What acid deposition is
Air pollutants deposited as acidic wet (pH below 5.6) or dry deposition.
Acid deposition is a mix of air pollutants that deposit from the atmosphere as acidic wet deposition (with a pH below 5.6) or acidic dry deposition. ("Acid rain" is the most familiar form, but acid deposition is the broader, correct term.)
The pH figure matters: pure rainwater is naturally slightly acidic (about pH 5.6) because carbon dioxide dissolves in it. Rain below pH 5.6 is therefore counted as acidic deposition caused by pollution.
The two types:
| Type | What it is | Examples |
|---|---|---|
| Wet deposition | Acids dissolved in precipitation, falling to the ground | Acidic rain, snow, hail and fog |
| Dry deposition | Acidic particles and gases that settle directly, later forming acid when they meet water | Acidic dust and gases |
A cross-border problem. Acid-forming gases can travel long distances on the wind before they fall, so a country that burns a lot of fossil fuels can cause acid deposition in neighbouring countries ("transboundary pollution"). This is why managing acid deposition (7.3) often needs international cooperation.
- Acid deposition = acidic wet deposition (pH below 5.6) or dry deposition.
- Wet: acidic rain, snow, hail, fog. Dry: acidic dust and gases.
- Pure rain is ~pH 5.6 (dissolved COβ); below that is pollution-caused.
- 'Acid deposition' is the correct broad term, not just 'acid rain'.
- Gases travel on the wind, so it is a cross-border (transboundary) problem.