Atmospheric Toluene and Benzene Mole Fractions at Cape Town and Cape Point and an Estimation of the Hydroxyl Radical Concentrations in the Air above the Cape Peninsula, South Africa

Benzene and toluene, emitted into the atmosphere from a number of common anthropogenic activities, pose a significant human health risk. The mole fractions of toluene and benzene were measured at two urban locations (Foreshore and Potsdam) in Cape Town and one background site at Cape Point, South Africa over the period of July-November 2017. The analysis of the mole fractions of benzene and toluene at two sampling sites in the city of Cape Town gave an indication of the probable anthropogenic sources of the air masses sampled at these sites. We propose that a traffic source dominated at the Foreshore site, while industrial processes dominated at the Potsdam site. The analysis of wind rose plots of benzene and toluene and the elevated mole fractions observed at the remote "clean air" sampling site, Cape Point, suggest that polluted air from Cape Town is the major source of the benzene and toluene mole fractions observed at Cape Point. Hydroxyl (OH) radical concentrations were estimated for Cape Town from the difference in T/B ([toluene]/[benzene]) ratios between Cape Town and Cape Point. The Cape Town OH estimations displayed a mean of (7.2 ± 3.5) × 10⁶ molecules cm^-3 at the Foreshore site and (9.1 ± 4.4) × 10⁶ molecules cm^-3 at the Potsdam site, without consideration of dilution reducing to (5.4 ± 3.4) × 10⁶ molecules cm^-3 for the Foreshore site and (7.4 ± 4.6) × 10⁶ molecules cm^-3 for the Potsdam site for the period of July-November 2017 when dilution was considered. The estimated Cape Town OH concentrations are on the high side but consistent with the results from other urban studies and may suggest a role for OH recycling following biogenic emissions between Cape Town and Cape Point.