Long-term measurements of concentration and dry deposition of atmospheric inorganic gaseous species at Cape Point, South Africa
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Atmospheric aerosols and trace gases are emitted into the atmosphere by various anthropogenic and natural sources, which are removed through chemical transformation, as well as wet- and dry deposition. These atmospheric species not only affect the radiative budget and climate of the earth, but also influence the natural cycle and availability of chemical compounds that serve as essential nutrients in various ecosystems. Air pollutants can cause adverse effects on human- and animal health. The aim of this study was to assess long-term measurements of inorganic gaseous species conducted at the Cape Point Global Atmosphere Watch station (CPT GAW) from 1995 to 2013 in order to establish inter-annual and seasonal trends, as well as dry deposition of these species. The CPT GAW station is located in a nature reserve approximately 60 km south of Cape Town in South Africa, which is locally and globally considered as an important atmospheric monitoring site due to its position at the south-western tip of Africa. The CPT GAW site is predominantly affected by clean background maritime air masses that are indicative of the Southern Hemisphere. In addition, it is also affected by local sources of atmospheric pollutants, which include the greater Cape Town conurbation and other industrial activities in this region. The primary measurements conducted at the CPT GAW involve the monitoring of greenhouse gases. Other continuous measurements include total gaseous mercury, 222Rn, solar radiation, precipitation chemistry and meteorological parameters. In addition, passive diffusive sampling of inorganic species is also performed at the CPT GAW. Passive sampling and precipitation collection are performed within the IGAC (International and Global Atmospheric Chemistry) endorsed DEBITS (Deposition of Biogeochemically Important Trace Species) programme, of which the African part, IDAF (IGAC/DEBITS/AFRICA), was initiated in 1994. Sulphur dioxide (SO2), nitrogen dioxide (NO2), ammonia (NH3) and ozone (O3) were measured at CPT GAW with passive diffusive samplers from 1995 to 2013 resulting in a 19-year data record for these species, while HNO3 measurements commenced in 2003 resulting in an 11-year data record. For these measurements, duplicate sets of passive samplers were exposed for a period of one month, replaced, sealed and sent to the Atmospheric Chemical Research Group (ACRG) of the North-West University’s Potchefstroom Campus (NWUPC) for analysis. These samplers were analysed using ion chromatography (IC) and ultraviolet-visible (uv/vis) spectroscopy. Mathematical software was used to calculate overlay back trajectories of air mass movement prior to arrival at the station using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model developed by the National Oceanic and Atmospheric Administration’s (NOAA) Air Resource Laboratory (ARL). Angular histograms of wind direction frequency were also compiled. No long-term trends were observed for O3, NH3 and HNO3, which indicated small cyclic fluctuations. SO2 and NO2 did indicate distinct decreases up until 2002, after which an increase in annual average concentrations was observed. These increases were attributed to economic growth and the increasing population in South Africa. Distinct seasonal patterns were observed for SO2, NO2 and O3. NO2 peaked from April to August, while O3 revealed elevated levels from July to October. SO2 had higher concentration during two periods of the year, i.e. January to February and July to August. The SO2, NO2 and O3 peaks observed during the winter months (June-August) were partially attributed to an increase in the long-range transport of pollutant species that was indicated by an increase of air mass movement from the industrialised interior of South Africa arriving at the CPT GAW. Meteorological data also indicated greater effects of air masses passing over the Cape Town conurbation. Fire event frequencies indicated that increased burning during January and February could contribute to elevated SO2 concentrations measured during these two months. An increase in NO2 concentrations during the wet season was also attributed to increased microbial activity occurring with the onset of the wet season. Gaseous deposition calculated with deposition velocities obtained in literature indicated that sulphur (S) deposition (SO2) ranged between 0.6±0.5 and 1.4±1.2 kgS.ha-1.yr-1, while total nitrogen (N) dry deposition (NO2 + NH3 + HNO3) was estimated to range between 3.1±1.0 and 4.0±1.3 kgN.ha-1.yr-1. O3 deposition was calculated to range between 11.7±2.2 and 57.1±10.6 kg.ha-1.yr-1. Estimated S dry deposition at CPT compared well with the other IDAF sites, with the exception of the industrially impacted Amersfoort, where S deposition was two times higher. NO2 and HNO3 dry N depositions were within the same range at all the IDAF sites. Relatively large differences were observed for N deposition associated with NH3. NH3 fluxes at the CPT GAW were higher compared to other southern African sites, but lower compared to NH3 deposition at sites in forests in central Africa. NH3 had the highest contribution to total N deposition fluxes measured at the CPT GAW.