Which pollutants initiate the formation of acid rain?

• A. Carbon dioxide and methane
• B. Sulfur dioxide and nitrogen oxides
• C. Nitrogen oxide and ozone
• D. Hydrogen sulfide and nitrogen

Answer: (B)

Acid rain starts when certain gases in the atmosphere begin the chemical steps that form acidity in rainwater. The gases that kick off this process are sulfur dioxide and nitrogen oxides. When these gases are released from burning fossil fuels and vehicle exhaust, they mix with moisture, oxygen, and sunlight in clouds. Through oxidation and hydration, they become sulfuric acid and nitric acid, which then ride down with rain as acid rain. This initiating pair is the reason—because they directly produce the strong acids responsible for lowering rain pH and causing the environmental and material damage associated with acid rain.

Other choices don’t start this same atmospheric chemistry. Carbon dioxide can form a weak carbonic acid but isn’t a primary driver of acid rain. Methane is a greenhouse gas with no direct, dominant role in forming acidic rain. Nitrogen oxides are involved, but ozone isn’t the initiating pollutant for acid rain in the same way; and hydrogen sulfide with nitrogen isn’t the primary pathway either. The key idea is that sulfur dioxide and nitrogen oxides are the air pollutants that initiate the acid-creating reactions leading to acid rain.