Calculator Inputs
Chemistry · Deposition · BufferingProvide emissions and local rainfall assumptions. Conversion and deposition fractions help approximate how much acid reaches your selected area.
Example Data Table
These examples show how different inputs can change estimated pH and loads.
| Scenario | SO₂ (t/yr) | NOₓ (t/yr) | Rain (mm) | Area (km²) | Alkalinity (mg/L) | Conversion (SO₂/NOₓ) | Deposition frac | Expected trend |
|---|---|---|---|---|---|---|---|---|
| Urban plume, low buffer | 1800 | 1400 | 700 | 15 | 8 | 0.70 / 0.60 | 0.28 | Lower pH, higher equivalents |
| Coastal rains, moderate buffer | 900 | 650 | 1100 | 40 | 22 | 0.55 / 0.45 | 0.18 | Near baseline pH, lower loads |
| Industrial corridor, strong buffer | 2400 | 1700 | 950 | 30 | 45 | 0.65 / 0.55 | 0.25 | Buffer offsets acidity, pH rises |
Formula Used
- Rain volume:
V(L) = (Rain_mm / 1000) × Area_m² × 1000 - SO₂ moles:
n(SO₂) = Emission_g / 64.066 - NOₓ moles (as NO₂):
n(NO₂) = Emission_g / 46.0055 - Acid formed locally:
n(H₂SO₄)=n(SO₂)×fₛ×f_dep,n(HNO₃)=n(NO₂)×fₙ×f_dep - Hydrogen ions added:
n(H⁺)=2·n(H₂SO₄)+1·n(HNO₃) - Alkalinity (as CaCO₃):
eq/L = (mg/L ÷ 1000) ÷ 50,eq_total = eq/L × V(L) - Background acidity:
[H⁺]_bg = 10^(−pH_bg) - Net acidity:
n(H⁺)_net = n(H⁺)_bg + n(H⁺)_added − eq_total - pH:
pH = −log₁₀( n(H⁺)_net / V(L) )
How to Use This Calculator
- Enter annual SO₂ and NOₓ emissions for your source region.
- Set rainfall and the area you want to evaluate.
- Choose conversion fractions to reflect atmospheric chemistry.
- Use a deposition fraction to approximate local impacts.
- Add alkalinity to represent neutralization from bases or dust.
- Submit to view pH, loads, and acid equivalents above the form.
- Download CSV or PDF to share results in reports.
SO₂ and NOₓ as acid rain precursors
Acid rain forms when sulfur dioxide and nitrogen oxides oxidize to strong acids. In this calculator, SO₂ converts to sulfuric acid (H₂SO₄) and NOₓ converts to nitric acid (HNO₃). Sulfuric acid yields two hydrogen ions per mole, while nitric acid yields one. Typical screening conversion fractions range from 0.3–0.8 for SO₂ and 0.2–0.7 for NOₓ, depending on sunlight, oxidants, and transport. Higher conversion raises the acid equivalents deposited per hectare each year.
Rainfall volume and dilution effects
Rainfall controls dilution because concentration is calculated as net moles of H⁺ divided by annual rainwater volume. Volume uses V = rain(m) × area(m²), then converts to liters for molarity. For example, 900 mm over 25 km² equals 22.5 million m³ of water. When rain increases while emissions stay constant, acidity can weaken even if total deposited mass stays similar.
Buffering using alkalinity as CaCO₃
Neutralization is represented with alkalinity in mg/L as CaCO₃. The calculator converts mg/L to equivalents using 50 g per equivalent for CaCO₃. A value of 20 mg/L equals 0.0004 eq/L, meaning 0.0004 moles of H⁺ can be neutralized per liter of rainwater. Dust-rich regions or sea-salt aerosols often increase alkalinity and can offset acidity substantially.
Local deposition fraction and area-based loading
Not all formed acids fall on the same place, so a local deposition fraction scales the portion reaching your selected area. The output reports acid load as kilograms per hectare and acid equivalents as eq/ha. These area-normalized values help compare sites, seasons, or mitigation actions. A small area with the same deposition fraction can show higher kg/ha than a large area because the same mass is distributed over fewer hectares.
Interpreting pH bands for screening decisions
pH is computed as −log₁₀([H⁺]) after background acidity and buffering are applied. Natural rain often sits near pH 5.6 from dissolved CO₂, while values below 5.0 can stress sensitive lakes and soils. The tool labels pH below 4.0 as severe, 4.0–5.0 as acidic, and 5.0–5.6 as slightly acidic. Use these bands for screening, then validate with monitoring and deposition networks. Pair this estimate with local deposition monitoring and seasonal wind-rose analysis for confidence.
FAQs
1) What does this calculator estimate?
It estimates rainwater pH and acid loading using SO₂/NOₓ emissions, chemistry conversion fractions, local deposition fraction, rainfall volume, and alkalinity buffering. Results are screening-level indicators for comparison and planning.
2) Why is background pH set near 5.6?
Rain naturally absorbs atmospheric CO₂ and forms weak carbonic acid, commonly giving pH around 5.6. Your site may differ, so the calculator lets you set an alternative baseline.
3) How should I choose conversion fractions?
Use local studies, modeling, or conservative screening values. In brighter, oxidant-rich air, SO₂ and NOₓ convert faster to acids. For quick checks, start around 0.6 for SO₂ and 0.5 for NOₓ.
4) What does alkalinity represent here?
Alkalinity approximates neutralizing capacity from bases and particles, expressed as mg/L as CaCO₃. Higher alkalinity means more H⁺ can be neutralized, raising predicted pH and lowering net acidity.
5) What is “local deposition fraction”?
It is the fraction of formed acids assumed to deposit on your selected area. Lower values represent more transport away from the area; higher values represent stronger local impact near sources.
6) Are CSV and PDF exports calculated server-side?
The exports are generated in the browser from the current inputs and the displayed results. They are useful for sharing, but you should retain your original assumptions and monitoring references alongside the files.