It looks like power plants are also a significant source of nitrogen oxides in the atmosphere, though I don't know what the major/top sources are. From the US EPA [1]:

  NO2 primarily gets in the air from the burning of fuel. NO2 forms from emissions from cars, trucks and buses, power plants, and off-road equipment.

As I say, I'm no expert, bu my understanding is that NOx compounds can be formed whenever nitrogen and oxygen are mixed at sufficiently high temperatures, e.g. when burning stuff in air.

[1] https://www.epa.gov/no2-pollution/basic-information-about-no...