Since last Saturday, residents of Johannesburg, Pretoria and surrounding cities in South Africa’s Gauteng region have complained about a pungent, sulfuric smell in the air, with many saying it smells like fart.
The region’s weather service has warned that the air quality is unhealthy for sensitive groups, including children, elderly, asthmatics and people suffering from lung and heart disease.
As the wind direction has been from the Highveld region to the southeast, the area with one of the largest concentrations of sulfur emitting coal power and industrial plants, many have pointed at the country’s two main industrial emitters, Eskom and SASOL, as the suspects.
SASOL operates two enormous coal-to-oil plants in the area. The company has argued that South Africa’s air quality standards for sulfur dioxide haven’t been breached and its facilities are operating normally, with emissions within their permitted limits.
Looking at official air quality monitoring data, sulfur dioxide levels did spike on Saturday and on Wednesday, but remained far below the country’s air quality standards.
Since essentially none of the power plants and industrial plants do anything to control their SO2 emissions – South Africa’s air emission standards are among the very weakest in the world, despite being a major coal consumer – there’s no scope for those emissions to increase dramatically. All the SO2 generated during coal-burning goes into the air. If they were equipped with SO2 control devices, a device failure could cause a sudden spike.
More importantly, sulfur dioxide can only be smelled at extremely high concentrations, and it smells like burnt matches, not fart. The measured SO2 levels are far below the odor threshold.
There is, however, another sulfurous pollutant that smells more like fart – hydrogen sulfide (H2S). It can be smelled at much lower concentrations, there is no air quality monitoring for it, and the largest emitter of hydrogen sulfide is, you guessed it, SASOL.
Gasification of coal to produce synthetic fuel produces a lot of hydrogen sulfide. SASOL’s pollution control performance for H2S is extremely poor, with its flue gases containing up to 10,000 kilograms per hour of the gas. In China, the only other country where coal-to-oil technology is used at scale, the maximum allowed emissions are less than 3 kilograms per hour per facility – less than 1/1000th of SASOL’s levels.
The company’s annual emissions from the Secunda plant, a mere 120 km from Johannesburg, are a whopping 80,000 tonnes per year. SASOL relies on very high stacks and the high temperature of the flue gas to disperse the pollution, but this doesn’t work in all atmospheric conditions.
To assess whether SASOL’s routine emissions could cause the sulfuric smell, I ran a dispersion simulation using actual weather data from last Friday to Sunday, using the HYSPLIT model developed by the U.S. National Oceanic and Atmospheric Administration (NOAA).
Modeling shows that weather conditions in the past days, and particularly Saturday morning, have seen weather conditions that are exactly right for high-altitude releases from Secunda to cause spikes in Johannesburg-Pretoria. If the plant has been emitting at average rates, the concentrations predicted by the model are far above the odor threshold.
This doesn’t prove that Secunda is the source, and certainly doesn’t show that emissions haven’t exceeded routine levels, but it offers one plausible explanation.
Smell test for pollution regulation
Most Gauteng residents don’t realize how dramatic the impact of these major emissions sources is on air quality in the city – coal and its pollution are something far away, in the neighboring province.
Most of the time, the SO2 and H2S turn into sulfate particles, a form of PM2.5 pollution, before reaching Gauteng. Ironically, this particle pollution is even more dangerous to health than the gases from which it is formed – but not as smelly.
Eskom and SASOL have managed to both delay the implementation of South Africa’s new emission standards and to water them down, at a major long-term cost to public health and the economy of the country. The companies have done this by downplaying the impacts of their emissions, claiming that the health benefits from installing air pollution controls would be minor and costs would be too high to justify the investment.
Now these claims literally don’t pass the smell test.