[YPCrumble]

This is a great answer, thank you. However this assumed the atmosphere is 45 km high and it seems very unlikely that these gases are light enough to go that high. If you assume the gases only go 1km in the air wouldn’t that assume a radius roughly 40x your initial estimate, or at least 1800 km to be within the 1 ppm threshold.

[bombcar]

That's why I did a little bit of a sleight of hand - worked out how many cubic meters it would need to be to disperse, and then converted that to square meters (so assumed a depth of one meter).

If I had done a sphere (half a sphere to be precise) you'd get a half-dome only 1.5 km across.

[nickjj]

I wish more concrete data were shared. Having half the US affected is a big deal.