I would expect that when the spans came down, the air under them had to be ‘blown out’ as the concrete spans fell...this might give the appearance of smoke etc. as the air rushed out from underneath the collapsing segments.
If I remember my physics correctly, an object will fall and accellerate at 9.8meters/sec2. So, if the bridge was 65 feet up it would take approx 2-2.5seconds to fall that distance so the speed at impact would be 100feet/second(?) so the air would have been blowing out pretty good, moving all sorts of dust and trash..
Just my really rough calcs...