This is a sequel I didn’t look forward to writing, but in a repeat of the “smelly air pollution” episode in Johannesburg which I analysed in 2021, people in the city reported a nasty sulphuric smell in early June this year, particularly in the morning of June 8.
The source of the smell seemingly caused confusion, with the country’s environment department quoted as saying:
“Given the nature of this odour, it is likely to be due to a combination of above-average levels of sulphur dioxide and/or hydrogen sulphide in the air. Unfortunately, these are very common air pollutants that are emitted from many sources in the affected areas and in adjacent provinces.”
There are indeed a lot of sources of sulfur dioxide (SO2) around Johannesburg – the neighboring Mpumalanga province is home to the most polluting coal power plant fleet in the world, due to the fact that the plants lack any controls for sulfur dioxide. However, even a quick look at the air quality measurements in Johannesburg over the smelly episodes shows that while elevated, sulfur dioxide levels were far far below the odor threshold – humans aren’t very good at smelling SO2.
What we are good at smelling, in contrast, is the rotten stench of hydrogen sulfide, and that narrows down the potential sources a lot. Ordinary coal combustion produces very little H2S. There is only one major industrial source of H2S in the area – coal gasification. The oil company Sasol uses the process to convert coal into liquid fuels, at two different gigantic plants near Johannesburg.
I ran a plume simulation to see if plume dispersion from Sasol could explain the smell incidents and, bingo: here is the predicted pollution plume from the Sasol Secunda plant in the morning of June 8.
There was a very concentrated plume that hit Joburg on that morning from Secunda. The simulation is run at the reported average routine H2S discharge rate from Secunda and the predicted peak concentrations (yellow areas) in Johannesburg reach 100 ug/m3 which is above the odor threshold for H2S (the plot is in kg/m3).
Whenever these pollution episodes happen, the first reaction seems to be to question the industrial polluters on whether they have had accidents or abnormal releases of the pollutants that would explain the sudden smell. It seems impossible for the public in South Africa to fathom that these releases are in fact, routine, normal and fully authorized.
The routine, reported H2S emissions from Sasol’s operations are huge. The facility relies on very high stacks and high temperature gas (thermal buoyancy) to disperse the emissions so as not to cause high local spikes in concentrations. So most of the time the discharges are dispersed wide enough that people don’t know to complain, even if the chronic exposure is affecting their health. However, during unfavorable conditions, it is possible for the plume to reach the ground level in high enough concentration to cause the smell.
Furthermore, most of the time, the H2S is oxidized into SO2 and further into sulfate aerosols (a part of PM2.5). While these pollutants don’t smell as bad, they are even more harmful to health, elevating the risk of heart attacks, strokes, lung cancer and many other deadly diseases. So the lack of effective regulation and controls for pollution from coal is a severe, ongoing health problem for the city and for the country — the smell is an occasional reminder.