Explanation

Photochemical smog:

• NOX is one of the primary causes of photochemical smog.
• Photochemical smog is a condition that develops when primary pollutants (such as oxides of nitrogen and volatile organic compounds) interact under the influence of sunlight to produce a mixture of secondary pollutants.
• Thus photochemical smog is mainly composed of oxides of nitrogen, volatile organic compounds (primary pollutants), ozone, and peroxyacetyl nitrate (PAN) (secondary pollutants).

Note:

It depends on primary pollutants as well as the formation of secondary pollutants.

Primary pollutants include

1. Oxides of nitrogen, particularly nitric oxide (NO) and nitrogen dioxide (NO2)
2. Volatile organic compounds.

Secondary pollutants include

1. Peroxyl-acyl nitrates (PAN)
2. Tropospheric ozone
3. Aldehydes

An important secondary pollutant for photochemical smog is ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NOx) combine in the presence of sunlight.