A molecule of three oxygen atoms (O3); it forms a layer in the stratosphere that absorbs ultraviolet light, while near the ground it is treated as a pollutant that irritates the airways.
You can't directly smell anything in space. But astronauts report a distinctive odor when returning to the spacecraft after spacewalks—described as 'gunpowder,' 'brake pads,' 'seared steak,' 'ozone,' and 'walnuts.'