Acid Rain Formation

Coal plants, smelters, and vehicle engines release sulfur dioxide (SO₂) and nitrogen oxides (NO_x) into the atmosphere. These gases do not fall straight down. They drift with wind currents, sometimes for hundreds of kilometers, while they slowly oxidize and react with water vapor in the air. SO₂ reacts with water to form sulfurous acid, which oxidizes further to sulfuric acid (H_2SO₄). NO_x reacts with water to form nitric acid (HNO₃). Both acids dissolve into cloud droplets, and when those clouds release precipitation, the rain, snow, or fog carries the acid down to the ground, often in a different region or even a different country from the smokestack that produced it.

This delay and distance is the whole story behind transboundary acid rain: a power plant in one country can measurably lower the pH of lakes and forests in a neighboring one, because the pollution travels as gas before it ever becomes acid. Tall smokestacks make this worse, not better. Building a stack higher reduces local ground-level pollution by diluting it in a larger volume of air, but it also pushes SO₂ and NO_x into faster-moving upper winds, carrying the problem farther downwind and spreading deposition over a wider area instead of concentrating it near the source.

Acid rain connects back to the rest of the unit's story about what sunlight and the atmosphere do to smokestack emissions: just as NO_x and volatile organic compounds cook into ozone and smog under sunlight, SO₂ and NO_x cook into acids through reactions with water vapor, showing the same lesson that where pollution is emitted and where it does damage can be very different places.