Calculator Inputs
Quick Interpretation
- 0–50: Good
- 51–100: Moderate
- 101–150: Unhealthy for Sensitive Groups
- 151–200: Unhealthy
- 201–300: Very Unhealthy
- 301–500: Hazardous
- > 500: Beyond the AQI
Formula Used
The Air Quality Index converts a pollutant concentration into a standardized scale using linear interpolation:
I = (IHi − ILo) / (BPHi − BPLo) × (Cp − BPLo) + ILo
- Cp: truncated SO₂ concentration (ppb).
- BPLo, BPHi: the SO₂ breakpoints surrounding Cp.
- ILo, IHi: the AQI values that match those breakpoints.
SO₂ Breakpoints Used
| AQI Range | Category | SO₂ 1-hour (ppb) | SO₂ 24-hour (ppb) |
|---|---|---|---|
| 0–50 | Good | 0–35 | — |
| 51–100 | Moderate | 36–75 | — |
| 101–150 | Unhealthy for Sensitive Groups | 76–185 | — |
| 151–200 | Unhealthy | 186–304 | — |
| 201–300 | Very Unhealthy | — | 305–604 |
| 301–400 | Hazardous | — | 605–804 |
| 401–500 | Hazardous | — | 805–1004 |
| > 500 | Beyond the AQI | — | > 1004 (extended) |
Unit Conversion (Optional)
If you enter µg/m³, the calculator converts to ppb assuming 25°C and 1 atm using: ppb = (µg/m³ × 24.45) / MW, where MW(SO₂) = 64.066 g/mol.
How to Use This Calculator
- Choose your input unit: ppb or µg/m³ (assumed 25°C, 1 atm).
- Enter a 1-hour max value for typical daily AQI reporting.
- If available, also enter the 24-hour average for high pollution days.
- Use Auto mode for standard guidance behavior.
- Click Calculate AQI to see the result above the form.
- Use the CSV or PDF buttons to export the computed result.
Example Data Table
| # | Input | Mode Used | Computed AQI | Category |
|---|---|---|---|---|
| 1 | 1-hour 20 ppb | 1-hour | 29 | Good |
| 2 | 1-hour 60 ppb | 1-hour | 81 | Moderate |
| 3 | 1-hour 120 ppb | 1-hour | 121 | Unhealthy for Sensitive Groups |
| 4 | 1-hour 250 ppb | 1-hour | 178 | Unhealthy |
| 5 | 24-hour 400 ppb | 24-hour | 232 | Very Unhealthy |
| 6 | 24-hour 700 ppb | 24-hour | 348 | Hazardous |
Sulfur Dioxide AQI Guide
1) What SO₂ AQI represents
The Air Quality Index (AQI) converts sulfur dioxide (SO₂) concentrations into a single number from 0 to 500. This helps compare air quality across days and locations. SO₂ is a reactive gas produced by burning sulfur-containing fuels and by some industrial processes. Short spikes can irritate airways, especially for sensitive groups.
2) Why averaging time changes the index
SO₂ health effects depend on exposure duration. Daily reporting commonly uses the maximum 1-hour value at lower levels, because short-term peaks drive symptoms. At very high pollution, a 24-hour average is used to better represent sustained exposure and to align index values above 200 with longer-duration risk.
3) Breakpoint ranges used here
This calculator applies standard breakpoint tables for SO₂. For the 1-hour method, 0–35 ppb maps to AQI 0–50, 36–75 ppb to 51–100, 76–185 ppb to 101–150, and 186–304 ppb to 151–200. For higher days, the 24-hour method uses 305–604 ppb for 201–300, 605–804 ppb for 301–400, and 805–1004 ppb for 401–500.
4) Truncation and why it matters
To match reporting practice, SO₂ concentrations are truncated to an integer number of parts per billion (ppb) before interpolation. Truncation avoids overstating the index when measurements include decimals. For example, 75.9 ppb is treated as 75 ppb in the calculation, keeping results consistent with many published AQI methods.
5) Optional µg/m³ conversion assumptions
If your monitor reports mass concentration, the tool converts µg/m³ to ppb assuming 25°C and 1 atmosphere. It uses the ideal-gas factor 24.45 and the SO₂ molecular weight 64.066 g/mol. If conditions differ significantly (temperature, pressure, humidity), convert externally for the most precise ppb input.
6) Using category data for decisions
AQI categories translate numbers into risk language. “Good” and “Moderate” generally support normal activity. “Unhealthy for Sensitive Groups” suggests reducing prolonged exertion for people with asthma or other respiratory concerns. “Unhealthy” and above call for broader activity reduction and closer attention to local guidance.
7) Typical SO₂ patterns and sources
Elevated SO₂ often appears near power plants, refineries, shipping corridors, and metal smelting operations. Peaks may track fuel use, wind direction, and atmospheric stability. Cold, calm conditions can limit dispersion, while strong winds can rapidly lower concentrations. Combining AQI with basic meteorology improves interpretation.
8) Professional reporting with exports
The built-in CSV and PDF exports support audits, environmental reports, and daily operational logs. Save the concentration basis (1-hour or 24-hour), the truncated ppb value, the AQI, and the category together. For transparency, document instrument type, averaging method, and calibration status alongside exported results.
FAQs
1) Should I enter both 1-hour and 24-hour values?
Enter both when you have them. Auto mode uses 1-hour at lower levels and 24-hour at higher levels, improving accuracy for both typical days and severe events.
2) Why does the calculator truncate ppb to an integer?
Many AQI methods use truncation for SO₂ reporting. It prevents decimal precision from inflating the index and keeps results aligned with published breakpoint tables.
3) What if my 24-hour SO₂ value is below 305 ppb?
Below 305 ppb, SO₂ AQI is usually reported using the daily maximum 1-hour concentration. Auto mode will recommend a 1-hour value for best low-level reporting.
4) Is the µg/m³ conversion always correct?
The conversion assumes 25°C and 1 atm using an ideal-gas factor. If your conditions differ, convert to ppb with your local temperature and pressure for the most accurate index.
5) Why does SO₂ switch to 24-hour averaging at high AQI?
High SO₂ days can involve sustained exposure. Using the 24-hour average for AQI above 200 better represents longer-duration health risk and supports consistent public reporting.
6) Can the AQI exceed 500 in this tool?
The AQI scale is defined up to 500. For extreme concentrations above the final breakpoint, this calculator extends the last segment to show “Beyond the AQI” behavior.
7) What data should I store for compliance or audits?
Record the input concentration, averaging basis, truncated ppb value, AQI result, category, location, and timestamp. Add instrument details and calibration notes for stronger traceability.