This is a follow-up to our earlier article on Using Common Sense and Science in Expressing Gas Concentrations, inspired by a real-life adventure of one of our sales engineers.

As our hero was slogging through some e-mail inquiries, he came upon two units of measurement he had not seen before:

ppmv   and    µg/Nm3


ppmv is simply parts-per-million by volume. This notation would distinguish it from parts-per-million based on weight. As it happens, when one is working with solids or liquids—i.e. when one leaves the world of gas measurements—parts-per-million is understood to be by weight (strictly speaking by mass).

As discussed in the article referenced above, the most proper way to think of parts-per-million in the gas world is as molar concentration: µmoles of minor component / total moles in mixture. By tradition, and because ppmv would be the same as the molar concentration for ideal gases, many people still speak of gas measurements as being in parts-per-million by volume.


µg/Nm3 means micrograms per normal cubic meter (Nm3). The “normal” cubic meter is defined as being at 0°C (273.15°K) and 101.325 kPa or 760 mmHg (i.e. 1 atmosphere of absolute pressure). However, this notation is no longer appropriate unless the specific reference conditions are explicitly stated, since there are currently many different definitions of what constitutes standard reference conditions.

Standard reference conditions in current use
(Adapted from Wikipedia)
TemperatureAbsolute pressureRelative humidityPublishing or establishing entity
°CkPa% RH
0100.000 IUPAC (present definition)
0101.325 IUPAC (former definition), NIST, ISO 10780
15101.3250ISA, ISO 13443, EEA, EGIA
20101.325 EPA, NIST
25101.325 EPA
25100.000 SATP
15100.000 SPE
°Fpsia% RH 
6014.696 SPE, OSHA, SCAQMD
6014.73 EGIA, OPEC, EIA
5914.50378Army Standard Metro
5914.69660ISO 2314, ISO 3977-2

Here are the full names of the entities listed in the above table–

IUPACInternational Union of Pure and Applied Chemistry
NISTNational Institute of Standards and Technology
ISAICAO’s International Standard Atmosphere
ISOInternational Organization for Standardization
EEAEuropean Environment Agency
EGIAElectricity and Gas Inspection Act (of Canada)
EPAU.S. Environmental Protection Agency
SATPStandard Ambient Pressure and Temperature
CAGICompressed Air and Gas Institute
SPESociety of Petroleum Engineers
OSHAU.S. Occupational Safety and Health Administration
SCAQMDCalifornia’s South Coast Air Quality Management District
OPECOrganization of Petroleum Exporting Countries
EIAU.S. Energy Information Administration
Std. MetroU.S. Army’s Standard Metro (used in ballistics)

As you can see, by using the unit µg/Nm3, you are bound to be misunderstood—if not in the definition of normal or standard conditions, then by the difficulties inherent in using mass/volume units rather than parts-per-million.

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