NO2 is a gas with a pungent odor and specific brown color. It makes the smog look brown over smoky cities. NO2 is responsible for photochemical smog in places with heavy traffic. Nitrogen oxides are associated with the greenhouse effect and acid rain phenomena.
Nitrogen dioxide undergoes chemical reactions. Its penetration into the atmosphere results from both natural factors (e.g. electrostatic discharges, volcanic eruptions) and those related to human activity as a result of penetration into the atmosphere of car exhaust fumes, and pollution from industrial plants and agriculture e.g. related to the use of fertilizers.
The permissible average annual level of NO2 concentration in the atmosphere is 40 µg / m3,
In Polish cities (especially in Krakow, Warsaw and Wroclaw) it is exceeded many times.