Spatial and temporal deposition of selected biogeochemical important trace species in South Africa

Abstract

The concern of potential adverse environmental effects due to increased anthropogenic emissions to the atmosphere necessitates the need for long-term atmospheric deposition programmes. Wet and dry depositions of emitted chemical species to the earth’s surface play an essential role in controlling the concentration of gases and aerosols in the troposphere. The chemical content of atmospheric deposition is the signature of several interacting physical and chemical mechanisms such as emission and source amplitude; transport in and dynamics of the atmosphere; atmospheric chemical reactions; and removal processes. The importance of atmospheric deposition as a source of nutrients and key trace elements, i.e. nitrogen (N), sulphur (S), carbon (C) and base metals, is widely recognised (Duce et al., 2009), while it could also be a source of toxic species (Greaver et al., 2012). It is therefore important to establish atmospheric budgets of key chemical compounds to understand the functioning of ecosystems and biogeochemical cycles (Dentener et al., 2006; Davidson et al., 2012). The study of deposition therefore allows for the tracing of the temporal and spatial evolution of atmospheric chemistry and is a pertinent indicator to evaluate natural and anthropogenic influences. Atmospheric chemistry determines the natural and anthropogenic makeup and abundance of pollutant species such as aerosols and gases on a regional scale (Isaksen et al., 2009). It is important to monitor the rates of deposition in order to assess the impact of deposited pollutants on terrestrial and aquatic ecosystems. This study focused on the total dry gaseous deposition and wet deposition of selected biogeochemically important trace species in South Africa. This study also contributed greatly to the database available for scientists in this field, since limited data on this subject is available for South Africa. This study includes measurement data for gaseous species as well as rainwater species in an attempt to assist future global deposition estimations. The sites for the current study are Amersfoort (AF), Louis Trichardt (LT) and Vaal Triangle (VT) (located on the South African Highveld), and Skukuza (SK), which is situated in the South African Lowveld. These sites are considered to be regionally representative of the north-eastern interior of South Africa. Two of these sites are in the region where the major anthropogenic emission sources in South Africa are situated. Measurements of selected trace gases by using passive samplers were conducted at LT (1995-2014), AF (1997-2014), SK (2000-2014) and VT (2008-2014). Passive samplers were successfully deployed at SA DEBITS sites to measure monthly averages of atmospheric concentrations of sulphur dioxide (SO2), nitrogen dioxide (NO2), ammonia (NH3), ozone (O3) and nitric acid (HNO3), with 90% of all samplers deployed resulting in usable results. The data illustrates the value and necessity of long-term air quality measurements at background sites. The influence of a country’s environmental policies and global awareness and focus on air pollution/prevention could be seen from a reduction in emissions of S and N pollutant gases up to 2003/2004, as well as the influence of socio-economic growth and international trade (international accreditation). The rapid industrial, economic and consumption growth in SA from 2002 to 2004 resulted in an increase in the emissions of gases. This was followed by the global financial crisis in 2007/2008 that influenced the production of large companies in SA, resulting in observed declines in the concentrations of gaseous species. Since 2010, a more pronounced increase was observed in the annual average concentrations at all sites. The increases can be attributed to high economic growth rates, which did not compensate for certain improvements, such as the incorporation of more stringent legislative application and the electrification of informal settlements. Throughout the result section, it was evident that anthropogenic activities dominated at two sites, namely VT and AF, influencing the concentrations measured at these sites and indicating the impact of the industrial sector (e.g. coal-fired power generation, petrochemical industry and transport) on the country. The other two sites, SK and LT, showed more regional influences and indicated the effect of meteorological conditions on measurements (e.g. the anti-cyclonic circulation of pollutants from the two industrial sites). Annual total gaseous dry sulphur deposition (contributed by SO2) was 4.6, 7.1, 1.0 and 0.9 kgS/ha/a at AF, VT, SK and LT, respectively, correlating well with recent global assessments, with ranges between 4 and 12 kgS/ha/a (Vet, et al., 2014). Annual total gaseous dry nitrogen depositions (contributed by NO2, NH3 and HNO3) were 16.7, 10.2, 4.2 and 4.0 kgN/ha/a at VT, AF, SK and LT, respectively. Deposition estimates were higher than modelled observations in the recent global assessment (Vet et al., 2014) at VT (especially for NO2, which was estimated at 2-4 kgN/ha/a for Southern Africa) and both VT and AF were much higher compared to other African sites (Delon et al., 2010; Adon et al., 2013). This might be due to the strong industrial anthropogenic influence experiences at South African sites. Furthermore, it must be emphasised that bidirectional exchange was not taken into account and only the downwards deposition was considered. Rain samples were collected at all four the sites from 2009 to 2014. The annual volume weighted mean indicated that the concentration of anthropogenically associated species was much higher at the two sites that are in close proximity to anthropogenic activities, while the concentrations of maritime and terrigenous species were higher at the two sites not directly impacted by major anthropogenic sources. Back trajectory analysis, however, did indicate that these two remote sites are also impacted by air masses passing over the source region through anti-cyclonic recirculation. In general, increases in the wet deposition of S and N were observed at all the sites compared to previous results reported. In addition, an increase in the H+ concentration is observed at all the sites that are reflected in pH distributions, indicating more rain events with lower pH values. This could be ascribed to a significant increase in anthropogenic activities and population growth in this part of South Africa with an associated increase in energy demand. An overall increase of wet deposition fluxes of species associated with anthropogenic activities in South Africa, i.e. sulphate (SO4 2-), nitrate (NO3 -) and ammonium (NH4 +), was observed at the sites when the 2009 to 2014 results were compared to previous data reported by Mphepya et al. (2004; 2006). This increase can most likely be ascribed to the increase in anthropogenic activities in South Africa. Concurrently, the annual H+ concentration increased since the previous publications., which is reflected in a shift to more acidic rain events at all the South African IDAF sites. Acidic potential calculations indicated that only 22 to 42% of the measured H+ concentrations were neutralised by alkaline species at the various sites.