Nitrogen oxides (NOx) react with volatile organic compounds (VOCs) in the presence of sunlight to form ozone (O3). This process is known as photochemical smog.

Here's a simplified explanation:

1. Sunlight provides the energy to break down nitrogen dioxide (NO2), a major component of NOx.

2. This breakdown produces nitrogen monoxide (NO) and oxygen atoms (O).

3. Oxygen atoms quickly react with oxygen molecules (O2) to form ozone (O3).

4. VOCs contribute to the formation of ozone by reacting with NO to form peroxyacyl nitrates (PANs), which further decompose into ozone.

Ozone is a major component of smog and is a harmful air pollutant. It can cause respiratory problems, damage crops, and contribute to acid rain.