system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What is it I'm actually smelling as a thunderstorm approaches?

Just today I walked outside to overcast skies, and I could smell the storm coming. What did I smell?
gpt
An electrical charge—from lightning or a man-made source such as an electrical generator—splits atmospheric nitrogen and oxygen molecules into separate atoms. Some of these recombine into nitric oxide, and this in turn reacts with other atmospheric chemicals, occasionally producing a molecule made up of three oxygen atoms—ozone, or O3. (Most atmospheric oxygen is made up of two atoms—O2.) The scent of ozone heralds stormy weather because a thunderstorm's downdrafts carry O3 from higher altitudes to nose level.