Atmospheric Environment Pergamon Press 1913. Vol. 7, pp. 763-767. Printed in Great Britain.

DISCUSSIONS

A STUDY OF SUSPENDED PARTICULATE AIR POLLUTION AT TWO LOCATIONS IN TORONTO, CANADA*

WE XOTE with satisfaction that Munn, as in the work by BEZUGLAYA and SONKYN (1971), quoted by him, found a secondary maximumwhen considering the dust concentrations in Toronto in the range of velocities close to 5 m s-r. We should like to point out that according to the analysis performed by the same authors of the observational data in different cities, such secondary maxima almost always occur, not only for dustiness, as is seen from Munn’s work, but also for the gaseous pollutant con- centrations.

These results are strongly connected with the action of high sources. Note should be taken that they are close to our theoretical results (see for example Appendix to Technical Notes WMO No. 121, 1972), from which it is seen that the maximum of ground concentration from thermal power stations and powerful boiler-houses must occur at the critical or dangerous wind velocity which equals approximately 5 m s- r.

It is also necessary to note that Munn’s data concern 24-h observations. The above mentioned secondary maximum would be expressed more clearly if the data had to do with the daylight hours, when the turbulent exchange is usually developed and an intensive transportation of pollutants from the high sources to the boundary layer takes place.

Main Geophysical Observatory, Leningrad, U.S.S.R.

M. E. BERLYAND L. R. Z&-KIN

* R. E. MUNN (1973) Atmospheric Environment 7, 311-318.

RESIDENCE TIME OF PARTICLES IN URBAN AIR*

IN THEIR replyt to Fischer’s comments$ on their original paper, Esmen and Corn seem to have made a mechanical error. To find the ratio of the turbidity at time zero, K(O), to the turbidity at a later time, K(t), they have set up the ratio, cancelled, and then integrated, rather than integrate first: see their equation (4). Conceptually this amounts to neglecting the time change of the extinction cross-section, accounting only for the time change of the total number of particles. In effect they have not followed the instructions of their reference (HODGKINSON, 1966): “For a polydisperse suspension naE has to be integrated over the particle-size distribution . . .“. The purpose of this note is to recalculate the turbidity and thereby respond to Fischer’s comments.

Using the same notation as Esmen and Corn, we are to find the time dependence of the turbidity:

K(t) = J

aEln(r, t) dr. (1) 0

We assume that the size distribution can be approximated for the size-range of interest by the time- dependent Junge density function

where they suggest r1 = 1~ and + = 75 h. Based on his observations of the diurnal variation of turbidity, Fischer suggests that the “dry removal” time constant, I, is an order of magnitude smaller.

* N. A. ESMEN and M. CORN (1971) Atmospheric Environment 5, 571-578. t N. A. ENEN and M. CORN (1971) Atmospheric Environment 5, 1059-1060. : W. H. FISCHER (1971) Atmospheric Environment 5, 1059.

763