A weapon to rapidly change the Earth's atmosphere

What I am trying to imagine is a chemical reaction that would change Earths atmosphere rapidly into something else which would be more suitable for an alien race. This would no doubt make the Earth's atmosphere unbreathable for humans thus creating a ready made plan for Aliens to populate.

Ideally this would be a weapon that would start some sort of a chemical chain reaction in Earths atmosphere. It might only be a small change. Small enough to kill off the humans and thus making it breathable for the aliens.

*Update: Am amazed and delighted at the responses. Such good ideas and suggestions. Thanks everyone who has taken the time to comment and answer. *

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A weapon to rapidly change the Earth's atmosphere

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