µg/m³ to PPB (Air) Converter
Common Conversions
| µg/m³ | ppb (air) |
|---|---|
| 0.1 | 2.445/MW |
| 1 | 24.45/MW |
| 5 | 122.25/MW |
| 10 | 244.5/MW |
| 25 | 611.25/MW |
| 50 | 1222.5/MW |
| 100 | 2445/MW |
| 250 | 6112.5/MW |
| 500 | 12225/MW |
| 1000 | 24450/MW |
| 5000 | 122250/MW |
| 10000 | 244500/MW |
Why this conversion matters in chemistry
Ambient air-quality monitoring is a place this matters. A 125 µg/m³ ozone hourly average from a reference-method monitor (MW 48 for O₃) converts to about 64 ppb at 25 °C — the form ambient ozone standards and the AQI are both written in, so the conversion happens before the number means anything to a compliance reviewer. The conversion uses the ideal-gas molar volume (24.45 L/mol at 25 °C, 1 atm). What this is, really: the step between µg/m³ instrument output and the ppb form air-quality standards default to.
Formula
Where the factor comes from
No single factor will do the job here, because quantities of two different kinds sit on either side. µg/m³ is a mass concentration; ppb in air is a mole fraction — molecules of analyte per 10⁹ molecules of gas. Crossing between them takes the analyte's molar mass, to turn mass into moles, and the molar volume of the air, to turn cubic meters into moles. At 25.00 °C and 101325 Pa the ideal gas law gives V = RT/P = 24.4654 L/mol, a figure that follows from defined values alone now that R is fixed; the 24.45 in the working formula is that rounded, low by roughly 0.06%. Assemble the pieces: (10⁻⁶ g/m³ ÷ MW) divided by (1000 L/m³ ÷ 24.45 L/mol), scaled by 10⁹, leaves 24.45/MW. The soft parts are the model and the chemistry — molar masses rest on measured atomic weights, and real air is not an ideal gas.
Precision and significant figures
Three figures in 24.45 and three or four in a molar mass together support about three in the answer, and the input rarely earns that many. A 46 µg/m³ NO₂ hourly value gives 46 × 24.45 ÷ 46.01 = 24.4 ppb; writing 24.44 overstates the measurement. The larger term is not rounding but the conditions the molar volume assumes. At 20 °C it is 24.055 L/mol, 1.7% below the 25 °C value, and at 0 °C it is 22.414 L/mol, roughly 8% below. Reporting conventions differ over which reference conditions apply, so applying 24.45 to data referenced elsewhere is a systematic bias, not a rounding choice.
Worked Examples
About a trace CO ambient reading.
About a typical urban ambient NO₂ level.
About a moderate ambient ozone reading.
About a typical indoor formaldehyde level.
Common mistakes
The formula applied to particulate matter
PM2.5, PM10 and filter-collected metals have no ppb equivalent, because a mole fraction counts molecules dispersed in the gas and a particle is not one of them. Semivolatile species complicate matters further by splitting between the gas and particle phases. A µg/m³ figure for anything collected on a filter stays in µg/m³.
Molar volume left at sea level
The 24.45 assumes 101325 Pa. A monitor at 2000 m sees roughly 79 kPa, where the molar volume runs about 31.4 L/mol — a 28% shift that no amount of care with the molar mass will recover. Stack gas at elevated temperature moves it further still. Recompute RT/P for the actual conditions rather than reaching for the table value.
Reported species and measured species differ
Ambient NOx is conventionally expressed as NO₂ whatever the actual split, and hydrocarbon totals are quoted as methane, propane or hexane depending on the program. Using NO at 30.01 where the figure was reported as NO₂ at 46.01 shifts the result by 53%. Take the molar mass from the reporting convention printed on the data.