PPB (Air) to µg/m³ Converter
Common Conversions
| ppb (air) | µg/m³ |
|---|---|
| 0.1 | MW/244.5 |
| 1 | MW/24.45 |
| 5 | MW/4.89 |
| 10 | MW/2.445 |
| 25 | MW/0.978 |
| 50 | MW×2.044 |
| 100 | MW×4.089 |
| 250 | MW×10.224 |
| 500 | MW×20.45 |
| 1000 | MW×40.90 |
| 5000 | MW×204.5 |
| 10000 | MW×409.0 |
Why this conversion matters in chemistry
EPA NAAQS compliance work runs through this constantly. The 70 ppb 8-hour ozone primary standard reads as 137 µg/m³ on a Federal Reference Method analyzer's output trace. That 24.45 factor in the denominator is what ties the two together: it's the molar volume in L/mol of an ideal gas at 25 °C and 1 atm, and any deviation from that reference state — high altitude, cold winter measurement campaigns — needs you to recompute it from PV = nRT before reusing the formula.
Formula
µg/m³ = (ppb × MW) ÷ 24.45
Worked Examples
1 ppb CO (MW 28) = 1.145 µg/m³
Carbon monoxide — the lightest of the criteria pollutants.
50 ppb O₃ (MW 48) = 98.16 µg/m³
Ozone — about the WHO interim target for ambient air.
10 ppb NO₂ (MW 46) = 18.82 µg/m³
Ambient nitrogen dioxide — about a typical urban-monitoring reading.
100 ppb SO₂ (MW 64) = 261.8 µg/m³
Sulfur dioxide — about the EPA short-term primary standard.
Frequently Asked Questions
How do I convert ppb to µg/m³?
µg/m³ = (ppb × MW) / 24.45. For 50 ppb of ozone (MW 48): (50 × 48) / 24.45 = 98.2 µg/m³. The 24.45 factor is the ideal-gas molar volume in L/mol at 25 °C and 1 atm.
Why do air-quality standards use different units?
US EPA reports in ppb — a mole ratio, independent of MW. WHO uses µg/m³ — mass per volume, dependent on MW. Each unit serves a different purpose; both are needed for cross-jurisdictional compliance.
Does altitude affect the conversion?
Yes. Lower atmospheric pressure at altitude increases the molar volume, which changes the 24.45 factor. The conversion is reference-state dependent — recalculate molar volume from PV = nRT for any deviation from 25 °C and 1 atm.