Environmental Science

Step 1: Understanding the Concept:
"Criteria air pollutants" is a term used (originally by the US Environmental Protection Agency, but the concept is applied globally) for a group of six common air pollutants that are widespread and known to be harmful to public health and the environment. The Air Quality Index (AQI) is primarily based on the concentrations of these pollutants.

Step 2: Identifying the Criteria Pollutants:
The six criteria air pollutants are:

Ground-level Ozone (O )
Particulate Matter (PM and PM )
Carbon Monoxide (CO)
Sulfur Dioxide (SO )
Nitrogen Dioxide (NO )
Lead (Pb)

Step 3: Analyzing the Options:
- (A) Sulfur Dioxide (SO ): This is a criteria pollutant.
- (B) Lead (Pb): This is a criteria pollutant.
- (C) Carbon dioxide (CO ): This is a major greenhouse gas responsible for climate change, but it is not classified as a criteria air pollutant because it is not directly toxic to humans at typical ambient concentrations.
- (D) Ozone (O ): Ground-level ozone is a criteria pollutant.

Step 4: Final Answer:
Carbon dioxide (CO ) is not on the list of criteria pollutants used for calculating the AQI. Therefore, option (C) is the correct answer.

Step 1: Understanding the Concept:
Photochemical smog, also known as Los Angeles-type smog, is a type of air pollution that is common in modern industrial cities with a large number of automobiles. The question asks for its defining characteristics.

Step 2: Analyzing each statement:
- Statement A: True. Photochemical smog is characterized by high concentrations of oxidizing agents, primarily ozone (O ). This makes it an oxidizing smog.
- Statement B: True. The primary pollutants that lead to photochemical smog are nitrogen oxides (NOx, including NO ) and volatile organic compounds (VOCs), which are hydrocarbons. These are mainly emitted from vehicle exhaust.
- Statement C: False. This is the opposite of statement A. Reducing smog, also called classical or London-type smog, is characterized by high levels of sulfur dioxide (SO ) from burning coal and has a reducing chemical nature.
- Statement D: True. The term "photochemical" indicates that sunlight is required. Ultraviolet (UV) radiation from the sun provides the energy to drive the chemical reactions between NOx and VOCs, leading to the formation of secondary pollutants like ozone.

Step 3: Final Answer:
The correct statements are A, B, and D. Therefore, option (A) is the correct choice.