Which of the oxide of nitrogen is not a common pollutant?

The correct option is N₂O₅.

Let us understand why dinitrogen pentoxide (N₂O₅) is not considered a common atmospheric pollutant compared to other oxides of nitrogen.

1. Understanding Common Nitrogen Oxide Pollutants:
The primary oxides of nitrogen that act as major air pollutants are collectively referred to as NO_x, which mainly includes nitric oxide (NO) and nitrogen dioxide (NO₂). These gases are released in large quantities into the atmosphere from combustion processes in automobile engines, industrial boilers, and power plants. Nitrous oxide (N₂O), also known as laughing gas, is a well-known greenhouse gas and contributor to ozone depletion.

2. Properties and Behavior of N₂O₅:
Dinitrogen pentoxide (N₂O₅) is a highly reactive, unstable compound. Under atmospheric conditions, it exists in equilibrium with nitrogen dioxide and nitrate radicals as described by the following chemical equation:

$$N_2{O}_5\rightleftharpoons N{O}_2+N{O}_3$$

Furthermore, N₂O₅ is extremely moisture-sensitive. When it encounters water vapor or moisture present in the air, it rapidly hydrolyzes to form nitric acid (HNO₃):

$$N_2{O}_5+H_{2}O\to 2HN{O}_3$$

3. Conclusion:
Because of this high reactivity and rapid conversion into nitric acid in the presence of atmospheric moisture, N₂O₅ does not persist in the atmosphere for long durations in its gaseous form. Consequently, it is not classified as a common ambient air pollutant, unlike NO, NO₂, and N₂O.