Skyhook One
The planning around chemical / fallout scenarios was easier.
As the delivery munitions were targeted at ground forces or centres of population / transport hubs.
So the affected area was generally at lower levels to ground level. So the prevailing winds were easier to estimate. As well as lower level turbulent mixing would keep the affected area more localised.
With this bad boy, there will be some lower level fall out, but the fact that the dust is starting from higher levels, would suggest that it is going to cover a large area very quickly, before the grains start to make their way down.
Just my thought why modelling the scenario is so much more difficult.