Consider the following: 1. Carbon monoxide 2. Nitrogen oxide 3. Ozone 4. Sulphur dioxide Excess of which of the above in the environment is/are cause(s) of acid rain?

Answer: B. Only NITROGEN OXIDES (2) and SULPHUR DIOXIDE (4) cause acid rain. ACID RAIN is precipitation with abnormally LOW pH (typically < 5.6), formed when atmospheric pollutants dissolve in water vapour and form acids. The two PRIMARY PRECURSORS are: (2) NITROGEN OXIDES (NOx, primarily NO and NO2): emitted from combustion (vehicles, power plants, industrial furnaces) and lightning. In the atmosphere, NOx reacts with water and oxygen to form NITRIC ACID (HNO3): 4NO2 + O2 + 2H2O → 4HNO3 Nitric acid in rain → acid rain. (4) SULPHUR DIOXIDE (SO2): emitted mainly from coal-burning power plants, smelters, and volcanoes. In the atmosphere, SO2 oxidises to SO3, which reacts with water to form SULPHURIC ACID (H2SO4): 2SO2 + O2 → 2SO3 SO3 + H2O → H2SO4 Sulphuric acid is the dominant acid rain component globally. Why others do NOT cause acid rain: (1) CARBON MONOXIDE (CO) is a poisonous pollutant from incomplete combustion. It is NOT acidic and does NOT form an acid in the atmosphere. (Carbon DIOXIDE forms weak carbonic acid, giving rain its natural pH ~5.6, but pure CO2 is not what makes rain abnormally acidic.) (3) OZONE (O3) is a strong oxidant and a photochemical pollutant in the troposphere. While it contributes to smog and can convert SO2/NOx to acids faster, ozone itself does NOT form an acid and is NOT itself a cause of acid rain. So only SO2 and NOx are the acid-rain precursors among the options. Source: NCERT Class 11 Chemistry Chapter 14 'Environmental Chemistry' / Class 12 Biology 'Environmental Issues'.