AP Syllabus focus:
‘Acid rain (acid deposition) forms when nitrogen oxides and sulfur oxides are present in the atmosphere from both human and natural sources.’
Acid deposition is a form of atmospheric pollution in which acids or acid-forming compounds are delivered from the air to Earth’s surface. It links emissions of nitrogen and sulfur oxides to changes in precipitation chemistry.
Core idea: what “acid deposition” means
Key term and pH context
Acid deposition: The transfer of acidic substances from the atmosphere to Earth’s surface as wet deposition (rain, snow, fog) and dry deposition (particles and gases).
Photograph-based figure showing an emission source associated with acid deposition precursors and a caption describing how emitted sulfur and nitrogen oxides are transformed into strong acids before deposition. It reinforces the idea that the same emitted gases can contribute to both wet and dry deposition pathways that ultimately lower environmental pH. Source
Acid deposition is often discussed using pH, a measure of how acidic a solution is; lower pH indicates higher acidity.
Map of precipitation pH across U.S. monitoring sites (2002), illustrating how acidity varies regionally. It provides a concrete example of how atmospheric chemistry and pollutant transport can produce lower-pH precipitation in some regions than others. Source
In the atmosphere, acids form when certain gases are chemically transformed into strong acids that dissolve in water droplets or attach to particles.
Wet vs. dry deposition
Wet deposition
Acids dissolved in precipitation (rain, snow, sleet) or in cloud/fog droplets that later contact surfaces.
Can fall far from where precursor gases were released because pollutants can be transported in air masses.
Dry deposition
Acidic gases (e.g., SO₂, HNO₃) and particles (e.g., sulfate and nitrate aerosols) settle or are absorbed directly onto leaves, soil, buildings, and water surfaces.
Often becomes more acidic after contact with moisture (dew, surface water films), effectively “activating” deposited compounds.
Chemical basis: how NOx and SOx create acids
Precursor gases
Acid deposition begins with nitrogen oxides (NOx) and sulfur oxides (SOx) in the air. These are called precursors because they are not always acidic when emitted, but they react in the atmosphere to form acids and acidic particles.
Precursor pollutant: A substance released to the atmosphere that can undergo reactions to form secondary pollutants, including acids and aerosols.
Secondary formation matters because it depends on atmospheric conditions (oxidants, sunlight, and water), not just emission rates.
Simplified formation pathways (conceptual)
The dominant strong acids produced are sulfuric acid (H₂SO₄) and nitric acid (HNO₃). These can form in gas-phase reactions and within cloud droplets.
= sulfur dioxide (gas)
= sulfuric acid (aqueous aerosol/cloud droplet)
= nitrogen dioxide (gas)
= nitric acid (gas or dissolved)
These equations are simplified “net” reactions; in reality, multiple oxidants and intermediate steps can be involved, especially inside cloud water.
Where the NOx and SOx come from (broad categories only)
Human and natural sources (as required by the syllabus)
The specification emphasizes that NOx and SOx in the atmosphere come from both human and natural sources, so acid deposition is not purely anthropogenic, even though human activity can greatly increase it.
Human-related (anthropogenic)
High-temperature combustion and industrial processes can add substantial NOx and SOx to the air.
Natural
Natural combustion and geologic/atmospheric processes can emit NOx and SOx without human involvement.
For AP Environmental Science, the key point is the presence of these oxides in the atmosphere; their subsequent chemical conversion drives acid deposition.
What “acid” actually reaches ecosystems and structures
Delivered forms
Acidity can arrive as:
Dissolved strong acids in raindrops or snowmelt (wet deposition).
Sulfate () and nitrate () aerosols that later dissolve in water on surfaces.
Acidic gases (notably nitric acid) that directly contact surfaces (dry deposition).
Why it is treated as deposition (not just “acid rain”)
“Acid rain” refers mainly to acidic precipitation, but acid deposition is broader because:
Dry deposition can contribute a large fraction of total acidic input in some areas.
Fog and cloud water can be more acidic than rain and deposit efficiently on vegetation and buildings.
The same precursor chemistry can acidify atmospheric moisture even when no rain falls.
FAQ
Fog droplets are very small and have a high surface-area-to-volume ratio, so they absorb gases efficiently.
They can also persist near emission plumes, allowing more time for acids and acidic particles to dissolve into the droplets.
Wet-only collectors open only during precipitation, capturing rain/snow for pH and ion analysis.
Dry deposition is estimated using a mix of methods (e.g., surface collectors, inferential models using air concentrations and deposition velocities), because it does not “fall” in a single easy-to-catch event.
Even unpolluted rain dissolves atmospheric $\mathrm{CO_2}$ to form weak carbonic acid, so it is slightly acidic.
Background pH varies by location and atmospheric composition, but it is not typically pH 7.
Yes. Airborne bases (notably ammonia-containing particles and mineral dust) can partially neutralise acids in droplets or on particles.
This can shift deposition from strong acids toward salts (e.g., ammonium sulfate), changing measured pH and chemistry.
After acids form, they frequently dissociate in water into ions, so monitoring focuses on $\mathrm{SO_4^{2-}}$ and $\mathrm{NO_3^-}$.
These ions persist in precipitation and aerosols, helping identify the contribution of sulphur- and nitrogen-derived acidity.
Practice Questions
Define acid deposition and name its two main forms. (2 marks)
Correct definition: transfer of acidic substances from atmosphere to Earth’s surface (1)
Names both forms: wet deposition and dry deposition (1)
Explain how nitrogen oxides and sulphur oxides in the atmosphere lead to acid deposition. In your answer, refer to precursor pollutants and distinguish between wet and dry deposition. (5 marks)
States that NOx and SOx act as precursor pollutants that form acids/secondary pollutants in the atmosphere (1)
Links SOx (e.g., ) to formation of sulphuric acid and/or sulfate aerosols (1)
Links NOx (e.g., ) to formation of nitric acid and/or nitrate aerosols (1)
Explains wet deposition as acids dissolved in precipitation/fog/cloud water reaching the surface (1)
Explains dry deposition as acidic gases/particles settling or being absorbed by surfaces (1)
