Wet deposition at a regional background site in South Africa : influence of air mass origin and rain intensity on chemical composition on chemical composition
The chemical composition of rainwater is an integral aspect of atmospheric chemistry. Wet deposition gives a good indication of the general ambient air quality and contributes to the understanding of the temporal and spatial evolution of atmospheric processes. There are numerous micro-physical, chemical and temporal processes that contribute to the eventual precipitation chemistry. In order to accurately assess the chemical contributions to rainwater, the environments where both the cloud formation and the rain event occurred need to be taken into account. In this study, wet deposition was measured at a regional background site in South Africa from December 2014 to July 2016. For this sampling period, two 10 month periods could be identified that represented two separate rain seasons. Ionic concentrations of nitrate (NO3-), sulfate (SO42-), chloride (Cl-), fluoride (F-), acetic acid (CH3COO-), formic acid (HCOO-), oxalic acid (C2O42-), propionic acid (C3H5O2-), ammonium (NH4+), sodium (Na+), potassium (K+), calcium (Ca2+) and magnesium (Mg2+) were analysed. In addition, the pH and electrical conductivity (EC) was also determined. A pilot method was developed to relate event-based cloud- and below-cloud air mass histories to the precipitation chemistry. In this method, back trajectory analysis was performed with the HYSPLIT v4.8 model using event-based parameters, i.e. cloud base height (CBH) and rain intensity measurements. The influence of major pollution point sources and source regions on the precipitation chemistry was investigated through event-based comparison of the chemical composition of the rainwater with the associated air mass histories. The precipitation chemistry results indicated that SO42- had the highest volume weighted mean (VWM) concentration, with NO3-, Ca2+ and NH4+ having the second, third and fourth highest VWMs, respectively. SO42- concentrations were similar to industrially influenced South African DEBITS (Deposition of Biogeochemical Important Trace Species) sites, i.e. Amersfoort and Vaal Triangle, where previous precipitation chemistry studies have been conducted. The concentrations of NO3- and NH4+ were higher compared to concentrations thereof at the South African DEBITS sites. The mean pH of 4.65 indicated that precipitation was acidic. pH frequency distributions indicated that 88% of rain events had pH levels < 5.7, i.e. the natural pH of rain. Source group contributions were estimated with Spearman correlations and empirical calculations. Fossil fuel combustion had the largest source contribution to the precipitation chemistry, while marine and terrigenous source groups had slightly lower contributions than fossil fuel. Agricultural source contributions were notable, with biomass burning contributions having the smallest influence. The ionic concentrations and pH of the rainwater increased over the dry austral winter months, which were attributed to prominent low-level inversion layers and anticyclonic recirculation of air masses. The second drier sampling period indicated higher pH levels, which could be attributed to neutralisation by increased levels of Ca2+ and Mg2+ associated with wind-blown dust. In addition, Ca2+ was also indicated as the most important neutralising factor at Welgegund. Although a relatively small source contribution was determined from biomass burning, which was attributed to the veld fire season (June to mid-October) in South Africa not coinciding with the wet season (mid-October to April), the influence of veld fires was also indicated by the correlations between K+, Cl- and organic acids. The air mass histories associated with the CBH and below-cloud base level were related to the precipitation chemistry. Rain events with similar CBH- and below-cloud air mass histories indicated a prominent influence of point sources and/or source regions over which these air masses have passed. Rain events for which back trajectory sets for both the CBH and the below-cloud related air masses passed over the major pollution point source region east of Welgegund, indicated elevated concentrations of anthropogenic related pollutants (i.e. SO42-, NO3-). In contrast, lower ionic concentrations were measured for rain events where both back-trajectory sets originated from the relatively cleaner western sector. The efficient scavenging and washout effect of rain was demonstrated for two rain events with similar air mass histories occurring on two consecutive days. The build-up of pollutants during winter was also indicated by the ionic concentrations measured for winter rain events. The influence of wildfires close to and distant from Welgegund was also indicated. The results obtained in this pilot study clearly highlighted the key influence of air mass history on rainwater chemistry. The method must be further developed by including more event-based parameters such as synoptic weather patterns, precipitation type and rain intensity. Long-term wet deposition studies will improve the statistical significance of the results presented in this study. Statistical analysis of a larger dataset, as well as the inclusion of meteorological parameters should allow greater insight into the relationships of the complex integrated processes that influence rainwater chemistry.