On 28 September 2016, I was staying at my daughter’s house in East road, Sandton, Johannesburg, South Africa. Our driver came around 7am in the morning, and I was getting ready to drop our grandsons at Crawford School.
That time, we saw that our car was fully covered with white and black innumerable spots. My long experience in environmental research revealed to me that this is something interesting related to air pollution, and I photographed the whole horizontal area of the car body. Those 3 photographs are enclosed here.
This was cold and calm morning, almost no wind, relative humidity about 85% and sky was partially cloud-covered. What are these spots? To understand this event, we know that South Africa is producing large percentage of power by thermal power plants. The coal used in such plants has sulphur content about 1.0 to 1.5 percent. These plants are spewing thousands of
The sulphur dioxide, if temperature is low and relative humidity is high, turns into sulphuric acid which reacts with elements like magnesium, calcium etc, present in the form of airborne particles, to form sulphate compound. In the same way,the nitrogen oxides resulting from vehicular emission form nitric acid which produces nitrates.
These airborne particulates normally are called aerosols. If the environmental conditions are conducive, these particles become heavy and starts undergo gravity -settling on the ground or any material surface and leave white and black spots. These acidic spots corrodes any metallic and nonmetallic surfaces.
Professor T. S. Verma
Department of Physics, University of Botswana