Calc 1: Blowing up Emerald Forest
At some point after a clash between Alpha and a goon, Emerald Forest gets its shit fucked so let's calc that.
Emerald Forest is about 10 km^2 in area, leaving us with a radius of 1.58113883008419 km.
((1.58113883008419/0.28)^3)/2 = 90.03387106437851 kilotons, Town level+
Calc 2: Blowing up Emerald City
Emerald City is approximately 1980 km^2, giving us a radius of 22.24859546128699 km.
(((22.24859546128699/0.28)^3)/2)/1000 = 250.8439949284133 megatons, Mountain level
So yeah, there you go.
EDIT: I decided to have the Emerald Forest be compared to the size of an actual forest. For this, I'll use the Ocala National Forest in Florida, which is 673 square miles or 1743 square kilometers.
This gives us a radius of 20.874625745148103133692327317502 km.
(((20.874625745148103133692327317502/0.28)^3)/2)/1000 = 207.18199181050213967990232390218 megatons, Mountain level