A Discussion on recent research in air pollution - The dispersion of pollutants in the free atmosphere by the large scale wind systems

Abstract

The travel and dispersion of pollutants in the free atmosphere m ay be investigated by the direct measurement of the distributions of tracer materials such as water vapour, ozone and radioactive substances. Another method is to study the spread of pollutants from a constant point source or the expansion of large clusters, by using air trajectories found by tracking balloons or estimated from sequences of wind values obtained from synoptic charts. So far these latter techniques have usually only taken horizontal motions into account since the balloons are normally maintained at constant levels and the winds taken from the charts have been assumed to be geostrophic. In principle the effect of large (synoptic) scale vertical motions can be included by using the component wind fields given at the different time steps of a numerical forecast integration to construct suitable three-dimensional trajectories. A pilot study of this type at the 900, 700, 500 and 300 m bar pressure levels (90, 70, 50 and 30kN m ~2) using the results of a 24 h numerical forecast by the Meteorological Office’s 10 level model is described. In the case studied the use of constant level trajectories gave horizontal dispersions (variances of the trajectory end points relative to their centre of gravity) which differed by only small amounts from those due to the three dimensional trajectories. The zonal variances exceeded the meridional variances by a small factor and both were 4 to 6 orders greater than those of the corresponding variances in the vertical. In each case for at least 12 to 18 h they were all roughly proportional to the square of the time after release (the ‘short time’ case). The large scale clusters rapidly distorted at rates which increased with their initial size and also with the deformation components of the wind field. At these scales deformation plays a major role in the apparent dispersion and the mean values of total deformation so obtained agreed satisfactorily with those calculated from a kinematic analysis of the horizontal wind field.