Acidic deposition emanating from the South African highveld – a critical levels and critical loads assessment

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dc.contributor.advisor Prof. H. J. Annegarn en
dc.contributor.author Josipovic, Miroslav
dc.date.accessioned 2009-12-08T10:26:20Z
dc.date.available 2009-12-08T10:26:20Z
dc.date.issued 2009-12-08T10:26:20Z
dc.date.submitted 2009
dc.identifier.uri http://hdl.handle.net/10210/3017
dc.description Ph.D. (Environmental Science) en
dc.description.abstract A number of studies to detect effects of atmospheric acidic deposition on regional environments have been conducted in South Africa in the past, without finding any clear evidence of adverse effects. Despite these studies, scepticism remained that acidic deposition could accumulate on the decadal scale and reach a point where the natural buffering capacities of soils and water bodies would be exceeded. Against this background, this study was conceived to make direct measurements of atmospheric concentrations of the acidifying gases, over as an extensive area as possible, to provide an objective basis to the question of whether or not acidic deposition pose immediate or long-term threats to regional ecosystems. As an integral study, the research strategy was designed to address the question as to whether ozone, as a secondary pollutant, would exceed the tolerance of the same ecosystems. A passive monitoring network was devised to measure monthly mean atmospheric concentrations of three trace gas species, SO2, NO2 and O3. The network comprised of 37 monitoring sites at remote locations over the northern and eastern portions of South Africa, at 1º grid intervals (0.5 º for several sites). Trace gases were sampled monthly over two complete annual cycles at each site, using passive diffusive samplers exposed for approximately 30 days. Samples were chemically analysed at an internationally accredited laboratory (Atmospheric Chemistry Research Group, North-West University) and mean monthly results were recorded. The collated database of trace gas concentrations enabled assessment in terms of standards, time and spatial distributions. Concentrations were evaluated in a critical levels assessment against several sets of South African and international standards and guidelines. The dry deposition rates were calculated from measured ambient trace gas concentrations, using an inferential model. For estimates of wet deposition, long-term acidic rainfall measurements were multiplied by the cumulative precipitation within the sampling period amount for each project site. These total dry and wet deposition estimates (for sulphur and nitrogen compounds), and concentrations of ozone were compared with results from two regional-scale studies of modelled concentration and deposition. The total acidic deposition estimates were adjusted with mitigating base cation deposition estimates, dry and wet, and the derived net acidity loads were subtracted from soils acid buffering (sensitivity) capacity loads to determine critical loads exceedance. Two ancillary tasks were undertaken, which contribute to the central aim. The first accessed the most recent measured regional lighting strokes data and adjusted it for intra/inter-cloud strokes to estimate the lightning produced NOx (LNOx) budget. High and a low lightning NOx budget estimates were then compared with three relevant anthropogenic emission inventories compiled for the Highveld region. The second task obtained regional SO2, NO2 and O3 remote sensing information and compared them with ground level concentrations from the passive sampling network. Remote sensing total column densities were converted and partitioned through atmospheric pressure averaging equations to derive tropospheric boundary concentrations appropriate for comparisons. vi The main findings were: • Concentration distributions for acidic gases SO2 and NO2 show prevailing high concentrations over the industrial Highveld. The areas downwind show substantially decreased concentrations, while remote areas show very low concentrations. Ozone concentrations are spatially uniform, with slightly increased concentrations located away from the central Highveld pollution source area, both north and south. However, ozone distributions and origins could not be directly related to the Highveld industrial emissions. No health-based exceedance (of long-term standards) was recorded for SO2, NO2 or O3. Three areas of critical exceedances were found for SO2, one in the industrial Highveld for sensitive lichen and semi-natural and forest vegetation; and two downwind, for sensitive lichen only. No ecosystem exceedance was established of NO2 and O3 critical levels. • Acidic deposition distributions showed a direct relationship to high atmospheric concentration distributions, with exceptions for those areas which had high annual rainfall, and for coastal and escarpment areas where wet deposition prevailed. No concurrent regional modelling had been performed by other parties. Two non-concurrent regional modelling studies were selected for comparison of measured and modelling results. Substantial disparities were identified between the respective results and, although general patterns were followed, the modelling for dry and wet acidic compounds swayed from overestimates to underestimates for sulphur and nitrogen species. In contrast, modelling of ozone mainly overestimated measured concentrations. In terms of spatial distributions, ozone modelling matched a wide band of elevated concentrations in northern area of South Africa (Limpopo Province). • Derived net acidic deposition loads were matched to critical loads from soil types in the region. This revealed areas with high deposition and highly sensitive local soils within the greater industrial Highveld with critical load exceedance. An area north-west of the central Highveld showed exceedance of the highest critical load, while several smaller areas downwind to the south-east showed exceedances of lower critical loads only. • The lightning NOx budget estimation showed that lightning NOx is a significant contributor to overall regional NOx budget and a major natural contributor. Depending on lightning count assumptions applied, annual LNOx production varied from a 1/10th to 1/3rd of the NOx anthropogenic trace gases emissions, taken from the most comprehensive of three recent inventories. However, when seasonal lightning budgets were examined, no distinct seasonal increases in ground-level NOx were found in areas of high lightning frequency, suggesting that much of LNOx is generated and transported above the boundary layer. • Comparisons of remote sensing concentrations results for SO2 and O3 have not revealed very good spatial and temporal agreement, although both showed agreement within an order of magnitude between measured and remotely sensed concentrations. NO2 agreement was much better in both spatial and temporal comparisons. The mean ratio of remotely sensed to measured NOx is 0.77, an encouraging result given the broad assumptions made in deriving the comparison. Disparities in the horizontal distributions between ground level and satellite products for NOx indicate a need for better information on the vertical distributions of NOx vii and other trace gases – the assumption of a uniformly mixed boundary layer is not robust enough for such comparisons. From findings of all individual studies an overall answer to the central question is reached that pollution from acidic gases and their compounds is not a current or medium term threat to regional ecosystems (beyond the central pollution source area), at current rates of emission. Results from the control site, positioned specifically within the well-researched industrial Highveld, confirmed the known situation in this sub-region, while serving well to put in perspective the entire region. Ozone distributions indicate uniform regional distributions, with most monthly means below 30 ppb, and highest single month 43 ppb. Now that this project has established a detailed measured set of acidic trace gas concentrations, limiting factors in determining critical loads are (i) more detailed maps of soil sensitivity (acidic buffering capacity); and (ii) base cation deposition measurements, including dry deposition of dust, and wet deposition, both at more sites and over similar periods used in the current study for the acid components. en
dc.language.iso en en
dc.subject Acid deposition en
dc.title Acidic deposition emanating from the South African highveld – a critical levels and critical loads assessment en
dc.type Thesis (D. Phil) en

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