Aerosol behaviour on the South African highveld

Abstract

Five atmospheric aerosol species have been monitored over a period of five years at up to 15 stations simultaneously on the South African Highveld, where a major industrial area on the Southern African subcontinent is situated. Stacked filters were exposed for 24 h and then analysed to determine the concentrations of fluorides, phosphates, chlorides, nitrates and sulfates (F −, PO 4 3−, Cl −, NO 3 − 3 and SO 4 2−. Theseasonal variation of the primary aerosols (F −1, Cl −1 and PO 4 3−1) was found to be mainly determined by the seasonal variation of the frequency and intensity of the subtropical subsidence. In contrast to findings in other areas of investigation, the seasonal variation of the nitrates is well developed and the maximum occurs during late winter and early spring. The seasonal variation of sulfate is less pronounced than that of the nitrates with a maximum occurring during summer. Stagnating conditions over several weeks are dominant throughout the year and sulfate concentrations above 20 μg m −3 a can occur at some sites. Strong evidence for the existence of a pool of pollutants on the Highveld which is persistent for most of the year has been found. Based on the behavioural pattern of the sulfate concentration in the network area, it was estimated that this pool is removed by ventilation about once to twice a month and that refilling takes about 10 to 20 days. Besides stagnating conditions and ventilation, atmospheric processes such as downmixing from a postulated pool of pollutants aloft and washout events have a marked impact on sulfate concentrations. One of the sites was situated on top of a hill some 300 m above the surrounding terrain. Average concentrations of sulfate at this elevated site were found to be about twice as high as in the remainder of the network, with peaks of up to 70 μg −3. Furthermore, it was found that transport mechanisms dominate the individual and seasonal variations of all aerosol species, irrespective of their primary or si;condary nature. Their seasonal variation displays a bimodal pattern, which could be linked to the seasonal cycle of atmospheric processes over the subcontinent. A hypothesis explaining regularly occurring peak episodes at the hilltop station was proposed for synoptic-scale atmospheric flow patterns, which transport the polluted air mass from the source area and back again.