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
Where the factor comes from
Two different kinds of quantity are being equated, which is why a molecular weight has to appear at all. Air-phase ppb is a mole ratio — one part analyte per 10⁹ parts of air, counted in molecules — while µg/m³ is a mass loading. The molar volume bridges them. At 25 °C and 101325 Pa the ideal gas law gives RT/P = 24.465 L/mol, so a cubic meter of air holds about 40.9 mol of gas; multiply by the mole ratio for moles of analyte, then by the molar mass for mass. The 24.45 in the working formula is the rounded value long entrenched in air-quality practice. None of that makes the factor exact: ideality is an assumption, the reference state is a convention rather than a discovery, and the molar mass you supply rests on standard atomic weights, which carry published uncertainties of their own.
Precision and significant figures
The reference state dominates every other source of error here. Computed at 25 °C and one atmosphere the molar volume is 24.465 L/mol, and the conventional 24.45 sits about 0.06% below that — negligible beside everything else in the calculation. Shift the reference to 20 °C, as some jurisdictions do, and the molar volume becomes 24.055 L/mol, moving every result by 1.7%. Sampling at altitude or in cold weather moves it further still, and the correction is a straightforward recalculation from PV = nRT. Given all that, three significant figures on the output is honest and a fourth is decoration. State the reference temperature and pressure with the number, or it cannot be converted back.
Worked Examples
Carbon monoxide — the lightest of the criteria pollutants.
Ozone — about the WHO interim target for ambient air.
Ambient nitrogen dioxide — about a typical urban-monitoring reading.
Sulfur dioxide — about the EPA short-term primary standard.
Common mistakes
Reference state assumed rather than stated
A µg/m³ figure means nothing without the temperature and pressure it was computed at, and the 25 °C and 20 °C conventions differ by 1.7% in the molar volume. Datasets from different sources get plotted together as though the axis were common. If the reference state is absent from the metadata, the number can be reported but it cannot honestly be compared.
Mass expressed as a surrogate species
Some measurements are conventionally reported as an equivalent species rather than the one present — nitrogen oxides expressed on an NO₂ basis, for instance, even where NO dominates the actual mixture. Using the molar mass of the species you assume is present instead of the one the reporting convention specifies puts the result out by the ratio of the two masses. Check what the basis is before choosing MW.
Particulate matter has no ppb equivalent
Suspended particles are reported in µg/m³ and nothing else, because they are not a gas: there is no single molar mass and no molar volume to divide by. Converting a particulate mass loading into ppb, or back the other way, produces a number with no physical referent. The formula on this page applies only to gas-phase species.