The effect of emission height in very large areas of emission

Abstract

It is important to study the effect of emission height when many sources and very large areas are concerned. Idealized situations have been taken in which emissions are assumed to be uniformly distributed within areas of emission and in which normally continuous variables have been represented by a number of fixed values. Thus three city sizes, five mixing layer depths (including 0 and ∝), six effective chimney heights and two atmospheric stability regimes have been taken and studied over distances up to 1000 km. Any wind speed and any uniform emission rate per unit area can be allowed for. The increase in concentration with distance over an area of emission has been calculated and also the decrease of concentration with distance outside the area. It is shown that while the concentration produced by an area of emission becomes independent of emission height at great distances, the maximum concentration at any distance decreases steadily with increase in emission height. It is shown that tall chimneys need to be taller for larger areas of emission, but that for all practical situations they provide effective control of ground level concentration. The reasons are: 1. 1. Tall chimneys (with buoyant emission) greatly increase the critical wind velocity for a given rate of emission. 2. 2. They put a finite limit to the increase of concentration caused by a decrease in the height of a stable layer above the ground. 3. 3. A large part of any significant concentration is usually due to local sources. 4. 4. They limit the increase in concentrations caused by increasing atmospheric stability. Even in the case of very large areas of emission, sources with tall chimneys account only for about 7 per cent of the highest widespread concentrations. The successful application of 90 per cent SO 2 removal on these sources would reduce overall pollution by about 6 per cent if chimney heights and flue gas temperatures were maintained. If they were not, there would be no measurable improvement in widespread concentrations and localized concentrations would be greatly increased. Sulphur face fuel for low level sources would reduce concentrations by about 55 per cent.