Wikipedia says

[...] Within the SCR catalyst, the NOx are catalytically reduced by the ammonia [...]

NO2 is nitrogen dioxide, which is singular. x can take any one of a range of values, so NOx means

nitrous oxide or nitrogen dioxide or dinitrogen pentoxide or other alternatives

and so is, logically, singular. If we regard NOx as a chemical of variable composition then this makes sense. x cannot logically have different values at once, but I think NOx is being used in this sentence to mean

nitrous oxide and nitrogen dioxide and dinitrogen pentoxide and other alternatives, all mixed together

It is possible the grammar is also influenced by NOx sounding plural as it is pronounced "knocks".

So can we say "The NOx are" or not?

  • If anyone knows how to change the title to say NO\<sub>*x*\</sub> (without the backslashes!),please can they do so? Sep 18, 2018 at 21:37
  • 1
    So, if it were H2O molecules would you want to say "the waters is" or "the waters are"?
    – Hot Licks
    Sep 18, 2018 at 21:55
  • 1
    @DavidRobinson I don't think you can do that in titles on this site. Sep 18, 2018 at 21:56
  • Is this like saying "the gases are" ? Sep 18, 2018 at 21:57
  • 1
    My point is that it's a lot clearer and less grammatically confusing to say "the NOx molecules are ...".
    – Hot Licks
    Sep 18, 2018 at 22:53

4 Answers 4


Yes, but only if you are referring to Nitrogen Dioxide as a category. If you are referring specifically to the gas, the answer is no.

For example, "All types of NOx are converted in the reaction" - valid because it is referring to the types of NOx you have, which is multiple.

"All of my NOx is converted in the reaction" is referring to NOx as a mass noun, so there is no plural in that sense.

  • 5
    In both your examples, you have taken NOx out of the picture as far as verb agreement goes. All types are, All is. In other words, your examples don't have anything to do with the question or your explanations.
    – Phil Sweet
    Sep 19, 2018 at 1:05
  • 1
    @PhilSweet for the second case, that's not true. There's still verb agreement with the main noun in this construction: "all of my sugar is sweet" vs "all of my sugar cubes are sweet". Sep 19, 2018 at 9:23

Yes, I figure you can. It just means the different nitrogen oxides. I was surprised to read that it also includes Nitrous oxide, considering the 2 Nitrogen atoms, but in any case 'oxides' would take a plural form.

Wikipedia says the following:

In atmospheric chemistry, NOx is a generic term for the nitrogen oxides that are most relevant for air pollution, namely nitric oxide (NO) and nitrogen dioxide (NO2).[1][2] These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone.

NOx gases are usually produced from the reaction among nitrogen and oxygen during combustion of fuels, such as hydrocarbons, in air; especially at high temperatures, such as occur in car engines.[1][2][3] In areas of high motor vehicle traffic, such as in large cities, the nitrogen oxides emitted can be a significant source of air pollution. NOx gases are also produced naturally by lightning.

The term NOx is chemistry shorthand for molecules containing one nitrogen and one or more oxygen atom. It is generally meant to include nitrous oxide (N2O),[1] although nitrous oxide is a fairly inert oxide of nitrogen that has many uses as an oxidizer for rockets and car engines, an anesthetic, and a propellant for aerosol sprays and whipped cream. Nitrous oxide plays hardly any role in air pollution, although it may have a significant impact on the ozone layer,[4] and is a significant greenhouse gas.

In the same article NOx is used both in plural and in singular forms depending on whether it refers to various nitrogen oxides or to singular nitrogen oxide, e.g.

Ott noted that the lightning-produced NOx is typically found at altitudes greater than 5 km, while combustion and biogenic (soil) NOx are typically found near the sources at near surface elevation (where it can cause the most significant health effects).

  • As for the two N atoms, these are stoichiometic formulae - just concerned with the ratios, not the amounts. So N2O counts as the same as NO½. N2O5 is also included as this counts as NO2½. The other problem, which does have linguistic significance is that N2O does not count as a "nasty substance". Whereas the others are nasty pollutants, and indeed the acceptance of the term NOx may have been influenced by the word noxious which is apposite, N2O is not noxious. It is laughing gas which is quite pleasant to breathe and is used in aerosol whipped cream, so is often disregarded as a NOx. Sep 18, 2018 at 23:33
  • The two plural references I can see are for "NOx gases are", where NOx is used as an adjective. The singular references are when talking about the term "NOx" rather than what it represents ("Nox is a generic term" and "The term NOx is chemistry shorthand"). So "NOx" itself, as a shorthand for "nitrogen oxides" is not used as the subject of a verb at all, so we can't make any judgment based on that passage. Sep 19, 2018 at 9:28
  • @Phil M Jones. This is the kind of sentence I was referring to in the article: NOx are typically found near the sources at near surface elevation (where it can cause the most significant health effects). NOx here refers to oxides and is not being used adjectivally. In any case I'll add it to the post.
    – S Conroy
    Sep 19, 2018 at 14:25
  • @SConroy Hmm, that certainly brings it into the same category as sheep and fish. Nice find. Sep 20, 2018 at 10:30

NOx is an abbreviation for what is referred to as "nitrogen oxides" in environmental regulations (e.g., 40 CFR 51.165(a)(1)(v)(E), 42 USC 7403(c)(3)(A)), so you can see why the Wikipedia entry chose this formulation. However, it's awkward. In actual usage in the environmental profession, people just use NOx like a mass noun (no article, singular agreement).


tl;dr- Yes, "the NOx are" is a correct construction when referring to various nitrogen oxides, as the quoted excerpt from Wikipedia is.

Earlier in that same paragraph, Wikipedia says:

The excess of air necessarily leads to generation of nitrogen oxides (NOx), which are harmful pollutants, from the nitrogen in the air.

This makes it pretty clear that they're using "NOx" as shorthand for "nitrogen oxides".

Substituting that into the sentence you're asking about, it's:

Within the SCR catalyst, the [nitrogen oxides] are catalytically reduced by [...]

This all looks good to me.

Separately, the singular can also be correct in other contexts. For example, when "NOx" is being used to refer to a specific nitrogen oxide in a general way, then the singular can be correct. Of course, in the excerpt from Wikipedia, they're pointing out the catalytic conversion of various nitrogen oxides rather than just one sort of nitrogen oxide, so the plural's correct there.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.