Photo chemical smog always contains

1. A

   Phosphorus

   Option (a) claims that phosphorus is always present in photochemical smog. However, phosphorus is not a standard component or precursor of this type of air pollution.

2. B

   Ozone

   Option (b) claims that ozone is always present in photochemical smog. This option is correct because ozone is a primary constituent formed through complex photochemical reactions involving nitrogen oxides and volatile organic compounds under the influence of sunlight, making it the essential key.

3. C

   Methane

   Option (c) claims that methane is a mandatory component of photochemical smog. Although methane is a greenhouse gas and a volatile organic compound, it is relatively unreactive in the formation of tropospheric ozone compared to other hydrocarbons.

4. D

   Aluminium ion

   Option (d) claims that aluminium ion is always found in photochemical smog. Aluminium is a metallic element and not a gaseous or particulate pollutant typically associated with photochemical reactions in the atmosphere.

Summary. Official key is (b) Ozone. Photochemical smog is a mixture of pollutants that are formed when nitrogen oxides and volatile organic compounds react to sunlight, creating a brown haze above cities. Ozone is the most abundant and critical secondary pollutant generated during this atmospheric process. The other options such as phosphorus, methane, and aluminium ions do not form the defining chemical characteristic of this specific type of smog. Understanding this mechanism helps differentiate photochemical smog from reducing smog.