The Air Quality Health Index is computed using the amount of various air pollutants like nitrogen dioxide, ozone, and fine particulate matter. The amount of all these pollutants is different during the day and the night. This can be attributed to several reasons.
Ozone is formed at ground level by chemical reactions that take place between oxygen molecules due to ultra violet radiation in sunlight. As the UV radiation is not present at night, the amount of ozone formed is reduced. Nitrogen dioxide and fine particulate matter produced by smaller vehicles is also higher during the day.
Some reasons why the amount of pollutants might increase at night include the cooling of the air that moves air pollutants present at height altitudes towards the ground. Also, some sources of pollution like power stations work throughout the night. As the number of heavy vehicles traveling is increased at night, this can also increase the level of some pollutants at night.