Ozone in the atmosphere

- Electric discharges from lightening splits diatomic oxygen in the air

- Combine with other diatomic oxygen molecules to form ozone

- Carried down from high altitudes

http://www.compoundchem.com/2014/05/14/thesmellofrain/

http://www.bbc.com/earth/story/20150115-how-the-smell-of-rain-happens

More particles are produced with light or moderate rainfall compared to downpours, then spread by the wind