HNO₃ plays an important role in the chemistry of NO_x in the atmosphere. Three principal pathways exist for the production of HNO₃ from NO₂:

1.  Hydroxyl radical attack

During the day, when OH is more abundant, nitric acid is produced. NO₂ is attacked by the hydroxyl radical, leading to an important loss of NO₂:

NO2 + OH

→

HNO3

(5)

However, during the day HNO₃ is photolysed very quickly to NO₂ and OH.

2.  Dinitrogen pentoxide intermediary

At night HNO₃ is produced via the combination product dinitrogen pentoxide, N₂O₅, in a heterogenous reaction (reaction 8, below) which can take place, for example, on fog droplets:

NO2 + O3	→	NO3 + O2	(6)
NO2 + NO3	→	N2O5	(7)
N2O5 + H2O	→	2 HNO3	(8)

3.  Absorption of NO₂ in water

A further mechanism for the production of HNO₃ acid from NO₂ is the direct absorption of NO₂ in atmospheric water. However, owing to the low solubility of NO₂ in water this process produces only small amounts of HNO₃:

2 NO2(gas) + 2 H2O(liquid)

→

HNO2(aq) + HNO3(aq)

(9)

However, the absorption of NO₂ in a water droplet constitutes a real sink for NO_x when the droplet is lost through precipitation.

The HNO₃ from these reactions should be viewed as a reservoir rather than a terminal product, since it can be the source of radical (or NO_x) regeneration. An example of this is the rapid daytime photolysis of HNO₃ to NO₂ and OH, followed by the rapid reaction of OH with CH₄ or other hydrocarbons. HNO₃ therefore acts as a sink for NO₂ more during the night than during the day.

Apart from its role as a reservoir for NO_x, HNO₃ precipitation itself is a pollutant which together with sulphuric acid is responsible for the phenomenon of acid rain. Every year acid rain causes damage to a large number of rivers and lakes. In the plant world, damage caused by acid rain can be seen in leaves and fruit. In humans, acid rain can cause respiratory problems. Acid rain also damages buildings, sculptures and monuments.