Atmospheric SO₂ and NO₂ flux measurements at a savannah-grassland-agriculture landscape in South Africa
Stenden, Franciscus Hubertus
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Deposition measurements of SO2 and NO2 in South Africa are mainly limited to passive sampling where a constant deposition velocity is assumed for a specific land use category. Gaseous deposition velocities in South Africa are a major uncertainty as no direct deposition velocity for SO2 and NO2 have been measured. Considering that South Africa houses one of the most diversified trade and industry sectors within Africa – which include mining, agriculture and fisheries, vehicle manufacturing, food processing and energy production – the impact of pollutant species is an increasing concern for environmental health. In an effort to reduce the uncertainties associated with deposition derived from passive sampling measurements and modelled deposition velocities for South African DEBITS sites, SO2 and NO2 flux measurements were performed at Welgegund measuring station for a one-year period with a quantum cascade laser (QCL) instrument. SO2 and NO2 monthly concentrations determined from passive samplers were correlated with the SO2 and NO2 flux measurements. It is anticipated that the active monitoring of SO2 and NO2 fluxes will significantly reduce the uncertainties associated with depositions derived from modelled deposition velocities. This type of study is also the first for South Africa where dry deposition velocities for relevant atmospheric gaseous species were directly measured with active online instrumentation. SO2- and NO2 fluxes were determined with a fast-response QCL instrument for one year at Welgegund. SO2 fluxes and deposition velocities determined in this study was in the same order of magnitude compared to SO2 fluxes and deposition velocities calculated with inferential models in South Africa, with a mean flux and deposition value for the sampling period of .-0.01 µg.m-2.s-1 and 0.16 cm. s-1 respectively. Welgegund can be considered a net sink of atmospheric SO2, as a net downward flux is observed for atmospheric SO2.Inferential models indicated distinct seasonal patterns, which was not observed from micrometeorological measurements with net SO2 fluxes remaining relatively similar throughout the year. In addition, marginal diurnal patterns were determined with relatively higher SO2 concentrations corresponding to break-up of inversion layers. NO2 fluxes and deposition velocities determined in this study were compared between two studies in South Africa in which an inferential model (Mpepya, 2002, Phala, 2015) was used to calculate NO2 fluxes and – deposition velocities. NO2 deposition fluxes calculated from modelled deposition velocities generally overestimate N deposition for these regions, since N upward fluxes are not considered in inferential models. The mean flux and deposition velocities value for the sampling period was 0.005 µg.m-2.s-1 and - 0.16 cm. s-1 respectively. Welgegund can be considered to be a net NO2 emitter, as a net upward flux for atmospheric NO2 was observed. Distinct seasonal patterns were observed for NO2 fluxes with highest NO2 fluxes corresponding to the warm and wet months due to increased temperature, soil temperature, precipitation and biogenic activity.