersion modelling of SO2, NO2 and O3 using regional-scale monitoring resultsSource: Clean Air Journal = Tydskrif vir Skoon Lug 18, pp 3 –8 (2010)More Less
Results of a regional-scale monitoring campaign were compared with two meso-scale to sub-continental modelling studies, for SO2 and NO2 and O3 respectively (Fourie, 2006, Zunckel et al., 2006, van Tienhoven et al., 2006, Van Tienhoven and Zunckel, 2004). However, a direct validation of the monitored results with modelled results could not be carried out, as available modelling studies dealt with different periods from the monitoring study.
For this study, three monitoring sites were selected for comparison with modelling results. These sites were strategically selected to be representative of the entire region. Site Elandsfontein in the centre of the industrial Highveld, site Amersfoort, downwind from the central pollution source region and site Louis Trichardt, a remote site. Sulphur, nitrogen and ozone species comparisons were considered in turn. The comparisons were carried out for equivalent annual (and seasonal) cycles. The comparisons produced mixed results. For sulphur and nitrogen species in most cases, depending on site and season, modelling results ranged between significant underestimates to overestimates. Ozone modelling almost always overestimated the concentrations compared to the measured results.
Despite several limiting factors, constraining the reliability of the comparisons between the modelled and measured results, they were important as the distribution of the gases showed patterns that imply understanding of the source and fate of these pollutants. The uncertainty in the magnitude of the model inaccuracies as well as margin of error of the measured data remained. Thus a modelling validation is recommended using the concurrent period with fewer uncertainties.
Source: Clean Air Journal = Tydskrif vir Skoon Lug 18, pp 10 –14 (2010)More Less
Landfill gas (LFG) consisting of 50-60 % v/v CH4 contributes to global greenhouse gas emissions as well as to local air pollution and nuisance odours; in addition, the uncontrolled subsurface migration of LFG can pose an explosion hazard. LFG is explosive mostly due to its CH4 content. CH4 is explosive at concentrations of 5-15 % in air. Venting of the gas to the atmosphere prevents any explosion risk; however, the concern lies with the lateral migration of CH4 through soil and along cracks and its subsequent accumulation. This highlights the importance of subsurface LFG monitoring. In this study, subsurface LFG generation is measured at a solid waste disposal site situated approximately 20 km west of Johannesburg. The results of three first-order kinetic models (to estimate LFG generation) for the site are compared. The three models are LandGEM, GasSim and the IPCC model contained in the 2006 UNFCCC 2006 National Inventory Guidelines for waste. High LFG concentrations are recorded along the northern boundary of the site (exceeding 60% v/v). Modelled LFG generation simulations are slightly higher from LandGEM whilst the IPCC Waste Model predicts the lowest concentrations.
Source: Clean Air Journal = Tydskrif vir Skoon Lug 18, pp 17 –21 (2010)More Less
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, CO2, 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/CO2 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.
The forcing of a photochemical air quality model with atmospheric fields simulated by a regional climate model dataSource: Clean Air Journal = Tydskrif vir Skoon Lug 18, pp 22 –28 (2010)More Less
Within the context of climate change over southern Africa, little is understood about the potential local response of air quality to changes in the larger scale environment. Under future climate forcing, there may be significant changes in the thermodynamic structure of the atmosphere over southern Africa, and in the circulation dynamics of the region. For example, there is evidence that more intense surface inversion layers may occur over the central interior of South Africa in response to the enhanced greenhouse effect (e.g. Engelbrecht et al, 2009). Such changes are likely to influence the future transport and chemistry of air pollutants over the region. The complexity in which climate change may affect regional air quality is evident.
Sophisticated numerical models are required to describe such complexity, and need to take into account all meteorological and emission changes, as well as apply relevant dispersion and chemistry to solving variation in pollutant concentration. The capacity to force atmospheric chemistry models with the output of regional climate models exists within CSIR NRE in the form of the photochemical air quality model CAMx forced by the MM5 regional climate model. This is useful for retrospective studies in air quality however the aspect of future climate forcing on air pollution is not addressed. This paper describes the development of new modelling capacity suitable for the simulation of photochemistry over southern Africa under both current and future anthropogenic forcing. The model CAMx was nested within the output of the regional climate model CCAM, which is applied at the NRE to obtain detailed projections of regional climate change. This new configuration may potentially be applied for photochemistry modelling at all time-scales, but the emphasis is on the very long integrations suitable to describe photochemistry characteristics over southern Africa at the climate-change time scale.
Here, the new CCAM-CAMx configuration was tested by simulating a selected previously modelled ozone episode, serving to demonstrate the capability of incorporating CCAM data into CAMx as well as investigating how well the new system performs. Results show the importance of preparing appropriate cloud and precipitation data from the CCAM output, as well as a need for finer vertical resolutions in CCAM. In general, CCAM may be appropriately used to force CAMx, illustrating the potential of the new system to simulate photochemistry and air quality over southern Africa under conditions of current and future anthropogenic forcing.