Source Apportionment. We used the CAMx OSAT source apportionment tool to track NO2 from several emission source sectors as listed in Table . To select individual electric generating units (EGUs) in our 0.444 km CAMx domain for NO2 tracking we used a threshold of 0.8 tons per day of NOx emissions. This threshold identified nine EGUs shown in the first nine rows of Table . We also selected on-road mobile, railyards, shipping and the George Bush Intercontinental (KIAH) and William P. Hobby (KHOU) airports for NO2 tracking. All remaining NOx emissions were tracked together in the "other" category. Relevant for this project, average weekday NOx emissions for the 0.444 m domain were 372.9 tons per day (123 Gg/yr).