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). These gases contribute to the
formation of smog and acid rain, as well as affecting tropospheric
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. 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), 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, 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).