A 100m tall chimney stack emits hydrogen chloride (density = 1.64kg/m3 ) at a rate of 1m3 /s. The plume initially rises a further 5m directly above the exit before being convected horizontally by a wind blowing at a speed of 10m/s under neutral atmospheric conditions. The terrain has a roughness length of zo = 0.03m. A small housing development commences at a location which is a distance of x = 1500m downwind of the stack and y = 500m from the centre-line of the plume.

 (a) What is the pollution concentration (kg/m3 ) at ground level at the start of the housing development? [Answer: 43 µg/m3 ]

 (b) If the wind direction changed so that the plume axis pointed directly towards the housing development, what would be the new ground level concentration at the same location as before ? (Assume that the wind speed and the rate of emission remain unchanged). [Answer: 1537 µg/m3 ]