2NO + O 2 → 2NO 2
■ 11 A.M.– 4 P.M.
As the sunlight becomes more intense, nitrogen dioxide is broken down and
the concentration of ozone increases:
uv
NO 2 → NO + O
O 2 + O → O 3
Nitrogen dioxide also reacts with water vapor to produce nitric acid and
nitric oxide:
3NO 2 + H 2 O → 2HNO 3 + NO
Nitrogen dioxide can also react with VOCs released by vehicles, refineries,
gas stations, and so on to produce toxic PANs (peroxacyl nitrates):
NO 2 + VOCs → PANs
■ 4 P.M.–Sunset
As the sun goes down, the production of ozone is halted.
Net result: NO + VOCs + O 2 + uv → O 3 + PANs