An assessment of the atmospheric nitrogen budget on the South African Highveld

Abstract

Atmospheric reactive nitrogen concentrations on the South African Highveld have become a growing concern, with satellite images indicating very high nitrogen dioxide (NO₂) concentrations in the region. This study investigated the nitrogen budget on the Highveld through the analysis of the concentration of the atmospheric nitrogen species on a temporal scale as well as the atmospheric conversion, transport and removal of these species. Data were collected at Eskom's Elandsfontein ambient air quality monitoring site, which is centrally located on the industrialised Highveld. A year's dataset from 2005 and 2006 was analysed and it was found that nitrogen oxide (NO_x) concentrations were higher in winter as a result of stable atmospheric conditions, as well as prevalent westerly and north-westerly airflow, which transported emissions directly from the nearby power station sources to the monitoring site. Nitrate (NO₃) concentrations also peaked during winter, with a distinct biomass burning peak during August 2005. Diurnally, NO_x concentrations indicated a tall-stack industrial source, where concentrations peaked at midday. The NO₃ concentrations were higher at night than during the day; during the day the NO₃ radical is rapidly photolysed and nitrates cannot be produced. Case studies indicated that the conversion rate of nitric oxide (NO) to NO₂ was highly variable as a result of varying atmospheric factors, which include time of day, dispersion, stability and regional atmospheric chemistry. These rates ranged from 11% to 59% per hour. Rates of dry deposition of NO, NO₂ and NO₃ were generally higher during winter as a result of higher atmospheric concentrations and increased atmospheric stability. Nitrogen was predominantly deposited as NO₂ throughout the year, except during spring when NO₃ deposition dominated. The total annual amount of nitrogen that was deposited to the Mpumalanga Highveld region was in the range of 6.7 kg/ha - 13.1 kg/ha per year, which is well below the stipulated critical load value. Such deposition, therefore, should not pose significant threats to the natural environment on the Highveld. Between 4% and 15% of the total emitted nitrogen from power generation on the Highveld was deposited to the surface via wet and dry deposition. The remainder was advected out of the region.