Nitrogen Dioxide Forms Nitric Acid in the Atmosphere

Steps:

• Identify acid rain as precipitation with pH below 5.6, caused by acidic pollutants reacting with water.
• Recall primary pollutants: sulfur dioxide (SO2) forms sulfuric acid; nitrogen oxides (NOx) form nitric acid.
• Match choices to NOx: nitrogen dioxide (NO2) is a key NOx component from vehicle emissions and combustion.
• Eliminate non-acidic or indirect contributors like CO, CO2, and hydrocarbons.

Why B is correct:

• Nitrogen dioxide (NO2) reacts with water and oxygen to form nitric acid (HNO3): 3NO2 + H2O → 2HNO3 + NO, directly acidifying rain.

Why the others are wrong:

• A. Carbon monoxide (CO) is toxic but does not form acids; it's from incomplete combustion.
• C. Carbon dioxide (CO2) forms weak carbonic acid (H2CO3) but is not a major acid rain pollutant.
• D. Hydrocarbons contribute to smog and ozone but not directly to acid formation.

Final answer: B