Brown smog is caused by the chemical process called photochemical
oxidation
Main ingredients are : Nitric Oxide (NO) + Volatile Organic Compounds
(VOCs) + heat + sunlight
Most NO are emitted from the combustion of fossil fuel in motor vehicles
Main resultant pollutants are : Ground-level ozone (O3) +
peroxyacetylnitrates (PANs) + aldehydes + other photochemical oxidants