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6. Given the conditions of Problems 20.4, calculate the ground level downwind concentration at 3km, class F with a smooth terrain. (5 pts) 20.4 At noon on a sunny summer day with a surface wind speed of 4 m/s,SO2 is released over rough terrain from a 90m stack at a rate of 400g/s. Assume that the plume rise is 60m. Calculate the ground-level concentration: (a) 3000m downwind; (b) 3000 m downwind and 100m crosswind; and (c) 3000m downwind and 500m crosswind