Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2023-05-11.
Converting molecule count into a mass concentration is straightforward in principle: one molecule of a given gas always weighs the same, and in practice you calibrate the instrument against a standard gas of known concentration rather than counting molecules. The subtlety is the volume, because the same number of molecules occupies different volumes at different temperatures and pressures. In-situ instruments measure where the molecules actually are, so the question does not arise; extractive instruments measure in a cell whose temperature and pressure generally differ from those at the sampling point. The convention is therefore to normalise the volume to a standard state — in industry almost always 0 °C and 1 atm, in air quality 20 °C in Europe and 25 °C in the United States — giving what is called a normalised or standard volume. Industrial mass concentrations are usually quoted in mg/m³ or g/m³, ambient ones in µg/m³. Volume concentrations in ppm or ppb follow from molecular mass and molar volume, and only from those.
In China the same normalisation logic runs through the emission monitoring system: CEMS data are reported at standard state, 273.15 K and 101.325 kPa, on a dry basis and referenced to the oxygen level that the industry standard requires, while ambient air quality under GB 3095 is assessed at actual temperature and pressure in µg/m³. Those are two different bases, and comparing across them without care produces wrong conclusions. Before quoting or comparing a concentration we check three things: the basis — which temperature, dry or wet, oxygen-corrected or not; the conversion factor — it follows the molecular weight, so SO2 and NO2 cannot share one; and the unit of the calibration gas, since many instruments are calibrated in ppm and report in mg/m³. For international work, note that EU and US ambient systems use 20 °C and 25 °C while industrial emissions use 0 °C, so the basis should be stated on the cover of any data package that crosses systems.
Gas Concentration Units and Unit Conversions — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2023-05-11. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
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