Acid Rain: How Smokestacks Sour the Sky

Why even clean rain is acidic

The number to anchor on is pH 5.6 — not 7. Pure water in equilibrium with the atmosphere is never neutral, because carbon dioxide dissolves into it and forms a weak acid:

• CO₂(g) ⇌ CO₂(aq)
• CO₂(aq) + H₂O ⇌ H₂CO₃ (carbonic acid, pKa₁ = 6.35)
• H₂CO₃ ⇌ H⁺ + HCO₃⁻

At the preindustrial 280 ppm CO₂, this set rainwater at pH ≈ 5.6; at today's ≈ 420 ppm the carbonic-acid floor is a hair lower still. So "acid rain" is defined operationally as rain below about pH 5.0–5.2, meaningfully more acidic than the CO₂ baseline. Because pH is logarithmic, a drop from 5.6 to 4.6 is a 10× increase in H⁺, and the worst recorded event — pH 2.1 at Wheeling, West Virginia, in 1979 — carried roughly 3,000× the H⁺ of clean rain. The strong acids responsible, H₂SO₄ and HNO₃, dissociate essentially completely, unlike the weak carbonic acid they overwhelm.

From smokestack to sulfuric acid

Coal and heavy fuel oil contain sulfur — anywhere from 0.5% to over 4% by mass, locked in pyrite (FeS₂) and organic sulfur. Combustion oxidizes it to sulfur dioxide:

• S (in fuel) + O₂ → SO₂
• 4 FeS₂ + 11 O₂ → 2 Fe₂O₃ + 8 SO₂

SO₂ itself is only mildly soluble and weakly acidic. The damage comes from oxidizing it to sulfur trioxide / sulfuric acid, and this is the slow, rate-controlling step. It runs by two routes. The gas-phase route is initiated by the hydroxyl radical, HO•:

• SO₂ + HO• → HOSO₂•
• HOSO₂• + O₂ → SO₃ + HO₂•
• SO₃ + H₂O → H₂SO₄

The aqueous route, inside cloud droplets, dominates and is far faster. SO₂ dissolves to bisulfite (HSO₃⁻), which is oxidized by dissolved hydrogen peroxide (H₂O₂) or ozone:

• SO₂ + H₂O ⇌ H⁺ + HSO₃⁻
• HSO₃⁻ + H₂O₂ + H⁺ → H₂SO₄ + H₂O

The H₂O₂ pathway is pH-independent and lightning-fast, which is why sulfate forms most efficiently in the humid interiors of clouds. The end product, sulfuric acid, is a strong diprotic acid (pKa₁ ≈ −3, pKa₂ = 1.99) — each molecule delivers two protons to the rain.

The nitric acid half of the problem

The second acid needs no sulfur at all — its raw material is the air itself. At the high temperatures inside engines and boilers (>1,500 °C), atmospheric N₂ and O₂ combine in the thermal (Zeldovich) mechanism:

• N₂ + O → NO + N
• N + O₂ → NO + O

The product is nitric oxide (NO), which oxidizes in air to nitrogen dioxide (2 NO + O₂ → 2 NO₂). Collectively NO + NO₂ are called NOₓ. During daylight, NO₂ is converted to nitric acid by the same hydroxyl radical that drives sulfate:

• NO₂ + HO• + M → HNO₃ + M

At night, when HO• is scarce, an alternate route through the nitrate radical and dinitrogen pentoxide takes over:

• NO₂ + O₃ → NO₃• + O₂
• NO₃• + NO₂ ⇌ N₂O₅
• N₂O₅ + H₂O → 2 HNO₃

Because the gas-phase NO₂ + HO• step is fast (minutes), HNO₃ forms and deposits closer to the source than sulfate does, and it stays a gas or fine aerosol that rains out readily. As coal scrubbing has cut SO₂ dramatically, the nitrogen share of acid deposition — from power plants, vehicles, and increasingly agriculture (via ammonia) — has grown in relative importance.

Wet, dry, and the long ride downwind

Acid reaches the ground two ways. Wet deposition is the classic acid rain, snow, or fog — droplets scavenge acids and aerosols and fall out. Dry deposition is the direct settling of SO₂ gas and sulfate/nitrate particles onto surfaces, later mobilized by the next rain; near sources it can carry as much acidity as wet deposition. Two features make acid deposition a political as well as a chemical problem:

• Tall stacks export it. The 200–300 m "superstacks" built in the 1960s–70s to fix local air quality injected SO₂ high enough that it traveled 500–1,000 km before falling — turning a Midwest problem into a New England and Scandinavian one.
• Oxidation takes time. Sulfate formation has a chemical lifetime of hours to a couple of days; a plume can cross several states before its SO₂ finishes converting to H₂SO₄.

Sweden and Norway received large loads of sulfur emitted in Britain and continental Europe; Canada received a share of the U.S. Ohio Valley's. This transboundary transport is why acid rain became a defining case for international emissions treaties.

What the acid does to lakes, soil, and stone

The harm depends almost entirely on buffering capacity — the local rock's ability to neutralize incoming acid. Watersheds floored by limestone (CaCO₃) shrug it off, because carbonate consumes protons: CaCO₃ + 2 H⁺ → Ca²⁺ + H₂O + CO₂. But granite and thin, quartz-rich soils have almost no acid-neutralizing capacity, which is exactly why the Adirondacks, Scandinavia, and the Canadian Shield were hit hardest.

• Aluminum mobilization. Below about pH 5, protons pry Al³⁺ out of soil clays: Al(OH)₃(s) + 3 H⁺ → Al³⁺ + 3 H₂O. Dissolved Al³⁺ is directly toxic to fish, clogging gills and disrupting ion balance — it kills more fish than the low pH alone.
• Nutrient stripping. Incoming H⁺ displaces Ca²⁺ and Mg²⁺ from soil, which then wash away, starving trees. Red spruce dieback in the Appalachians was linked to this calcium loss.
• Stone decay. Marble and limestone monuments dissolve, and worse, react with SO₂ to form gypsum: CaCO₃ + H₂SO₄ + H₂O → CaSO₄·2H₂O + CO₂. Gypsum's larger volume and solubility flake off building surfaces — the "black crust" and pitting seen on cathedrals across Europe.

Acid also corrodes metals, and it accelerates the leaching of lead and copper from old plumbing, a public-health link to soft, poorly buffered water.

How the sky was un-soured

Acid rain is the rare pollution story that was largely solved, and the fix was pure chemistry. The 1990 U.S. Clean Air Act Amendments created a cap-and-trade market for SO₂ that made cutting emissions cheaper than anyone forecast. The two workhorse technologies:

• Flue-gas desulfurization ("scrubbers"). A limestone or lime slurry is sprayed into the exhaust, capturing SO₂: CaCO₃ + SO₂ → CaSO₃ + CO₂, then oxidized to CaSO₄·2H₂O — synthetic gypsum sold for wallboard. Modern scrubbers remove >95% of SO₂.
• Selective catalytic reduction (SCR) for NOₓ: injected ammonia reduces NO over a vanadium/titanium catalyst — 4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O — turning nitrogen oxides back into harmless N₂.

The payoff is measurable. U.S. SO₂ emissions fell from ~31 million tons in 1970 to under 2 million by 2020, sulfate in rain dropped more than 90% across the Northeast, and average precipitation pH climbed by roughly 0.5–1 unit. Sensitive lakes are slowly re-alkalizing, though soils — stripped of their calcium reserves over decades — recover more slowly than the water, a reminder that the chemistry of damage runs faster than the chemistry of repair.

The two acids of acid rain compared with the natural carbonic-acid background of clean rain.

Property	Sulfuric acid path	Nitric acid path	Clean-rain background
Primary source	SO₂ from coal/oil S content	NOₓ from high-T combustion	Atmospheric CO₂ ≈ 420 ppm
Acid formed	H₂SO₄ (diprotic, strong)	HNO₃ (monoprotic, strong)	H₂CO₃ (weak, pKa₁ 6.35)
Rate-limiting step	SO₂ oxidation (aqueous, hours)	NO₂ + OH• (gas, minutes)	CO₂ dissolution (fast)
pKa	−3 and 1.99	−1.4	6.35 and 10.33
Share of U.S. acidity	~⅔ historically	~⅓ and rising relatively	sets the 5.6 floor