And it leaves a trail of smoke (OMG, maybe NO2) The NO2 theory requires a microwave plasma or rocket fuel as the source.
Microwaves don't have enough energy to create NO2https://aip.scitation.org/doi/figure/10.1063/1.4996790
Plasma - yes
Which rocket fuel were you thinking of?https://en.wikipedia.org/wiki/Dinitrogen_tetroxide
Nuclear explosion - yes
Meteorite - yesHow do meteorites create NO2?
Lightning strike - yesIn quantities too small to be considered here?
Chemical reactions - yesBut not relevant to the DPI?
The link is referring to a specific experiment which uses concentrated microwaves to heat up matter to very high temperatures and the matter then turns into a hot plasma, which can create NO2. It would be very difficult and close to incredible to do this naturally. Microwaves themselves dont have rhe energy. Where would the microwaves come from?
dinitrogen tetroxide makes a good oxidiser as a part of a two component system - fuel and oxidiser, and it was used by russia during DPI period. But how is the rocket fuel delivered to the area?
Anything that can create sufficiently high temperatures in the atmosphere can cause oxygen and nitrogen to react to cause nitrogen dioxide. A nuclear blast, or a meteor. They are both very high concentrated energy events.