ESA

A mysterious high-altitude layer of sulphur dioxide discovered by ESA’s Venus Express has been explained. As well as telling us more about Venus, it could be a warning against injecting our atmosphere with sulphur droplets to mitigate climate change.
 
Venus is blanketed in sulphuric acid clouds that block our view of the surface. The clouds form at altitudes of 50–70 km when sulphur dioxide from volcanoes combines with water vapour to make sulphuric acid droplets. Any remaining sulphur dioxide should be destroyed rapidly by the intense solar radiation above 70 km.

So the detection of a sulphur dioxide layer at 90–110 km by ESA’s Venus Express orbiter in 2008 posed a complete mystery. Where did that sulphur dioxide come from?