Volcano

 Some volcanic eruptions can temporarily “clean” parts of the atmosphere — but usually as a side effect, not in a healthy or permanent way.

Here’s what happens:

• Large eruptions release huge amounts of sulfur dioxide (SO₂) high into the atmosphere.
• That gas reacts with water vapor and forms tiny sulfate aerosols.
• These particles reflect sunlight back into space and can cool the planet for months or even years.  
• The aerosols can also interact with atmospheric chemistry and remove or alter certain pollutants.

A famous example is Mount Pinatubo in 1991. It injected around 15 million tons of sulfur dioxide into the stratosphere and cooled global temperatures by roughly 0.5°C for about two years.  

But volcanoes are not actually “good” air purifiers overall:

• They also release ash, toxic gases, and greenhouse gases.
• Volcanic aerosols can damage the ozone layer in some cases.  
• The cooling effect is temporary, while the local environmental damage can be severe.

So the phrase “accidentally started cleaning the atmosphere” is partly true in the sense that volcanic particles can temporarily reduce warming and alter atmospheric pollutants — but it comes with major trade-offs.