Editing Parts-per notation (section)

=== Mass fraction vs. mole fraction vs. volume fraction ===
Another problem of the parts-per notation is that it may refer to [[Mass fraction (chemistry)|mass fraction]], [[mole fraction]] or [[volume fraction]]. Since it is usually not stated which quantity is used, it is better to write the units out, such as kg/kg, mol/mol or m<sup>3</sup>/m<sup>3</sup>, even though they are all dimensionless.<ref>{{cite journal |last1=Schwartz |first1=S.E. |last2=Warneck |first2=P. |title=Units for use in atmospheric chemistry (IUPAC Recommendations 1995) |journal=Pure and Applied Chemistry |date=1995 |volume=67 |issue=8–9 |pages=1377–1406 |doi=10.1351/pac199567081377|url=http://www.iupac.org/publications/pac/1995/pdf/6708x1377.pdf| s2cid = 7029702}}</ref> The difference is quite significant when dealing with gases, and it is very important to specify which quantity is being used. For example, the conversion factor between a mass fraction of 1&nbsp;ppb and a mole fraction of 1&nbsp;ppb is about 4.7 for the greenhouse gas [[CFC-11]] in air (Molar mass of CFC-11 / Mean molar mass of air = 137.368 / 28.97 = 4.74). For volume fraction, the suffix "V" or "v" is sometimes appended to the parts-per notation (e.g. ppmV, ppbv, pptv).<ref>{{cite web |title=EPA on-line tools for site assessment calculation: Indoor air unit Conversion |publisher=[[United States Environmental Protection Agency|Environmental Protection Agency]] |url=http://www.epa.gov/athens/learn2model/part-two/onsite/ia_unit_conversion.html}}</ref><ref>{{cite book |author=Beychok, Milton R. |year=2005 |article=Air dispersion modeling conversions and formulas |title=Fundamentals of Stack Gas Dispersion |edition=4th |publisher=Milton R. Beychok |isbn=0964458802 |url=https://archive.org/details/fundamentalsofst0000beyc |url-access=registration}}</ref> However, ppbv and pptv are usually used to mean mole fractions{{dash}}"volume fraction" would literally mean what volume of a pure substance is included in a given volume of a mixture, and this is rarely used except in the case of [[alcohol by volume]].

To distinguish the mass fraction from volume fraction or mole fraction, the letter "w" (standing for "weight") is sometimes added to the abbreviation (e.g. ppmw, ppbw).<ref>{{cite web |title=Units |series=Introduction to green engineering |date=2012-08-23 |df=dmy-all |publisher=[[University of Virginia]] |url=https://pages.shanti.virginia.edu/CE_2050_F12/2012/08/23/units/}}</ref>

The usage of the parts-per notation is generally quite fixed within each specific branch of science, but often in a way that is inconsistent with its usage in other branches, leading some researchers to assume that their own usage (mass/mass, mol/mol, volume/volume, mass/volume, or others) is correct and that other usages are incorrect. This assumption sometimes leads them to not specify the details of their own usage in their publications, and others may therefore misinterpret their results. For example, [[electrochemistry|electrochemists]] often use volume/volume, while [[Chemical engineering|chemical engineers]] may use mass/mass as well as volume/volume, while [[chemist]]s, the field of [[occupational safety]] and the field of [[permissible exposure limit]] (e.g. permitted [[gas]] exposure limit in [[air]]) may use mass/volume. Unfortunatelly, many academic publications of otherwise excellent level fail to specify their use of the parts-per notation, which irritates some readers, especially those who are not experts in the particular fields in those publications, because parts-per-notation, without specifying what it stands for, can mean anything.{{Citation needed|date=October 2021}}