How are Oxides of Nitrogen (NOx) defined in the NEI?

For all source categories, NOx is compiled from emissions measurements that express NOx mass based on the molecular weight of nitrogen dioxide (NO2), which is 46.  Even though emission streams initially consist mainly of NO from most sources, this expression of NOx by NO2 MW is appropriate due to the fast rate of transformation of NO to NO2 under ambient conditions or when the emissions effluent is exposed to any type of a oxidant.  While NOx is made up of NO2, nitrogen oxide (NO), and for mobile sources nitrous acid (HONO), the combination of these by mass is more simply  done using a single molecular weight.  If computing the moles of NO2, NO, and/or HONO from NEI NOx data and speciation factors, we recommend that one should therefore use the molecular weight of NO2.