Acid Rain pH Calculator

Calculate pH of rainwater from atmospheric CO2 and SO2/NOx pollutant concentrations.

Current atmosphere: 420ppm
Clean air: <1ppb, Polluted: 10-100ppb
Urban air: 10-100ppb
Please check your inputs and try again.

Why Rain Is Acidic Even When Clean

Unpolluted rain is not pH 7. Atmospheric carbon dioxide dissolves into falling droplets and forms carbonic acid, which partially dissociates and releases hydrogen ions. At present-day CO2 levels this sets the natural baseline at roughly pH 5.6, and that value is the reference against which acid rain is defined.

CO2 + H2O → H2CO3 → H+ + HCO3−

Because carbonic acid is weak, only a small fraction dissociates — which is why the baseline sits near 5.6 rather than dropping much lower. Sulfur and nitrogen oxides behave entirely differently. SO2 oxidises to sulfuric acid and NOx to nitric acid, both strong acids that dissociate essentially completely. A far smaller concentration therefore produces a far larger pH change.

SpeciesAcid formedStrengthEffect on rain pH
CO2Carbonic, H2CO3WeakSets the ~5.6 natural baseline
SO2Sulfuric, H2SO4Strong, diproticLargest single contributor to acid rain
NOxNitric, HNO3StrongSignificant, and rising where SO2 has been controlled

Reading the Result

pHInterpretation
5.6Natural baseline from atmospheric CO2 alone
5.0–5.6Slightly acidified — modest pollutant loading
4.3–5.0Acid rain — typical of industrialised regions
<4.3Severely acidified — comparable to the worst historical episodes

Remember that pH is logarithmic. A fall from 5.6 to 4.6 is a tenfold increase in hydrogen ion concentration, and from 5.6 to 3.6 is a hundredfold. Small-looking pH differences represent very large chemical changes, which is why a drop of one unit matters so much ecologically.

Worked Examples

Example 1: Clean atmosphere: 420ppm CO2, no SO2/NOx
pH from CO2 only
Result: pH=5.6 — naturally acidic from carbonic acid
Natural rain is always slightly acidic
Example 2: Polluted: 420ppm CO2, 50ppb SO2, 30ppb NOx
Additional H+ from SO2 and NOx
Result: pH drops to ~4.3 — acid rain
SO2 from coal power plants; NOx from vehicles
Example 3: Sulfur controls in effect
420 ppm CO2, 5 ppb SO2, 25 ppb NOx
Result: pH ≈ 4.9
With SO2 largely controlled by scrubbing, NOx from vehicles becomes the dominant acidifier — the pattern now seen across much of Europe and North America.
Example 4: The logarithmic point
pH 5.6 compared with pH 4.6
Result: 10× the hydrogen ion concentration
[H+] rises from 2.5 × 10−6 to 2.5 × 10−5 mol/L. The pH number changes by one; the chemistry changes by an order of magnitude.
Example 5: Pre-industrial baseline
280 ppm CO2, no SO2 or NOx
Result: pH ≈ 5.7
Lower CO2 gives slightly less acidic rain. The shift from 5.7 to 5.6 from CO2 alone is small — pollutant acids, not CO2, drive acid rain.

Common Mistakes

⚠️
Assuming clean rain should be pH 7

Dissolved atmospheric CO2 makes unpolluted rain naturally acidic at around pH 5.6. Acid rain is defined against that baseline, not against neutrality.

⚠️
Reading pH differences linearly

The scale is logarithmic. pH 4.6 has ten times the hydrogen ion concentration of pH 5.6, and pH 3.6 has a hundred times. A one-unit fall is a large change, not a small one.

⚠️
Treating SO2 and CO2 as comparable per unit

Sulfuric acid is strong and dissociates fully; carbonic acid is weak and barely dissociates. Parts per billion of SO2 can outweigh hundreds of parts per million of CO2.

⚠️
Ignoring buffering by the receiving environment

Rain pH is only half the story ecologically. Catchments on limestone neutralise incoming acidity, while granite catchments have almost no buffering capacity and acidify rapidly.

Frequently Asked Questions

Why is natural rain acidic?
CO2 dissolves in water forming carbonic acid H2CO3, which partially dissociates giving H+. At 420ppm CO2, equilibrium pH≈5.6. This is normal — 'acid rain' is defined as pH<5.6 (below natural CO2 equilibrium).
Acid rain ecological effects?
pH<5.5: fish die, salmon cannot reproduce. pH<4.5: most fish dead, aluminum mobilized from soil (toxic to roots). Acid neutralizing capacity (ANC) of watershed determines vulnerability. Limestone regions buffered; granite regions vulnerable.
Why is natural rainwater acidic?
Atmospheric CO2 dissolves in water droplets forming carbonic acid, which partially dissociates and releases hydrogen ions. This sets a natural baseline near pH 5.6.
What pH counts as acid rain?
Rain below about 5.6 — the natural CO2 baseline. Values between 4.3 and 5.0 are typical of industrialised regions, and below 4.3 indicates severe acidification.
Why do SO2 and NOx matter more than CO2?
They form strong acids that dissociate completely, whereas carbonic acid is weak and barely dissociates. Parts per billion of SO2 can lower pH more than a large change in CO2.
Does a drop of one pH unit matter?
Substantially. The scale is logarithmic, so one unit is a tenfold change in hydrogen ion concentration — enough to affect fish reproduction and mobilise aluminium from soils.
Has acid rain improved?
In North America and Europe, yes — sulfur emission controls significantly raised rain pH from the 1990s onward. NOx from transport has proved harder to reduce and is now the larger share in many regions.

Formula Explorer connections

Interpretation: This relationship connects hydrogen-ion activity, dissociation, conjugate ratios or titration stoichiometry to acid–base behavior. Assumption: Concentration approximates activity mainly in dilute solutions. Temperature, ionic strength, acid strength and equilibrium approximations affect accuracy.

Buffer Capacity Calculator →Buffer pH Calculator →Common Ion Effect Calculator →Chemistry Formula Explorer →