Ozone is a colorless gas with a sweet odor. Ozone is not a direct emisson from transportation sources. It is a secondary pollutant formed when VOCs and NOx combine in the presence of sunlight. Ozone is associated wiht smog or haze conditions. ALthough the ozone in the upper atmosphere protects us from harmful ultraviolet rays, ground-level ozone produces an unhealth environment in which to live. Ozone is created by human and natural sources.