Modelling the effects of biogenic NOx and industrial H2S emissions on the South African Highveld and Waterberg regions

Abstract

A comprehensive deposition and dispersion model was built for the South African Highveld and Waterberg areas using CALPUFF with the aim of studying the effects of biogenic NOx emissions on sulphur and nitrogen deposition. The effect of industrial H2S emission on sulphur deposition was also investigated for the Highveld. Emission sources inventoried or the Highveld and Waterberg area included industrial sources, vehicle exhaust emissions, household fuel burning emissions and emissions from power stations. The Highveld model was the most extensive. Three scenarios were modelled: average rainfall year (2001), below average rainfall year (2003) and above average rainfall year (2010). The modelling domain was 350 km × 350 km. The Waterberg priority area was only modelled for 2006 and the domain size was 130 km ×100 km. To quantify biogenic soil NOX emissions, models was constructed for both areas using land use data from CALMET, rainfall data and atmospheric ground level temperatures covering each modelling domain. Use was made of work done by Yienger and Levy (1995). To accommodate CALPUFF each area was divided into smaller area sources, each with a specific hourly NOX emission rate. The biogenic NOx emitted made up 3.96 %, 4.14 % and 3.34 % of total released NOx for 2001, 2003 and 2010 respectively. This is significantly more than is released by household fuel burning, small industrial sources and biomass burning. Dry nitrogen deposition rates were affected most, adding between 1.69 - 6.19 % at various receptor locations. Wet deposition rates were affected very little (0.13 % to 0.75 %). Effect on total nitrogen deposition rates ranged from 0.32 % to 1.77 %. CALPUFF was unable to account for H2S conversion to SO2 in its reaction scheme model, therefore conversion rates had to be approximated from observations made on the Highveld by Igbafe (2007). Assuming different conversion percentages for each season, and inputting the converted emissions rates as SO2 emissions sources into CALPUFF, it was predicted that H2S contributes an average of 4.85 %, an average of 5.95 %, and an average of 5.15 % for wet S, dry S and total S deposition respectively Highveld dispersion and deposition predictions are reported on for the three modelling periods of 2001, 2003 and 2010. The modelled biogenic emissions were included in the model. Spatial plots for wet, dry and total S and N deposition were produced. Wet, dry and total S and N deposition rates at specific receptor locations are reported on. Waterberg biogenic emission are only 2.3 % of total NOx emissions for the Waterberg area and would affect nitrogen deposition values very little compared to the nitrogen deposition produced by the emissions from Matimba and Medupi power stations. Because of this it was decided not to run a CALPUFF dispersion and deposition model for the Waterberg area.