https://twitter.com/HeshmatAlavi/status/1290683991369097216 [camera operator probably would have died in the second blast]
Is that what set it off? Or is that what ammonium nitrate sounds like burning?
The cloud accompanying the big explosion has a very distinct red tint to it, indicating that it's a result of the ammonium nitrate going off. That before it doesn't, which suggests that the ammonium nitrate wasn't burning at the time. If you compare the Texas City disaster, it's noted that spectators were drawn towards the site by the unusual smoke clouds before the big explosion happened.
There's a lot of details that aren't clear yet, but a variety of materials being stored in a warehouse catching fire, with fireworks going off first and the ammonium nitrate going off last, is consistent with the known facts. It would, however, be exceptionally bad safety practices to store ammonium nitrate anywhere near something like fireworks. Then again, storing a large amount in a warehouse right next to downtown Beirut is itself insane, considering it has a history of being implicated in the largest explosions not involving nuclear weapons.
The deflagration of solid AN may produce NOx (an orange plume)...
Thermal decomposition produces nitrous oxide (laughing gas) and water.
[1]https://www.sciencedirect.com/science/article/pii/S001021801...
I did check and it was quickly geo-located -