A pollutant that is mostly associated with acid rain is

Step 1: Analyze the question The question asks which pollutant is mostly associated with acid rain.

Step 2: Explain the concept Acid rain forms when acidic gases dissolve in atmospheric water to form acids. The main gases responsible are:

• Sulphur (IV) oxide (SO₂), mainly from burning fossil fuels and from industrial smelting
• Nitrogen oxides (NO, NO₂), from vehicle exhausts, gas flaring and high-temperature combustion

Reactions:

• SO₂ + H₂O → H₂SO₃ (sulphurous acid), which is further oxidised to H₂SO₄ (sulphuric acid)
• 4NO₂ + 2H₂O + O₂ → 4HNO₃ (nitric acid)

Step 3: Check each option

• A. Nitrogen (IV) oxide (NO₂): It dissolves in rainwater to form nitric acid. ✔ It is a direct cause of acid rain.
• B. Ozone: It is linked to the ozone layer, smog and respiratory problems, not mainly to acid rain. ✘
• C. Fluorine: It is not a major atmospheric pollutant linked to acid rain (CFCs are the ones associated with ozone depletion). ✘
• D. Carbon (IV) oxide (CO₂): It is mainly linked to global warming. It makes rain only very weakly acidic (carbonic acid, natural rain at about pH 5.6), so it is not considered the acid rain pollutant. ✘

Step 4: Answer

Exam tip: Remember "S and N make rain sour": Sulphur oxides and Nitrogen oxides cause acid rain. CO₂ means global warming, CFCs mean ozone depletion. If SO₂ is not among the options, go for the nitrogen oxide.