By Dr Nicolaas C Steenkamp
25 February 2021 – Residents in parts of Gauteng, South Africa, recently complained about the smell of sulphur hanging in the air for more a week. The smell was especially pungent in Pretoria and surrounding areas and could even be smelled in Johannesburg for a few days.
Despite several claims and different theories, this peculiar phenomenon was probably the result of a change in dominant wind direction and the topography of Pretoria.
Although the origin of the sulphur smell has not been determined yet, it has been suggested that it was blown in by winds from the Secunda region and from the coal mining towns in Mpumalanga. Sulphur dioxide is a nonflammable, colorless gas with a very strong, pungent odor. Most people can smell sulfur dioxide at levels of 0.3 to 1 ppm.
Normally the dominant wind direction in Pretoria is from the north and northeast, but a few weeks ago the wind changed direction and blew from the southeast. One valid explanation is that the recent tropical cyclone Eloise, which caused the formation of a tropical low-pressure system over Mozambique and a clockwise movement of air, was responsible for the unique occurance. As this system passed over the mining, petrochemical and wood pulp factories of Mpumalanga, pungent gasses emitted by these operations were blown towards Pretoria in small amounts. This circulation pattern could be clearly seen in the meteorological satellite imagery displayed as live passes on Pinkmatter Solutions FarEarth Observer online platform.
The topography of Pretoria is defined by a series of quartzite ridges, with a high slope angle, of the Transvaal Supergroup, that run parallel east-west, with a slight crescent shape towards the north as it wraps around the Bushveld Complex. These ridges are broken north-south by faults that created the passes. The valleys between the ridges and the passes channel the winds from the east. These can clearly be seen in 3D topographical renderings of a digital elevation model (DEM) generated on the FarEarth Change Monitor from stereo satellite imagery.
Sulphur dioxide is heavier than air and will tend to settle towards the ground when the winds become less turbulent or is funneled into a wind-shaded area or basin areas (such as between the ridges), that in turn creates transient atmospheric reservoirs. Once enough sulphur dioxide accumulate in these atmospheric reservoir areas, the levels increase to the point where the smell becomes potent enough to be detected by humans. The smell disappeared as the normal wind circulation was restored and the accumulated gasses dissipated.