A power plant burns 9 tons of coal per h. The sulfur content of the coal is 2.5%. The effective stack height is 100m, and the wind speed at the effective stack height is 2.8 m/s. It is one hour before sunrise, and the sky is clear. (1) Calculate the ground level concentration ( 0.5h sampling) at 5km directly downwind from the stack; (2) Determine the highest ground level concentration ( 0.5h sampling), and the corresponding distance directly downwind from the stack