Atmospheric Environment Pergamon Press 1969. Vol. 3, pp. 221-223. Printed in Great Britain

LIDAR MEASUREMENT OF BACKSCATTER AND ATTENUATION OF

ATMOSPHERIC AEROSOL

(First received 4 October 1968 and in final form 20 November 1968)

Abstract-Vertical pro&s of the volume backscatter and extinction coefficients of the atmos- pheric aerosol have been derived from lidar m~ure~nts. The analysis assumes a ho~on~~y uniform atmosphere, in which case the attenuation between the lidar and a ilxed height is pro- portional to cosec 6, where 8 is the angle of elevation. Observations at two angles of elevation thus su&e to determhre the attentuation and hence the true backscatter coefhcient. Two examples, based on observations at several angles of elevation, are presented to illustrate the technique.

Sc~rraar~~ and extinction ~~rnen~ are well established as a technique for studying the con- centration and size distribution of small particles. With the advent of lidar it has now become possible to make such measurements remotely. In this note lidar backscatter measurements (at wavelength @69 w) in the lowest thousand me&s of the atmosphere are analysed to yield vertical pro&a of the volume lx&scatter and extinction coethcients.

A simple form of the Mar equation [see, for example, C~LLI?J (1966) or B~ruurrr and BEN-DOV (1967)] is

G%= 8. = j?exp(-2 s zudr)

where P = received power (W), G = system sensitivity (W m), 8, = observed or apparent volume backscatter coetbcient (m-l), fi = true volume backscatter coeEcit?nt (m-‘), u = extinction coefE- cient (m-l) and r = range (m). This equation may be used to deduce values of 8. from measurementa of P, if the sensitivity G is known. Attempts have been made to determine this by measuring the signals returned from solid targets of known reflectivity, but this calibration technique has led to values of /I consistently a factor three lower than those found in the literature. The values reported here have therefore been adjusted by this factor.

Since extinction is not in general a unique function of backscatter it is not usually possible to go on and derive /I directly from equation (1). However, in the special case of a horizontally uniform atmos- phere, it is possible to obtain vertical profiles of/l and o from measurements at two or more angles of elevation. If an observation is made at angle 6 in such an atm~ph~, then equation (1) may be re- written

where h is the height (m). Takhrg logarithms,

(3)

If observations are now made at several different angles of elevation and log /I. is plotted against cose~ 0,

the points will fall on a straight line with intercept log fi at cosec 8= 0 and slope - 2 log e s

li &. Vertical

profiles of @ and u may thus be derived. The extent to which the technique can be appli&i in practie depends on the horixontal uniformity of the real atmosphere

Two examples are presented. Both sets of measurements were made at Tilbury. The observation at each angle of elevation extended from a minimum range of 300 m to a maximum of 3 km. On 14 December 1967, the sky was overcast with cloud at 550 m and the visual range was about 3 km in a

221

222 Technical Notes

light NW wind. FIGURE l(a) shows the plots of log 8. against cosec 0 for a number of heights: these appear to justify the assumption of horizontal uniformity. The derived profiles of B and u appear in FIG. l(b). The concentration of particles decreased from the ground up to a height of 250 m and then started increasing again towards the cloud base. The coefficients are roughly proportional, with

(0)

b\ * + \ * x

\\ +

408-n L! v x

+

65m

85m

S\

*\

x

480m kl

125m

250 60m 4 p

I\

800 j-- (b)

,- , IO 20 30 40

cosec 8 m-’

FIG. 1. Observations at 1730 GMT 14 December 1967. (a) Plots of observed volume back- scatter coethcient at various heights against cosec 8. (b) Derived protiles of true volume back-

scatter and extinction coetbcients.

(a)

600

(b)

cosec e In-’

FIG. 2. Observations at 1910 GMT 31 May 1968. (a) Plots of observed volume backscatter coefficient at various heights against cosec 6. (b) Derived profiles of true volume backscatter and

extinction coefficients.

Technical Notes 223

On the second occasion, 31 May 1968, the sky was clear and visibility was 10 km in a light k? breeze. The plots of log 8. against cosec 0 are shown in FIG 2(a). The extinction is much less than on the first occasion, but profIfes of @ and Q have been derived and are shown in Fro. 2(bj. There was a We& defined turbid layer extending &om the ground up to 350 m. Again, 8 and d vary in a similar manner, but with u-2&

Acknowledgement-This paper is published by permission of the Central Electricity Generating Board.

REFERENCES

XQaam-r E. W. and BEN-Dov 0. (1967) Application of the Lidar to Air Polfution M~s~ern~~. J. a&. Metear., 6,500-515.

COLLW R. T. H. (1966) Lidar : a new atmospheric probe, Q. Ji R. met. Sac. 92,220-230.

Central Electricity Research Laboratories, ~~~r~~~ Swz&

P. M. RmT0N