THE ANSWER IS: "Stack height = 310 m"

The management of the smelter in Problem 6.6 has been informed that the concentration calculated in that problem at that location and for those conditions is twice the allowable. They propose to remedy this situation by installing a higher stack. How high must this stack be so that the estimated concentration will be exactly one-half that in Problem 6.6? (The plume rise is the same as in Problem 6.6.)

(For reference -- 6.6. A large, poorly controlled copper smelter has a stack 150 m high and a plume rise of 75 m. It is currently emitting 1000 g/s of S02. Estimate the ground-level concentration of S02 from this source at a distance 5 km directly downwind when the wind speed is 3 m/s and the stability class is C. The answer to 6.6 is 638 g/m^3, so half of that concentration would be 319 g/m^3.)