| dc.description.abstract | Adverse effects of acid deposition ascribed to increased atmospheric emission rates of acid-forming pollutants on terrestrial and aquatic ecosystems are a global concern. This is largely accorded to the emission of sulphur dioxide (SO2) and nitrogen oxides (NOX) into the atmosphere. These acidic species of sulphur and nitrogen are the main contributors to soil acidification and subsequent leaching of nutrient base cations, which may lead to surface-water acidification (Hicks & Kuylenstierna, 2009; Stevens, Dise & Cowling, 2009). Rapid industrial development in South Africa due to the abundance of mineral resources may have contributed largely to the acidification of regional ecosystems. Kuylenstierna and Hicks (2002) reported that acid deposition may exceed deposition loads of ecosystems over the Mpumalanga Highveld region, and emphasised the need for research studies to make a direct link between atmospheric concentrations of acid-forming pollutants, deposition fluxes, and changes to terrestrial and aquatic ecosystems. Research studies focusing over eastern South Africa have previously reported that atmospheric deposition fluxes of sulphur and nitrogen do not pose an immediate threat to regional ecosystems (Mphepya, 2002; Bird, 2011; Mabhaudhi, 2014). Thus, direct measurements of selected acid-forming gaseous species and water-soluble aerosols were taken to estimate if terrestrial and aquatic ecosystems in industrial, background and remote sites under study are at risk to possible acidification effects since industrial SO2 and NOX emission rates have increased continuously over the years (Pretorius et al., 2015). Ecosystems susceptible to acid deposition as a result of increased emission rates are likely to show adverse effects in the future (Kuylenstierna et al., 2001). Therefore, deposition fluxes need to be quantified to assess their impact on regional ecosystems.
This work reports on wet (June 2015 to November 2016) and dry deposition (December 2010 to September 2016) at selected industrial sites (Elandsfontein, Lephalale), background sites (Cathedral Peak, Vaalwater) and a remote (Knysna) site. The Knysna site is in a remote area away from industrial facilities. This site was included to compare results with the inland sites that are affected predominantly by industrial emissions over north-eastern South Africa. The six study sites in the Lephalale region (L1 to L6) were chosen solely for the monitoring of ambient gaseous concentrations and dry deposition (2010 to 2016). The other four study sites (at Elandsfontein, Cathedral Peak, Vaalwater and Knysna), referred to as “SANCOOP” (South Africa – Norway Research Co-operation) sites, were monitored simultaneously for chemical characterisation of wet-and-dry atmospheric deposition (2015 to 2016). Rain-water samples were collected using automated wet-only (AeroChemetric) samplers based on event sampling, and analysed for mineral ions (H+, NO3─, SO42─, Na+, Cl─, F─, NH4+, K+, Mg2+ and Ca2+), organic ions (CH3COO─, HCOO─, C3H5O2─, C2O42─) and total carbonates (HCO3─ and CO32─). Ionic and conductivity balance was used for data quality assessment of rain-water samples, according to the World Meteorological Organisation (WMO) and Deposition of Biogeochemically Important Trace Species (DEBITS) analytical protocols. The gaseous species of SO2, NO2 and O3 were monitored annually using the Swedish Environmental Research Institute (IVL) passive samplers, which were exposed in pairs for data reliability. These samplers are widely used within the DEBITS network and are suitable for sampling gaseous species in tropical and subtropical regions. The gaseous species (after sampler elution) and rain-water samples were analysed using ion chromatography. The wet deposition fluxes were estimated using rain-water ionic concentration values of chemical species and rain depth. The inferential method was used for estimating dry deposition fluxes of SO2, NO2 and O3 by using averaged ambient concentrations and applicable deposition velocities (Mphepya, 2002; Zhang, Brook & Vet, 2003).
The annual Volume-Weighted Mean (VWM) concentration of sulphate (SO42─) was highest at Elandsfontein (40.89 μeq/L), followed by Vaalwater (39.50 μeq/L) and Cathedral Peak (29.25 μeq/L), with the lowest at Knysna (15.66 μeq/L). Similarly, the highest annual concentration of nitrate (NO3─) was measured at Elandsfontein (22.82 μeq/L), followed by Vaalwater (22.63 μeq/L), Cathedral Peak (20.88 μeq/L), and lowest at Knysna (4.68 μeq/L). This trend in SO42─ and NO3─ concentrations changed for ammonium (NH4+), where the highest annual concentration was measured at Cathedral Peak (25.23 μeq/L), and followed by Vaalwater (23.85 μeq/L) and Elandsfontein (19.04 μeq/L), with the lowest at Knysna (18.30 μeq/L). The lowest annual concentration values for SO42─, NO3─ and NH4+ were measured at Knynsa. The highest annual VWM concentration of organic acids (HCOO─, CH3COO─, C3H5O2─, C2O42─) was measured at Vaalwater (3.29 μeq/L) and Cathedral Peak (1.73 μeq/L), with the lowest at Elandsfontein (0.96 μeq/L) and Knysna (0.56 μeq/L). The VWM concentrations of mineral and organic rain-water ionic species were generally highest during Spring and Summer.
The annual average ambient concentration of SO2 (9.01 ppb) at the Elandsfontein site was greater than the highest annual ambient concentration of 5.82 ppb measured at Lephalale site L3. The annual average NO2 (4.36 ppb) and O3 (18.81 ppb) ambient concentrations measured at the Elandsfontein site were comparable respectively to the highest annual ambient concentrations of NO2 (L4 = 4.52 ppb) and O3 (L3 = 16.82 ppb) measured at Lephalale, which is also an industrial area. The annual average gaseous concentrations of SO2 and NO2 measured at Cathedral Peak, Vaalwater and Knysna were no greater than 1.42 ppb. The highest annual average ambient concentration of O3 was measured at Cathedral Peak (25.15 ppb) and Vaalwater (21.02 ppb). The lowest annual O3 concentration of all SANCOOP sites, measured at Knysna (16.46 ppb), was closely comparable with the annual O3 concentration at Lephalale (L3 = 16.82 ppb) but slightly lower compared with Elandsfontein (18.81 ppb). The highest seasonal concentrations of SO2, NO2 and O3 at the Lephalale sites were measured in Spring. The seasonal variations in concentrations of SO2 and NO2 were not as pronounced at the SANCOOP sites. The highest seasonal concentration of O3 at the SANCOOP sites was generally observed in Summer and Spring.
The rain-water composition was analysed primarily for marine, crustal and anthropogenic activities using the sum of the source-apportioned ionic concentrations (μeq/L) of K+, Ca2+, Mg2+, Cl─ and SO42─. The percentage contributions of the marine, crustal and anthropogenic sources to rainwater composition were estimated respectively at Elandsfontein (44 %, 10 %, 46 %), Cathedral Peak (30 %, 11 %, 59 %), Vaalwater (21 %, 20 %, 59 %) and Knysna (87 %, 4 %, 9 %). The relative contribution of biomass burning to rain-water composition, estimated using organic acids (CH3COO─, HCOO─, C3H5O2─, C2O42─), was estimated at Elandsfontein (6 %), Cathedral Peak (12 %), Vaalwater (17 %) and Knysna (10 %), respectively. The annual wet deposition flux of (S)O42─ was highest at Cathedral Peak (3.92 kg/ha/yr) and Elandsfontein (3.80 kg/ha/yr), and lowest at Knysna (2.26 kg/ha/yr) and Vaalwater (1.94 kg/ha/yr). The total wet deposition flux of nitrogen, calculated using (N)O3─ and (N)H4+, was highest at Cathedral Peak (5.41 kg/ha/yr) and Elandsfontein (3.41 kg/ha/yr), and lowest at Knysna (2.90 kg/ha/yr) and Vaalwater (2.00 kg/ha/yr). The contribution of rain depth and annual concentration values to wet deposition fluxes was clearly observed.
The total (wet + dry) annual deposition flux of sulphur, calculated using (S)O42─ and (S)O2, was highest at Elandsfontein (10.69 kg/ha/yr) and Cathedral Peak (4.46 kg/ha/yr). The lowest total annual deposition fluxes of sulphur were estimated at Vaalwater (2.05 kg/ha/yr) and Knysna (2.39 kg/ha/yr). Similar observations were made for total annual deposition fluxes of nitrogen, calculated using (N)O3─, (N)H4+ and (N)O2. Cathedral Peak (5.61 kg/ha/yr) and Elandsfontein (4.68 kg/ha/yr) were study areas of highest annual nitrogen deposition fluxes, followed by Knysna (3.02 kg/ha/yr) and Vaalwater (2.42 kg/ha/yr). Total annual deposition fluxes (kg/ha/yr) of sulphur and nitrogen measured at Elandsfontein are comparable with large regions of Europe and North America (Vet et al., 2014).
The total annual deposition of sulphur oxides (SOX) and NOX at the SANCOOP sites was in the range 20 to 89 meq/m2/yr. These acid deposition fluxes of SOX and NOX are lower than the acid deposition fluxes previously reported for Europe and North America (200 to 400 meq/m2/yr) that led to stringent policies and monitoring programmes being initiated to reduce emission rates of acid-forming pollutants (Hettelingh et al., 1991).
The net annual deposition loads of sulphur and nitrogen in this study were compared with critical load exceedance maps for regional soil-buffering rates compiled by Josipovic (2009). The net deposition load estimated in this study at Elandsfontein (70.64 meq/m2/yr) is comparable with the ranges of 51 to 74 meq/m2/yr and 76 to 93 meq/m2/yr estimated at the western and central Highveld region (Josipovic, 2009). The net deposition loads measured in this study at Cathedral Peak (43.60 meq/m2/yr) and Vaalwater (19.65 meq/m2/yr) are higher respectively in comparison with the nearby Escourt and Vaalwater study sites reported by Josipovic (2009). The net deposition load estimated at Knynsa (< 1 meq/m2/yr) is the lowest of all study areas. In conclusion, the possibility of acidification effects to terrestrial and aquatic ecosystems over north-eastern South Africa is acknowledged | en_US |
